A fifth moon for Pluto!

By Phil Plait | July 11, 2012 10:12 am

Astronomers have just announced that tiny Pluto has a fifth moon! It was discovered using the Hubble Space Telescope:

You can see it in that image (click to enhadesenate) in the green circle. Pluto was targeted by HST for several observations in late June and early July, and P5 – also called S/2012 (134340), the moon’s designation until it gets a proper name – was seen moving around the tiny world. This image is from July 7.

As moons go, it isn’t much: it’s probably only about 10 – 25 kilometers (6 – 15 miles) across, making it one of the smallest moons detected in the entire solar system. That’s actually pretty amazing, given Pluto was 4.7 billion km away (2.8 billion miles) when these images were taken!

Pluto was observed in part to look for more moons. In 2015, the New Horizons probe will zip past Pluto, and scientists want to know as much about the system as they can before it gets there. The odds are low of them hitting any of those moons – space is big, and the moons and spacecraft are small – but a) better safe than sorry, and 2) if there are more targets to observe we want to know now so they can be added to the itinerary!

Observations like this are good for discovering moons and getting their locations, but size is a different matter. Literally. We know how far away the moon is, and how bright, but it’s far too small to directly get the size. Its diameter has to be estimated by assuming how reflective the surface is. If it’s dark like coal, it has to be bigger to be so bright, and if it’s shiny like ice, it’s smaller. That’s why we don’t know P5’s size to even within a factor of 2! But once New Horizons zips past, it may be able to nail down the size far better.

The first moon of Pluto, Charon, was discovered in 1978. Nix and Hydra were found using Hubble in 2006, and the fourth moon just last year, in 2011.

As for the argument about Pluto being a planet or not, this will no doubt provide grist for the mill. However, number of moons does not a planet make; Mercury and Venus have none and they’re planets. Mars has twice as many as Earth does, but it’s not twice the planet! And many very small asteroids have moons, too.

My feelings about this are on record: the word "planet" is not and can not be defined; it’s a concept, not a definition. It’s like the word "continent": it’s more of an idea than something you can rigidly define. There is no sharp border that you can use to divide objects into planet and not planet.

So I actually don’t care if you call Pluto a planet or not. It is what it is: a very cool object, perhaps the biggest in the Kuiper Belt of frozen icy comet-like bodies past Neptune. It’s an oddity, since it’s so bright, and yes, has so many moons.

And it’s absolutely worthy of study, no matter what you call it.

Image credit: NASA, ESA, and M. Showalter (SETI Institute)


Related Posts:

Pluto has another moon!
The unbearable roundness of being
BAFact math: How bright is the Sun from Pluto? (and the followup, How big does the Sun look from Pluto?)
Shining shoes for NASA

Comments (123)

  1. Messier Tidy Upper

    Superluminous (beyond merely brilliant!) news! :-D

    I am jubilant at this latest felictious find. Always love hearing about my favourite planet & its realm has just expanded yet again and become even more wonderful than it already was. Which was very.

    Pluto is a remarkable, astounding little world, small and eccentric as it may be. Pluto will always be a planet to me. :-)

    Five moons now. Five – Charon, Hydra, Nyx, P4 as yet unnamed and now P5.

    Guess the New Horizons team just got busier – and can’t wait until 2015 to see it all close up! ;-)

    *****

    “…Marc Buie can very easily imagine what it must be like to walk around on Pluto: with less than 1% of your weight on Earth because of the low gravity, at temperatures of 230 degrees below zero, in the twilight because the Sun is nothing more than a dazzling star in the black sky, across snowfields of methane ice and transparent crystals of frozen nitrogen and with a gigantic moon hanging overhead – at least if you are on the right side of the planet.”
    – Page 61, ‘The Hunt For Planet X’, Govert Schilling, Copernicus Books, 2009.

    “During its summer, the frozen nitrogen on Pluto evapourates to
    create a temporary atmosphere. With the onset of winter the nitrogen turns to frost and falls back to the surface. On Pluto the winter weather doesn’t merely deteriorate – it completely disappears.”
    – Page 19, ‘The Planets’, McNab & Younger, BBC Worldwide Ltd., 1999.

    “Pluto spins on its axis once in 6.3 Earth days and this means that a Plutonian year has 14,164 Plutonian days.”
    – Sir Patrick Moore, Page 34,“Puzzling Pluto” in ‘Astronomy Now’ magazine, February 2011.

  2. Ori Vandewalle

    Phil, isn’t Eris the biggest KBO?

  3. Mikey

    How precisely do we define “moon” for that matter? Each pebble or boulder zooming around a planet doesn’t count as a moon, or else the rings of Saturn would provide it with billions.

    For that matter, how big is the second largest natural object orbiting the Earth? Is there anything between the Moon and a speck of dust?

  4. andy

    also called S/2012 (134340),

    Surely that should be S/2012 (134340) 1?

  5. The dwarf planets are certainly worth study. There is a lot of weird stuff going with the things they capture in their gravitational pull.

    It’s like if our moon had a natural satellite. I don’t care what you call it, it would be neato.

  6. Messier Tidy Upper

    FWIW. My preferred planet definition is that a planet is an astronomical body that :

    1) Is round or rounded through its own gravity and thus not a comet or asteroid.

    2) Has never been capable of being self-luminous at visible wavelengths due to core nuclear fusion thus not a star or brown dwarf.

    3) Is not directly orbiting another planet and thus is not a moon.

    Three fairly simple, easily & quickly determined criteria.

    As opposed to the IAU’s “orbital clearence” rubbish one which violates Occams razor given it requires enormous clarifications, and the reductio ad absurdum logical test and is immensely unfair on planets like Pluto in larger orbits and overly generous on worlds like Mercury in smaller ones. The, in my opinion utterly ridiculous, IAU definition of planet basically depends far too much on where you orbit and not enough on what an object actually is.

    Orbital clearance also depends on other variable factors such as how much material a given planetary system has to begin with and what stage in its history youobserve it and has many other flaws as well.

    If using my preferred – although not original to me definition – that means we have many more planets in our solar system than we used to think – most of them ice dwarfs like Pluto as opposed to rock dwarfs like Earth and gas giants like Saturn then, hey, what’s really wrong with that?

    @2. Ori Vandewalle : “Phil, isn’t Eris the biggest KBO?”

    Both are actually about the same size diameter~wise, Eris has a tiny smidgin more mass and it would seem is vastly more reflective even than Enceladus.

    I’ll also note that since Eris no ice dwarf worlds larger than or equal to Pluto have been discovered in the Edgeworth-Kuiper Cometary belt region.

    Plus if you run through a checklist of features you’d expect of planets Pluto meets virtually all of them – moons – now 5 of them 3 more than Mars and five more than Venus and mercury, an atmosphere witha nitrogen compositionreminiscentof our own and precipitation even, geologically diffferentiated and possibly active, maybe rings even and more.

    If it looks like a duck and quacks like a duck .. ;-)

  7. Mike

    Hi Phil,

    Could the chaotic workings of Pluto, Charon and theses other bodies have picked up this new body from the Kuiper belt through some chaotic inter (dwarf)planetary transport network? In other words, do you think it been there since we started observing Pluto?

    -Mike

  8. andy

    Phil, isn’t Eris the biggest KBO?

    (136199) Eris is not a Kuiper Belt Object, it belongs to the Scattered Disc.

  9. beanfeast

    I was just pondering the planet definition and I now wonder if Earth would be classified as a planet if it occupied Pluto’s orbit and what is the minimum size a body would have to be, to be classified as a planet in Pluto’s orbit?

  10. Tyler Hatch

    We need some kind of classification system for planets. Something like:

    Class I: All Jupiter-sized planets and larger ones
    Class II: Neptune and Uranus sized planets
    Class III: Anything about the size of Earth
    Class IV: Mercury, Ganymede, Titan
    Class V: Most other moons and Pluto
    Class VI: Anything smaller, including asteroids

  11. Frank

    A distant lump of dirty ice has been discovered next to another distant lump of dirty ice. Truly, this is a momentous day.

  12. At some point, Phil, you’re going to run out of “Click to enlarge.” variants…
    #ButTodayIsNotThatDay
    #HashtagUsedOutsideOfTwitter
    #BadRichard!

  13. Renee Marie Jones

    Planets are like Big Brother.

    Winston to interrogator: “Does Big Brotber even exist?”
    Interrogator: “Of course he exists.”
    Winston: “No, I mean, does he exist like you or me?”
    Interrogator: “You do not exist.”

  14. Johnny

    So we can see distant galaxies in more detail than Pluto. Something doesn’t add up.

  15. Keith Bowden

    As for Pluto, of course it’s a planet. It’s right there in the name: dwarf planet. Dwarf is the adjective, planet the noun, ergo: it’s a planet. =}

  16. Irrespective of what you might call it, Pluto has a fascinating system of moons in tow. And they’re all so tiny, too… Also Pluto now has two moons which still need names. Personally, I’d vote to call them Cerberus and Persephone!

  17. andy

    1) Is round or rounded through its own gravity and thus not a comet or asteroid.

    2) Has never been capable of being self-luminous at visible wavelengths due to core nuclear fusion thus not a star or brown dwarf.

    3) Is not directly orbiting another planet and thus is not a moon.

    Three fairly simple, easily & quickly determined criteria.

    Ok, bearing in mind the various dependencies of the onset of deuterium fusion, please classify the following objects as planets/not-planets according to your rule 2:

    HD 217786b: only known companion of an F8V star at an orbital distance of 2.38 AU. Minimum mass 13.0±0.8 times Jupiter

    HAT-P-13c: second substellar companion in order of distance from a G4-type star in a hierarchical system of 2 planets. Mass at least 14.5±1.0 times Jupiter.

    Upsilon Andromedae c: second substellar companion in order of distance from an F8V star in a system of 4 planets. Mass 14.57 times Jupiter.

    BD+20°2457 b and c: a 2:1 resonant system consisting of objects of at least 12.47 and 21.42 times Jupiter orbiting a 2.8 solar mass red giant star.

  18. Frank

    We’ve known about the Kuiper Belt for years now, and not one KBO has been named “Yuggoth.” I’m going to have to ask the astronomical community to hand in its collective geek badge.

  19. Darren

    @14 johnny “So we can see distant galaxies in more detail than Pluto. Something doesn’t add up.”

    At its closest, it’s about 30AU from sun or 29AU from earth. I don’t know about an average galaxy, but we would have little trouble imaging galaxies at 10Mparsecs. That means the galaxy is about 10^11 times farther away.

    Our galaxy is 100k light years across, while pluto is something around 2500km. That works out to a factor of 10^14 times larger. So on a size/distance calculation, the galaxy is 1000 times larger than pluto.

  20. Dragonchild

    Dear Pluto,

    I’m happy to hear the news that you have yet another member of your family. This is a joyous occasion for all. Yes, yes, it’s a special miracle and a little bundle of joy and all that. But if I may be so bold as to ask a favor, please refrain from making any more of these announcements WHILE I’M STILL DRIVING TO YOUR GODDAMN BABY SHOWER FOR P4.

    In jest,

    The Guy Coding New Horizons’ Observation & Approach Maneuvers
    New Horizons Project Planning Team

    /not really

  21. AliCali

    A lot of people are hung up on the definition of planet. As Dr. Plait pointed out, it’s like defining a continent. The definition is more historical (and leads to hysteria) than anything else.

    The time to hung up on definitions is if classification leads to something useful. For instance, we classify mammals, egg-laying creatures, plants, etc., and that’s useful when figuring out how they evolved and learning about various functions of the organisms. We classify spectra of stars, and once the classification was correct, we got a lot of useful information such as brightness, lifespan, method of death, etc. We’re looking for patterns by grouping items together.

    For the classification of planet vs. dwarf planet or asteroid or whatever, does it lead to anything useful? Will we see patterns because we’ve grouped Mercury through Neptune together sans asteroids, or does it make no real difference? Classifications can change around until we see patterns and get useful information, such as gas giant vs. rocky planet. Since we only have one system of planets that we can study in good detail, it’ll be hard to know how to classify these objects yet.

    At least getting the public worked up about something in astronomy means more public attention to the field and maybe causes more people to look up.

  22. amphiox

    1) Is round or rounded through its own gravity and thus not a comet or asteroid.

    How round is round? How far off spherical does an object need to be before it is not considered round?

    Does Vesta, which is oblong (hence, “rounded”) but fairly far off spherical (hence, not “round”) count?

    What happens to a world flattened by a very rapid rotation rate, like Saturn but more extreme?

    What about a gas giant pulled into an egg shape by tidal forces due to a very close orbit around its star?

    What about two planets on the bottom end of the scale, of equal mass, but different composition, one mostly ices and the other mostly rock, with the icier one rounded by its gravity, but the rocky one, being made of stiffer material, not? Is one a planet then and the other not?

    Everything gets complicated at the edges.

  23. Steve D

    As a geologist, I like the continent analogy. Even classically, the continents were arbitrary. Europe-Asia-Africa are one connected land mass as are the Americas. In plate tectonic terms, a continent is underlain by continental crust and bounded either by oceanic crust or a plate boundary. Greenland, New Zealand and Madagascar are continents and New Guinea is part of Australia, since the intervening sea is underlain by continental crust. Corsica and Sardinia are microcontinents, detached pieces of the Alps, but Sicily is part of Africa. Spain was a continent for much of the last 150 million years but now seems firmly attached to Europe. Iceland is not a continent, but a thick mass of oceanic crust, and Hawaii is a hot spot volcanic land mass. Cuba and the Greater Antilles are accumulated masses of material at former subduction zones. That’s one way continents grow, but they’re not continents. Same with much of Indonesia. Borneo, the Philippines and Japan are now detached pieces of Asia. Continents? Arguably. Technically Baja California and coastal California count, but they’re not often called a microcontinent (microplate, all the time, though.)

    Considering that we have about 800 exoplanets, it should be obvious that the definition of a planet is far from settled. Is an Earth mass object in a young solar system that has not cleared its orbital neighborhood a planet? What about a recently impacted large object that has not yet reached hydrostatic equilibrium?

  24. Peter B

    Regarding definitions of planets, I can see that it’s always going to be tricky to include a mass definition for them, as objects in space presumably exist at every mass from “grain of dust” up to “large enough to be a star”.

    However, I was wondering, why is there no mention of the ecliptic in planetary definitions? The inner eight planets all orbit the Sun within a couple of degrees of each other, while Pluto’s orbit is tilted at about 30 degrees. To me that’s a good enough reasonto separate ‘planets’ from ‘others’.

  25. Peter B

    Frank @ #11 said: “A distant lump of dirty ice has been discovered next to another distant lump of dirty ice. Truly, this is a momentous day.”

    Well, it moved [i]you[/i] enough that you wrote about it. :-)

    Anyway, you’re the first person to mention “momentous”. The Bad Astronomer didn’t. He talked about how “…scientists want to know as much about the system as they can before [New Horizons] gets there…” Would it perhaps be momentous if New Horizons discovers incontrovertible evidence of an alien presence on P5?

    And anyway 2, what’s wrong with celebrating the discovery of another moon of a distant planet (or dwarf planet, or whatever)? Or are you one of these people who thinks all space exploration should be stopped so we can build more hospitals?

  26. Alan D

    The could have been diplomatic, especially given the impossibility of a true definition for “planet,” and simply grandfathered Pluto in as a planet.

    Clear skies, Alan

  27. Dragonchild

    @22. AliCali
    “A lot of people are hung up on the definition of planet. As Dr. Plait pointed out, it’s like defining a continent. The definition is more historical (and leads to hysteria) than anything else.”

    What bugs me more than anything else is the REASON — that otherwise we’d have “too many planets”. Wuh? You mean like when there were only four elements?

    Did we re-classify the rare earths because otherwise there’d be too many elements? Did we re-classify the bones because, wow, if you think 30+ planets are a lot, there are 206 bones in the human body! There are THOUSANDS of species; we ought to re-define life so we don’t have keep track of so many.

    I think for all the excuses, the real issue is that an easy-to-remember fact that people were taught in school has been forever destroyed. If anyone’s getting upset about that, they probably shouldn’t be involved in science.

  28. Grand Lunar

    I like how you put it, regarding the term “planet”, Phil, with it being more of a concept than a definition.

    I’m in league with Neil Tyson, referring to Pluto as “king of the comets” (though I personally prefer it being one of the largest KBOs) rather than “puniest planet”.

  29. Messier Tidy Upper

    @23. amphiox :

    (BTW.Numbering your questions for convenience in reference later, hope that’s okay.)

    I.) How round is round? How far off spherical does an object need to be before it is not considered round?

    Very discernibly to the unaided human eye. If at a glance it looks round or ellipsoidal for fast rotators then it qualifies provided this roundness is due to sufficent mass and thus gravity (Ie. hydrostatic equilibrium) rather than structural co-incidental morphology. (eg. Ida’s moon Dactyl.)

    II) Does Vesta, which is oblong (hence, “rounded”) but fairly far off spherical (hence, not “round”) count?

    Vesta is certainly right on that boarderline between visibly round enough and not quite there. It appears to have internally differentiated geologically and been active and some astronomers are certainly willing to call it a planet – one did in an Astronomy magazien article recently if I recall right. I’d err on the side of being more inclusive than not given the strangeness and diversity of the universe we live in so, yes, I’m prepared to provisionally class Vesta as a planet.

    III)What happens to a world flattened by a very rapid rotation rate, like Saturn but more extreme?

    It counts as a planet as should be clear from the phrasing used. Another analogous example here are stars such as Achernar, Regulus and Altair that are egg-shaped due to their extreme rotation rates but remain being called stars nonetheless!

  30. amphiox

    re #24;

    If one applies the current IAU criteria plus the same logic user for classification of stars, then an earth sized object in a young solar system that has not yet cleared its orbit, but would be able to given time and assuming no unforeseen disruption, should be considered a protoplanet, just as protostars have not yet achieved fusion in their cores, but will given time, and just as earth itself was a protoplanet before it finished clearing its orbit prior to the moon creating impact with Theia, even though it would have already been almost Venus-sized at that point.

  31. amphiox

    re #30;

    Personally I would prefer a lower limit definition set as a specific mass cutoff, using the hydrostatic equilibrium of an idealized planet of uniform composition absent rotation or outside gravitational influence. It’s a little more complicated at the start in coming up with the definition but once that is settled subsequent classification becomes easier.

    After all, for practical purposes that is how we are differentiating brown dwarfs from giant planets – if it is bigger than 14J we’re essentially calling it a brown dwarf without making any attempt to figure out if it ever managed deuterium fusion or not in it’s history.

  32. amphiox

    Another analogous example here are stars such as Achernar, Regulus and Altair that are egg-shaped due to their extreme rotation rates but remain being called stars nonetheless!

    Ah, but hydrostatus equilibrium isn’t part of our definition for stars. If we somehow discovered a star that was square-shaped due to some alien super-tech, it would still be a star by lieu of its fusion, irrespective of its shape.

    The star analog in this scenario would be a case of a star that had previously been fusing happily away but somehow had some external force cause fusion to stop in its core, perhaps the case of say a very small red dwarf star getting some of its mass sucked off by a companion blackhole or neutron star, just enough to drop it below the approximate 80J threshold such that hydrogen fusion stops in its core. Would it still be a star then, or does it get reclassified into a brown dwarf?

    What if instead of a dense companion permanently drawing away its mass, it was impacted by a planet in such a way that some of its mass was blown off temporarily into a ring around it, just enough to drop it below the mass threshold necessary for fusion and stopping fusion in its core, but with the ring eventually falling back down onto the star, putting its mass back over the threshold and letting it start fusing again. Does it stay a star throughout or should it change from star to brown dwarf and back to star, or from star to protostar and back to a main sequence star?

  33. Continued Part II @23. amphiox :

    NB. Link to one source for the flattened stars – ‘The Lion’s pumpkin-shaped heart
    Rapid rotation flattens Regulus”
    by Ken Croswell — Published: January 19, 2005 Astronomy magazine online – in my name for this comment.

    IV)What about a gas giant pulled into an egg shape by tidal forces due to a very close orbit around its star?

    Yes, I would still count that or indeed a rock dwarf or ice dwarf type planet as a planet in those circumstances. A planet that’s natural state has been distorted by external forces but if all things are equal an object would be round, non-luminous by core fusion and no moon then, yeah, planet.

    V) What about two planets on the bottom end of the scale, of equal mass, but different composition, one mostly ices and the other mostly rock, with the icier one rounded by its gravity, but the rocky one, being made of stiffer material, not? Is one a planet then and the other not?

    I would say so, yes, one of those boarderline objects would be a planet and the other would miss out. YMMV natch.

    Although as you’ve know I’m prepared to err on the generous side where there’s doubt such as the case of Vesta.

    Everything gets complicated at the edges.

    At fine enough levels of resolution yes. You are right there. :-)

    There are certainly boaderline cases and perhaps we need to consider reinstating the word ‘Planetoid’ for objects like Vesta that fall into them at the small end.*

    Pluto however is, in my view, very definitely *inside* the planet category and, in fact, a relatively large planet being the largest or equal largest of the ice dwarf category of planet notably larger than the planets Haumea, Sedna, Ceres, etc ..

    ———————————————————–

    * For the large end maybe the term ‘sub-star’ best fits for objects such as Upsilon Andromedae c.

  34. amphiox

    It also occurs to me that, since planet has always been a “I know it when I see it” concept like continents, there actually isn’t really that much scientific merit or utility in trying to pigeon hole a restrictive definition for the entire category of “planet”.

    Far more useful would have been if the IAU spent their time developing definitions for sub-classes of planets that would be useful in exoplanet classification (Categories like Jovians, Ice Giants, Super-Earth’s, Terrestrials, Ice dwarfs, etc)

    A distinction should be made between objects that gravitationally dominate their local space (moons and Lagrange-point companions would therefore not affect the definition) and objects that do not and are just a part of a much larger swarm of material, but such a distinction can easily be made among sub-classes, and all of them can still be planets.

  35. amphiox

    re #34;

    Wow. Achernar looks arguable even more oblong than Vesta!

    It’s kind of surprising that a star could spin itself that fast to flatten itself that much and not tear itself apart….

  36. @ ^ amphiox : Yep, sure is! That’s one of my favourite astronomical illustrations & facts. It gets mind bending trying to imagine such non-spherical stars! :-)

    From similar astronomical artworks based on sound science I gather that, among other examples, Sheliak (Beta Lyrae – Kaler link in name here) is similarly distorted although in its case times two due to close gravitational orbit of two B-type stars like your exmaple /question IV. Spica and many others may be similar although wherever they ever reach Achernarean extremes of flatness I don’t know. Mira and some red giants are also distinctly misshapen which again, strikes me as rather wonderful thing to try and visualise. 8)

    @Part II – continued @23. amphiox :

    A couple of other key things to note about my preferred definition of planet – it doesn’t ignore exoplanets or demand a planet needs ato orbit a star – actually very specifically only our Sun – as the current IAU definition does and thus includes the many “rogue” / “orphan” / “ejected” exoplanets that may NOT orbit stars at all but be free-floating in space.

    The way the IAU definition rules out exoplanets completely quite literally by definition and its being so pre-Copernican, principle of Medicority violating always has me facepalming at just how ridiculous their definition is. :-o

    The universe being so diverse and bizarre and surprising; a definition that allows most room for flexibility and inclusion especially at the equivalent of the animal-mineral-vegetable stage clearly is to be preferred surely?!

  37. puppygod

    Is Charon even a Pluto’s moon? I mean, shouldn’t they be considered rather a binary planets system? It’s not so much that Charon orbits Pluto, as both of them orbit around the common center of mass – which is outside of Pluto. As far as I know, it’s the one and only such case in the whole solar system. Even Earth-Moon system (second largest in solar system when it comes to planet-moon ratio) has common center of mass located under the surface of Earth.

    So, instead of pushing for calling the Pluto planet, maybe push for calling Pluto-Charon binary planets? That’s way more awesome.

  38. Nigel Depledge

    Ori Vandewalle (2) said:

    Phil, isn’t Eris the biggest KBO?

    IIUC, Eris is just a smidge smaller than Pluto, although it was initially thought to be larger than Pluto.

  39. ctj

    putting the whole “planet” debate aside, is there any coherent theory as to how pluto (or the pluto/charon system) is acquiring all of these moons? i know the kuyper belt is theorized to contain billions of comets, but at that distance, i believe it’s more sparsely populated than the inner solar system.

    so how is pluto, with such a tiny mass, capturing all of these objects? alternatively, they seem to lie in the same orbital plane, suggesting they formed together (and pluto would seem to have insufficient mass to force them into equatorial orbits by tidal action) – so how did a system with so many moons yet with such low mass form in such an odd orbit and so far from the sun? and if pluto’s orbit was caused by an interaction with neptune or some other giant, how did the system’s solar orbit get so perturbed without disrupting the system?

    i can’t wait for new horizons to get there. pluto is just so weird and fascinating!

  40. Nigel Depledge

    MTU (6) said:

    FWIW.

    Oh, dear.

    I must confess I was delighted to read your post #1, because you left aside the defintion of planet altogether.

    But you couldn’t let it lie, could you?

    My preferred planet definition is that a planet is an astronomical body that :

    1) Is round or rounded through its own gravity and thus not a comet or asteroid.

    How round is round? The Earth is not a perfect sphere, and neither is it a perfectly oblate spheroid. It has mountains, therefore it’s not perfectly round. Mars has even bigger mountains, so it’s even less perfectly round. Where do you draw the line? Well, obviously, at some arbitrary point.

    I raise this objection here because it is logically identical to your objection to the gravitational clearance criterion. It is hypocritical to accept the gravitational roundness criterion while simultaneously rejecting the gravitational clearance criterion.

    A line must be drawn somewhere, and that line is drawn at some aribtrarily-defined point. Certainly this is the case for hydrostatic equilibrium, because our solar system contains objects of many different sizes.

    The gravitational clearance criterion, however, actually recognises a naturally-existing demarcation among solar-system objects. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Ouranos and Neptune are self-evidently a different class of objects from Pluto, Eris, Ceres and so on.

    2) Has never been capable of being self-luminous at visible wavelengths due to core nuclear fusion thus not a star or brown dwarf.

    3) Is not directly orbiting another planet and thus is not a moon.

    I agree with these two. Mostly.

    Three fairly simple, easily & quickly determined criteria.

    Poppycock.

    Your first criterion is a minefield of arbitrary delineations. How sure are we that Ceres, for example, really is round? I think we are pretty sure about that, since it is by far the largest main-belt object. How about some of the larger KBOs, like Eris, Haumea and Makemake? How sure are we that these are gravitationally round? Or at least, round enough to meet your criterion?

    In the IAU definition, it doesn’t really matter – they’re KBOs and some of them might also count as dwarf planets. In your definition, whether they are planets or not would hinge on detailed knowledge of the shape of these objects. Knowledge that we don’t currently have. So, what use is your definition? Your first criterion certainly isn’t simple, and it certainly isn’t easily-determined.

    As opposed to the IAU’s “orbital clearence” rubbish one which violates Occams razor given it requires enormous clarifications, and the reductio ad absurdum logical test and is immensely unfair on planets like Pluto in larger orbits and overly generous on worlds like Mercury in smaller ones.

    First off, the gravitaionl clearance criterion no more violates Occam’s razor than does the gravitational roundness criterion, which you unquestioningly accept. IOW, they both violate it to some extent (and it may be argued that the gravitaional roundness criterion violates it more, although I’d prefer not to go there).

    Second, what has fairness got to do with whether an object counts as a planet or not. Are you afraid to hurt its feelings? Or do you believe that Pluto has a stake in what we puny humans call it? No, this is animism of the worst kind. Fairness is irrelevant. Usefulness matters. Any definition that counts Pluto in the same class of objects as Earth and Jupiter is not very useful.

    The, in my opinion utterly ridiculous, IAU definition of planet basically depends far too much on where you orbit and not enough on what an object actually is.

    And, as I have pointed out previously, the context in which an object orbits matters. This argument is empty, and your attempt to fill that void with rhetoric only highlights the hollowness of the argument.

    Orbital clearance also depends on other variable factors such as how much material a given planetary system has to begin with and what stage in its history youobserve it and has many other flaws as well.

    No, because the IAU definition applies solely and specifically to our solar system. For a very good reason : ours is the only system for which we have adequate knowledge to be able to apply a definition.

    If using my preferred – although not original to me definition – that means we have many more planets in our solar system than we used to think – most of them ice dwarfs like Pluto as opposed to rock dwarfs like Earth and gas giants like Saturn then, hey, what’s really wrong with that?

    In and of itself, nothing. Apart from the fact that you end up with a host of KBOs whose categorisation remains unknown until we have more mission to the Kuiper Belt to take the relevant measurements.

    You have succeeded only in persuading me that the IAU is onto something by the poor logic and emotional appeals in your arguments against the IAU definition.

  41. Messier Tidy Upper

    @30.MTU :

    Vesta is certainly right on that boarderline between visibly round enough and not quite there. It appears to have internally differentiated geologically and been active and some astronomers are certainly willing to call it a planet – one did in an Astronomy magazien article recently if I recall right.

    Aha! Found it – Jim Bell, professor of Astronomy and Planetary Science, Universityof Arizona :

    “Many planetary scientists refer to Vesta as the “smallest terrrestrial planet” despite the fact that the International Astronomical Union has not officially designated it as even a dwarf planet.”

    Source : Page 32, “Dawn’s Early Light : A Vesta Fiesta” article by Jim Bell in ‘Australian Sky & Telescope’ Feb/ March 2012.

    This is echoed by Keith Cooper who notes :

    Some people have described Vesta as the smallest terrestrial planet and what theymean by that is it has a core,mantle and crust like Mercury,Venus, Earth and Mars.”

    Source : Page326, “Visiting Vesta” article by Keith Cooper in ‘Astronomy Now’magazine, July 2011.

    I’ll also add that Isaac Asimov has refered to Ceres as a planet as well notably here :

    “… he had left out a planet. It was not his fault; everyone leaves it out. I leave it out myself when I list the nine planets, because it is the four-and-a-halfth planet. I’m referring to Ceres; a small but respectable world that doesn’t deserve the neglect it receives.”
    – Page 63, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by
    Isaac Asimov, Mercury Press, 1973.

    Looking at recent illustrations and maps it seems that certainly from some angles Vesta appears very much spehrical although somewhat less so from others.

  42. Messier Tidy Upper

    @41. Nigel Depledge :

    You have succeeded only in persuading me that the IAU is onto something by the poor logic and emotional appeals in your arguments against the IAU definition.

    Well that’s a shame and also erroneous on your part not mine. How is my logic “poor” and my arguments merely “emotional” exactly? Clearly we’re not going to agree on this question and we have argued it at length before on other threads here but I really do not understand how you can draw such contrary conclusions from what I’ve said on this. It baffles and frustrates me that you can seriously say this after my detailed, reasonable, in depth logical deconstruction of your arguments for the IAU’s absurd definition.

    How round is round? The Earth is not a perfect sphere, and neither is it a perfectly oblate spheroid. It has mountains, therefore it’s not perfectly round. Mars has even bigger mountains, so it’s even less perfectly round. Where do you draw the line? Well, obviously, at some arbitrary point.

    If you see a cricket ball and it doesn’t appear round to you then you have a problem. Same here. If a planet is visibly spherical to the human eye or an approximation of that due to its own gravity then its round enough to qualify. No roundness isn’t perfect, yes, there are mountains and seas and stuff like that but we all know what is meant by round and even the IAU adopted that as part of their definition too. If you want to get mega-technical and call it being in ‘hydrostatic equilibrium’ then okay but it means the same basic thing.

    It is hypocritical to accept the gravitational roundness criterion while simultaneously rejecting the gravitational clearance criterion.

    No it isn’t . Nowhere close. Being round is an intrinsic easily determined quality whereas “orbital clearance” is much harder to define and determine and depends on a lot more extrinsic variable factors such as

    A – How far away a planet orbits,
    B – How old the age of the planet or system is.
    C – How much material was there to start off with.
    D – How clear is clear and what about NEOs, sungrazing comets, trojans, etc ..
    E – The influence of other nearby Planets and planetary histories as far as pertubations etc goes. Say for instance a wandering exoplanet strays into our solar system from interstellar space – does every planet whose orbit it crosses from Jupiter to Earth – automatically cease to be a planet thereby?
    This all seems so immensely obvious that I am amazed you, an otherwise intelligent person,actually put that comparison of these two criteria forward with a straight face – or are you just winding me up? :-(

  43. Oh & to that alphabetically ordered list add the probability of double planets existing – scaled up versions of our Earth-Moon & Pluto-Charon systems which are sometimes called “double planets” themselves.

    Remember that we’re discovering exoplanets in surprisingly close (& getting closer) proximity to each other all the time. See for instance :

    http://www.abc.net.au/science/articles/2012/06/22/3531203.htm

    for an astoundingly close Super-Venus and Hot Neptune duo around Kepler 36.

    Plus :

    http://www.space.com/8840-alien-planets-gather-close-dying-star.html

    For HD 200964 and 24 Sextantis – separate cases btw.

    & my favourite example :

    http://kencroswell.com/HD45364.html

    HD 45364 b & c, exo-Saturns orbiting an orange dwarf star in a way analogous to Neptune and Pluto ‘s orbital relationship.

    Oh & before ruling that planetary collisions are just hypothetical try and recall where our planets’ unusually large Moon came from! ;-)

    (Additionally too, remember that we also have observed evidence of excess dust created by planetary collisions around other stars fro just one instance in the case of the white dwarf NLTT 43806 – linked to my name here.)

  44. Messier Tidy Upper

    Part II @41. Nigel Depledge :

    Your first criterion is a minefield of arbitrary delineations. How sure are we that Ceres, for example, really is round? I think we are pretty sure about that, since it is by far the largest main-belt object. How about some of the larger KBOs, like Eris, Haumea and Makemake? How sure are we that these are gravitationally round? Or at least, round enough to meet your criterion?

    Because we can both observe and calculate their morphologies. I’ve already answered this in #43 btw so you are repeating yourself, Nigel.

    Yeah, we can tell whether something is round or not and if we’re not sure well we put a question mark over it then go check using our spacecraft and telescopes and our science. Pluto is round (or hydrostatic equilibrium) as is Ceres and Eris. How sure are we? Very.

    If, Mr Depledge, you are – seemingly desperately and emotionally – disputing this known roundness of Ceres and Pluto’s and Makemake, etc .. which is based on repeated calculations and observations and accepted by the IAU hence them qualifying for their current status, then its up to you to show cause why they’re not round. Plus to explain how the HST among other key scientific instruments and observatories got it wrong and back up your extraordinary claim.

    In your definition, whether they are planets or not would hinge on detailed knowledge of the shape of these objects. Knowledge that we don’t currently have.

    Not that detailed actually and yes we do. Look at images of Ceres and Pluto and the others. Again, are you really *seriously* going to argue that these worlds aren’t in hydrostatic equilibrium – round enough – when even the IAU accepts they are?

    First off, the gravitaionl clearance criterion no more violates Occam’s razor than does the gravitational roundness criterion, which you unquestioningly accept. IOW, they both violate it to some extent (and it may be argued that the gravitaional roundness criterion violates it more, although I’d prefer not to go there).

    Really? really? :roll:

    See my response at # 43 which says it all. Orbital clearance is violating Occams law and raises a whole slew of unnecessary superflous questions demanding problematic and arbitrary answers that roundness – a visble immediate intrinsic trait does not.

  45. Nigel Depledge

    Beanfeast (9) said:

    I was just pondering the planet definition and I now wonder if Earth would be classified as a planet if it occupied Pluto’s orbit and what is the minimum size a body would have to be, to be classified as a planet in Pluto’s orbit?

    If an Earth-sized object occupied the orbit of Pluto, it probably would not meet the gravitational clearance criterion. I have no idea what mass of object would be the minimum to meet the criterion, but Neptune certainly would – as it is, Neptune’s gravity has a strong influence on Pluto’s orbit. Neptune’s mass is pretty close to 10^26 kg.

    However, this does not stop anyone from calling Pluto a planet in casual conversation. The IAU definition only applies to the technical work of professional astronomers.

  46. Messier Tidy Upper

    Continued – Part III @41. Nigel Depledge :

    Second, what has fairness got to do with whether an object counts as a planet or not.

    Fairness here = consistency and making reasonable, logical divisions not unfiar IOW inconsistent and illogical ones. It is unfair –inconsistent and unreasonable – to say that planet orbiting at 1 AU becomes a non-planet if simply if it orbits at 30 AU or 100 AU. It is unfair – inconsistent and illogical – to say a planet ceases to become a planet if another object wanders across its path. Do you understand what is meant by “fair” there now?

    Are you afraid to hurt its feelings? Or do you believe that Pluto has a stake in what we puny humans call it?

    No. Neither of those.

    Pluto is an inaminate lump of rock and ice with certain physical and chemical and orbital properties. Of course – and equally, of course, people are human.

    I care about astronomy and astronomers and I think this debacle has hurt us by making astronomers look ridiculous – because it was a ridiculous decision. :-(

    Astronomers and others defending a ridiculous decision and saying it wasn’tas absurd as it clearly was only makes astronomers look ever sillier. Correcting it and admitting the IAU got it wrong would help and would be in the best interests of astronomy and its relations with the public. :-(

    Thinking of emotions about pluto though I could and, heck, will pose the opposite question to you : Why are you so keen to bash Pluto and its supporters? Are you and is your side in this being objective or are you perhaps more influenced by your emotions than you care to admit?

    No, this is animism of the worst kind.

    What the .. ? Animism? What’s animism got to do with anything? :roll:

    Where do you pull that non-sequiteur from and what do you have to back that extraordinary claim up with?

    Fairness is irrelevant. Usefulness matters. Any definition that counts Pluto in the same class of objects as Earth and Jupiter is not very useful.

    What basis do you have for saying that? Saying Pluto, Earth and Jupiter are planets is equally as useful as saying Barnards Star, our Sun and Eta Carinae are stars and Halley’s , McNaughts and Shoemaker-Levy-9 are all comets. Its a starting point in the fundamental astronomical classifications which we can then move on to finer distinctions in. Jupiter and Pluto are different types of planets just as Barnard’s Star is a different typoe of star from Eta Carinae and Shoemaker-levy9 is -or rather was – a different sort of comet to Comet McNaught. But the star-planet-comet division is a useful starting point.

    How is it more useful to misclassify Pluto as an asteroid or comet which it is, in fact, very much NOT?

  47. Nigel Depledge

    Johnny (14) said:

    So we can see distant galaxies in more detail than Pluto. Something doesn’t add up.

    Well, if a distant galaxy is 100,000,000 times farther away, but 100,000,000,000 times larger than Pluto, we would expect to be able to see more of its structure.

    Let’s see . . .

    Pluto’s diameter is 2274 km (from nineplanets.org).

    A light-year is about 10^12 km, or about 440,000,000 Pluto diameters.

    Our galaxy is about 100,000 light-years across, or 44 trillion times the size of Pluto.

    So, most galaxies will be about 10^13 – 10^14 Pluto diameters in size.

    Pluto’s average distance from the Sun is roughly 5.9 billion km (again from nineplanets.org).

    The nearest large galaxy (the great spiral of Andromeda) is about 2,000,000 light years away, or about 339 million times as far away from the sun as Pluto’s average distance.

    Most other galaxies, therefore, will be about 10^9 – 10^12 times as far away from us as Pluto is. So even a relatively small galaxy (10^13 Pluto diameters) at a very great distance (10^12 Pluto orbital radii) will be an easier target in which to resolve detail telescopically than is Pluto itself.

    What doesn’t add up?

  48. Messier Tidy Upper

    @41. Nigel Depledge :

    “The, in my opinion utterly ridiculous, IAU definition of planet basically depends far too much on where you orbit and not enough on what an object actually is.”- MTU
    And, as I have pointed out previously, the context in which an object orbits matters.

    If you accept the IAU definition which for reasons I’ve already explained I don’t.

    A planet is a planet whether it orbits at 1 AU or 100 AU or whether it lacks a star at all. I don’t see why – other than being a moon of a larger body – orbital distance and location in space should define a planet. I don’t think it is reasonable to call Mercury a planet at less than 1 AU but not at 30 AU. Same applies for Earth and the same applies for Pluto.

    I don’t think it is fair to say that if Venus crossed Earth’s orbit for a time then during that time neither of those planets – Earth or Venus – would actually be planets. I don’t think that make sense. Ditto if (or *when* by many models!) Saturn crosses Neptune’s orbit or Pluto crossed Mars’ one.

    I don’t think it is consistent to define a dwarf star as a proper star but a dwarf planet as a non-planet.

    I think all those things are unreasonable and therefore the IAU definition which does those things or leads to those conclusions is unreasonable and thus needs to be rejected.

    I don’t understand why you seem to have so much trouble following and accepting this logic. And the above is definitely logic not mere “hollow rhetoric” as you insultingly and wrongly describe it.

  49. Shepard

    That’s no moon! It’s the Charon Mass Relay waiting to be discovered…

  50. #38 Puppygod:
    You’re correct; Pluto and Charon are indeed the only pair of bodies in the Solar System, whose barycentre lies in empty space. So whether you choose to call Pluto a planet or a dwarf planet, it and Charon should really be called a binary planet/dwarf planet, rather than a planet and satellite.

  51. Nigel Depledge

    MTU (43) said:

    Well that’s a shame and also erroneous on your part not mine.

    You have claimed this before, but repeatedly failed to show it to be so.

    How is my logic “poor”

    I will come to this.

    and my arguments merely “emotional” exactly?

    That you say it is unfair to exclude Pluto while including Mercury is an appeal to emotion.

    The fact that your arguments are littered with rhetoric – such as the frequent derogatory adjectives you use in reference to the IAU definition – is a debating trick to frame the debate emotionally rather than objectively and it has no place in science.

    Clearly we’re not going to agree on this question and we have argued it at length before on other threads here but I really do not understand how you can draw such contrary conclusions from what I’ve said on this.

    When the IAU announced their definition, I really didn’t much care one way or the other.

    Your frequent IAU-bashing tirades on the topic made me pay attention.

    As I read and began to understand some of the points used to counter your arguments, I began to notice that you had no substantive answer to the counter-arguments and your preferred definition is no better logically than the one that you so passionately criticise.

    It baffles and frustrates me that you can seriously say this after my detailed, reasonable, in depth logical deconstruction of your arguments for the IAU’s absurd definition.

    Here you go again. You call the IAU’s definition absurd but you have not shown this.

    You have often used hypothetical scenarios to construct absurdly-defined planets and not-planets, but you ignore the fact that the IAU’s definition applies solely and specifically to our solar system, rendering these scenarios irrelevant.

    I have deconstructed all the points you have raised in detail, and not one of your points withstands critical scrutiny. If you were to raise an argument that I could not refute, then I would seriously consider that your position on this topic might be the stronger one. As it is, the very flaws in your arguments have turned me to supporting the IAU.

    You have attempted to answer the points I make, but not one of your answers has the power to convince.

    How round is round? The Earth is not a perfect sphere, and neither is it a perfectly oblate spheroid. It has mountains, therefore it’s not perfectly round. Mars has even bigger mountains, so it’s even less perfectly round. Where do you draw the line? Well, obviously, at some arbitrary point.

    If you see a cricket ball and it doesn’t appear round to you then you have a problem.

    Irrelevant. Cricket balls don’t come in the same wide array of sizes and shapes as planetary bodies. For the bodies of the solar system, there will be a grey area in which bodies are of a size to be almost-round, and there must be an arbitrary boundary. That this boundary must occur at different masses (and diameters) for rocky and icy bodies does not help your case.

    Same here. If a planet is visibly spherical to the human eye or an approximation of that due to its own gravity then its round enough to qualify. No roundness isn’t perfect, yes, there are mountains and seas and stuff like that but we all know what is meant by round and even the IAU adopted that as part of their definition too. If you want to get mega-technical and call it being in ‘hydrostatic equilibrium’ then okay but it means the same basic thing.

    I have heard the term, but bodies with dynamic surfaces (such as Earth and Io) may or may not actually be at hydrostatic equilibrium.

    You seem to be addressing the detail while bypassing the point. The point is that there has to be a dividing line between round and not-round. Because of the range of sizes of objects in the solar system, this dividing line does not have a natural position, but must be arbitrary. So, for example, Earth and Earth’s moon are both close enough to count as round. Phobos and Vesta (for example) are clearly not round. But there are objects between these sizes, and it seems reasonable to expect that there are asteroids and KBOs still to be discovered (or discovered but still to be fully characterised) that indeed fall into this grey area.

    Thus, the dividing line between round and not round is arbitrary.

    You have objected to the gravitational clearance criterion because it applies an arbitrary division between clear and not clear. Or, to take a more pragmatic approach to the same criterion, between an orbital region that is gravitationally dominanted by one body and an orbital region that is not.

    Ceres, for instance, is round (or pretty close to round), but it does not dominate the region in which it orbits, whereas Earth, Jupiter, Saturn and even Mercury do gravitationally dominate the respective regions in which they orbit. You have raised trivial objections regarding Jupiter’s Trojans, but their orbit is quite obviously dictated by Jupiter’s gravity, similar to the way in which the orbits of Phobos and Deimos are dictated by Mars’s gravity.

    The orbits are not perfectly clear, in the same way that Earth is not perfectly round. The imperfections on Earth’s surface are several orders of magnitude smaller than the size of Earth itself, and the same applies to Mars, Venus and Mercury. Similarly, the masses of bodies that share the orbital region of each of the eight planets is several orders of magnitude smaller than the mass of the dominant body (in fact, Earth’s moon comes closest to being of comparable mass to its dominant body, and is still more than 2 orders of magnitude less massive than Earth – and the moon’s orbit is quite clearly dictated by Earth’s gravity).

    However, there are sure to be bodies in the solar system for which the deviations from perfect roundness (or from being a perfect oblate spheroid) are perhaps only 1 order of magnitude less than the diameter of the body. In other words, there almost certainly exists a continuum of objects that transitions the range from round to not-round. Whereas for orbits that are clear versus not-clear there is an obvious demarcation. Earth and its moon are on one side of a gap (an orbit where Earth dominates everything that orbits within that region but where one of those other bodies is of substantial mass, being just under 1% of Earth’s own mass), while objects such as Pluto and Ceres are on the other side of the gap (objects that do not dominate the other bodies that orbit within that region). And the gap is a perfectly natural division within our solar system.

    Since the two division have approximately the same basis (on the one hand, an arbitrary distinction between round enough and not round enough; and, on the other, an arbitrary distinction between gravitationally dominated and not gravitationally dominated, albeit informed by a natural demarcation that could be mere chance), any objection to one criterion on the basis of its logic is perforce an objection to the other criterion too.

    It is hypocritical to accept the gravitational roundness criterion while simultaneously rejecting the gravitational clearance criterion.

    No it isn’t . Nowhere close. Being round is an intrinsic easily determined quality

    So you claim, but this is not so.

    For example, how many KBOs are in hydrostatic equilibrium? The answer is we have no idea. Probably several of the known KBOs are indeed round enough to meet our criteria. You mentioned earlier that if an object looks round to the eye, that is good enough, but this ignores the three-dimensional nature of solar-system bodies when compared to our often two-dimensional views of them. Some of those distant bodies may be round from one viewing angle, but not round from another. We just do not know. So your claim that this is an easily-determined criterion is false.

    whereas “orbital clearance” is much harder to define and determine and depends on a lot more extrinsic variable factors such as

    A – How far away a planet orbits,
    B – How old the age of the planet or system is.
    C – How much material was there to start off with.
    D – How clear is clear and what about NEOs, sungrazing comets, trojans, etc ..
    E – The influence of other nearby Planets and planetary histories as far as pertubations etc goes. Say for instance a wandering exoplanet strays into our solar system from interstellar space – does every planet whose orbit it crosses from Jupiter to Earth – automatically cease to be a planet thereby?

    But this is stuff we pretty much already know. We know what other large objects orbit in the same regions as the eight planets, therefore we have the information to make this distinction.

    So this is easier to determine than your roundness criterion. We already know that, irrespective of how large an as-yet-to-be-dioscovered KBO is, it does not gravitationally dominate the region in which it orbits.

    Comets, NEOs and Trojans are subsumed in the pragmatic application of the clearance criterion, being mostly pretty small objects, in the same way that we are able to dismiss small deviations from roundness in the gravitational roundness criterion. “How clear is clear” is a direct parallel to “how round is round”, from the point of view of their logical basis.

    For example, IIRC, Jupiter’s trojans are collectively only about one ten-thousandth the mass of Jupiter. Whereas Pluto shares its orbital region with several other objects that are each substantial fractions of Pluto’s own mass. Sure, Pluto has moons whose orbits are dictated by Pluto’s gravity, but that’s only a tiny portion of Pluto’s orbital region. As a parallel situation, we are able to dismiss Mars’s mountains as deviations from roundness because they are so small in relation to the planet’s diameter, whereas the lumps and bumps and protrusions on the surface of Phobos are so large relative to the object itself that the object is not round.

    Regarding a wandering exoplanet – well, such an object is likely, if large enough, to disrupt existing orbits and throw the whole solar system into chaos, but if such an event were to occur, I daresay that those IAU members who wish to could vote to redefine the term “planet” as applicable. The main point about the definition is that it is useful now.

    This all seems so immensely obvious that I am amazed you, an otherwise intelligent person,actually put that comparison of these two criteria forward with a straight face – or are you just winding me up?

    Right back atcha with this question.

    How come the obvious and natural demarcation in orbital context between, on the one hand, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Ouranos and Neptune and, on the other, Pluto, Eris, Ceres etc.escapes you as relevant to defining a term for technical discussions among professional astronomers?

  52. Nigel Depledge

    MTU (45) said:

    See my response at # 43 which says it all. Orbital clearance is violating Occams law and raises a whole slew of unnecessary superflous questions demanding problematic and arbitrary answers that roundness – a visble immediate intrinsic trait does not.

    No. Just, no.

    Roundness is not obvious. Our solar system is three-dimensional and our views of solar-system bodies are too often only two-dimensional. See my long post above (currently #51) for more detail here.

  53. Nigel Depledge

    MTU (45) said:

    If, Mr Depledge, you are – seemingly desperately and emotionally – disputing this known roundness of Ceres and Pluto’s and Makemake, etc .. which is based on repeated calculations and observations and accepted by the IAU hence them qualifying for their current status, then its up to you to show cause why they’re not round.

    I’m not disputiong this for any specific individual body – but there’s a whole bunch of bodies in the Kuiper Belt that have either not been discovered or have been discovered and not well characterised.

    The point here being that, under your definition, we don’t know whether these are planets or not until we point a whopping big telescope at them or until we visit them. Whereas under the IAU definition we already know.

  54. Nigel Depledge

    MTU (47) said:

    Fairness here = consistency and making reasonable, logical divisions not unfiar IOW inconsistent and illogical ones. It is unfair –inconsistent and unreasonable – to say that planet orbiting at 1 AU becomes a non-planet if simply if it orbits at 30 AU or 100 AU. It is unfair – inconsistent and illogical – to say a planet ceases to become a planet if another object wanders across its path. Do you understand what is meant by “fair” there now?

    Really? So you are redefining the word “fair” as “consistent and logical” rather than (for example) as equitable and even-handed.

    And here you go again with the hypotheticals.

    When you have developed the technology to shift Mercury from its present orbit to one 39.5 AUs out, let me know. Until then, keep your imaginary scenarios out of the discussion.

  55. Messier Tidy Upper

    Finally Part V @41.Nigel Depledge :

    No,because the IAU definition applies solely and specifically to our solar system.

    Which is ridiculous. We know thousands – at least and likely a nigh uncountable number of planets exist. Almost all orbiting other stars than our own. To try and hand-wave that reality away or ignore it seems just nuts.

    Yes, of course we’re going to keep studying and learning more about exoplanets but these studies aren’t going to make *all* these new planets disappear or turn out to be non-existent even if very occassionally the odd one such as Gliese 581 g will turn out to be illusory!

    For a very good reason : ours is the only system for which we have adequate knowledge to be able to apply a definition.

    I don’t think that reason is either good or true. We’re learning more all the time and we certainly enough to adequately say these bodies are other planets which is, well the whole point really.

    you end up with a host of KBOs whose categorisation remains unknown until we have more mission to the [Edgeworth-ed.]Kuiper Belt to take the relevant measurements.

    …And? Even assuming that’s true – and I don’t think it is because we can calculate fairly well from the observed data whether an Edgeworth-Kuiper Cometary belt object is likely to qualify for planethood or not – so what? It provides us with an extra incentive to go out and study these things and that ain’t bad.

    Here’s a thought too. From our inner solar system the outer solar system – the realm of the ice dwarfs like Pluto may seem a crowded zone with a lot of similar worlds orbiting together but from the perspective of the ice dwarf realm, the inner planets are in a similar position. From the Cometary disk and Oort cloud, it is the *inner* solar system that seem to have “uncleared” close together orbits, all these similar worlds in populations of gas and rock jammed in tight against the daytime star! ;-)

    Clearly, Nigel, we’re each seeing this issue from very different perspectives. It might be an idea for you to consider looking at things a bit differently too!

    @38. puppygod :

    “So, instead of pushing for calling the Pluto planet, maybe push for calling Pluto-Charon binary planets? That’s way more awesome.

    Agreed and this was one of the possible results of an earlier superior draft IAU planet definition which would have made both Charon and Pluto planets along with Eris and Ceres.

    @17. andy :

    Ok, bearing in mind the various dependencies of the onset of deuterium fusion, please classify the following objects as planets/not-planets according to your rule 2:
    [HD 217786b, HAT-P-13c, Upsilon Andromedae c, etc ..]

    Good question, I’ll consider it and get back to you on that later.

  56. @54. Nigel Depledge :

    And here you go again with the hypotheticals. …keep your imaginary scenarios out of the discussion.

    You know what were hypothetical and imaginary once?

    The neutrino, Einsteins theory of relativity, the Higgs boson, landing on the Moon and planets around other stars.

    Thought experiments and ideas Nigel. These do kinda have a place in science, y’know? :roll:

    When you have developed the technology to shift Mercury from its present orbit to one 39.5 AUs out, let me know.

    It wouldn’t necessarily take technology on or any action on part. Future gravitational interactions may well end up shifting Mercury’s orbit very dramatically!

    See the link in my name which notes one no, make that two studies predicting a possibility that Mercury could be ejected from our solar system into interstellar space – or impact with Earth or Venus – and that Mars too could one day be ejected as well.

    Two new computer simulations of long-term planetary motion–one by Jacques Laskar (Paris Observatory), the other by Konstantin Batygin and Gregory Laughlin (University of California, Santa Cruz)–have both reached the same disturbing conclusion.
    – Linked article “Will Mercury Hit Earth Someday?” by Ken Croswell, published on SkyandTelescope(dot)com, 24th April 2008.

    Low odds maybe but not impossible.

    BTW. Pretty sure I’ve provided that link for you before backing up similar claims inpast discussions on this. Did you miss it? Forgotten it? Or .. ?

    Then too, there’s the possibility a world very similar to Mercury in the relevant respects mass and size may well be discovered in the Edgeworth-KuiperCometary belt or Oort cloud one day.

    So you are redefining the word “fair” as “consistent and logical” rather than (for example) as equitable and even-handed.

    Could be mistaken but I’m fairly ( ;-) ) sure that the word ‘fair’ does already actually mean or imply all those things! Yes, I speak english. :-P

  57. KC

    I’ve never understood the poor criteria of hydrostatic equilibrium. No one seems to have a good answer to the question “How round is round?”

    The Earth itself is not perfectly round – most planets are not – they are oblate spheroids. Earth has a flattening of 1/300, Jupiter is more elongated and is 1/15.

    So where is the dividing line? If the Dawn spacecraft finds Ceres has a flattening of 1/14 its not a planet but if its 1/15 yes it is??

  58. TheBlackCat

    Which is ridiculous. We know thousands – at least and likely a nigh uncountable number of planets exist. Almost all orbiting other stars than our own. To try and hand-wave that reality away or ignore it seems just nuts.

    Yes, of course we’re going to keep studying and learning more about exoplanets but these studies aren’t going to make *all* these new planets disappear or turn out to be non-existent even if very occassionally the odd one such as Gliese 581 g will turn out to be illusory!

    Amongst these thousands of planets, do we know of any that don’t conform to the IAU definition? If not than this is irrelevant.

    See the link in my name which notes one no, make that two studies predicting a possibility that Mercury could be ejected from our solar system into interstellar space – or impact with Earth or Venus – and that Mars too could one day be ejected as well.

    Neither ejecting a planet from the solar system nor smashing it into another planet in any way contradict the IAU definition.

    Then too, there’s the possibility a world very similar to Mercury in the relevant respects mass and size may well be discovered in the Edgeworth-KuiperCometary belt or Oort cloud one day.

    First, if we come across such a body, we have learned something about solar systems and can improve our definition. Perhaps “a body that originally cleared its orbital area, but may have changed orbits or had other objects enter its orbit later”.

    Second, it is easy to imagine corner cases for hydrostatic equilibrium as well, even the obvious problem of finding a body at the border between hydrostatic equilibrium and not.

    What about a body that originally formed round under hydrostatic equilibrium, but had a glancing collision that dug out enough material that it no longer maintained hydrostatic equilibrium, and now has a gouge in it a significant fraction of its diameter? This is not different than your hypothetical wandering Mercury, in that it was a body that would have been under the planetary definition but now isn’t.

    There are a ton of issues with small, cold bodies, since they can have various mixtures of rock, ice, and liquid hydrocarbons that could be in various combinations of shapes (like a body with a very deep hydrocarbon ocean that is spherical, but a rocky core that isn’t). Or what about a hot body with a round sea of liquid rock and an irregular metal core?

    Where do you draw the line between atmosphere and core? Gas giants can have irregular solid cores. Only the gas is in hydrostatic equilibrium. What if the atmosphere is the same thickness as the core, with the atmosphere being thick and dense enough to be spherical but the core not? How do you decide when the atmosphere is big enough that you should consider it instead of the solid core?

    So if you are willing to play this game with hypothetical corner cases, it can be used to invalidate your definition at least as easily as the IAU one.

  59. andy

    Before the discovery of the Kuiper belt the criterion of an object not being a member of a belt population was one of the basic rules of thumb for assessing planethood. The IAU definition seems to be a poorly-worded approximation of this rule-of-thumb. This rule-of-thumb seems to serve pretty well even when measured up against known exoplanetary systems: the planet formation process seems to consistently produce a handful of large objects (the major planets) and remnant small body reservoirs (e.g. the asteroid and Kuiper belts).

    And no, the eight planets do not constitute a belt population: if the distribution of planets were anything like analogues to the asteroid or Kuiper belts the system should have dozens of objects in the 1-10 Earth masses range in the region between Mercury and Neptune. If you want to say that you can’t tell the difference between the planets and the asteroid belt then please explain where the missing super-Earths are.

    And no, Jupiter does not fit in with the population of the rest of the objects that share its orbit: there is a mass gap of over 8 orders of magnitude between Jupiter and the largest Trojan asteroid (624) Hektor. If you want to argue that there is no reason to treat Jupiter+Jupiter co-orbitals as anything different to the asteroid belt, please explain where the missing intermediate-mass co-orbitals are that would bridge this mass gap.

    Remember: planetary systems form and evolve as a system. If you want to consider planet-planet scattering to get Earth out to the Kuiper belt, bear in mind that the alteration of the planetary orbits as a result of the scattering are going to have knock-on effects on the small body population in the region, above and beyond what an Earth-mass planet that formed there in situ would have.

  60. Darren

    @50: “You’re correct; Pluto and Charon are indeed the only pair of bodies in the Solar System, whose barycentre lies in empty space. So whether you choose to call Pluto a planet or a dwarf planet, it and Charon should really be called a binary planet/dwarf planet, rather than a planet and satellite.”

    Does that imply that as our moon recedes, when it crosses the point that the barycenter moves beyond the earth’s surface that it gets an upgrade?

    Power up!

  61. amphiox

    Does that imply that as our moon recedes, when it crosses the point that the barycenter moves beyond the earth’s surface that it gets an upgrade?

    Would our moon ever actually get to that point in its expected lifespan?

  62. #60 Darren, #61 Amphiox:

    “Would our moon ever actually get to that point in its expected lifespan?”

    That’s a very good question – and the answer, I think, is debatable!
    The Moon has 1/81 x the Earth’s mass, so for the barycentre to be outside the Earth, its distance would have to increase to more than 81 Earth radii, or about 518000 km. Its current mean distance is 382000 km, so it would have to recede by a further 136000 km.
    I believe its rate of recession is about 3 cm per year, so to recede by 136000 km would take 136000 x 1000 x 100 / 3 = approximately 4.5 billion years!
    This is roughly equal to the age of the Earth, and to the time we have left before the Sun goes red giant. So the Moon may or may not reach that position while it and the Earth still exist – but if it does, it won’t make much practical difference!!!

  63. Messier Tidy Upper

    @17. andy :

    Ok, bearing in mind the various dependencies of the onset of deuterium fusion, please classify the following objects as planets/not-planets according to your rule 2:

    HD 217786b: only known companion of an F8V star at an orbital distance of 2.38 AU. Minimum mass 13.0±0.8 times Jupiter

    HAT-P-13c: second substellar companion in order of distance from a G4-type star in a hierarchical system of 2 planets. Mass at least 14.5±1.0 times Jupiter.

    Upsilon Andromedae c: second substellar companion in order of distance from an F8V star in a system of 4 planets. Mass 14.57 times Jupiter.

    BD+20°2457 b and c: a 2:1 resonant system consisting of objects of at least 12.47 and 21.42 times Jupiter orbiting a 2.8 solar mass red giant star.

    That is a tricky issue but I would say that if they ever shone by core fusion – even briefly -then, yeah, technically they count as brown dwarfs. So did they once shine with their own light due tocure nuclear fusion? This fromyour linked paper :

    Even though, for most proto-brown dwarf conditions, 50% of the initial deuterium will burn if the object’s mass is ~(13.0 ± 0.8) MJ, the full range of possibilities is significantly broader. For models ranging from zero-metallicity to more than three times solar metallicity, the deuterium-burning mass ranges from ~11.0 MJ (for three times solar metallicity, 10% of initial deuterium burned) to ~16.3 MJ( for zero metallicity, 90% of initial deuterium burned).

    Suggests they possibly did. If so, however planet-like they are they count as brown dwarfs under my core fusion criteria. If not, then they’re very Superjovian planets instead.

    Like Vesta these are right on the boarderline of planet and yes, in many respects brown dwarfs and the very largest asteroids overlap with that category.

    Nature builds a whole continuuum of objects from the largest hypergiant stars through dimred dwrafs and brown dwrafs down through gas giants, gas dwarfs, rocky or Hot Ice-y superEarths, Terrestrial planets or rok dwrafs, ice dwarfs and asteroids and meteroids and interplanetary /interstellar dust grains.

    Sometimes its easy to classify where something fits sometimes it isn’t. Those are tricky one’s under my definition but Pluto isn’t tricky and clearly is a planet. As is Ceres, Sedna, etc..

  64. Messier Tidy Upper

    Continued : @17. Andy – Part II

    As for HD 217786b, HAT-P-13c, Upsilon Andromedae c, and BD+20°2457 b and c, well, yeah, it isn’t so clear. I would say that if they ever shone by core fusion – even briefly – then, yeah, technically they count as brown dwarfs.

    So did they once shine with their own light due to core nuclear fusion? If so, however planet-like they may be are they count as brown dwarfs under my core fusion criteria. If not, then they’re very Superjovian planets instead.

    We’d need to get their spectra, establish their metallicity and calculate whether they did shine or not to determine for certain.

    Tentatively & until we determine this I’d say that :

    HD 217786b with its minimum mass of 13.0±0.8 times Jupiter is a brown dwarf since it seems it will burn 50% of its core deuterium in nuclear reactions and thus was self-luminous for a time. I’ll also note the mass is a minimum and could indeed be higher.

    Same applies even more so to HAT-P-13c with its 14 Jupiter masses – minimum. Brown dwarf albeit a very Superjovian like one in some respects.

    Likewise Upsilon Andromedae c well over the 13 Jupiter mass limit and very definitely BD+20°2457 c which is far in excess of the deuterium fusion limit at 21.42 times bulk.

    OTOH, BD+20°2457 b with its mere twelve and a half Jupiter masses almost certainly scrapes under the line to be one of the most massive Superjovians and exoplanets known.

    Makes sense right?

    Mind you we could change any of these these classifications if it turns out from spectrographic or other evidence that they did or didn’t shine by core fusion as we think.

  65. @59. andy :

    Before the discovery of the Kuiper belt the criterion of an object not being a member of a belt population was one of the basic rules of thumb for assessing planethood. The IAU definition seems to be a poorly-worded approximation of this rule-of-thumb.

    Actually then we’d have to disqualify all planets based on their being a belt of inner planets and a belt of gas giants all crammed in together at well under one Plutonian Astronomical Unit! (PAU – distance from Sun to Pluto equalling about 30 Earth AU.) ;-)

    What is a belt is clearly a vexed question isn’t it and depends on external factors outside of a planet candidates control.

    So by that reasoning if the Sun contained only Pluto and Earth in theri present orbits Pluto would be a planet.

    If we added Ceres and Eris then all these would still be planets still not being in a “belt” and changing nothing in that regard if we added Jupiter and venus ditto and so on up till we reach .. well, which point exactly?

    Would it be a “belt” when we add Neptune because it and Pluto have overlapping orbits? I don’t think that criteria is strong enough to alter things because we’d hardly do the same if it was Venus and Earth overlapping orbits would we?

    Would it become a belt when we added our tenth planet or our twentieth or our hundreth -and seen from what distance / perspective?

    So things are not so clear cut and obvious after you perform this “belting” excercise are they? ;-)

    (Another reason why that belt “rule of thumb” and its even worse IAU criterion are just wrong.)

  66. @57. KC :

    I’ve never understood the poor criteria of hydrostatic equilibrium. No one seems to have a good answer to the question “How round is round?”

    You know what we need?

    One of those testing devices the umpires use in cricket to check whether or not the ball (see link in my name here) has gone out of shape or not! You know those ring things the umpires put the ball through to test it ‘s shape hasn’t changed too much by the battering its been recieving. Take one of those devices, scale it up to astronomical size, fit it with rocket engines and off we go! Planet circularity testing mechanism aquired! ;-)

  67. Messier Tidy Upper

    @58. TheBlackCat :

    Amongst these thousands of planets, do we know of any that don’t conform to the IAU definition? If not than this is irrelevant.

    All of them fail the absurd IAU definition because none of them orbit our Sun which is exactly the point I was making there. Sigh.

    Neither ejecting a planet from the solar system nor smashing it into another planet in any way contradict the IAU definition.

    Actually *both* those circumstances do indeed contradict the ridiculous IAU definition because a planet needs to have a clear orbit (Pluto’s “sin” by that decree) and a planet on a collision course with other by definition has an unclear orbital path or orbit a star – indeed to very specifically orbit just one star out of the countless googolillions of stars out there to qualify.

    Black Cat, you’re smart enough to see that surely?

    What about a body that originally formed round under hydrostatic equilibrium, but had a glancing collision that dug out enough material that it no longer maintained hydrostatic equilibrium, and now has a gouge in it a significant fraction of its diameter? This is not different than your hypothetical wandering Mercury, in that it was a body that would have been under the planetary definition but now isn’t.

    No, it is very different – in one case we have a planet that is identical to Mercury shifted intoa different position but not physically altered and the other we have a planet that has been physically changed and reduced in mass by an impact.

    Yes, planets can be destroyed or transformed by impacts into asteroids but transforming a planet into a non-planet by merely placing it in a different orbit – unless it is captured as a moon – seems a different thing to me. My definition allows planets to change based on such circumstances & Ithink thatis reasonable. I do NOT consider it reasonable to say that for example Mercury, Pluto or, for that matter, Earth is a planet if t orbits at point X but not if it orbits at point Y.

  68. Messier Tidy Upper

    @52. Nigel Depledge :

    MTU (45) said : “See my response at # 43 which says it all. Orbital clearance is violating Occams law and raises a whole slew of unnecessary superflous questions demanding problematic and arbitrary answers that roundness – a visble immediate intrinsic trait does not.”
    No. Just, no. Roundness is not obvious.

    Well I disagree, I’d say yes, just yes – roundness is immediately pretty obvious once you see an object in enough detail.

    Ceres is clearly round as is Pluto. It is something that is apparent at a glance.

    From that we can say that anything Ceres sized or over is likely to be round or at least in hydrostatic equilibrium and hence it is a criterion of both the IAU definition and my own prefered one.

    Another alternative simple definition would perhaps be to define a planet as something with a mass between 12 Jupiter masses and the mass of Ceres (or maybe Vesta) provided it is not a moon or an object that shone by core nuclear fusion, eg. stellar remnants.

    Plus you seem to have missed the point and ignored the rest of what I wrote at #43 showing that roundness *is* far better and less problematic than the idea of “orbital clearing” as a criterion for defining planet.

    So, I take it you are now finally conceeding that point, Nigel?

    @53. Nigel Depledge :

    MTU (45) said: “If, Mr Depledge, you are – seemingly desperately and emotionally – disputing this known roundness of Ceres and Pluto’s and Makemake, etc .. which is based on repeated calculations and observations and accepted by the IAU hence them qualifying for their current status, then its up to you to show cause why they’re not round.”
    I’m not disputiong this for any specific individual body ..

    Right. So you accept Pluto and Ceres are clearly round I take it. Yes? Good. That we means we know or at least can pretty safely predict that objects with Ceres mass and above meet the roundness criterion.

    .. but there’s a whole bunch of bodies in the Kuiper Belt that have either not been discovered or have been discovered and not well characterised.

    Agreed. Next step – we find out. :-)

    We observe and study and send spacecraft and learn about them. If these Edgheworth-Kuiper Cometary belt objects are sufficently massive and round and otherwise qualify as planets – variety : ice dwarf – then add them to the list. Characterise and classify them.

    If they aren’t then add them to another category -the asteroids and cometary nuclei making up the Cometary disk and cloud.

    If we’re not sure then put a question mark against them until we are.

    Your problem with that is ..what?
    The point here being that, under your definition, we don’t know whether these are planets or not until we point a whopping big telescope at them or until we visit them. Whereas under the IAU definition we already know.

  69. Messier Tidy Upper

    Durnnit. More editing time (& probably sleep) required.

    My comment was supposed to finish at the “Your problem with that is .. what?”

    @53.Nigel Depledge :

    The point here being that, under your definition, we don’t know whether these are planets or not until we point a whopping big telescope at them or until we visit them. Whereas under the IAU definition we already know.

    So you’d rather a mistaken bad classification basedon illogical and incosnistent criteria that is unreasonable rather than uncertainty then?

    I would not. I’d rather we said we’re not sure if these are planets or not yet and will try to find out with classifications having a question mark or asterisk against them until we’re sure.

    @46. Nigel Depledge :

    If an Earth-sized object occupied the orbit of Pluto, it probably would not meet the gravitational clearance criterion.

    Which tells me that the gravitational criterion is a ludicrous idea that does NOT work.

    Because Earth is not a dwarf planet and I don’t think we’d call it one even if was in that situation.

    Planets are planets wherever they are and however small they are. A small planet is a small planet. Earth we know is a plaet -and so is Jupiter and so is,yes, Pluto. Adefinition that calls or would call them something else just for orbiting more distant is a definition that fails.

    I have no idea what mass of object would be the minimum to meet the criterion, but Neptune certainly would – as it is, Neptune’s gravity has a strong influence on Pluto’s orbit. Neptune’s mass is pretty close to 10^26 kg.

    Or, IOW, Neptune has 17 Earth massses.

    I wouldn’t be so sure though – Neptune’s orb it isn’t technically clear.

    However, this does not stop anyone from calling Pluto a planet in casual conversation. The IAU definition only applies to the technical work of professional astronomers.

    So it doesn’t apply to me or I think you and most of the rest of us then? ;-)

    Adefinition that most people reject and that is not used by anybody but a tiny handful of folks is NOT what I’d consider a very good definition!

  70. MaDeR

    @MTU:

    “Plus if you run through a checklist of features you’d expect of planets Pluto meets virtually all of them – moons – now 5 of them 3 more than Mars and five more than Venus and mercury”
    You STILL are speaking like having moons contribute in any way to planethood? Like body with two moons are more planet than body with no moon? Good grief, this is idiotic. And lying, by the way.

    “Yes, planets can be destroyed or transformed by impacts into asteroids but transforming a planet into a non-planet by merely placing it in a different orbit”
    Aren’t you cute. You know that Mercury ejected from our solar system (yes, this is “merely” change of orbit) would cease to be planet ALSO in YOUR definition of planet, right?

    Nice to see how you measure different definitions with different marks. Somehow your favourite definition can get away with thing that is ih so codemning other definition. There was one word in english to describe this kind of behaviour.

  71. Messier Tidy Upper

    @70. MaDeR :

    You STILL are speaking like having moons contribute in any way to planethood? Like body with two moons are more planet than body with no moon? Good grief, this is idiotic. And lying, by the way.

    How so?

    Also no, that is NOT exactly what I am saying.

    Pluto isn’t a planet because it has more moons than all the inner planets do put together any more than Jupiter is a planet because it has well over a half century of moons mostly tiny but four large.

    Pluto is a planet because, by my definition, it is round, never shone by core nuclear fusion and is NOT a moon.

    Having moons is not a defining planetary trait – however it is a secondary suggestive trait of planets.

    A planet can be a planet without moons but you would expect to find a moons mostly around planets so if an astronomical body has a lot of moons – as Pluto does – then,yes, that is certainly something suggestive and supportive of the case for its planethood.

    Therefore noting Pluto’s growing number of moons is supporting evidence of its claim to planethood and that’s no lie at all.

    What about that do you fail to understand MaDeR?

    Aren’t you cute. You know that Mercury ejected from our solar system (yes, this is “merely” change of orbit) would cease to be planet ALSO in YOUR definition of planet, right?

    Wrong. Wrong once again, MaDeR and showing a very massive reading comprehension FAIL on your part.

    Under my definition orbiting a star – any star – is NOT required and I’m pretty sure I’ve pointed this out in more than a few comments here.

    If Mercury were hypothetically ejected from our solar sytem then under my definition it would remain a planet. It would join a whole probably very numerous class of planets we have been starting to discover which are NOT attached to any star but rather are free-floating rogue /ejected / orphan planets drifting through interstellar space.

    Nice to see how you measure different definitions with different marks. Somehow your favourite definition can get away with thing that is ih so codemning other definition. There was one word in english to describe this kind of behaviour.

    There’s one word in English to describe your comment here too MaDeR – Wrong! ;-)

    Or we could also say it was incorrect, erronous, abusive, misinformed and false too.

    You owe me an apology.

  72. Messier Tidy Upper

    @29. Grand Lunar :

    I’m in league with Neil Tyson, referring to Pluto as “king of the comets” (though I personally prefer it being one of the largest KBOs) rather than “puniest planet”.

    Except that it is wrong to call Pluto a comet. Pluto is something very different and much larger and lacking in tails and boasting five moons and so on. Yes N.d.G. Tyson is wrong about that – and his meanness and belittling of those disagree with him over Pluto cost him a lot of respect from me.

    @59. andy :

    the planet formation process seems to consistently produce a handful of large objects (the major planets) and remnant small body reservoirs (e.g. the asteroid and Kuiper belts).

    Really? We know of a lot of exoplanetary systems that come in a lot of varieties. Some have Superjovian planets, many have Hot Jupiters and Eccentric Orbiters, a few are somewhat analogous to our own and some have us baffled.

    Pluto is one type of small planet – so is Earth and, depending on how you define it even Neptune and Jupiter could be viewed as small.

    And no, the eight planets do not constitute a belt population..

    As seen from Pluto’s perspective I’d argue they actually kinda might do.

    How do you define a belt population? What number of objects do you need in how big a space? You want to argue one limited narrow definition of it that you haven’t explictly put to us or said why we should prefer it over alternative ideas of what a “belt population” means.

    If you want to say that you can’t tell the difference between the planets and the asteroid belt then please explain where the missing super-Earths are.

    Huh? Where the SuperEarths are – why one didn’t form in our solar system is oen interesting question. What this has to do with how we best define planet is something that is a different and unrelated question entirely.

    And no, Jupiter does not fit in with the population of the rest of the objects that share its orbit: there is a mass gap of over 8 orders of magnitude between Jupiter and the largest Trojan asteroid (624) Hektor. If you want to argue that there is no reason to treat Jupiter+Jupiter co-orbitals as anything different to the asteroid belt, ..

    That’s not what I’m arguing and you seem to have missed the point.

    Which is that how “clear” an orbit is and what “clear” means is,well, unclear!

    All the planets in our solar system have unclear orbits whether they are crossed by sun-grazing comets, long and short-period comets, asteroids, shared with trojans, intersected with gas giants, intersected by ice dwarfs or whatever. A clear orbit is a murky concept and an unreasonable expectation especially as you orbit further out and therefore have more space to clear.

    This means that using orbital clearence to define planet adds superflous questions – needless extra metaphorical epicycles – that a good scientific definition in keeping with Occam’s law must dispense with.

    Orbital clearance depends on too many extrinsic variables that are outside a planet’s control (such as amount of material in a system, age of a system, distance of orbit, etc ..) and thus should NOT be used to define whether something is a planet or not.

    Remember: planetary systems form and evolve as a system.

    Almost always sure – provided we’re not talking about an individual free-floating planet that formed from an interstellar nebula directly and alone! How is that relevant though to how we define what a planet is?

    If you want to consider planet-planet scattering to get Earth out to the Kuiper belt, bear in mind that the alteration of the planetary orbits as a result of the scattering are going to have knock-on effects on the small body population in the region, above and beyond what an Earth-mass planet that formed there in situ would have.

    Again, fair enough but not really relevant to how we define planet in my view.

  73. Nigel Depledge

    MTU (67) said:

    @58. TheBlackCat :

    Amongst these thousands of planets, do we know of any that don’t conform to the IAU definition? If not than this is irrelevant.

    All of them fail the absurd IAU definition because none of them orbit our Sun which is exactly the point I was making there. Sigh.

    You keep calling the IAU definition “absurd” but you still have not shown this. As TBC points out, several of the arguments youi use against the IAU definition can equally be used against your preferred definition (which is why I keep reiterating the point about the logical difficulty of accepting the gravitational roundness criterion while simultaneously rejecting the gravitational clearance criterion).

    You have not yet made a convincing argument that the IAU’s restriction of the term planet to our solar system is flawed. And you dismiss, rather than address, the argument in favour of this criterion, i.e. that we know plenty about our solar system, but very little about exoplanets. For instance, we currently cannot detect exoplanets smaller than Earth, or with periods that are comparable to those of Ouranos, Neptune, Pluto or Eris. We thererfore have no idea about such things as asteroid belts or parallels to our Edgeworth-Kuiper Belt in other solar systems.

    All we can say for sure is that many stars have planetary bodies orbiting them. Although we can deduce the morphology and have measurements of the composition (gross components, anyhow) of some exoplanets, we have no idea of the kind of context in which those objects orbit their parent star.

    If an alien civilisation were to use techniques like our own to view our solar system from (say) 100 light-years away, over the kind of time over which we have been observing exoplanets, they might just have detected Earth, and they might have recently got enough data to confirm Jupiter. They would not have confirmation on Saturn’s existence, and they would not yet have noticed Ouranos or Neptune, except perhaps as one-off anomalies in their data. They would not have detetcted Mercury, Venus or Mars. Or the asteroid belt.

    The difference in the level of knowledge is substantial.

    MTU (56) said:

    Thought experiments and ideas Nigel. These do kinda have a place in science, y’know?

    Yes, I know.

    But you are not conducting thought experiments, you are inventing scenarios in order to break the IAU definition, when that definition is specifically intended to apply only to our solar system as it exists currently. To say “if Mercury were in the EKB, it wouldn’t count as a planet” kind-of misses the fact that Mercury isn’t in the EKB.

    You also speculate about “well, if X event happens in the future some time, the IAU definition fails”. Well, sure, and the IAU is, of course, free to change the definition as and when new knowledge requires it to change. This is not at issue. Just because the definition is useful here and now doesn’t mean it is set in stone.

    By the same token, you say “so what?” to my comment about your choice of definition leaving us with potential hordes of undefined objects. Again, it’s about utility. The IAU definition is useful to professional astronomers here and now. whereas your choice isn’t.

    Finally (yes, I know I have not yet addressed half of what you have written, but you seem to have more online time than I these days – I’ll get to it as and when I can), no-one is forcing you to use the IAU definition. I just wish you’d stop jabbering on about it, because the arguments you use are so poor, and you frame the debate emotionally (yes, your frequent use of adjectives such as absurd and ridiculous does thus serve to frame the debate) and in such a way as to polarise opinion rather than use reason to come to some sort of coexistence.

    I don’t care whether you like the IAU definition or not. I don’t care whether you use it or not. I just want you to stop attacking the IAU’s definition every time the BA posts about Pluto. Partly because it is becoming tiresome, partly because you use rhetoric to deride opposing views and partly because you commonly dismiss or ignore the many perfectly valid points that have been made (by me and by others) that refute, counter or, at least, challenge your arguments.

  74. Messier Tidy Upper

    @51. Nigel Depledge Says:

    MTU (43) said : “Well that’s a shame and also erroneous on your part not mine.”
    You have claimed this before, but repeatedly failed to show it to be so.

    I have shown it multiple times and done my best to explain it to you. Your disagreement doesn’t mean I’m wrong – merely that you still disagree. You don’t accept my arguments well, ok, a lot of people don’t accept arguments for a lot of things. For instance evolution or the existence of Dark Matter or the best toppings for a pizza. It doesn’t make those people right. Onthis issue, youare not right in my view. Guess we’ll have to agree to disagree -unless of course you see sense and agree with me one day! ;-)

    @73. Nigel Depledge :

    You keep calling the IAU definition “absurd” but you still have not shown this.

    To your satisfaction maybe. In fact I think I think Ihave shown it more than adequately. Your failure to appreciate and accept this is now, well, your failure to appreciate it, not mine.

    MTU (56) said: “Thought experiments and ideas Nigel. These do kinda have a place in science, y’know?”
    Yes, I know.

    Then maybe, here’s a thought, stop attacking people for using them and attacking them a sjusthypothetcial or whatever hey?

    But you are not conducting thought experiments, you are inventing scenarios in order to break the IAU definition, when that definition is specifically intended to apply only to our solar system as it exists currently.

    Which, as I’ve already argued is one good reason to reject it because it violates the Copernican principle. Sigh. How many times do I have to say this..??

    Also, yeah, I’m conducting thought experiments, I’m asking now “what if” and thinking through what the implications might be. I really, really don’t see your problem with this is and it is immensely frustrating to me.

    ..the IAU is, of course, free to change the definition as and when new knowledge requires it to change.

    I’m saying that time is right now. I’m saying that this IAU definition is a bad one that should never have been adopted to begin with and the time to change it, frankly, was yesterday.

    What we know already is enough to tell me the IAU got it badly wrong.

  75. Nigel Depledge

    MTU (68) said:

    Well I disagree, I’d say yes, just yes – roundness is immediately pretty obvious [only after] you see an object in enough detail.

    FTFY.

    My point, that you appear to be ignoring, was that there are plenty of objects that we have not yet seen in enough detail.

    Ceres is clearly round as is Pluto. It is something that is apparent at a glance.

    For objects the size of Ceres, yes.

    But for the gravitational roundness criterion there exists a grey area. As I pointed out in a previous comment, and you seem again to be ignoring, it is very likely that there are plenty of objects in the EKB that are within that range of sizes where roundness is either not immediately obvious or where roundness is apparent but not adequate to indicate hydrostatic equilibrium.

    And you also seem to ignore the possibility that some objects may be round from one viewing angle and not-round from other viewing angles.

    From that we can say that anything Ceres sized or over is likely to be round or at least in hydrostatic equilibrium and hence it is a criterion of both the IAU definition and my own prefered one.

    But in your definition, it is a hinge-point among objects with heliocentric orbits, whereas the IAU definition uses it to supplement the gravitational clearance criterion.

    And, sure, we can say that any Ceres-sized (or larger) object is round and therefore ticks that box. But this still ignores the possibilities that (a) we may soon discover objects in the transitional size range between round and not-round and (b) some objects already known may be in this transitional size range but we simply don’t yet have the observations to decide if they are round or not-round. Our knowledge of the main asteroid belt is not exhaustive, and our knowledge of the EKB is patchy at best.

  76. Nigel Depledge

    MTU (74) said:

    Which, as I’ve already argued is one good reason to reject it because it violates the Copernican principle. Sigh. How many times do I have to say this..??

    And you continue to ignore the reason that the IAU adopts a pragmatic approach and applies a definition that is useful here and now.

    Sure, one day it would be great to have a universal definition that can be applied anywhere and at any time. We’re a very long way from having enough knowledge to make that definition.

    You have come up with hypothetical scenarios that break the IAU definition, and TBC and I have both shown how easy it is to do exactly the same thing to your preferred definition. But all this byplay misses two key points:

    1. That, because the IAU definition applies only to our solar system and , there is no possibility of near-future discoveries in exoplanet systems rendering the definition meaningless; and
    2. that, because the IAU definition includes the gravitational clearance criterion, there is no possibility of near-future discoveries in the EKB rendering the definition meaningless.

    Limiting the scope of the definition in this way renders it robust enough to be useful.

  77. Nigel Depledge

    MTU (74) said:

    I have shown it multiple times and done my best to explain it to you. Your disagreement doesn’t mean I’m wrong – merely that you still disagree. You don’t accept my arguments well, ok, a lot of people don’t accept arguments for a lot of things. For instance evolution or the existence of Dark Matter or the best toppings for a pizza. It doesn’t make those people right. Onthis issue, youare not right in my view. Guess we’ll have to agree to disagree -unless of course you see sense and agree with me one day!

    Well, this is one way of interpreting it.

    My own view is that your arguments against the IAU definition contain flaws that renders them powerless. I have repeatedly pointed out these flaws and you have repeatedly refused to accept that they are flaws.

    Some things, such as arguments about pizza toppings, are purely a matter of personal preference. Except about anchovies, which are just evil.

    Other things, such as the arguments about the existence of DM, are still undecideable in terms of there not being an adequate preponderance of evidence to render a final conclusion.

    Other things again are purely a matter of reason and pragmatism. So, you don’t like the IAU definition. Fine, I get that. No-one is forcing you to use it. But you seem to have taken it upon yourself to convince everyone else in the world that the IAU is “wrong”, when most people really don’t care how professional astronomers apply the term planet. And the arguments you have used contain – on my view – flaws that I cannot leave be.

    If you had come up with some arguments that did not contain such obvious flaws, I’d probably be supporting your view, not arguing against it. It’s not so much that I think the IAU definition is the best thing since sliced bread: instead it seems to me that it is the most useful compromise anyone could come up with on the basis of current knowledge.

    Sure, it denies Pluto full “planetary” status, but fortunately Pluto doesn’t care. There have been good reasons for a couple of decades to consider Pluto to be in a different category from the eight IAU planets. The discovery of 2003UB313 (which I think is the same object as is now called Eris) sparked off the whole debate, because it’s around the same size as Pluto, emphasising Pluto’s lack of uniqueness among EKB objects. And it really would be meaningless to classify every last EKB object that happens to be large enough to look round in the same category as Jupiter, Earth and so on.

  78. MaDeR

    “Having moons is not a defining planetary trait – however it is a secondary suggestive trait of planets. ”
    First I heard about some nebulous “secondary trait”. Essentially you are claiming body with moon is more planet than body without. This is pure bunk.
    Using “it have lotsa moons” as an argument for Pluto planethood is lie, because neither IAU nor your definition says anything about moons. You just grasp at straws to find something that will appear as argument for Pluto planethood. Pitful.

    “Under my definition orbiting a star – any star – is NOT required”
    *rereads* …damn. Okay, that was brainfart and I apologize for this particular mistake. My general opinion about you still stands, unfortunately.

    Oh, and about other discussions…
    “Orbital clearance depends on too many extrinsic variables that are outside a planet’s control (such as amount of material in a system, age of a system, distance of orbit, etc ..) and thus should NOT be used to define whether something is a planet or not. ”
    Being round depends on a lot of variables too (spinning speed, density of body, composition of body – ice rounds easier than rock – etc). No, redefining it as “hydrostatic equlibrum” will not help. No, “outside planet control” is lamest shadow of pretender to excuse that I ever seen. It does not matter if planet changes on its own or from external forces.
    How come variableness is not problem for your definition, but is problem for IAU definition? Again you know word for that.

    Both definitions use distinctions that are equally arbitrary. For example, certain ratio of mass of body versus mass of every speck dust on its orbit (to fill “clearance” criteria) is no more arbitrary than “it is round because it looks round to human eye”. If anything, “looks round to human eye” is MORE arbitrary, because it is more uncertain (everyone have a little different eyes) than some particular cutout number as ratio.

  79. Messier Tidy Upper

    @73. Nigel Depledge :

    As TBC points out, several of the arguments youi use against the IAU definition can equally be used against your preferred definition (which is why I keep reiterating the point about the logical difficulty of accepting the gravitational roundness criterion while simultaneously rejecting the gravitational clearance criterion).

    Did you read what I said in comment #67 & #43 here? I don’t see it as difficult or inconsistent at all and have explained already why that is. Roundness doesn’t have the same severe problems that orbital clearance does.

    We know when we see something round that it is round and that roundness is an internal intrinsic property of a given body. Orbital cleraance OTOH, well, hell, re-read what I’ve read already. It isn’t clear (metaphorically spekaing!), it raises unnecessary questions and it isn’t areasonable criterion to use. Whereas roundness is quote reasonable because you can tell instantly, prettymuch,whether somethingisround or not.

    We seem to keep going around this issue with me explaining it and you apparently failing to get it. Dunno why. I think you’re a smart person and don’t understand why it gives you such trouble to grasp this reality. :-(

    Once more then. Say it aloud. Speak slowly. Repeat until you get it.

    Round – you can see. Is property of actual body. Relies on less external variables to aquire. Works.

    Orbital clearance – cannot be determined easily. Is external to the actual planet candidate.
    Depends on awhole huge range of avriable factors. Raises extra questions . Does NOT work.

    Comprendez? Sigh.

    You have not yet made a convincing argument that the IAU’s restriction of the term planet to our solar system is flawed

    Oh for pity’s sake! Yes, I have. Say it with me now : Co-per-nican Principle. Principle of mediocrity. Our Earth, our solar system is not unique, not the centre of the cosmos or special. (Except to us.)

    Knowledge that, yes, other stars have planets and some planets even travel through space without stars. Sheesh! How many times?! Do you think we should ignore the Copernican principle and ignore the universe beyond our solar system? Are you really and truly going to argue that?

    If an alien civilisation were to use techniques like our own to view our solar system from (say) 100 light-years away, over the kind of time over which we have been observing exoplanets, they might just have detected Earth, and they might have recently got enough data to confirm Jupiter. They would not have confirmation on Saturn’s existence, and they would not yet have noticed Ouranos or Neptune, except perhaps as one-off anomalies in their data. They would not have detetcted Mercury, Venus or Mars. Or the asteroid belt. The difference in the level of knowledge is substantial.

    So what? That has no relevance to the question of how we define planet. It is relevant when we’re talking about what sort of planet that hypothetical aliens can see and what sort of planets we can see but that is a separate question to what a planet actually is.

    By the same token, you say “so what?” to my comment about your choice of definition leaving us with potential hordes of undefined objects. Again, it’s about utility. The IAU definition is useful to professional astronomers here and now. whereas your choice isn’t.

    Yes it is. Clearly another point we’re just going to have to agree to disagree over. Mydefinition works in allowing astronomers to determine that something is a planet in a consistent logical way using simple easily detrmined criteria. The IAU definition fails at usefulness because it is NOT useful and not logical and not consistent even with basic english where dwarf stars are stars but dwarf planets supposedly don’t count.

    If youwant to argue otherwise, well how about you tell me how many papers have cited the useage of the IAU definition? How exactly has it helped astronomers work? Who even refers to the IAU eight as “classical planets” or combines comets and asteroids into the even clunkier “small solar system bodies” and so on?

    no-one is forcing you to use the IAU definition.

    Oh don’t worry I don’t use it! ;-)

    I just wish you’d stop jabbering on about it, because the arguments you use are so poor, and you frame the debate emotionally (yes, your frequent use of adjectives such as absurd and ridiculous does thus serve to frame the debate)

    No, my arguments aren’t poor – only your opinion of them which is wrong.

    Jabbering – gee that’s an emotional adjective to describe what & how I’ve been arguing here. Also an insulting and false one.

    Absurd and ridiculous are good accutrate and applicable adjectives to describe the IAU defintion and decision because it was absurd for them to come up with that ridiculous definition.

    Yes, being human I do have emotions and do feel very strongly about this but no, the fact that I’m passionate about this issue doesn’t make me wrong or invalidate what I say.

  80. Messier Tidy Upper

    Finally, @73. Nigel Depledge :

    I just want you to stop attacking the IAU’s definition every time the BA posts about Pluto.

    Well, tough. I wish I didn’t have to discuss it because the IAU had corrected their dreadful mistake or had never made it in the first place. Until they do they are making astronomers look silly and trying to impose an absurd definition of planet that is simply ridiculous. I oppose this and shall continue to oppose it.

    If you really want this Pluto planethood question to go away and find it so tiresome then you’ll have to lobby the IAU to see sense and change their current definition to one that is actually reasonable.

    Partly because it is becoming tiresome,

    I find it tiresome too and extremely exasperating when you seem to constantly argue for something that is the astronomical equivalent of saying “mice aren’t mammals because there’s a lot of them and they move in funny ways!” If you find it so tiresome maybe you could just say you disagree with me and leave it at that?

    ..partly because you use rhetoric to deride opposing views ..

    Not rhetoric, Nigel, reason and logic and patience and valid arguments. Even though you will no doubt dismiss them as invalid which is your erroneous opinion.

    ..and partly because you commonly dismiss or ignore the many perfectly valid points that have been made (by me and by others) that refute, counter or, at least, challenge your arguments.

    I disagree with you that I have “ignored” or “dismissed” the anti-ice dwarf IAU factions arguments or that they have many if any valid points against my position here – I have demolished them here repeatedly and thoroughly. It is *your* side here that has been refuted, countered and challenged whether you recognise and accept that or not.

  81. Nigel Depledge

    MTU (72) said:

    Except that it is wrong to call Pluto a comet. Pluto is something very different and much larger and lacking in tails and boasting five moons and so on. Yes N.d.G. Tyson is wrong about that – and his meanness and belittling of those disagree with him over Pluto cost him a lot of respect from me.

    But you use exactly the same rhetorial ploys when you deride the IAU’s definition.

  82. Nigel Depledge

    Further to my #81 above :

    How does that make you right and Tyson wrong?

    BTW, compositionally, Pluto is probably far more like a comet than it is like any of the eight planets. Also, of the 9 objects that ’til but recently were called planets, Pluto is the only one with significant orbital eccentricity. It is the only one that orbits with an inclination more than 5° from the ecliptic.

    If, as you insist, its orbital context does not matter regarding its definition, then the only difference between a comet and Pluto is that Pluto is the largest “dirty snowball” out there. Put Pluto near the sun, and it would develop a halo and tail. That it is in hydrostatic equilibrium is a fluke of timing. Given a couple of million years of closer exposure to the sun and it probably would lose enough mass to cease being round.

    See? We can use your “hypotheticals” argumentation technique to show that Pluto is a comet.

    But the fact remains that Pluto doesn’t get close enough to the sun to lose mass and develop a halo and a tail, so it isn’t a comet. It instead remains the largest known KBO.

  83. Nigel Depledge

    MTU (80) said:

    Well, tough. I wish I didn’t have to discuss it because the IAU had corrected their dreadful mistake or had never made it in the first place.

    No-one is forcing you to discuss it.

    Your blathering about it here in Phil’s blog isn’t going to make the IAU take any notice.

    Until they do they are making astronomers look silly

    Not really. Most people either don’t know or don’t care how the IAU defines planet for technical discussions among professional astronomers.

    and trying to impose an absurd definition of planet that is simply ridiculous.

    Wow. you’ve used both absurd and ridiculous in the same sentence. I can’t refute that!

    Oh, wait, I already have.

    If you are having this definition imposed on you, I would like to know exactly who it is imposing it and how they are enforcing that imposition. ‘Cos, AFAICT, no-one is forcing you to use the IAU definition.

    I oppose this and shall continue to oppose it.

    Fine, go tell it to the IAU. I have no influence with them, and I’m bored with your weak arguments and your refusal to acknowledge valid refutations of those arguments.

  84. Frank

    @26, Peter B – Just being a bit of a snot. Not entirely serious.

    As for the “planet” thing, who bloody well cares? It’s just a random word that got slapped onto some objects that circle the sun and not others. It originally referred to any moving heavenly object (including the sun), and then it got applied to eight or nine objects that don’t have a whole lot in common with each other (see: Jupiter and Mercury) except that they circle the sun. Historically, “planet” has meant little more than “a thing in space that isn’t a star.”

    So why all the hoopla? Who cares if this confusing, ill-defined (or entirely un-defined), practically-useless word gets applied to eight objects, or nine, or a million? Is there some special club that we don’t want to leave Pluto out of? Does it clarify anything to call it a planet or not? Does calling it a “planet” tell us anything that “Kuiper belt object” doesn’t? Does it do anything at all but make that old mnemonic about “my very eager mother” work once again? Is it some magic word that makes an object’s residence in our solar system “official?”

    No. The historical meaning was “a wandering celestial body,” which applies to pretty much everything in the universe. They’ve tried to narrow down the definition over the years, but any definition they arrive at is going to be entirely arbitrary. So who cares? Who cares if they restrict the word “planet” to rocky worlds like Earth and Mars, or any round thing that circles the sun, or go back to the original meaning and start calling the sun, moon, and comets “planets?” What practical or informational difference does it make? Why do we even need the word “planet” at all?

    We may as well come up with a new word, like “forberglabber,” and define it as “any object in space that isn’t anything else.” Then we can all have long, emotional, impassioned, and oh-so-productive arguments about whether or not Pluto is a forberglabber.

  85. andy

    That’s not what I’m arguing and you seem to have missed the point.

    Which is that how “clear” an orbit is and what “clear” means is,well, unclear!

    I never said anything about how clear the orbit is. You seem to be labouring under the delusion that the only alternative to your preferred definition is the IAU one, and therefore missing my point. Requiring that an object is not a member of a belt population is a far less restrictive criterion than requiring the orbit to be “clear”, which I think we both agree is an inadequate criterion.

    The eight major planets do not comprise a belt population: the mass distribution is completely different. If the major planets were drawn from a scaled up version of the asteroid belt or Kuiper belt, our system should be awash with super-Earths. In fact if you care to take a look at it, it is not. Calling the major planets a belt is an utterly misleading description.

    And as for this being “unfair” on objects located far from their star, so what? The gravitational roundness criterion is itself “unfair” on objects that form close to the star as they will form from greater proportions of rocky material that is stronger than ice and therefore requires higher masses to become gravitationally rounded.

    And your proposed fusion criterion is demonstratably utterly inadequate when confronted with systems like Upsilon Andromedae or Nu Ophiuchi (the latter a system of objects with minimum masses 21.9 and 24.5 times Jupiter in a 2:1 resonance), which provide a strong indication that nature is quite capable of assembling deuterium-burning planets. Similarly observations of open clusters indicate that the star formation process produces objects down to a few Jupiter masses, which implies the existence of stars that never undergo core nuclear fusion. In short, defining categories based on the ability of an object to undergo nuclear fusion leads to utter confusion.

  86. john

    FIRST, let’s define “is” then proceed to the larger concepts.

    btw: this is comment 89 !
    and i still don’t know what “enhadesenate” means.

  87. Messier Tidy Upper

    @ ^ 89. john : ‘enhadesenate’ is a word made up by the BA to replace ‘enlarge’ which is his custom here for some reason.

    This time it is from ‘en’ to enlarge and ‘hades’ the Greek equivalent of the Roman god Pluto after whoem the eponymous planet was named and the ~ate suffix and just means click to get a larger view of the discovery image of Pluto and its five moons.

    @88. andy :

    You seem to be labouring under the delusion that the only alternative to your preferred definition is the IAU one, and therefore missing my point.

    No, I’m aware there are other possible alternatives to both my preferred definition and the IAU one. In fact, I’ve already noted one such alernative based on an arbitrary mass cut-off in an earlier comment here.

    Requiring that an object is not a member of a belt population is a far less restrictive criterion than requiring the orbit to be “clear”, which I think we both agree is an inadequate criterion.

    Well I’m gald we agree on that second part. What is your personal preferred definition then?

    The eight major planets do not comprise a belt population: the mass distribution is completely different.

    Define belt population! As seen from Pluto all the inner planets orbit in what could sure;y be considered a belt from Mercury to at least Saturn as i’ve noted before. How many objects do you need for a belt? Why that number?Over how big an area exactly? Why should it involve mass distribution at all? No, the whole belt population doesn’t seem that much better than orbital clearance to me, a slight step up maybe but not enough of one.

    If the major planets were drawn from a scaled up version of the asteroid belt or Kuiper belt, our system should be awash with super-Earths.

    Why so? I don’t think that actually follows at all. I don’t think you are quite following my argument which isn’t that the inner planets are exactly like the asteroid belt but that seen from Pluto, they form a population of approximately similar bodies in approximately the same narrow orbital zone – well under 1 P-AU thus appear as a belt population. Its a different perspective.

  88. Messier Tidy Upper

    Continued – @ 88. andy :

    Calling the major planets a belt is an utterly misleading description.

    Calling dwarf planets non-planets is utterly misleading in my view just as much as calling dwarf stars non-stars would be.

    And as for this being “unfair” on objects located far from their star, so what?

    Its misleading that’s what if Earth was located out in the Oort cloud would it really be any less a planet? Same for applies for Pluto – put it inside the orbit of Mars and it’ d clearly be a planet and one of the brighter ones in our sky.

    The gravitational roundness criterion is itself “unfair” on objects that form close to the star as they will form from greater proportions of rocky material that is stronger than ice and therefore requires higher masses to become gravitationally rounded.

    Something having a certain mass is fair as a dividing point between planet and asteroid – in that it is a direct property of that object and rather than tothe environment in which it is found which is a less fair criteria. Surely a reasonable definition must define a planet by its own nature and internal properties – a planet is a planet whether it orbits a star close in far out or not at all.

    To illustrate this with a biological analogy; a frog is a frog whether it is located in a swamp or a desert. (BTW. We do indeed have frogs living in both places!) The IAU anti -Plutonians seem to think by analogy that a frog is only a frog when it is located in its swamp and not if it is found in a desert!

    Yeah, there is that complication right at the boundary of mass but that’s only right at that boundary which Pluto isn’t mass~wise. Pluto is larger than Ceres which is distinctly round so no trouble on that score. As I noted when discussing Vesta I’m inclined to say we should err on the side of generousity with rocky bodies sitting right on that limit as apparently Vesta is. Nor am I alone in thinking this with many professional astronomers rejecting the IAU definition and calling Vesta the smallest terrestrial planet – eg. Jim Bell, professor of Astronomy and Planetary Science, Universityof Arizona – as cited in comment #42.

    (It should also be remembered that perhaps the expert in the field Alan Stern referred to the IAU definition as “idiotic” his words not mine.)

    And your proposed fusion criterion is demonstratably utterly in adequate when confronted with systems like Upsilon Andromedae or Nu Ophiuchi.

    Fixed It For You. Did you miss my comments 66 & 67 above here where I answered your questions in that regard? Yerah, its abit tough with objects right at the borderline but we can find out and assign categories using my definition.

    Under which deuterium burning planets are brown dwarfs that are masquearading -very well in many cases -as Superjovian planets. There is a continuum of objects in nature.

  89. Messier Tidy Upper

    D’oh!

    That was supposed to read :

    And your proposed fusion criterion is demonstratably utterly in adequate when confronted with systems like Upsilon Andromedae or Nu Ophiuchi

    Of course. Dangnabbit!

    *Now* I’ve fixed it for you! ;-)

    Again, it if shines by core fusion it makes more sense to say it is a brown dwarf than a Superjovian planet.

    Nature builds a continuum of objects in all sizes from supermassive stars all the way down to dust grains. Brown dwarfs are in that intermediate stage beqween stars and planets and gas giants are intermediate again between brown dwarfs and rock dwarfs such as Earth.

    Which ever definition you use is always going to find some challenging boarderline cases at the overlapping points. Vesta is one example at one extreme, Upsilon Andromedae c is another at the other end of that spectrum.

    Nu Ophiuchi the latter a system of objects with minimum masses 21.9 and 24.5 times Jupiter in a 2:1 resonance .(bracket removed for clarity.)

    FWIW, I’d say that Nu Ophiuchi (b & c I presume?) as described there was *very* clearly a pair of brown dwarfs given both are well in excess (minimum!) of the eleven or so Jupiter mass limit for any deuterium fusing based on your linked paper. (Paper found in comment # 17.)

    Just as stars can orbit other stars , and do planets and brown dwarfs. I don’t have any issue with saying that many brown dwarfs are very hard to tell apart from planets other than the fact that they do or did once fuse deuterium which is the defining trait of a brown dwarf and not something that even the most Superjovian of gas giants can do.

    Similarly observations of open clusters indicate that the star formation process produces objects down to a few Jupiter masses, which implies the existence of stars that never undergo core nuclear fusion.

    Or, alternatively, that gas giants like stars can be directly formed from such nebulae. It seems silly to me to say that an object with the mass of a gas giant is a star just because of how it was born if it is un-star-like in so many other key respects.

    In short, defining categories based on the ability of an object to undergo nuclear fusion leads to utter confusion.

    I disagree. I think that helps clarify and make critical distinctions between objects with other overlapping traits.

  90. Messier Tidy Upper

    @86. Nigel Depledge :

    No-one is forcing you to discuss it.

    Equally Nigel, no one is forcing you to read and respond to what I say.

    Your blathering about it here in Phil’s blog isn’t going to make the IAU take any notice.

    Well, you never know! ;-)

    Some pretty impressive people do read the Bad Astronomy blog after all and hopefully if there’s a critical mass of smart people thinking and discussing the question it may hopefully lead somewhere. Beats the alternative in my opinion.

    Not really. Most people either don’t know or don’t care how the IAU defines planet for technical discussions among professional astronomers.

    That is your opinion anyhow.

    Bear in mind that it may well be at variance with reality and isn’t mine. A lot of people have asked me about Pluto’s status and a lot of the public find it baffling, sad and objectionable even if they won’t argue it as passionately as I will. Many people do care – and given your vigorous and relentless arguing for the IAU definition it appears to me like you are among them too.

    Wow. you’ve used both absurd and ridiculous in the same sentence. I can’t refute that! Oh, wait, I already have.

    Only in your own mind. As I’m sure you already realise I think you’ve failed toprove your case here just as much as you seem to think I’ve failed to prove mine. We’re just going to have to disagree on this one.

    If you are having this definition imposed on you, I would like to know exactly who it is imposing it and how they are enforcing that imposition. ‘Cos, AFAICT, no-one is forcing you to use the IAU definition.

    I don’t. The IAU clearly are the one’s trying to impose a definition by virtue of their political authority over astronomers everywhere – I think there’s a fallacy named for that action of theirs. ;-)

    Fine, go tell it to the IAU. I have no influence with them, and I’m bored with your weak arguments and your refusal to acknowledge valid refutations of those arguments.

    Your assessment of the strength of my arguments and yours is your own – and is, I think, erroneous.

    I think I’ve answered allyour argumenst and refuted tyour claims against what I’ve said.

    I think we both have about the same minimal say when it comes to the IAU but FWIW I *have* actually emailed them on this issue.

  91. Messier Tidy Upper

    @85. Nigel Depledge :

    BTW, compositionally, Pluto is probably far more like a comet than it is like any of the eight planets. Also, of the 9 objects that ’til but recently were called planets, Pluto is the only one with significant orbital eccentricity. It is the only one that orbits with an inclination more than 5° from the ecliptic. If, as you insist, its orbital context does not matter regarding its definition, then the only difference between a comet and Pluto is that Pluto is the largest “dirty snowball” out there.

    Have you already forgotten that Pluto has moons – five of them we now know – plus possibly rings and perhaps a subterreanean ocean and that Pluto can and does retain an atmosphere and is almost certainly geologically differentiated unlike comets and much more?

    Yep, there’s plenty of differences ‘twixt Pluto and comets there! ;-)

    No Pluto isn’t just a big cometary nucleus.

    Put Pluto near the sun, and it would develop a halo and tail. That it is in hydrostatic equilibrium is a fluke of timing. Given a couple of million years of closer exposure to the sun and it probably would lose enough mass to cease being round.

    There are Hot Jupiter gas giants and Mustafar -type rock dwarf exoplanets for which this is also true because they they are close enough to their stars to have their outer layers stripped off and blown away. Would you call them “comets” too? I don’t think so.

    Indeed, this may very well one day be true of Earth and Venus as our Sun balloons into a red giant and expands to engulf most of the inner planets.

    When Pluto or Venus or Osiris or one of the hotter and more imperiled Kepler worlds lose too much mass to retain hydrostatic equilibrium then they’ll become ex-planets sure – but until they’re round they count in my book.

    But the fact remains that Pluto doesn’t get close enough to the sun to lose mass and develop a halo and a tail, so it isn’t a comet. It instead remains the largest known KBO.

    It remains a planet, an ice dwarf variety of planet, the commonest and perhaps leats understoodf and well known type of planet but a planet nonetheless. That’s my view and one shared by many astronomers and others.

  92. Messier Tidy Upper

    @87. Frank :

    As for the “planet” thing, who bloody well cares?

    Many people do – me for one.

    It’s [Planet-ed] just a random word that got slapped onto some objects that circle the sun and not others. It originally referred to any moving heavenly object (including the sun), and then it got applied to eight or nine objects that don’t have a whole lot in common with each other (see: Jupiter and Mercury) except that they circle the sun. Historically, “planet” has meant little more than “a thing in space that isn’t a star.”

    Planet is from the Greek word for wandering star – planetes. It referred originally to those moving lights in the sky other than Earth and Sun and Moon. Other than the “hairy stars” or comets too.

    Eventually though we discovered the asteroids , built “brick moons” the artificial satellites including Sputnik, the International Space Station and Hubble Space Telescope and so many more. Plenty of moving lights in the sky now that aren’t stars – or planets either.

    They’ve tried to narrow down the definition over the years, but any definition they arrive at is going to be entirely arbitrary.

    Well kinda but not really. No more than for any words.

    If we have a definition that is logical and consistent and doesn’t result in absurdities such as Earth not being a planet if it happened to be in a more distant Plutonian orbit or exoplanets not technically being counted as planets at all and not contradicting other astronomical terminology such as dwarf stars counting as stars still then surely that’s not exactly just “arbitarary “and is much better for understanding things, right?

    @84. Nigel Depledge :

    MTU (72) said : “Except that it is wrong to call Pluto a comet. Pluto is something very different and much larger and lacking in tails and boasting five moons and so on. Yes N.d.G. Tyson is wrong about that – and his meanness and belittling of those disagree with him over Pluto cost him a lot of respect from me.”

    But you use exactly the same rhetorial ploys when you deride the IAU’s definition.

    Not true. I may mock the arguments of the other side but not the people adn I don’t think it is fair to characterise my behaviour here as “mean” whereas N.d.G. Tyson mocked and belittle dschoolchildren writing to protest his dumping of Plutofrom a display. There was an interview I saw on one documentary which cost Tyson a huge amount of “decent human being” respect in my eyes. Here were kids being interested and passionate in ascientific issue and Tyson was being just plain cruel to them.

    @ « Close-up of a solar monster
    “How the Universe Works” Season 2 premier tonight! »
    A fifth moon for Pluto!
    Astronomers have just announced that tiny Pluto has a fifth moon! It was discovered using the Hubble Space Telescope:

    You can see it in that image (click to enhadesenate) in the green circle. Pluto was targeted by HST for several observations in late June and early July, and P5 – also called S/2012 (134340), the moon’s designation until it gets a proper name – was seen moving around the tiny world. This image is from July 7.

    As moons go, it isn’t much: it’s probably only about 10 – 25 kilometers (6 – 15 miles) across, making it one of the smallest moons detected in the entire solar system. That’s actually pretty amazing, given Pluto was 4.7 billion km away (2.8 billion miles) when these images were taken!

    Pluto was observed in part to look for more moons. In 2015, the New Horizons probe will zip past Pluto, and scientists want to know as much about the system as they can before it gets there. The odds are low of them hitting any of those moons – space is big, and the moons and spacecraft are small – but a) better safe than sorry, and 2) if there are more targets to observe we want to know now so they can be added to the itinerary!

    Observations like this are good for discovering moons and getting their locations, but size is a different matter. Literally. We know how far away the moon is, and how bright, but it’s far too small to directly get the size. Its diameter has to be estimated by assuming how reflective the surface is. If it’s dark like coal, it has to be bigger to be so bright, and if it’s shiny like ice, it’s smaller. That’s why we don’t know P5′s size to even within a factor of 2! But once New Horizons zips past, it may be able to nail down the size far better.

    The first moon of Pluto, Charon, was discovered in 1978. Nix and Hydra were found using Hubble in 2006, and the fourth moon just last year, in 2011.

    As for the argument about Pluto being a planet or not, this will no doubt provide grist for the mill. However, number of moons does not a planet make; Mercury and Venus have none and they’re planets. Mars has twice as many as Earth does, but it’s not twice the planet! And many very small asteroids have moons, too.

    My feelings about this are on record: the word “planet” is not and can not be defined; it’s a concept, not a definition. It’s like the word “continent”: it’s more of an idea than something you can rigidly define. There is no sharp border that you can use to divide objects into planet and not planet.

    So I actually don’t care if you call Pluto a planet or not. It is what it is: a very cool object, perhaps the biggest in the Kuiper Belt of frozen icy comet-like bodies past Neptune. It’s an oddity, since it’s so bright, and yes, has so many moons.

    And it’s absolutely worthy of study, no matter what you call it.

    Image credit: NASA, ESA, and M. Showalter (SETI Institute)

    ——————————————————————————–

    Related Posts:

    – Pluto has another moon!
    – The unbearable roundness of being
    – BAFact math: How bright is the Sun from Pluto? (and the followup, How big does the Sun look from Pluto?)
    – Shining shoes for NASA

    537 435Share Share91July 11th, 2012 10:12 AM Tags: Hubble Space Telescope, planet, Pluto
    by Phil Plait in Astronomy, Cool stuff, Piece of mind, Pretty pictures, Science | 95 comments | RSS feed | Trackback >

    95 Responses to “A fifth moon for Pluto!”
    1. Messier Tidy Upper Says:
    July 11th, 2012 at 10:18 am
    Superluminous (beyond merely brilliant!) news!

    I am jubilant at this latest felictious find. Always love hearing about my favourite planet & its realm has just expanded yet again and become even more wonderful than it already was. Which was very.

    Pluto is a remarkable, astounding little world, small and eccentric as it may be. Pluto will always be a planet to me.

    Five moons now. Five – Charon, Hydra, Nyx, P4 as yet unnamed and now P5.

    Guess the New Horizons team just got busier – and can’t wait until 2015 to see it all close up!

    *****

    “…Marc Buie can very easily imagine what it must be like to walk around on Pluto: with less than 1% of your weight on Earth because of the low gravity, at temperatures of 230 degrees below zero, in the twilight because the Sun is nothing more than a dazzling star in the black sky, across snowfields of methane ice and transparent crystals of frozen nitrogen and with a gigantic moon hanging overhead – at least if you are on the right side of the planet.”
    – Page 61, ‘The Hunt For Planet X’, Govert Schilling, Copernicus Books, 2009.

    “During its summer, the frozen nitrogen on Pluto evapourates to
    create a temporary atmosphere. With the onset of winter the nitrogen turns to frost and falls back to the surface. On Pluto the winter weather doesn’t merely deteriorate – it completely disappears.”
    – Page 19, ‘The Planets’, McNab & Younger, BBC Worldwide Ltd., 1999.

    “Pluto spins on its axis once in 6.3 Earth days and this means that a Plutonian year has 14,164 Plutonian days.”
    – Sir Patrick Moore, Page 34,“Puzzling Pluto” in ‘Astronomy Now’ magazine, February 2011.

    2. Ori Vandewalle Says:
    July 11th, 2012 at 10:42 am
    Phil, isn’t Eris the biggest KBO?

    3. Mikey Says:
    July 11th, 2012 at 10:43 am
    How precisely do we define “moon” for that matter? Each pebble or boulder zooming around a planet doesn’t count as a moon, or else the rings of Saturn would provide it with billions.

    For that matter, how big is the second largest natural object orbiting the Earth? Is there anything between the Moon and a speck of dust?

    4. andy Says:
    July 11th, 2012 at 10:54 am
    also called S/2012 (134340),

    Surely that should be S/2012 (134340) 1?

    5. Mark Anderson Says:
    July 11th, 2012 at 11:05 am
    The dwarf planets are certainly worth study. There is a lot of weird stuff going with the things they capture in their gravitational pull.

    It’s like if our moon had a natural satellite. I don’t care what you call it, it would be neato.

    6. Messier Tidy Upper Says:
    July 11th, 2012 at 11:05 am
    FWIW. My preferred planet definition is that a planet is an astronomical body that :

    1) Is round or rounded through its own gravity and thus not a comet or asteroid.

    2) Has never been capable of being self-luminous at visible wavelengths due to core nuclear fusion thus not a star or brown dwarf.

    3) Is not directly orbiting another planet and thus is not a moon.

    Three fairly simple, easily & quickly determined criteria.

    As opposed to the IAU’s “orbital clearence” rubbish one which violates Occams razor given it requires enormous clarifications, and the reductio ad absurdum logical test and is immensely unfair on planets like Pluto in larger orbits and overly generous on worlds like Mercury in smaller ones. The, in my opinion utterly ridiculous, IAU definition of planet basically depends far too much on where you orbit and not enough on what an object actually is.

    Orbital clearance also depends on other variable factors such as how much material a given planetary system has to begin with and what stage in its history youobserve it and has many other flaws as well.

    If using my preferred – although not original to me definition – that means we have many more planets in our solar system than we used to think – most of them ice dwarfs like Pluto as opposed to rock dwarfs like Earth and gas giants like Saturn then, hey, what’s really wrong with that?

    @2. Ori Vandewalle : “Phil, isn’t Eris the biggest KBO?”

    Both are actually about the same size diameter~wise, Eris has a tiny smidgin more mass and it would seem is vastly more reflective even than Enceladus.

    I’ll also note that since Eris no ice dwarf worlds larger than or equal to Pluto have been discovered in the Edgeworth-Kuiper Cometary belt region.

    Plus if you run through a checklist of features you’d expect of planets Pluto meets virtually all of them – moons – now 5 of them 3 more than Mars and five more than Venus and mercury, an atmosphere witha nitrogen compositionreminiscentof our own and precipitation even, geologically diffferentiated and possibly active, maybe rings even and more.

    If it looks like a duck and quacks like a duck ..

    7. Mike Says:
    July 11th, 2012 at 11:09 am
    Hi Phil,

    Could the chaotic workings of Pluto, Charon and theses other bodies have picked up this new body from the Kuiper belt through some chaotic inter (dwarf)planetary transport network? In other words, do you think it been there since we started observing Pluto?

    -Mike

    8. andy Says:
    July 11th, 2012 at 11:13 am
    Phil, isn’t Eris the biggest KBO?

    (136199) Eris is not a Kuiper Belt Object, it belongs to the Scattered Disc.

    9. beanfeast Says:
    July 11th, 2012 at 11:16 am
    I was just pondering the planet definition and I now wonder if Earth would be classified as a planet if it occupied Pluto’s orbit and what is the minimum size a body would have to be, to be classified as a planet in Pluto’s orbit?

    10. Tyler Hatch Says:
    July 11th, 2012 at 11:28 am
    We need some kind of classification system for planets. Something like:

    Class I: All Jupiter-sized planets and larger ones
    Class II: Neptune and Uranus sized planets
    Class III: Anything about the size of Earth
    Class IV: Mercury, Ganymede, Titan
    Class V: Most other moons and Pluto
    Class VI: Anything smaller, including asteroids

    11. Frank Says:
    July 11th, 2012 at 11:39 am
    A distant lump of dirty ice has been discovered next to another distant lump of dirty ice. Truly, this is a momentous day.

    12. Richard Drumm The Astronomy Bum Says:
    July 11th, 2012 at 12:12 pm
    At some point, Phil, you’re going to run out of “Click to enlarge.” variants…
    #ButTodayIsNotThatDay
    #HashtagUsedOutsideOfTwitter
    #BadRichard!

    13. Renee Marie Jones Says:
    July 11th, 2012 at 12:30 pm
    Planets are like Big Brother.

    Winston to interrogator: “Does Big Brotber even exist?”
    Interrogator: “Of course he exists.”
    Winston: “No, I mean, does he exist like you or me?”
    Interrogator: “You do not exist.”

    14. Johnny Says:
    July 11th, 2012 at 1:12 pm
    So we can see distant galaxies in more detail than Pluto. Something doesn’t add up.

    15. Keith Bowden Says:
    July 11th, 2012 at 1:33 pm
    As for Pluto, of course it’s a planet. It’s right there in the name: dwarf planet. Dwarf is the adjective, planet the noun, ergo: it’s a planet. =}

    16. Invader Xan Says:
    July 11th, 2012 at 1:49 pm
    Irrespective of what you might call it, Pluto has a fascinating system of moons in tow. And they’re all so tiny, too… Also Pluto now has two moons which still need names. Personally, I’d vote to call them Cerberus and Persephone!

    17. andy Says:
    July 11th, 2012 at 2:01 pm
    1) Is round or rounded through its own gravity and thus not a comet or asteroid.

    2) Has never been capable of being self-luminous at visible wavelengths due to core nuclear fusion thus not a star or brown dwarf.

    3) Is not directly orbiting another planet and thus is not a moon.

    Three fairly simple, easily & quickly determined criteria.

    Ok, bearing in mind the various dependencies of the onset of deuterium fusion, please classify the following objects as planets/not-planets according to your rule 2:

    HD 217786b: only known companion of an F8V star at an orbital distance of 2.38 AU. Minimum mass 13.0±0.8 times Jupiter

    HAT-P-13c: second substellar companion in order of distance from a G4-type star in a hierarchical system of 2 planets. Mass at least 14.5±1.0 times Jupiter.

    Upsilon Andromedae c: second substellar companion in order of distance from an F8V star in a system of 4 planets. Mass 14.57 times Jupiter.

    BD+20°2457 b and c: a 2:1 resonant system consisting of objects of at least 12.47 and 21.42 times Jupiter orbiting a 2.8 solar mass red giant star.

    18. Frank Says:
    July 11th, 2012 at 2:13 pm
    We’ve known about the Kuiper Belt for years now, and not one KBO has been named “Yuggoth.” I’m going to have to ask the astronomical community to hand in its collective geek badge.

    19. Darren Says:
    July 11th, 2012 at 2:53 pm
    @14 johnny “So we can see distant galaxies in more detail than Pluto. Something doesn’t add up.”

    At its closest, it’s about 30AU from sun or 29AU from earth. I don’t know about an average galaxy, but we would have little trouble imaging galaxies at 10Mparsecs. That means the galaxy is about 10^11 times farther away.

    Our galaxy is 100k light years across, while pluto is something around 2500km. That works out to a factor of 10^14 times larger. So on a size/distance calculation, the galaxy is 1000 times larger than pluto.

    20. Hubble descubre una quinta luna de Plutón « Series divergentes Says:
    July 11th, 2012 at 2:56 pm
    […] A fifth moon for Pluto!, Bad Astronomy Sharéalo:Share on TumblrCorreo electrónicoMe gusta:Me gustaBe the first to like this. […]

    21. Dragonchild Says:
    July 11th, 2012 at 4:29 pm
    Dear Pluto,

    I’m happy to hear the news that you have yet another member of your family. This is a joyous occasion for all. Yes, yes, it’s a special miracle and a little bundle of joy and all that. But if I may be so bold as to ask a favor, please refrain from making any more of these announcements WHILE I’M STILL DRIVING TO YOUR GODDAMN BABY SHOWER FOR P4.

    In jest,

    The Guy Coding New Horizons’ Observation & Approach Maneuvers
    New Horizons Project Planning Team

    /not really

    22. AliCali Says:
    July 11th, 2012 at 5:32 pm
    A lot of people are hung up on the definition of planet. As Dr. Plait pointed out, it’s like defining a continent. The definition is more historical (and leads to hysteria) than anything else.

    The time to hung up on definitions is if classification leads to something useful. For instance, we classify mammals, egg-laying creatures, plants, etc., and that’s useful when figuring out how they evolved and learning about various functions of the organisms. We classify spectra of stars, and once the classification was correct, we got a lot of useful information such as brightness, lifespan, method of death, etc. We’re looking for patterns by grouping items together.

    For the classification of planet vs. dwarf planet or asteroid or whatever, does it lead to anything useful? Will we see patterns because we’ve grouped Mercury through Neptune together sans asteroids, or does it make no real difference? Classifications can change around until we see patterns and get useful information, such as gas giant vs. rocky planet. Since we only have one system of planets that we can study in good detail, it’ll be hard to know how to classify these objects yet.

    At least getting the public worked up about something in astronomy means more public attention to the field and maybe causes more people to look up.

    23. amphiox Says:
    July 11th, 2012 at 5:41 pm
    1) Is round or rounded through its own gravity and thus not a comet or asteroid.

    How round is round? How far off spherical does an object need to be before it is not considered round?

    Does Vesta, which is oblong (hence, “rounded”) but fairly far off spherical (hence, not “round”) count?

    What happens to a world flattened by a very rapid rotation rate, like Saturn but more extreme?

    What about a gas giant pulled into an egg shape by tidal forces due to a very close orbit around its star?

    What about two planets on the bottom end of the scale, of equal mass, but different composition, one mostly ices and the other mostly rock, with the icier one rounded by its gravity, but the rocky one, being made of stiffer material, not? Is one a planet then and the other not?

    Everything gets complicated at the edges.

    24. Steve D Says:
    July 11th, 2012 at 5:43 pm
    As a geologist, I like the continent analogy. Even classically, the continents were arbitrary. Europe-Asia-Africa are one connected land mass as are the Americas. In plate tectonic terms, a continent is underlain by continental crust and bounded either by oceanic crust or a plate boundary. Greenland, New Zealand and Madagascar are continents and New Guinea is part of Australia, since the intervening sea is underlain by continental crust. Corsica and Sardinia are microcontinents, detached pieces of the Alps, but Sicily is part of Africa. Spain was a continent for much of the last 150 million years but now seems firmly attached to Europe. Iceland is not a continent, but a thick mass of oceanic crust, and Hawaii is a hot spot volcanic land mass. Cuba and the Greater Antilles are accumulated masses of material at former subduction zones. That’s one way continents grow, but they’re not continents. Same with much of Indonesia. Borneo, the Philippines and Japan are now detached pieces of Asia. Continents? Arguably. Technically Baja California and coastal California count, but they’re not often called a microcontinent (microplate, all the time, though.)

    Considering that we have about 800 exoplanets, it should be obvious that the definition of a planet is far from settled. Is an Earth mass object in a young solar system that has not cleared its orbital neighborhood a planet? What about a recently impacted large object that has not yet reached hydrostatic equilibrium?

    25. Peter B Says:
    July 11th, 2012 at 6:55 pm
    Regarding definitions of planets, I can see that it’s always going to be tricky to include a mass definition for them, as objects in space presumably exist at every mass from “grain of dust” up to “large enough to be a star”.

    However, I was wondering, why is there no mention of the ecliptic in planetary definitions? The inner eight planets all orbit the Sun within a couple of degrees of each other, while Pluto’s orbit is tilted at about 30 degrees. To me that’s a good enough reasonto separate ‘planets’ from ‘others’.

    26. Peter B Says:
    July 11th, 2012 at 7:08 pm
    Frank @ #11 said: “A distant lump of dirty ice has been discovered next to another distant lump of dirty ice. Truly, this is a momentous day.”

    Well, it moved [i]you[/i] enough that you wrote about it.

    Anyway, you’re the first person to mention “momentous”. The Bad Astronomer didn’t. He talked about how “…scientists want to know as much about the system as they can before [New Horizons] gets there…” Would it perhaps be momentous if New Horizons discovers incontrovertible evidence of an alien presence on P5?

    And anyway 2, what’s wrong with celebrating the discovery of another moon of a distant planet (or dwarf planet, or whatever)? Or are you one of these people who thinks all space exploration should be stopped so we can build more hospitals?

    27. Alan D Says:
    July 11th, 2012 at 7:17 pm
    The could have been diplomatic, especially given the impossibility of a true definition for “planet,” and simply grandfathered Pluto in as a planet.

    Clear skies, Alan

    28. Dragonchild Says:
    July 11th, 2012 at 7:18 pm
    @22. AliCali
    “A lot of people are hung up on the definition of planet. As Dr. Plait pointed out, it’s like defining a continent. The definition is more historical (and leads to hysteria) than anything else.”

    What bugs me more than anything else is the REASON — that otherwise we’d have “too many planets”. Wuh? You mean like when there were only four elements?

    Did we re-classify the rare earths because otherwise there’d be too many elements? Did we re-classify the bones because, wow, if you think 30+ planets are a lot, there are 206 bones in the human body! There are THOUSANDS of species; we ought to re-define life so we don’t have keep track of so many.

    I think for all the excuses, the real issue is that an easy-to-remember fact that people were taught in school has been forever destroyed. If anyone’s getting upset about that, they probably shouldn’t be involved in science.

    29. Grand Lunar Says:
    July 11th, 2012 at 8:48 pm
    I like how you put it, regarding the term “planet”, Phil, with it being more of a concept than a definition.

    I’m in league with Neil Tyson, referring to Pluto as “king of the comets” (though I personally prefer it being one of the largest KBOs) rather than “puniest planet”.

    30. Messier Tidy Upper Says:
    July 11th, 2012 at 11:55 pm
    @23. amphiox :

    (BTW.Numbering your questions for convenience in reference later, hope that’s okay.)

    I.) How round is round? How far off spherical does an object need to be before it is not considered round?

    Very discernibly to the unaided human eye. If at a glance it looks round or ellipsoidal for fast rotators then it qualifies provided this roundness is due to sufficent mass and thus gravity (Ie. hydrostatic equilibrium) rather than structural co-incidental morphology. (eg. Ida’s moon Dactyl.)

    II) Does Vesta, which is oblong (hence, “rounded”) but fairly far off spherical (hence, not “round”) count?

    Vesta is certainly right on that boarderline between visibly round enough and not quite there. It appears to have internally differentiated geologically and been active and some astronomers are certainly willing to call it a planet – one did in an Astronomy magazien article recently if I recall right. I’d err on the side of being more inclusive than not given the strangeness and diversity of the universe we live in so, yes, I’m prepared to provisionally class Vesta as a planet.

    III)What happens to a world flattened by a very rapid rotation rate, like Saturn but more extreme?

    It counts as a planet as should be clear from the phrasing used. Another analogous example here are stars such as Achernar, Regulus and Altair that are egg-shaped due to their extreme rotation rates but remain being called stars nonetheless!

    31. amphiox Says:
    July 12th, 2012 at 12:08 am
    re #24;

    If one applies the current IAU criteria plus the same logic user for classification of stars, then an earth sized object in a young solar system that has not yet cleared its orbit, but would be able to given time and assuming no unforeseen disruption, should be considered a protoplanet, just as protostars have not yet achieved fusion in their cores, but will given time, and just as earth itself was a protoplanet before it finished clearing its orbit prior to the moon creating impact with Theia, even though it would have already been almost Venus-sized at that point.

    32. amphiox Says:
    July 12th, 2012 at 12:17 am
    re #30;

    Personally I would prefer a lower limit definition set as a specific mass cutoff, using the hydrostatic equilibrium of an idealized planet of uniform composition absent rotation or outside gravitational influence. It’s a little more complicated at the start in coming up with the definition but once that is settled subsequent classification becomes easier.

    After all, for practical purposes that is how we are differentiating brown dwarfs from giant planets – if it is bigger than 14J we’re essentially calling it a brown dwarf without making any attempt to figure out if it ever managed deuterium fusion or not in it’s history.

    33. amphiox Says:
    July 12th, 2012 at 12:28 am
    Another analogous example here are stars such as Achernar, Regulus and Altair that are egg-shaped due to their extreme rotation rates but remain being called stars nonetheless!

    Ah, but hydrostatus equilibrium isn’t part of our definition for stars. If we somehow discovered a star that was square-shaped due to some alien super-tech, it would still be a star by lieu of its fusion, irrespective of its shape.

    The star analog in this scenario would be a case of a star that had previously been fusing happily away but somehow had some external force cause fusion to stop in its core, perhaps the case of say a very small red dwarf star getting some of its mass sucked off by a companion blackhole or neutron star, just enough to drop it below the approximate 80J threshold such that hydrogen fusion stops in its core. Would it still be a star then, or does it get reclassified into a brown dwarf?

    What if instead of a dense companion permanently drawing away its mass, it was impacted by a planet in such a way that some of its mass was blown off temporarily into a ring around it, just enough to drop it below the mass threshold necessary for fusion and stopping fusion in its core, but with the ring eventually falling back down onto the star, putting its mass back over the threshold and letting it start fusing again. Does it stay a star throughout or should it change from star to brown dwarf and back to star, or from star to protostar and back to a main sequence star?

    34. Messier Tidy Upper Says:
    July 12th, 2012 at 12:34 am
    Continued Part II @23. amphiox :

    NB. Link to one source for the flattened stars – ‘The Lion’s pumpkin-shaped heart
    Rapid rotation flattens Regulus” by Ken Croswell — Published: January 19, 2005 Astronomy magazine online – in my name for this comment.

    IV)What about a gas giant pulled into an egg shape by tidal forces due to a very close orbit around its star?

    Yes, I would still count that or indeed a rock dwarf or ice dwarf type planet as a planet in those circumstances. A planet that’s natural state has been distorted by external forces but if all things are equal an object would be round, non-luminous by core fusion and no moon then, yeah, planet.

    V) What about two planets on the bottom end of the scale, of equal mass, but different composition, one mostly ices and the other mostly rock, with the icier one rounded by its gravity, but the rocky one, being made of stiffer material, not? Is one a planet then and the other not?

    I would say so, yes, one of those boarderline objects would be a planet and the other would miss out. YMMV natch.

    Although as you’ve know I’m prepared to err on the generous side where there’s doubt such as the case of Vesta.

    Everything gets complicated at the edges.

    At fine enough levels of resolution yes. You are right there.

    There are certainly boaderline cases and perhaps we need to consider reinstating the word ‘Planetoid’ for objects like Vesta that fall into them at the small end.*

    Pluto however is, in my view, very definitely *inside* the planet category and, in fact, a relatively large planet being the largest or equal largest of the ice dwarf category of planet notably larger than the planets Haumea, Sedna, Ceres, etc ..

    ———————————————————–

    * For the large end maybe the term ‘sub-star’ best fits for objects such as Upsilon Andromedae c.

    35. amphiox Says:
    July 12th, 2012 at 12:36 am
    It also occurs to me that, since planet has always been a “I know it when I see it” concept like continents, there actually isn’t really that much scientific merit or utility in trying to pigeon hole a restrictive definition for the entire category of “planet”.

    Far more useful would have been if the IAU spent their time developing definitions for sub-classes of planets that would be useful in exoplanet classification (Categories like Jovians, Ice Giants, Super-Earth’s, Terrestrials, Ice dwarfs, etc)

    A distinction should be made between objects that gravitationally dominate their local space (moons and Lagrange-point companions would therefore not affect the definition) and objects that do not and are just a part of a much larger swarm of material, but such a distinction can easily be made among sub-classes, and all of them can still be planets.

    36. amphiox Says:
    July 12th, 2012 at 12:40 am
    re #34;

    Wow. Achernar looks arguable even more oblong than Vesta!

    It’s kind of surprising that a star could spin itself that fast to flatten itself that much and not tear itself apart….

    37. Messier Tidy Upper Says:
    July 12th, 2012 at 1:11 am
    @ ^ amphiox : Yep, sure is! That’s one of my favourite astronomical illustrations & facts. It gets mind bending trying to imagine such non-spherical stars!

    From similar astronomical artworks based on sound science I gather that, among other examples, Sheliak (Beta Lyrae – Kaler link in name here) is similarly distorted although in its case times two due to close gravitational orbit of two B-type stars like your exmaple /question IV. Spica and many others may be similar although wherever they ever reach Achernarean extremes of flatness I don’t know. Mira and some red giants are also distinctly misshapen which again, strikes me as rather wonderful thing to try and visualise.

    @Part II – continued @23. amphiox :

    A couple of other key things to note about my preferred definition of planet – it doesn’t ignore exoplanets or demand a planet needs ato orbit a star – actually very specifically only our Sun – as the current IAU definition does and thus includes the many “rogue” / “orphan” / “ejected” exoplanets that may NOT orbit stars at all but be free-floating in space.

    The way the IAU definition rules out exoplanets completely quite literally by definition and its being so pre-Copernican, principle of Medicority violating always has me facepalming at just how ridiculous their definition is.

    The universe being so diverse and bizarre and surprising; a definition that allows most room for flexibility and inclusion especially at the equivalent of the animal-mineral-vegetable stage clearly is to be preferred surely?!

    38. puppygod Says:
    July 12th, 2012 at 1:12 am
    Is Charon even a Pluto’s moon? I mean, shouldn’t they be considered rather a binary planets system? It’s not so much that Charon orbits Pluto, as both of them orbit around the common center of mass – which is outside of Pluto. As far as I know, it’s the one and only such case in the whole solar system. Even Earth-Moon system (second largest in solar system when it comes to planet-moon ratio) has common center of mass located under the surface of Earth.

    So, instead of pushing for calling the Pluto planet, maybe push for calling Pluto-Charon binary planets? That’s way more awesome.

    39. Nigel Depledge Says:
    July 12th, 2012 at 3:01 am
    Ori Vandewalle (2) said:

    Phil, isn’t Eris the biggest KBO?

    IIUC, Eris is just a smidge smaller than Pluto, although it was initially thought to be larger than Pluto.

    40. ctj Says:
    July 12th, 2012 at 3:04 am
    putting the whole “planet” debate aside, is there any coherent theory as to how pluto (or the pluto/charon system) is acquiring all of these moons? i know the kuyper belt is theorized to contain billions of comets, but at that distance, i believe it’s more sparsely populated than the inner solar system.

    so how is pluto, with such a tiny mass, capturing all of these objects? alternatively, they seem to lie in the same orbital plane, suggesting they formed together (and pluto would seem to have insufficient mass to force them into equatorial orbits by tidal action) – so how did a system with so many moons yet with such low mass form in such an odd orbit and so far from the sun? and if pluto’s orbit was caused by an interaction with neptune or some other giant, how did the system’s solar orbit get so perturbed without disrupting the system?

    i can’t wait for new horizons to get there. pluto is just so weird and fascinating!

    41. Nigel Depledge Says:
    July 12th, 2012 at 3:26 am
    MTU (6) said:

    FWIW.

    Oh, dear.

    I must confess I was delighted to read your post #1, because you left aside the defintion of planet altogether.

    But you couldn’t let it lie, could you?

    My preferred planet definition is that a planet is an astronomical body that :

    1) Is round or rounded through its own gravity and thus not a comet or asteroid.

    How round is round? The Earth is not a perfect sphere, and neither is it a perfectly oblate spheroid. It has mountains, therefore it’s not perfectly round. Mars has even bigger mountains, so it’s even less perfectly round. Where do you draw the line? Well, obviously, at some arbitrary point.

    I raise this objection here because it is logically identical to your objection to the gravitational clearance criterion. It is hypocritical to accept the gravitational roundness criterion while simultaneously rejecting the gravitational clearance criterion.

    A line must be drawn somewhere, and that line is drawn at some aribtrarily-defined point. Certainly this is the case for hydrostatic equilibrium, because our solar system contains objects of many different sizes.

    The gravitational clearance criterion, however, actually recognises a naturally-existing demarcation among solar-system objects. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Ouranos and Neptune are self-evidently a different class of objects from Pluto, Eris, Ceres and so on.

    2) Has never been capable of being self-luminous at visible wavelengths due to core nuclear fusion thus not a star or brown dwarf.

    3) Is not directly orbiting another planet and thus is not a moon.

    I agree with these two. Mostly.

    Three fairly simple, easily & quickly determined criteria.

    Poppycock.

    Your first criterion is a minefield of arbitrary delineations. How sure are we that Ceres, for example, really is round? I think we are pretty sure about that, since it is by far the largest main-belt object. How about some of the larger KBOs, like Eris, Haumea and Makemake? How sure are we that these are gravitationally round? Or at least, round enough to meet your criterion?

    In the IAU definition, it doesn’t really matter – they’re KBOs and some of them might also count as dwarf planets. In your definition, whether they are planets or not would hinge on detailed knowledge of the shape of these objects. Knowledge that we don’t currently have. So, what use is your definition? Your first criterion certainly isn’t simple, and it certainly isn’t easily-determined.

    As opposed to the IAU’s “orbital clearence” rubbish one which violates Occams razor given it requires enormous clarifications, and the reductio ad absurdum logical test and is immensely unfair on planets like Pluto in larger orbits and overly generous on worlds like Mercury in smaller ones.

    First off, the gravitaionl clearance criterion no more violates Occam’s razor than does the gravitational roundness criterion, which you unquestioningly accept. IOW, they both violate it to some extent (and it may be argued that the gravitaional roundness criterion violates it more, although I’d prefer not to go there).

    Second, what has fairness got to do with whether an object counts as a planet or not. Are you afraid to hurt its feelings? Or do you believe that Pluto has a stake in what we puny humans call it? No, this is animism of the worst kind. Fairness is irrelevant. Usefulness matters. Any definition that counts Pluto in the same class of objects as Earth and Jupiter is not very useful.

    The, in my opinion utterly ridiculous, IAU definition of planet basically depends far too much on where you orbit and not enough on what an object actually is.

    And, as I have pointed out previously, the context in which an object orbits matters. This argument is empty, and your attempt to fill that void with rhetoric only highlights the hollowness of the argument.

    Orbital clearance also depends on other variable factors such as how much material a given planetary system has to begin with and what stage in its history youobserve it and has many other flaws as well.

    No, because the IAU definition applies solely and specifically to our solar system. For a very good reason : ours is the only system for which we have adequate knowledge to be able to apply a definition.

    If using my preferred – although not original to me definition – that means we have many more planets in our solar system than we used to think – most of them ice dwarfs like Pluto as opposed to rock dwarfs like Earth and gas giants like Saturn then, hey, what’s really wrong with that?

    In and of itself, nothing. Apart from the fact that you end up with a host of KBOs whose categorisation remains unknown until we have more mission to the Kuiper Belt to take the relevant measurements.

    You have succeeded only in persuading me that the IAU is onto something by the poor logic and emotional appeals in your arguments against the IAU definition.

    42. Messier Tidy Upper Says:
    July 12th, 2012 at 3:33 am
    @30.MTU :

    Vesta is certainly right on that boarderline between visibly round enough and not quite there. It appears to have internally differentiated geologically and been active and some astronomers are certainly willing to call it a planet – one did in an Astronomy magazien article recently if I recall right.

    Aha! Found it – Jim Bell, professor of Astronomy and Planetary Science, Universityof Arizona :

    “Many planetary scientists refer to Vesta as the “smallest terrrestrial planet” despite the fact that the International Astronomical Union has not officially designated it as even a dwarf planet.”

    Source : Page 32, “Dawn’s Early Light : A Vesta Fiesta” article by Jim Bell in ‘Australian Sky & Telescope’ Feb/ March 2012.

    This is echoed by Keith Cooper who notes :

    Some people have described Vesta as the smallest terrestrial planet and what theymean by that is it has a core,mantle and crust like Mercury,Venus, Earth and Mars.”

    Source : Page326, “Visiting Vesta” article by Keith Cooper in ‘Astronomy Now’magazine, July 2011.

    I’ll also add that Isaac Asimov has refered to Ceres as a planet as well notably here :

    “… he had left out a planet. It was not his fault; everyone leaves it out. I leave it out myself when I list the nine planets, because it is the four-and-a-halfth planet. I’m referring to Ceres; a small but respectable world that doesn’t deserve the neglect it receives.”
    – Page 63, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by
    Isaac Asimov, Mercury Press, 1973.

    Looking at recent illustrations and maps it seems that certainly from some angles Vesta appears very much spehrical although somewhat less so from others.

    43. Messier Tidy Upper Says:
    July 12th, 2012 at 3:58 am
    @41. Nigel Depledge :

    You have succeeded only in persuading me that the IAU is onto something by the poor logic and emotional appeals in your arguments against the IAU definition.

    Well that’s a shame and also erroneous on your part not mine. How is my logic “poor” and my arguments merely “emotional” exactly? Clearly we’re not going to agree on this question and we have argued it at length before on other threads here but I really do not understand how you can draw such contrary conclusions from what I’ve said on this. It baffles and frustrates me that you can seriously say this after my detailed, reasonable, in depth logical deconstruction of your arguments for the IAU’s absurd definition.

    How round is round? The Earth is not a perfect sphere, and neither is it a perfectly oblate spheroid. It has mountains, therefore it’s not perfectly round. Mars has even bigger mountains, so it’s even less perfectly round. Where do you draw the line? Well, obviously, at some arbitrary point.

    If you see a cricket ball and it doesn’t appear round to you then you have a problem. Same here. If a planet is visibly spherical to the human eye or an approximation of that due to its own gravity then its round enough to qualify. No roundness isn’t perfect, yes, there are mountains and seas and stuff like that but we all know what is meant by round and even the IAU adopted that as part of their definition too. If you want to get mega-technical and call it being in ‘hydrostatic equilibrium’ then okay but it means the same basic thing.

    It is hypocritical to accept the gravitational roundness criterion while simultaneously rejecting the gravitational clearance criterion.

    No it isn’t . Nowhere close. Being round is an intrinsic easily determined quality whereas “orbital clearance” is much harder to define and determine and depends on a lot more extrinsic variable factors such as

    A – How far away a planet orbits,
    B – How old the age of the planet or system is.
    C – How much material was there to start off with.
    D – How clear is clear and what about NEOs, sungrazing comets, trojans, etc ..
    E – The influence of other nearby Planets and planetary histories as far as pertubations etc goes. Say for instance a wandering exoplanet strays into our solar system from interstellar space – does every planet whose orbit it crosses from Jupiter to Earth – automatically cease to be a planet thereby?
    This all seems so immensely obvious that I am amazed you, an otherwise intelligent person,actually put that comparison of these two criteria forward with a straight face – or are you just winding me up?

    44. Messier Tidy Upper Says:
    July 12th, 2012 at 5:05 am
    Oh & to that alphabetically ordered list add the probability of double planets existing – scaled up versions of our Earth-Moon & Pluto-Charon systems which are sometimes called “double planets” themselves.

    Remember that we’re discovering exoplanets in surprisingly close (& getting closer) proximity to each other all the time. See for instance :

    http://www.abc.net.au/science/articles/2012/06/22/3531203.htm

    for an astoundingly close Super-Venus and Hot Neptune duo around Kepler 36.

    Plus :

    http://www.space.com/8840-alien-planets-gather-close-dying-star.html

    For HD 200964 and 24 Sextantis – separate cases btw.

    & my favourite example :

    http://kencroswell.com/HD45364.html

    HD 45364 b & c, exo-Saturns orbiting an orange dwarf star in a way analogous to Neptune and Pluto ‘s orbital relationship.

    Oh & before ruling that planetary collisions are just hypothetical try and recall where our planets’ unusually large Moon came from!

    (Additionally too, remember that we also have observed evidence of excess dust created by planetary collisions around other stars fro just one instance in the case of the white dwarf NLTT 43806 – linked to my name here.)

    45. Messier Tidy Upper Says:
    July 12th, 2012 at 5:05 am
    Part II @41. Nigel Depledge :

    Your first criterion is a minefield of arbitrary delineations. How sure are we that Ceres, for example, really is round? I think we are pretty sure about that, since it is by far the largest main-belt object. How about some of the larger KBOs, like Eris, Haumea and Makemake? How sure are we that these are gravitationally round? Or at least, round enough to meet your criterion?

    Because we can both observe and calculate their morphologies. I’ve already answered this in #43 btw so you are repeating yourself, Nigel.

    Yeah, we can tell whether something is round or not and if we’re not sure well we put a question mark over it then go check using our spacecraft and telescopes and our science. Pluto is round (or hydrostatic equilibrium) as is Ceres and Eris. How sure are we? Very.

    If, Mr Depledge, you are – seemingly desperately and emotionally – disputing this known roundness of Ceres and Pluto’s and Makemake, etc .. which is based on repeated calculations and observations and accepted by the IAU hence them qualifying for their current status, then its up to you to show cause why they’re not round. Plus to explain how the HST among other key scientific instruments and observatories got it wrong and back up your extraordinary claim.

    In your definition, whether they are planets or not would hinge on detailed knowledge of the shape of these objects. Knowledge that we don’t currently have.

    Not that detailed actually and yes we do. Look at images of Ceres and Pluto and the others. Again, are you really *seriously* going to argue that these worlds aren’t in hydrostatic equilibrium – round enough – when even the IAU accepts they are?

    First off, the gravitaionl clearance criterion no more violates Occam’s razor than does the gravitational roundness criterion, which you unquestioningly accept. IOW, they both violate it to some extent (and it may be argued that the gravitaional roundness criterion violates it more, although I’d prefer not to go there).

    Really? really?

    See my response at # 43 which says it all. Orbital clearance is violating Occams law and raises a whole slew of unnecessary superflous questions demanding problematic and arbitrary answers that roundness – a visble immediate intrinsic trait does not.

    46. Nigel Depledge Says:
    July 12th, 2012 at 5:29 am
    Beanfeast (9) said:

    I was just pondering the planet definition and I now wonder if Earth would be classified as a planet if it occupied Pluto’s orbit and what is the minimum size a body would have to be, to be classified as a planet in Pluto’s orbit?

    If an Earth-sized object occupied the orbit of Pluto, it probably would not meet the gravitational clearance criterion. I have no idea what mass of object would be the minimum to meet the criterion, but Neptune certainly would – as it is, Neptune’s gravity has a strong influence on Pluto’s orbit. Neptune’s mass is pretty close to 10^26 kg.

    However, this does not stop anyone from calling Pluto a planet in casual conversation. The IAU definition only applies to the technical work of professional astronomers.

    47. Messier Tidy Upper Says:
    July 12th, 2012 at 5:35 am
    Continued – Part III @41. Nigel Depledge :

    Second, what has fairness got to do with whether an object counts as a planet or not.

    Fairness here = consistency and making reasonable, logical divisions not unfiar IOW inconsistent and illogical ones. It is unfair –inconsistent and unreasonable – to say that planet orbiting at 1 AU becomes a non-planet if simply if it orbits at 30 AU or 100 AU. It is unfair – inconsistent and illogical – to say a planet ceases to become a planet if another object wanders across its path. Do you understand what is meant by “fair” there now?

    Are you afraid to hurt its feelings? Or do you believe that Pluto has a stake in what we puny humans call it?

    No. Neither of those.

    Pluto is an inaminate lump of rock and ice with certain physical and chemical and orbital properties. Of course – and equally, of course, people are human.

    I care about astronomy and astronomers and I think this debacle has hurt us by making astronomers look ridiculous – because it was a ridiculous decision.

    Astronomers and others defending a ridiculous decision and saying it wasn’tas absurd as it clearly was only makes astronomers look ever sillier. Correcting it and admitting the IAU got it wrong would help and would be in the best interests of astronomy and its relations with the public.

    Thinking of emotions about pluto though I could and, heck, will pose the opposite question to you : Why are you so keen to bash Pluto and its supporters? Are you and is your side in this being objective or are you perhaps more influenced by your emotions than you care to admit?

    No, this is animism of the worst kind.

    What the .. ? Animism? What’s animism got to do with anything?

    Where do you pull that non-sequiteur from and what do you have to back that extraordinary claim up with?

    Fairness is irrelevant. Usefulness matters. Any definition that counts Pluto in the same class of objects as Earth and Jupiter is not very useful.

    What basis do you have for saying that? Saying Pluto, Earth and Jupiter are planets is equally as useful as saying Barnards Star, our Sun and Eta Carinae are stars and Halley’s , McNaughts and Shoemaker-Levy-9 are all comets. Its a starting point in the fundamental astronomical classifications which we can then move on to finer distinctions in. Jupiter and Pluto are different types of planets just as Barnard’s Star is a different typoe of star from Eta Carinae and Shoemaker-levy9 is -or rather was – a different sort of comet to Comet McNaught. But the star-planet-comet division is a useful starting point.

    How is it more useful to misclassify Pluto as an asteroid or comet which it is, in fact, very much NOT?

    48. Nigel Depledge Says:
    July 12th, 2012 at 5:42 am
    Johnny (14) said:

    So we can see distant galaxies in more detail than Pluto. Something doesn’t add up.

    Well, if a distant galaxy is 100,000,000 times farther away, but 100,000,000,000 times larger than Pluto, we would expect to be able to see more of its structure.

    Let’s see . . .

    Pluto’s diameter is 2274 km (from nineplanets.org).

    A light-year is about 10^12 km, or about 440,000,000 Pluto diameters.

    Our galaxy is about 100,000 light-years across, or 44 trillion times the size of Pluto.

    So, most galaxies will be about 10^13 – 10^14 Pluto diameters in size.

    Pluto’s average distance from the Sun is roughly 5.9 billion km (again from nineplanets.org).

    The nearest large galaxy (the great spiral of Andromeda) is about 2,000,000 light years away, or about 339 million times as far away from the sun as Pluto’s average distance.

    Most other galaxies, therefore, will be about 10^9 – 10^12 times as far away from us as Pluto is. So even a relatively small galaxy (10^13 Pluto diameters) at a very great distance (10^12 Pluto orbital radii) will be an easier target in which to resolve detail telescopically than is Pluto itself.

    What doesn’t add up?

    49. Messier Tidy Upper Says:
    July 12th, 2012 at 5:55 am
    @41. Nigel Depledge :

    “The, in my opinion utterly ridiculous, IAU definition of planet basically depends far too much on where you orbit and not enough on what an object actually is.”- MTU
    And, as I have pointed out previously, the context in which an object orbits matters.

    If you accept the IAU definition which for reasons I’ve already explained I don’t.

    A planet is a planet whether it orbits at 1 AU or 100 AU or whether it lacks a star at all. I don’t see why – other than being a moon of a larger body – orbital distance and location in space should define a planet. I don’t think it is reasonable to call Mercury a planet at less than 1 AU but not at 30 AU. Same applies for Earth and the same applies for Pluto.

    I don’t think it is fair to say that if Venus crossed Earth’s orbit for a time then during that time neither of those planets – Earth or Venus – would actually be planets. I don’t think that make sense. Ditto if (or *when* by many models!) Saturn crosses Neptune’s orbit or Pluto crossed Mars’ one.

    I don’t think it is consistent to define a dwarf star as a proper star but a dwarf planet as a non-planet.

    I think all those things are unreasonable and therefore the IAU definition which does those things or leads to those conclusions is unreasonable and thus needs to be rejected.

    I don’t understand why you seem to have so much trouble following and accepting this logic. And the above is definitely logic not mere “hollow rhetoric” as you insultingly and wrongly describe it.

    50. Shepard Says:
    July 12th, 2012 at 6:08 am
    That’s no moon! It’s the Charon Mass Relay waiting to be discovered…

    51. Neil Haggath Says:
    July 12th, 2012 at 6:10 am
    #38 Puppygod:
    You’re correct; Pluto and Charon are indeed the only pair of bodies in the Solar System, whose barycentre lies in empty space. So whether you choose to call Pluto a planet or a dwarf planet, it and Charon should really be called a binary planet/dwarf planet, rather than a planet and satellite.

    52. El Hubble descubre una quinta luna de Plutón | Noticias CEU Says:
    July 12th, 2012 at 7:20 am
    […] (Vía Bad Astronomy). […]

    53. El telescopio espacial Hubble descubre una quinta luna de Plutón | Noticias CEU Says:
    July 12th, 2012 at 7:20 am
    […] (Vía Bad Astronomy). […]

    54. Nigel Depledge Says:
    July 12th, 2012 at 8:12 am
    MTU (43) said:

    Well that’s a shame and also erroneous on your part not mine.

    You have claimed this before, but repeatedly failed to show it to be so.

    How is my logic “poor”

    I will come to this.

    and my arguments merely “emotional” exactly?

    That you say it is unfair to exclude Pluto while including Mercury is an appeal to emotion.

    The fact that your arguments are littered with rhetoric – such as the frequent derogatory adjectives you use in reference to the IAU definition – is a debating trick to frame the debate emotionally rather than objectively and it has no place in science.

    Clearly we’re not going to agree on this question and we have argued it at length before on other threads here but I really do not understand how you can draw such contrary conclusions from what I’ve said on this.

    When the IAU announced their definition, I really didn’t much care one way or the other.

    Your frequent IAU-bashing tirades on the topic made me pay attention.

    As I read and began to understand some of the points used to counter your arguments, I began to notice that you had no substantive answer to the counter-arguments and your preferred definition is no better logically than the one that you so passionately criticise.

    It baffles and frustrates me that you can seriously say this after my detailed, reasonable, in depth logical deconstruction of your arguments for the IAU’s absurd definition.

    Here you go again. You call the IAU’s definition absurd but you have not shown this.

    You have often used hypothetical scenarios to construct absurdly-defined planets and not-planets, but you ignore the fact that the IAU’s definition applies solely and specifically to our solar system, rendering these scenarios irrelevant.

    I have deconstructed all the points you have raised in detail, and not one of your points withstands critical scrutiny. If you were to raise an argument that I could not refute, then I would seriously consider that your position on this topic might be the stronger one. As it is, the very flaws in your arguments have turned me to supporting the IAU.

    You have attempted to answer the points I make, but not one of your answers has the power to convince.

    How round is round? The Earth is not a perfect sphere, and neither is it a perfectly oblate spheroid. It has mountains, therefore it’s not perfectly round. Mars has even bigger mountains, so it’s even less perfectly round. Where do you draw the line? Well, obviously, at some arbitrary point.

    If you see a cricket ball and it doesn’t appear round to you then you have a problem.

    Irrelevant. Cricket balls don’t come in the same wide array of sizes and shapes as planetary bodies. For the bodies of the solar system, there will be a grey area in which bodies are of a size to be almost-round, and there must be an arbitrary boundary. That this boundary must occur at different masses (and diameters) for rocky and icy bodies does not help your case.

    Same here. If a planet is visibly spherical to the human eye or an approximation of that due to its own gravity then its round enough to qualify. No roundness isn’t perfect, yes, there are mountains and seas and stuff like that but we all know what is meant by round and even the IAU adopted that as part of their definition too. If you want to get mega-technical and call it being in ‘hydrostatic equilibrium’ then okay but it means the same basic thing.

    I have heard the term, but bodies with dynamic surfaces (such as Earth and Io) may or may not actually be at hydrostatic equilibrium.

    You seem to be addressing the detail while bypassing the point. The point is that there has to be a dividing line between round and not-round. Because of the range of sizes of objects in the solar system, this dividing line does not have a natural position, but must be arbitrary. So, for example, Earth and Earth’s moon are both close enough to count as round. Phobos and Vesta (for example) are clearly not round. But there are objects between these sizes, and it seems reasonable to expect that there are asteroids and KBOs still to be discovered (or discovered but still to be fully characterised) that indeed fall into this grey area.

    Thus, the dividing line between round and not round is arbitrary.

    You have objected to the gravitational clearance criterion because it applies an arbitrary division between clear and not clear. Or, to take a more pragmatic approach to the same criterion, between an orbital region that is gravitationally dominanted by one body and an orbital region that is not.

    Ceres, for instance, is round (or pretty close to round), but it does not dominate the region in which it orbits, whereas Earth, Jupiter, Saturn and even Mercury do gravitationally dominate the respective regions in which they orbit. You have raised trivial objections regarding Jupiter’s Trojans, but their orbit is quite obviously dictated by Jupiter’s gravity, similar to the way in which the orbits of Phobos and Deimos are dictated by Mars’s gravity.

    The orbits are not perfectly clear, in the same way that Earth is not perfectly round. The imperfections on Earth’s surface are several orders of magnitude smaller than the size of Earth itself, and the same applies to Mars, Venus and Mercury. Similarly, the masses of bodies that share the orbital region of each of the eight planets is several orders of magnitude smaller than the mass of the dominant body (in fact, Earth’s moon comes closest to being of comparable mass to its dominant body, and is still more than 2 orders of magnitude less massive than Earth – and the moon’s orbit is quite clearly dictated by Earth’s gravity).

    However, there are sure to be bodies in the solar system for which the deviations from perfect roundness (or from being a perfect oblate spheroid) are perhaps only 1 order of magnitude less than the diameter of the body. In other words, there almost certainly exists a continuum of objects that transitions the range from round to not-round. Whereas for orbits that are clear versus not-clear there is an obvious demarcation. Earth and its moon are on one side of a gap (an orbit where Earth dominates everything that orbits within that region but where one of those other bodies is of substantial mass, being just under 1% of Earth’s own mass), while objects such as Pluto and Ceres are on the other side of the gap (objects that do not dominate the other bodies that orbit within that region). And the gap is a perfectly natural division within our solar system.

    Since the two division have approximately the same basis (on the one hand, an arbitrary distinction between round enough and not round enough; and, on the other, an arbitrary distinction between gravitationally dominated and not gravitationally dominated, albeit informed by a natural demarcation that could be mere chance), any objection to one criterion on the basis of its logic is perforce an objection to the other criterion too.

    It is hypocritical to accept the gravitational roundness criterion while simultaneously rejecting the gravitational clearance criterion.

    No it isn’t . Nowhere close. Being round is an intrinsic easily determined quality

    So you claim, but this is not so.

    For example, how many KBOs are in hydrostatic equilibrium? The answer is we have no idea. Probably several of the known KBOs are indeed round enough to meet our criteria. You mentioned earlier that if an object looks round to the eye, that is good enough, but this ignores the three-dimensional nature of solar-system bodies when compared to our often two-dimensional views of them. Some of those distant bodies may be round from one viewing angle, but not round from another. We just do not know. So your claim that this is an easily-determined criterion is false.

    whereas “orbital clearance” is much harder to define and determine and depends on a lot more extrinsic variable factors such as

    A – How far away a planet orbits,
    B – How old the age of the planet or system is.
    C – How much material was there to start off with.
    D – How clear is clear and what about NEOs, sungrazing comets, trojans, etc ..
    E – The influence of other nearby Planets and planetary histories as far as pertubations etc goes. Say for instance a wandering exoplanet strays into our solar system from interstellar space – does every planet whose orbit it crosses from Jupiter to Earth – automatically cease to be a planet thereby?

    But this is stuff we pretty much already know. We know what other large objects orbit in the same regions as the eight planets, therefore we have the information to make this distinction.

    So this is easier to determine than your roundness criterion. We already know that, irrespective of how large an as-yet-to-be-dioscovered KBO is, it does not gravitationally dominate the region in which it orbits.

    Comets, NEOs and Trojans are subsumed in the pragmatic application of the clearance criterion, being mostly pretty small objects, in the same way that we are able to dismiss small deviations from roundness in the gravitational roundness criterion. “How clear is clear” is a direct parallel to “how round is round”, from the point of view of their logical basis.

    For example, IIRC, Jupiter’s trojans are collectively only about one ten-thousandth the mass of Jupiter. Whereas Pluto shares its orbital region with several other objects that are each substantial fractions of Pluto’s own mass. Sure, Pluto has moons whose orbits are dictated by Pluto’s gravity, but that’s only a tiny portion of Pluto’s orbital region. As a parallel situation, we are able to dismiss Mars’s mountains as deviations from roundness because they are so small in relation to the planet’s diameter, whereas the lumps and bumps and protrusions on the surface of Phobos are so large relative to the object itself that the object is not round.

    Regarding a wandering exoplanet – well, such an object is likely, if large enough, to disrupt existing orbits and throw the whole solar system into chaos, but if such an event were to occur, I daresay that those IAU members who wish to could vote t

  93. Messier Tidy Upper

    ^ Arrrrrrggghhh! What the blazes?!? :-o

    That wasn’t supposed to happen and don’t know how the hades it did. Just copied and pasted from MaDeR’s #81 and got all that instead and out of editing time to fix it. Sorry. :-(

    I don’t suppose you could fix that last comment by remove the extra material that wasn’t supposed to be there by any chance Phil Plait? Please?

    For clarity :

    @81. MaDeR :

    “Under my definition orbiting a star – any star – is NOT required” -MTU -ed.
    *rereads* …damn. Okay, that was brainfart and I apologize for this particular mistake. My general opinion about you still stands, unfortunately.

    As does my general and very low opinion of you too, MaDeR.

    Sort of good to see you admit you messed up on that point but you need to apologise for calling me a liar as well. I know I have my faults, I can be wordy, stubborn and argumentative at times but lying is NOT one of them and I’d like to see you acknowledge that and apologise for your false acusation against me. :-(

    I think the rest of what you wrote there, MaDeR, I’ve already covered in other earlier comments.

  94. Messier Tidy Upper

    Ah well, once again @81. MaDeR in the remote hope that MaDeR might actually get what I’m saying now and to clarify to anyone whose confused :

    First I heard about some nebulous “secondary trait”. Essentially you are claiming body with moon is more planet than body without. This is pure bunk.

    Its pure bunk to say that that is what I’m claiming.

    Perhaps its too subtle for you, MaDeR but I’m noting that if there’s a checklist of features that you’d expect to see for a planet then moons would be on that checklist along with other things such as mass, an atmosphere, geological differentaition, etc .. Having a moon doesn’t make an object a planet but OTOH, if an object ticks a number of planet-feature boxes including moons then its more likely to be a planet than not. Comprehend now?

    Using “it have lotsa moons” as an argument for Pluto planethood is lie, because neither IAU nor your definition says anything about moons.

    Hence the whole secondary thing I noted that moons aren’t a defining trait of a planet but instead a secondary suggestive indication that object X may well be a planet if it has moons *and* other features including those meeting the defining criteria too.

    Being round depends on a lot of variables too (spinning speed, density of body, composition of body – ice rounds easier than rock – etc).

    Yes, roundness does depend on such variables and I’ve not argued that it doesn’t. I’ve actually made allowences for those such as with the rapid spin of ice dwarf planet Haumea. What those variables have in common though is that they are internal properties intrinsic to that body and part of what make it, it – NOT external factors such as its orbit.

    No, redefining it as “hydrostatic equlibrum” will not help.

    Actually for objects like Haumea as referred to above it will very much help.

    No, “outside planet control” is lamest shadow of pretender to excuse that I ever seen. It does not matter if planet changes on its own or from external forces.

    Why MaDeR what asurprise ( :roll ) that’s mere rhetoric and unsupported rude assertion from you NOT you providing any logical argument for why you think so or evidence to back that up.

    Again, define frog say. How do we do that? We do it by it having certain key features such as a particular body structure and morphology and skin type. We don’t do it by saying a frog is found only in one place – since we know they are found in many different environments and if we take a frog and mutate its DNA into something new (say turning its skin hard and scaly so it doesn’t have to be kept wet and we change its shape into a bipedal frog-like humanoid!) then we say we’ve changed it from being a frog into something else that isn’t a frog anymore.

    Or take a tree. We define a tree by certain features such as height, internal structures such as tree rings and so on. We don’t define trees by the places they’re found since they are found on all continents barring only Antartica and when we chop down a tree and turn it into something else it becomes timber or wood and an ex-tree.

    See the pattern? How then, consistently should we define planet?

    Surely by certain intrinsic properties of the planet itself just as frogs and trees are defined – for just as frogs and trees are found in a wide variety of places and environments so too are planets. Just as you’d expect to find certain types of trees in a jungle or a pine forest or a eucalyptus bushland or certain species of frogs in deserts or rainforests or swamps so too would you expect to find certain types of planets in certain environments. Like orbiting a star much nearer (eg. Hot Jupiters) or further out (Earth) or not at all. (Rogue planets.)

    As for extenal forces, again if we do something to a planet that makes it into something else that clearly is different from what it originally was and doesn’t act to define it when it is a planet. Strip off a planet’s outer layers and reduce its mass till its no longer round and it becomes something else. An ex-planet or asteroids or partof its star or scattered into the interstellar medium or whatever. Alderaan stopped beinga planet when it was exploded by the Death Star. Mercury will stop being a planet when it is engulfed in our red giant Sun. That’s reasonable.

    Saying something stops being something just because its found in unexpected places makes no sense just as it makes no sense to say a tree or frog found in a desert isn’t therefore actually a tree or forg when everything else tells you it is. Thatisn’t reasonable at all.

    Pluto is a planet that the IAU are saying shouldn’t count just because of where it is. They’re making the absurd mistake of saying a tree isn’t a tree or a frog not a frog because its found with a whole lot of other trees / frogs / planets in the wrong spot. Which as definitions goes, just doesn’t work.

  95. Nigel Depledge

    MTU (97) said:

    Perhaps its too subtle for you, MaDeR but I’m noting that if there’s a checklist of features that you’d expect to see for a planet then moons would be on that checklist along with other things such as mass, an atmosphere, geological differentaition, etc .. Having a moon doesn’t make an object a planet but OTOH, if an object ticks a number of planet-feature boxes including moons then its more likely to be a planet than not. Comprehend now?

    But what use is such a checklist, when none of the features by itself is indicative of planethood?

    Mercury has no moons, and effectively no atmosphere, so it fails to tick at least two of your boxes. Some asteroids have moons, and Titan has a rich, complex atmosphere complete with weather, so there are planety of non-planet objects that tick at least some of those boxes.

    So who gets to decide how many boxes must be ticked for planetary status to be “likely”? And how does this get anyone closer to making a decision about an object than having a clear definition?

  96. Nigel Depledge

    Frank (87) said:

    As for the “planet” thing, who bloody well cares? It’s just a random word that got slapped onto some objects that circle the sun and not others. It originally referred to any moving heavenly object (including the sun), and then it got applied to eight or nine objects that don’t have a whole lot in common with each other (see: Jupiter and Mercury) except that they circle the sun. Historically, “planet” has meant little more than “a thing in space that isn’t a star.”

    So why all the hoopla? Who cares if this confusing, ill-defined (or entirely un-defined), practically-useless word gets applied to eight objects, or nine, or a million? Is there some special club that we don’t want to leave Pluto out of? Does it clarify anything to call it a planet or not? Does calling it a “planet” tell us anything that “Kuiper belt object” doesn’t? Does it do anything at all but make that old mnemonic about “my very eager mother” work once again? Is it some magic word that makes an object’s residence in our solar system “official?”

    No. The historical meaning was “a wandering celestial body,” which applies to pretty much everything in the universe. They’ve tried to narrow down the definition over the years, but any definition they arrive at is going to be entirely arbitrary. So who cares? Who cares if they restrict the word “planet” to rocky worlds like Earth and Mars, or any round thing that circles the sun, or go back to the original meaning and start calling the sun, moon, and comets “planets?” What practical or informational difference does it make? Why do we even need the word “planet” at all?

    I mostly agree with this.

    The only reason I argue with MTU about it is that I consider his arguments to be poor, and I think it only reasonable that someone should quash his IAU-baching tirdaes from time to time. Obviously, he does not accept my arguments, and I equally obviously do not accept his, but I hope that other people who are interested will have seen both sidfes of the question and reach a decision about (for example) whether or not the IAU has the right to define the term planet for their use in technical discussion.

    At the end of the day, the IAU is not forcing any non-members to use the definition, and most members of the public (or even amateur astronomers) won’t care one way or the other about the IAU definition. The word planet, used in casual conversation, will continue to mean exactly what it has meant for most of the last 300 years.

  97. Nigel Depledge

    MTU (97) said:

    Hence the whole secondary thing I noted that moons aren’t a defining trait of a planet but instead a secondary suggestive indication that object X may well be a planet if it has moons *and* other features including those meeting the defining criteria too.

    Yeah, this kinda shoots your argument in the foot.

    If you have a clear definition of a term, you have no need for secondary characteristics. The IAU definition is clear and leads to no ambiguous cases that require additional characterisation or clarification.

  98. Nigel Depledge

    MTU (97) said:

    Yes, roundness does depend on such variables and I’ve not argued that it doesn’t. I’ve actually made allowences for those such as with the rapid spin of ice dwarf planet Haumea. What those variables have in common though is that they are internal properties intrinsic to that body and part of what make it, it – NOT external factors such as its orbit.

    Hmmm, yeah, but why?

    I mean, why do you dismiss the context within which an object orbits? Or, indeed, the orbital characteristics of the object?

    It is obviously the biggest difference between the four inner planets and Ceres, for example. It’s the main reason that, even before Eris et al. were discovered, some people were questioning Pluto’s status as a planet – because (as far as anyone could tell) Pluto was more like other KBOs than it was like other planets, and unlike Mercury to Neptune, Pluto did not dominate the behaviour of other objects in the same orbital region.

  99. Nigel Depledge

    MTU (97) said:

    Again, define frog say. How do we do that? We do it by it having certain key features such as a particular body structure and morphology and skin type. We don’t do it by saying a frog is found only in one place – since we know they are found in many different environments and if we take a frog and mutate its DNA into something new (say turning its skin hard and scaly so it doesn’t have to be kept wet and we change its shape into a bipedal frog-like humanoid!) then we say we’ve changed it from being a frog into something else that isn’t a frog anymore.

    This is wrong.

    Sure, 50 years ago, species were defined by sets of obvious characteristics, but that’s old hat.

    Now, species are defined by where they are within phylogentic trees.

    So, there was a time when a butterfly was distinguished from a moth by two main charactristics – clubbed (as opposed to feathered) antennae, and day-flying as opposed to night-flying behaviour. This is pretty crude, as some moths have clubbed antennae and some moths fly by day. Now, the edges of such broad categories are defined more by how a species relates to other species than by what individual characteristics that species has.

    Or take a tree. We define a tree by certain features such as height, internal structures such as tree rings and so on. We don’t define trees by the places they’re found since they are found on all continents barring only Antartica and when we chop down a tree and turn it into something else it becomes timber or wood and an ex-tree.

    Again, this is not quite right. Many species or sub-species are indeed defined – at least in part – by where they are found. You only have to browse through the Collins bird guide for 10 minutes to see many species that are named after the place where they are found, and are almost only distinguished from closely-related species by where they are found.

    Besides, your biological taxonomy analogy is crude at best. Biology hads to contend with the issue of individual species containing substantial variation, and with very blurry edges. Jupiter, for example, is not a species that contains intrinsic variation, it is a single object. Your taxonomy parallels rapidly fall down under scrutiny.

  100. Nigel Depledge

    MTU (97) said:

    See the pattern? How then, consistently should we define planet?

    Well, maybe we should ask an international body of professional astronomers . . .

    ;-)

    Surely by certain intrinsic properties of the planet itself just as frogs and trees are defined – for just as frogs and trees are found in a wide variety of places and environments so too are planets.

    As explained in my preceding post, the analogy is not valid.

    [. . . ]

    As for extenal forces, again if we do something to a planet that makes it into something else that clearly is different from what it originally was and doesn’t act to define it when it is a planet. Strip off a planet’s outer layers and reduce its mass till its no longer round and it becomes something else. An ex-planet or asteroids or partof its star or scattered into the interstellar medium or whatever. Alderaan stopped beinga planet when it was exploded by the Death Star. Mercury will stop being a planet when it is engulfed in our red giant Sun. That’s reasonable.

    And yet, despite the fact that we don’t have the capability to do these things, you seem to think it valid to move a planet from an orbit in which it is the single dominant object to an orbit in which it is no longer the dominant object and expect it still to be a planet.

    What if, for instance, Mercury were one of Jupiter’s Trojans? I don’t know for sure, but it seems to me that Mercury is small enough that its behaviour would be dictated by the mass of Jupiter. In your world, it would still count as a planet. Why, when it would have effectively ceased to be an independent major body and become a mere adjunct to the body that dominates that orbit?

    And why would you distinguish such an object from actual satellites, when the only significant difference between a moon of Jupiter and a Trojan of Jupiter is that a moon happens to be close enough to orbit the larger object directly?

    Saying something stops being something just because its found in unexpected places makes no sense just as it makes no sense to say a tree or frog found in a desert isn’t therefore actually a tree or forg when everything else tells you it is. Thatisn’t reasonable at all.

    Except that it is often how subspecies really are defined.

    Your analogy, aside from its intrinsic flaws, does not make a good parallel to our solar system. The duck-billed platypus has many characteristics in common with reptiles, but more in common with mammals, so we classify it with mammals not with reptiles. In fact, modern cladistics aside, the defining characteristic of a mammal is to produce milk: if it doesn’t make milk, it ain’t a mammal. Under your preferred definition, a planet has no single defining characteristic, but is just the accumulation of a collection of characteristics. Whereas under the IAU definition, there is a single defining characteristic: if it isn’t the dominant body within its orbit, it ain’t a planet.

    Sure, the other characteristics matter, just as the possession of skin and feet matter to a mammal, but, in the same way that reptiles and birds also have skin and feet, so too are there other solar-system bodies that are gravitationally round, and plenty of others with heliocentric orbits.

    Pluto is a planet that the IAU are saying shouldn’t count just because of where it is.

    No. This is your skewed interpretation, not the IAU definition. If Pluto dominated its orbital region, it would count as a planet, no matter how far out from the sun it orbits. The reason that Pluto isn’t a planet is that it shares its orbital space with lots of similar objects, and is therefore not a dominant object.

    They’re making the absurd mistake of saying a tree isn’t a tree or a frog not a frog because its found with a whole lot of other trees / frogs / planets in the wrong spot. Which as definitions goes, just doesn’t work.

    And, as I have pointed out, your analogy is the thing that does not work.

  101. Nigel Depledge

    MTU (95) said:

    If we have a definition that is logical and consistent and doesn’t result in absurdities such as Earth not being a planet if it happened to be in a more distant Plutonian orbit or exoplanets not technically being counted as planets at all and not contradicting other astronomical terminology such as dwarf stars counting as stars still then surely that’s not exactly just “arbitarary “and is much better for understanding things, right?

    Erm, maybe.

    Good luck with finding a definition that has no arbitrary cut-off between round and not-round, for example.

    BTW, your argument about “absurdities” has failed, and been shown to be purely superficial. It can show anything you want, just by dreaming up a scenario that doesn’t or couldn’t exist.

    For instance, if we launched the moon into an independent orbit in between jupiter and Saturn, it would count as a planet. Or if we piled up all the biggest asteroids until they merged into a single body, that would count as a planet. Or if we moved all the moons of Jupiter into the same orbit as Mercury, then Mercury wouldn’t count as a planet any more.

    Please stop using this argument.

    @84. Nigel Depledge :

    MTU (72) said :

    “Except that it is wrong to call Pluto a comet. Pluto is something very different and much larger and lacking in tails and boasting five moons and so on. Yes N.d.G. Tyson is wrong about that – and his meanness and belittling of those disagree with him over Pluto cost him a lot of respect from me.”

    But you use exactly the same rhetorial ploys when you deride the IAU’s definition.

    Not true. I may mock the arguments of the other side but not the people adn I don’t think it is fair to characterise my behaviour here as “mean” whereas N.d.G. Tyson mocked and belittle dschoolchildren writing to protest his dumping of Plutofrom a display. There was an interview I saw on one documentary which cost Tyson a huge amount of “decent human being” respect in my eyes. Here were kids being interested and passionate in ascientific issue and Tyson was being just plain cruel to them.

    A difference of magnitude only, not a difference of kind.

  102. Peter B

    Seeing as the ‘define planet’ discussion is still going strong, I was wondering if people might care to respond to the question I asked in post #25: “…I was wondering, why is there no mention of the ecliptic in planetary definitions? The inner eight planets all orbit the Sun within a couple of degrees of each other, while Pluto’s orbit is tilted at about 30 degrees. To me that’s a good enough reasonto separate ‘planets’ from ‘others’.”

  103. Nigel Depledge

    MTU (94) said:

    Have you already forgotten that Pluto has moons – five of them we now know – plus possibly rings and perhaps a subterreanean ocean and that Pluto can and does retain an atmosphere and is almost certainly geologically differentiated unlike comets and much more?

    Yep, there’s plenty of differences ‘twixt Pluto and comets there!

    No Pluto isn’t just a big cometary nucleus.

    Yeah, but as far as we can tell it’s made of the same kinds of stuff. That it may be differentiated and retains some atmosphere etc. is all coincident with the fact that Pluto is so big for a comet.

    So, as I said, compositionally, Pluto resembles comets more than it resembles the eight planets.

    My comments about Pluto’s orbit were just to show how easy it is to list points Pluto has in common with comets.

  104. Messier Tidy Upper

    @105. Peter B : Thing is that we know of many exoplanets as well as ice dwarfs in our own solar system that don’t orbit in the ecliptic plane.

    @106. Nigel Depledge :

    So, as I said, compositionally, Pluto resembles comets more than it resembles the eight planets.

    And compositionally Jupiterand Saturn resemble our Sun more closely than Earth or Pluto or Mercury but that doesn’t make gas giants into stars! ;-)

    Having some things in common isn’t necessarily saying things are the same.

    Yes, Pluto has some similarities with comets as do some exoplanets and other worlds in our solar system – but that doesn’t make Pluto a comet because it has those other key differences.

    (Hmm .. Earth has water and ice, so do comets ergo Earth is a comet by your logic, Nigel? ;-) )

    @103. Nigel Depledge :

    Well, maybe we should ask an international body of professional astronomers . . .

    Or better yet maybe we should ask astronomers who specialise in the area of planets like Jim Bell, professor of Astronomy and Planetary Science, University of Arizona who I quoted in comment #42 here saying Vesta is the smallest terrestrial planet!

    Or how about Alan Stern an expert on Pluto who stated the IAU definition was “idiotic” – his words not mine? ;-)

    Or the many other astronomers who strongly disagree with what one dubious and rushed and not very democratic or representative meeting decreed?

  105. Messier Tidy Upper

    @ 102. Nigel Depledge :

    Sure, 50 years ago, species were defined by sets of obvious characteristics, but that’s old hat.Now, species are defined by where they are within phylogentic trees.

    Well, the old definitions of species work for me and for the purposes of this analogy. Call me old fashioned if you like but I think my analogy and point still stands.

    Many species or sub-species are indeed defined – at least in part – by where they are found. You only have to browse through the Collins bird guide for 10 minutes to see many species that are named after the place where they are found, and are almost only distinguished from closely-related species by where they are found.

    Named sure, and individual species – but we’re talking about a braoder level of categorising here than that. If something is a (generic) frog or tree then that’s what it is regardless of where it is found so you’re badly missing the point.

    We’re not talking species – that’d be the next sub-division dwon where we talk about Hot Jupiters and Eccentric Orbiters and Ice Dwarfs we’re talking “kingdom” level divisions here eg. animals , plants, microbes level of classification.

    Btw. even at the species level a particular species found in different places will be the same identical species if its characteristics (incl. DNA phylogeny which, again, are properties of the actual creature or flora) say it is identical. Biologists then will need to find explanations for why species X is in location Y and Z simultaneously but they won’t just call species X something else because it’s found in a different place as well.

    @103. Nigel Depledge :

    As explained in my preceding post, the analogy is not valid.

    Yes, it is. You’re disgareeing with analogy X doesn’t make it invalid.

  106. Messier Tidy Upper

    @100. Nigel Depledge :

    If you have a clear definition of a term, you have no need for secondary characteristics. The IAU definition is clear and leads to no ambiguous cases that require additional characterisation or clarification.

    Nonsense. I’ve given multiple examples where the IAU definition is unclear and does violate Occams razor as you know. For instance, Earth and, for that matter, Neptune and Jupiter too, are only planets because they happen to be in certain rather atypical orbits around one particular star out of waaay more than octillions of trillions of stars out there.

    Put any planet almost elsewhere else in the cosmos and it automatically stops being a planet for that reason alone? Ridiculous!

    Or have an uncertain amount of extra material arbitrarilyand unclearly defined placed along the path of one planets orbit and it stops being a planet? Absurd!

    Or define away double planets and worlds in collision by definition making them non-planets when clearly they are? Ludicrous! That’s the IAU definition for you and, among other good reasons, why it fails.

    As for the secondary characteristics they’re a *separate* check list of planetary features or indicators and NOT defining criteria or part of my preferred planet definition. You are clearly confused here about what my definition actually is and isn’t. Maybe Nigel, you should pause and re-read what Ive said and reconsider making sure you actually understand what I’m arguing before you debate any further?

    @102. Nigel Depledge :

    Besides, your biological taxonomy analogy is crude at best. Biology hads to contend with the issue of individual species containing substantial variation, and with very blurry edges.

    Astronomy similarly has to contend with “blurry edges” such as classifying boaderline objects like Vesta and the overlapping brown dwarf / Superjovian margin and a huge variety of plantary types and characteristics as we’re increasingly discovering as we find more and more exoplanets.

    Its not an exact direct comparison hence the word analogy – its meant as a tool to help you and others understand what I’m saying here.

    Jupiter, for example, is not a species that contains intrinsic variation, it is a single object.

    Well its a good thing I never said Jupiter was a species then isn’t it! Sigh. :roll:

    OK, Nigel, we have a classification that runs upwards like this this :

    Jupiter is an individual planet.

    Jupiter is one of the gas giant type planets.

    The gas giants (incl. Jupiter) are one type of planet.

    Just as Pluto is an individual planet (kingdom) within a particular genus or planetary type – ice dwarf.

    Come on, its not hard! This is a really basic error you seem to be making here and where you get the impression that I’m saying something other than what I’m actually saying is beyond me. You’re normally smarter than that. Sheesh.

    Your taxonomy parallels rapidly fall down under scrutiny.

    No, that’d be your -Nigel Depledge’s – ability to read and comprehend what I’ve been saying here which falls down under scrutiny actually! ;-)

  107. Messier Tidy Upper

    Continued @103. Nigel Depledge :

    despite the fact that we don’t have the capability to do these things, you seem to think it valid to move a planet from an orbit in which it is the single dominant object to an orbit in which it is no longer the dominant object and expect it still to be a planet.

    Us not being able to do something (currently) isn’t the same as saying it cannot be done or occur naturally and in any case thought experiments are useful theoretical tools. I thought’y you’d conceeded that already earlier no?

    Hint : Einstein’s Relativity, neutrinos and black holes were all imagined as “what if” before they were actually discovered. Deja vu?

    What if, for instance, Mercury were one of Jupiter’s Trojans? I don’t know for sure, but it seems to me that Mercury is small enough that its behaviour would be dictated by the mass of Jupiter. In your world, it would still count as a planet. Why, when it would have effectively ceased to be an independent major body and become a mere adjunct to the body that dominates that orbit?

    Because it would be a round, non-luminous-from-core-nuclear-fusion, non-moon astronomical body that’s why! ;-)

    Yes, I’d still count Mercury in those cirumstances or other trojan planets as planets if they met the defining criteria. Why ever wouldn’t you? What if (when?) we do find such planets maybe Earth-sized or larger – don’t you think they should count?

    And why would you distinguish such an object from actual satellites, when the only significant difference between a moon of Jupiter and a Trojan of Jupiter is that a moon happens to be close enough to orbit the larger object directly?

    Because of that last key point in bold that means it isn’t a moon but a planet instead.

    Same reason a double planet is a double planet system and not a moon hence the specific wording in the definition to allow for such exotic remarkable planets not be unreasonably defined out of existence.

    Whereas under the IAU definition, there is a single defining characteristic: if it isn’t the dominant body within its orbit, it ain’t a planet.

    False. The IAU definition has a number of clauses just as my definition has – roundness and non-fusion being among them. In fact the IAU definition has further unnecessary, counter-productive and unscientific clauses in that they rule out all planets NOT orbiting not only a star but specifically *our* star. As you well know.

  108. @98. Nigel Depledge :

    But what use is such a checklist [of secondary planetray featires -separate tothe defining three criteria liste din comment #6 -ed.], when none of the features by itself is indicative of planethood? Mercury has no moons, and effectively no atmosphere, so it fails to tick at least two of your boxes. Some asteroids have moons, and Titan has a rich, complex atmosphere complete with weather, so there are planety of non-planet objects that tick at least some of those boxes. So who gets to decide how many boxes must be ticked for planetary status to be “likely”? And how does this get anyone closer to making a decision about an object than having a clear definition?

    Its really a separate question just as the familiar “if it looks like aduck and walks like a duck and quacks like a duck it probably is a duck” saying is separate from the biological classification of avian lifeforms of the family Anatidae. (Link in my name.)

    People know a duck when they see it just as they know a planet when they see it.

    How many features , how many boxes to tick off, well none are defining but clearly the more the better! It’s just another approach or way of looking at the issue.

    Just as there are other definitions of planet aside from both my preferred one as listed at #6 here and the IAU’s ridiculous absurdity involving the illogical and Occam’s razor violating “orbital clearance” bunk.

    @99. Nigel Depledge :

    The only reason I argue with MTU about it is that I consider his arguments to be poor, and I think it only reasonable that someone should quash his IAU-baching tirdaes from time to time.

    Good to see I’m not the only one who can’t type! ;-)

    As you know, I do consider your arguments poor and as for quashing tirades well, yeah, its ahot button issue for me. Something I feel very passionate about and will continue to argue.

    Tirades? Meh, that’s your opinion. My milage as you’d expect varies. I hope you’ve noticed that I always support what I say with evidence and what I consider very good reasons and that I’ll discuss this issue, politely and logically if forcefully.

    I’m not silly enough to think the IAU is flawless or always gets it right. I’m not sure why you seem to consider them infallible and it baffles me that you, Nigel Depledge, are so determined to defend what I consider their indefensible definition. (shrugs.)

    At the end of the day, the IAU is not forcing any non-members to use the definition, and most members of the public (or even amateur astronomers) won’t care one way or the other about the IAU definition. The word planet, used in casual conversation, will continue to mean exactly what it has meant for most of the last 300 years.

    Yet there are others such as itseems thepiar of us who do care about this topic.

    Plus you seem to be dismissive of the IAU definition there yourself by saying most people will just ignore it by which logic we should still count Pluto and the ice dwarfs as planets under the commonly accepted definition ignoring the IAU’s technical one.

    A bit like how most people call the tomato a ‘vegtable’ rather than a fruit as it technically is.

  109. @101. Nigel Depledge asked :

    I mean, why do you dismiss the context within which an object orbits? Or, indeed, the orbital characteristics of the object?

    Because an orbit doesn’t define a planet as a planet is – or should be called – a planet wherever its found. (Aside from not being a moon which I do include.)

    This allows for a planet being a planet wherever it is whether orbiting close to its star, far from its star or not around a star at all – just as an aircraft is still an aircraft whether it is in a hanger, on the runway or in flight and that’s true whether we’re talking about a small cessna or a jumbo jet.

    It’s the main reason that, even before Eris et al. were discovered, some people were questioning Pluto’s status as a planet – because (as far as anyone could tell) Pluto was more like other KBOs than it was like other planets, and unlike Mercury to Neptune, Pluto did not dominate the behaviour of other objects in the same orbital region.

    So how big is an orbital region? How do we define that? Why should it matter?

    Pluto does control or influence a lot of space – it has five moons one of them relatively large as well as being in an orbital relationship with Neptune analogous to that of a pair of massive gas giants around the orange dwarf star HD 45364. Should the junior of those exoplanets then be discounted from planetary status despite having thrice Neptune’s mass?

  110. Nigel Depledge

    Peter B (105) said:

    Seeing as the ‘define planet’ discussion is still going strong, I was wondering if people might care to respond to the question I asked in post #25: “…I was wondering, why is there no mention of the ecliptic in planetary definitions? The inner eight planets all orbit the Sun within a couple of degrees of each other, while Pluto’s orbit is tilted at about 30 degrees. To me that’s a good enough reasonto separate ‘planets’ from ‘others’.”

    I’m not sure it’s really necessary. The “gravitational clearance” criterion rules out Pluto, which (IIUC) is the only large solar-system body to have a steeply-inclined orbit. That’s my guess, anyhow.

  111. Nigel Depledge

    MTU (107) said:

    Yes, Pluto has some similarities with comets as do some exoplanets and other worlds in our solar system – but that doesn’t make Pluto a comet because it has those other key differences.

    (Hmm .. Earth has water and ice, so do comets ergo Earth is a comet by your logic, Nigel? )

    I agree that Pluto ain’t a comet. I was merely pointing out that it is easy to list similarities between Pluto and comets.

    In the same way, it is easy to list similarities between Pluto and the eight planets, but the same argument works both ways. The similarities between Pluto and comets don’t make it a comet. In exactly the same way, the similarities between Pluto and the eight planets don’t make it a planet.

  112. Nigel Depledge

    MTU (107) said:

    (Hmm .. Earth has water and ice, so do comets ergo Earth is a comet by your logic, Nigel? )

    Just realised I missed out addressing this bit in #114.

    No. In the same way that Pluto isn’t a comet, thoses similarities don’t make Earth a comet. In fact, you are stretching things a bit there, because Pluto contains many of the same volatile substances that comets contain, whereas Earth only shares some of the less volatile components (water and rock).

    However, you make a very good point seemingly without realising it.

    If that argument holds true about Earth and comets, i.e. the superficial resemblances not making Earth a comet, then it also holds true for some of the arguments you put forth about Pluto being a planet, i.e. the superficial resemblances to some of the planets (such as moons and weather that you often cite as planetary traits).

  113. Nigel Depledge

    MTU (107) said:

    Or better yet maybe we should ask astronomers who specialise in the area of planets like Jim Bell, professor of Astronomy and Planetary Science, University of Arizona who I quoted in comment #42 here saying Vesta is the smallest terrestrial planet!

    Or how about Alan Stern an expert on Pluto who stated the IAU definition was “idiotic” – his words not mine?

    Arguments from authority do not wash. Their arguments may be no stronger than the ones you propose.

    Or the many other astronomers who strongly disagree

    Hey, if a majority of IAU members disagreed with the definition, then I’m sure it would have changed in the last 6 years. The fact that it has not is strongly indicative that the majority of IAU members accept it.

    with what one dubious and rushed and not very democratic or representative meeting decreed?

    This is a separate issue that has nothing to do with whether or not the definition is valuable.

  114. Nigel Depledge

    MTU (112) said:

    Because an orbit doesn’t define a planet as a planet is – or should be called – a planet wherever its found. (Aside from not being a moon which I do include.)

    This is contradictory.

    You count the thing at the focus of the object’s orbit as relevant (i.e. does the object orbit a body that sustains nuclear fusion or not?), yet you dismiss what else shares the orbital region.

    If you were to be purely logical, then you must either include or exclude consideration of the object’s orbit. It is not logical to cherry-pick what aspects of an object’s orbit matter enough to be included in a definition. Either the object’s orbital context matters (including what thing the object orbits) or it does not.

    This allows for a planet being a planet wherever it is whether orbiting close to its star, far from its star or not around a star at all – just as an aircraft is still an aircraft whether it is in a hanger, on the runway or in flight and that’s true whether we’re talking about a small cessna or a jumbo jet.

    This, too, is an invalid analogy. Any manufactured item can be categorised according to sctrictly-known parameters. Solar system objects are naturally-formed, not manufactured. And they naturally form divisions. The four inner planets; the four gas / ice giants; a whole bunch of asteroids in the main belt; and a whole bunch of icy bodies in the EKB. Plus some other small stuff in eccentric orbits (mostly comets) or at stable points on the orbits of larger bodies (Trojans).

    It’s the main reason that, even before Eris et al. were discovered, some people were questioning Pluto’s status as a planet – because (as far as anyone could tell) Pluto was more like other KBOs than it was like other planets, and unlike Mercury to Neptune, Pluto did not dominate the behaviour of other objects in the same orbital region.

    So how big is an orbital region? How do we define that? Why should it matter?

    We define it pragmatically, in the same way we draw an arbitrary line between objects that are round and those that are not round.

    Alternatively, we could define that region by drawing two circles, with radii equal to the aphelion and perihelion distances of the object. At the end of the day, it doesn’t really matter very much, because any sensible definition of the object’s orbital region will give us the same result in terms of the definition of a planet. This is because the solar system has a natural and obvious demarcation between the eight objects that dominate their orbital regions and all the other objects that do not.

    Ask not “why should it matter?”. Ask “why should we dismiss it from consideration?”.

    Pluto does control or influence a lot of space – it has five moons one of them relatively large

    But it does share orbital space with other EKBs that it does not dominate, and Pluto is by far the junior partner in its relationship with Neptune. So, to a certain extent, Pluto’s orbital behaviour is dictated by Neptune’s gravity.

    as well as being in an orbital relationship with Neptune analogous to that of a pair of massive gas giants around the orange dwarf star HD 45364. Should the junior of those exoplanets then be discounted from planetary status despite having thrice Neptune’s mass?

    I am sure that the junior partner of the exoplanet pair exerts more influence over its sibling than Pluto exerts over Neptune. Neptune has something of the order of 10,000 times Pluto’s mass. Neptune quite obviously dominates Pluto, and Pluto will have almost no influence at all over Neptune’s movement.

  115. Looks like there’s a new Pluto classification & planet definition thread now – Attack of the Pluto! posted by the BA on the 25th July 2012 at 1:21 PM – with my comment there linked to my name here.

    Seems another planetary scientist supports Pluto’s planetary status too with this comment :

    “If you’re important enough to have aquired five satellites you’re a planet.”
    – Page 11, New Scientist , 21st July 2012.

    By Kevin Baines, a planetary scientist at JPL recorded in the latest issue of New Scientist magazine on a page long article on the discovery of Pluto’s fifth moon.

    Maybe the best definition of planet if we have to have one is to say that an object is a planet if it isn’t directly orbiting another planet (ie not a moon) and is between the mass of Vesta and the minimum mass for deuterium fusion of about eleven Jupiter’s allowing for metallicity?

    Sound good to y’all?

  116. Messier Tidy Upper

    114. Nigel Depledge :

    In the same way, it is easy to list similarities between Pluto and the eight planets, but the same argument works both ways. The similarities between Pluto and comets don’t make it a comet. In exactly the same way, the similarities between Pluto and the eight planets don’t make it a planet.

    Similarities are suggestive NOT definitive. I think we’re agreed on that, yes?

    What makes Pluto a planet isn’t its similarities although they are indicators of what it may be, what defines planet, well that’s the question! You know my preferred answer to that and why I disagree with the IAUs faulty one.

    @115. Nigel Depledge :

    the superficial resemblances to some of the planets (such as moons and weather that you often cite as planetary traits).

    You saying us having weather, oceans and an atmosphere is just superficial? Geez I’d hate to be without ‘em! ;-)

    Serious answer, well, see above. Suggestive indicators versus defining traits.

    @116. Nigel Depledge :

    Arguments from authority do not wash. Their arguments may be no stronger than the ones you propose.

    Or, worse yet, the ones *you* use! :-P

    Argument from authority eh? Funny, that’s how I see the IAU – unsuccessfully because fallaciously – justifying its rubbish definition!

    What the quote goes to show is that other professional expert astronomers like me think the IAU got it wrong and that smaller planets are planets nonetheless. I am not alone in my thinking here.

    Hey, if a majority of IAU members disagreed with the definition, then I’m sure it would have changed in the last 6 years. The fact that it has not is strongly indicative that the majority of IAU members accept it.

    Or that the IAU is simply ignoring the majority who disagree and that the IAU leadership is unrepresentative of the majority opinion.

    If the Royal Zoological Institite decreed that a duck was actually classed as a type of fish (y’know coz there’s a lot of ducks and fish and they all live in or on water!) would you just accept that even if it was only one loopy president of the RZI and a small cadre of his friends forcing that classification on the rest of us?

    This [the rushed undemocratic nature of the anti-Pluto IAU defining meeting-ed.] is a separate issue that has nothing to do with whether or not the definition is valuable.

    Yes so far as it goes.

    However, it does suggest that the process of coming up with the IAU definition was flawed and a flawed process is likely to lead to a flawed result as in this case it clearly did.

  117. Messier Tidy Upper

    @118. Nigel Depledge :

    MTU (112) said: “Because an orbit doesn’t define a planet as a planet is – or should be called – a planet wherever its found. (Aside from not being a moon which I do include.)”
    This is contradictory.You count the thing at the focus of the object’s orbit as relevant (i.e. does the object orbit a body that sustains nuclear fusion or not?), yet you dismiss what else shares the orbital region.

    Well there’s a pretty huge and clear difference between directly orbiting another planet and not doing so. That’s just axiomatic really.

    Also, no I don’t require a planet to orbit a star at all let alone our Sun like the IAU do so I’m not sure what you’re meaning there and you seem to (again) be misunderstanding me.

    This, too, is an invalid analogy. Any manufactured item can be categorised according to sctrictly-known parameters. Solar system objects are naturally-formed, not manufactured.

    So what? The analogy was illustrative of the general principle. Which is that something reemains thatsomething whether it is loctaed inone spot or another. If you prefer we could use ducks or birds instead for exactly the same point. A duck is a duck whether it is flying amidst a flock in the air or swimming alone in the creek or has been injuredand is sitting ina cage at the vets!

    Same way Pluto is – or should be – a planet whether it orbits where it doers with its flock of fellow ice dwarfs or was somehow found orbiting alone around another star or was not orbiting a star at all.

    And they [solar system bodies] naturally form divisions. The four inner planets; the four gas / ice giants; a whole bunch of asteroids in the main belt; and a whole bunch of icy bodies in the EKB. Plus some other small stuff in eccentric orbits (mostly comets) or at stable points on the orbits of larger bodies (Trojans).

    Or as I like to describe things we have three main types of planet in our solar system – the rocky, the gassy and the icy!

    Sure, there are divisions. Most of the inner planets form their own belt close into our star, theditto the gas giants have their zone and luckily don’t stray fromitas eccentric orbiter typeexoplanets do and the icedwrafs have their own realm beyond that. I don’t see otr think thatmake sice dwraf planets not planets. Planet is a very broad class just as star and galaxy are.

    We define it pragmatically, in the same way we draw an arbitrary line between objects that are round and those that are not round.

    Point is defining “orbital region” raises superfluous unnecessary questions and issues and thereby violates Occam’s razor. Why have it as a condition if we don’t have to and it unduly complicates things?

    But it [Pluto] does share orbital space with other EKBs that it does not dominate, and Pluto is by far the junior partner in its relationship with Neptune. So, to a certain extent, Pluto’s orbital behaviour is dictated by Neptune’s gravity.

    And toacertrainextent our Sun’s trajectory around the galaxy is dictated by Jupiter which doesn’t stop our Sun being a star! ;-)

    You think that sharing orbital space matters, I don’t.

    I am sure that the junior partner of the exoplanet pair exerts more influence over its sibling than Pluto exerts over Neptune.

    Perhaps but I don’t think that is a key factor. They do have the same basic orbital relationship and are analogous.

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