Planet pr0n

By Phil Plait | August 13, 2008 9:00 am

Before we get to the pr0n, I want to remind you of a prize fight with Tyson.

Is Pluto a planet?OK, so not Mike Tyson, but Neil DeGrasse Tyson, astronomer and chief dude at the Hayden Planetarium. He’ll be duking it out — verbally — with planetary astronomer Mark Sykes, and the topic will be how to define a planet, and it’s all part of a bigger conference called The Great Planet Debate to talk about the current state of planetary science, and hopefully to hammer out a better definition of the word "planet".

The debate takes place Thursday, August 14th at 4:30 p.m. EDT and will be streamed live over the web (and recorded for watching later). Other parts of the conference will be streamed and archived as well, so take a look around the site to see what leading scientists are thinking.

I wish I could see it, but seeing as how I’m on a boat floating near the Equator, I’m at something of a disadvantage.

I’m pretty sure the debate between Tyson and Sykes will wind up inevitably be drawn into the topic of Pluto — even if they start it off about planets in general, you just know it’ll go there. It’s a big part of what Neil is known for. He says it’s not a planet, and Sykes, I suspect, will hammer away at the silly and arbitrary "planet" definition created out of whole cloth by the International Astronomical Union a couple of years ago. We know enough about Pluto to know whether it’s a planet or not, but the problem is we don’t have a definition for what a planet is. Well, we do, but it’s silly.

I recently had lunch with Alan Stern, planetary scientist and Principle Investigator for the New Horizons Pluto mission, and our mutual friend and colleague astronomer Dan Durda. Alan and I agree on many things about planetary definitions. For example, under the current rules, for an object to be a planet it has to be able to gravitationally sweep clean its neighborhood. That makes the Earth a planet: any asteroid or whatever that has an orbit similar to Earth’s will, over time, either hit us or get slingshot away by our gravity.

But if you take Earth and put it way out in the solar system, way past Neptune, the volume of space is so much larger that an Earth-sized body cannot sweep up all the material. In that case, we could literally move Earth out a few billion kilometers, and according to the new rules it would no longer be a planet.

And that’s just dumb.

However, Alan and I had a short (but fun) discussion about the boundaries of such definitions. He says, for example, that a better definition is one where a planet is an object whose surface is substantially modified by gravity (note that this is already in the IAU rules). So, for example, an object made of iron and 500 km across has enough gravity that it will form something close to a sphere. But an object made of ice (which is softer than iron) will do that when it’s substantially smaller; maybe 300 km across. So you can have two objects, both the same size, and one will be a planet and one won’t. I think that’s a bit silly, but as Alan and Dan said, at the boundaries you have problems.

I’ve been chewing that over, and now I think that’s my very point. A definition is all about boundaries. How do you divide one thing from another? And with planets — even with stars — that boundary line is pretty fuzzy. Certainly, when you find a ball 12,000 km across, it’s a planet. But what if it’s 400 km across, and only kinda spherical? What do you call it?

Maybe, in the end, planets are like pornography (you were wondering when I’d get to that, weren’t you?): you know ‘em when you see ‘em. But that’s not very satisfying, is it? And if it’s bad for a Supreme Court to say that, it’s certainly no good for science. But I think that may be all we have, because making a definition for what a planet is will always be arbitrary, and the boundaries forever blurry.

On the other hand, after further conversation with Alan, I’m mulling over his point: the universe provides us with an incredible diversity of objects. Classifying them, labeling them, is a good way to be able to do a zoological analysis on them, if you will. When you look for similarities and differences across populations you start to see what makes things the way they are. Heck, I even gave talks on this when I was back at Sonoma State University: how would know the difference between mammals and reptiles if you didn’t label them? And note that you recognize both as being subsets of animals. You may find weird things (platypuses) but they can help you understand the classifications better, divide up the different kinds of animals. What works for biology works for astronomy, too.

A major point I make is that given this, you have to be careful. By labeling something, that puts it in a box in your head. You might miss some interesting feature if you only examine the object in the box. That happened to me years ago with brown dwarfs, when I thought of them as failed stars, and missed the fact that they can have weather like planets do.

Also, it’s hard to deny that the public plays a role in this. They pay the bills, and they want to know if Pluto, Ceres, Quaoar, and Eris are planets or not. I don’t think our science should be driven by public opinion — if they want Pluto to be a planet, that may be too bad if it doesn’t fit the bill. But it would be kinda nice to give them an answer to that, one that can be easily understood. I’m willing to go along with this, provisionally, as long as all the dark dusty corners of the definition are examined. And we have to mind our borders.

So Alan has swayed me. Sortof. But I very much look forward to talking about this with him. That discussion really was a lot of fun, and there is a lot of territory yet to cover.

CATEGORIZED UNDER: Astronomy, Science

Comments (108)

  1. Paul Clapham

    “What works for biology works for astronomy, too.”

    I’m not so sure about that. Biology works to produce species that are separate (at least with animals and plants, maybe not with bacteria so much.) But astronomy? Where’s the borderline between F and G stars? Where’s the mechanism that works to separate them? Well, there isn’t one. Stellar evolution isn’t anything like biological evolution. So that’s why it’s harder to define the borders in astronomy than in biology.

  2. When the planet cools down I’d call it a rock that used to be a planet.

    Whenever I think of a planet I think of an object that is formed mostly from its own gravity (as the current definition says) but also has some type of active geology/atmosphere that is heat-driven. And it has to orbit.

    Asteroids and all those hard, cold objects flying around out there are just rocks with a more complex name. Our moon, for example is a dusty rock.

    Just to be clear, I’m not classifying EVERYTHING as either planet or rock. Obviously there’s a whole lot of other stuff out there.

  3. BMcP

    Pluto will always be a planet in my heart, I just have a soft spot for it. I know that isn’t scientific or anything, but we all can have our weird little eccentricities and mine is the planet Pluto. ;)

  4. Gutav Nyström

    “playpuses”

    Is that “platypuses” you mean, or am I missing something?

  5. TMB

    playpuses? ;-) I can think of too many comments, but I’ll just bite my tongue…

  6. I believe we should destroy Pluto by any means possible in order that it can no longer be called/its name can no longer be changed from/to a planet, a dwarf planet, a plutoid, etcetera.

    I for one am a fan of the idea of sending a fleet of nuclear powered ion engines to encircle Pluto’s rotational equator and calibrated to fire whenever they swing around to the far side of Pluto, thus allowing them to apply constant thrust in the direction of the sun. We should then crash Pluto into Mars. Because it would be wicked cool.

  7. By labeling something, that puts it in a box in your head. You might miss some interesting feature if you only examine the object in the box.

    But the converse is also true: it’s only by classifying things carefully that we start to see patterns. Classifying life on Earth helped us understand evolution and genetics and classifying solar system bodies help us understand formation and on-going processes. It would be neigh impossible to understand things like stellar evolution if we had to treat each star as a new, unique body rather than part of a family of similar bodies. And that’s really the crux of the issue, isn’t it?

    As for the definition that the IAU settled on, it’s flawed to be sure. I think what they were aiming for is insisting that a planet be the gravitationally dominant body in it’s region, which seems reasonable. (It’s a way of saying that the body isn’t part of a swarm of similar bodies nearly the same size, for a start.)

    That said, this debate has become a matter of ego for a lot of people and it’ll never be settled until we turn over a generation of researchers I think. (It’s also gotten almost as bitter as a political campaign; I’ve seen some pretty disparaging things said about dynamicists, for example, because of the debate.)

    Paul: Biology may work to produce separate species, but in reality, that just doesn’t happen. There’s a huge problem trying to define “species” in a way that divides things as researchers want, very similar to defining “planet”.

  8. (Dammit, that first paragraph was supposed to be in a blockquote. Please mentally insert quotes around that bit I quoted from Phil. Grazie.

  9. Joseph Szczesniak

    The problem is, once you find a certain classification (planet) that you can apply to some stuff, like objects that crush themselves into spherical shapes, then you have to start classifying more stuff, like the objects that can’t quite do that, but have orbits like a planet or what ever else the laws of physics can create.

    This is probably good though, I’m hoping the great planet debate looks at more than just what a planet is, but what else we should start looking at again and reclassifying. We’re at a point in astronomy where we have a lot of powerful instruments and have been making a lot of great discoveries, and it’s time to reanalyze it all!

  10. Gustav Nyström

    Well, isn’t craptastic? I spelled my own name wrong!

  11. StevoR

    Here are no less than twelve good reasons I reckon Pluto should be classed as a proper planet :

    (BTW. Feel free to forward this on to Alan Stern or for that matter Tyson when discussing this debate folks! ;-) )

    _* 12 REASONS WHY PLUTO _IS_ A PLANET : *__

    1) The orbital clearing condition which is the reason for eliminating Pluto is fatally flawed because it is itself too hard to define – what is meant by “cleared” & how far from the planet must the orbit be “cleared”? Strictly speaking this eliminates any object in our solar system as all planets (except perhaps Mercury?) have objects – comets and asteroids crossing their orbits, Jupiter has Trojan asteroids, Neptune has Pluto crossing its orbit, Earth has numerous near-earth asteroids such as Eros and so forth. A consistent application of this criterion would exclude all the planets of our solar system!

    2) A reductio ad absurdum approach reveals that this criterion fails because it leads to absurd results ruling out objects we’d clearly consider planets based on their location – a Jupiter or Earth-type planet hypothetically located in the Oort cloud would be excluded yet we’d clearly still call it a planet otherwise! Why then draw the line at smaller objects that would otherwise fit the planetary description ie. rounded by their own gravity and directly orbiting the Sun? (Or their common centre of gravity for “double planets.”)

    3) In relation to forming planetary systems including historically our own, planetary orbits cross and interact in unpredictable ways. By the IAU’s “orbital clearance” criterion, these objects – even ones Jupiter sized and above – are NOT strictly planets because their orbits are not yet cleared – again failing the ‘reductio ad absurdum’ test. Eg : The earth before it was hit by the Mars-sized body that became our moon would NOT have been termed a “planet” because it had that Mars-sized object in its orbital path.

    4) From point 3 above, we see that by IAU definitions planets cannot collide because their neighbourhood then isn’t clear – nor can they exist as binaries or “double planets” by the same logic. This appears contrary to common-sense and consistency. It also has potential for creating trouble with exoplanets given the so-far hypothetical but quite probable possibility that some extrasolar planets may exist in this form – even potentially twin Neptunes or Jupiters. Given that some would describe the Earth-Moon system as well as the Pluto-Charon one as such a ‘double planet’ then a strict definition of the IAU rule may rule our Earth out of planetary status again clearly a ridiculous proposition!

    5) Inconsistency and inapplicability in regard to exoplanets – the IAU definition excluded planets of other stars. Yet surely planets orbiting other suns are no less planets for not orbiting our star! Even more tellingly, at least one of the Pulsar planets, PSR B 1257+12 e is tiny – smaller than our Moon and 1/5th Pluto’s size raising a glaring inconsistency. Given PSR1257+12 e is counted as an exoplanet then Pluto, equally, should equally count as a planet for the sake of consistency.

    6) The “dwarf planet – dwarf” star analogy – just as dwarf stars are still stars so surely are dwarf planets still planets. Extrapolating the “dwarf planets don’t count” line to stellar astronomy would imply the Sun is not a proper star nor are 99 % of all stars – those 90% on the main-sequence and the 10 % of “stellar corpses” such as white dwarfs and neutron stars. Moreover, as with stars, the smaller the object’s size the greater its numbers! Therefore calling a planet “dwarf” should NOT rule it out of being considered a proper planet.

    7) “Classical” problems with the “classical” planets term : the IAU defined “classical”; planets are restricted to our Earth’s solar system and it is hard to see how they apply to exoplanets or how the term can work usefully as a scientific description. Apart from differing immensely – Earth and Pluto are arguably far more similar worlds than Earth and Jupiter or Mercury or Neptune – they also clash with a previous understanding arguably much more apt of classical planets being those visible to the “classical” age peoples – the five original bright wanderers – Mercury, Venus, Mars, Jupiter, & Saturn. If that ‘classical’ term is retained, it seems best used in this sense as a historical and descriptive sense.

    8) Sentimental, cultural and historical reasons – noting Pluto’s long-established and culturally scientific place as a recognised planet from its discovery in 1930 until its (hopefully temporary) demotion in 2006. This also covers the slight to Clyde Tombaugh’s memory, widow and family plus the perceived political aspect of stripping from planetary status the sole planet discovered by an American.

    9) The undemocratic manner in which the IAU ruling was made. Of the 10,000 IAU members only 2,500 attended the Prague meeting that demoted Pluto and rejected the other planetary candidates, Eris, Charon and Ceres from planetary status. Furthermore, of those 2,500 only the merest handful – just 424 actually got to vote making therefore a very unrepresentative decision. Among those to excluded from voting and arguing their case in that last minute meeting were some highly relevant and articulate people – notably Pluto expert Alan S. Stern, head of the New Horizons mission. Stern’s summary of the IAU judgement was blunt : “ … idiotic. I have nothing but ridicule for this decision.” (Alan Stern, P.28, ‘Astronomy Now’, October, 2006.)

    10) The decision to demote Pluto has had a generally negative reception from the general public and on public perceptions of astronomers bringing them into disrepute and making astronomy fans and science writers lives harder!

    11) The first proposed IAU definition of ‘planet’ (that would have included Pluto, Eris and Ceres) was much better in terms of logical consistency and general application as well as being more easily explained, understand and applied – ie. two main criteria for planets are that they are objects circling a star directly which are not themselves stars or brown dwarfs and are rounded by their own gravity.

    12) Pluto is a complex world with the key aspects of planets – it dominates its own satellite system of three moons (Charon, Hydra & Nix), has its own atmosphere, has a complex geology and weather system (of nitrogen frosting based on HST images and theory) and meets all the criteria for planethood with the sole exception of the problematic and, I believe, absurd “orbital clearance” criterion.

    ***

    Hopefully, there won’t be too many more years that pass before the IAU’s decision is reversed and Pluto re-instated as a planet along with having Eris and Ceres added! I’d rather see a bigger solar system with a more inclusive definition than a smaller one with a more narrow-minded less inclusive definition.

    Go Alan Stern go! ;-)

    BRING BACK PLUTO!!! 8)
    ;-)

  12. Quiet Desperation

    Shhhhh. Shh shh shhhhhhh. Settle down. It’s not a planet. Let it go. Let it go. Shhhhhhh.

  13. Tod

    Phil, thanks for yet another fine insight into the world of astronomy, as well as for the info about the upcoming “fight,” which sound like it will be fun and informative.

    While you’re doing the boat ride across the equator thing, please check one thing for us: Does the water swirling down the drain suddenly change direction as you cross the equator (coriolis force)? That was supposedly a plot device in some obscure spy novel…but you and I know that it doesn’t happen that way!

  14. Mark Eret

    So, classification is all about patterns. Thankfully, we have a branch of mathematics devoted to weeding out patterns from data: statistics! Why not use a statistical definition to get a hold on this?

    One potential scheme that might work works something like this:
    1. List a bunch of parameters associated with objects in the solar system and use these parameters to calculate a correlation between various objects in the solar system.
    2. “Planets” are the minimal set such that the correlation of its members with all other objects in the solar system is sufficiently low and the correlation of its members with other members is sufficiently high.

    The eight planets that we typically think of should have a high correlation with the other planets, but are fairly unlike everything else. Pluto, on the other hand, would have have a low correlation with the other planets, but a high correlation with everything else.

  15. TMB

    StevoR,

    It looks to me like essentially all of your arguments are against the specific IAU adoption, and your conclusion is “the IAU adoption was bad and it excluded Pluto, therefore Pluto should not be excluded”, which is logically flawed. The IAU adoption also excluded my cat, but that doesn’t mean my cat is a planet (he is kind of getting spherical, but I don’t think it’s from his own self-gravity).

  16. StevoR

    My preference for a definition of “planet” (& yes, I do think we need one!) would be for a simple definition stating a planet is a spherical or near spherical object incapable of sustaining nuclear fusion directly orbiting a star.

    I’d then go on to list various types of planets :
    (Not that this is so much original to me as being plain common sense. ;-) )

    1) Gas giants composed mostly of Hydrogen and helium of about Jupiter’s size and nature. (A.k.a. “Jovian” planets, larger examples being superjovians, sub-set being “Hot Jupiter’s” ie. those like 51 Pegasi orbiting closer to their stars than Mercury.)

    2) Ice Giants composed of hydrogen & helium plus methane and water similar but smaller and usually colder than gas giants and which resemble Neptune in nature. (A.k.a. “Neptunean” planets, sub-set being “hot Neptune’s orbiting closer to their stars than Venus.)

    3) Terrestrial, rocky or earthlike planets being those composed largely of rocky materials with solid surfaces and including Earth, Venus, Mars & Mercury in our solar system.

    4) Ice dwarf planets which include most – but not all – of the current dwarf planets : Eris, Pluto, Sedna, Quaoar, etc … These would be the small, icy objects composed of methane-nitrogen & water ices at the fringes of a solar or stellar system and would be considered as legitimate planets provided they directly orbit their primary star.

    5) Asteroidean or rock dwarf planets or planetoids like Ceres and perhaps Juno, Pallas and Vesta if it turns out these are large enough to be very near spherical in shape. These would, generally, orbit closer to their primary than ice dwarfs and consist of rock and / or water ice with less methane and nitrogen than on the ice dwarfs.

    Smaller objects would then be comets and asteroids, natural satellites or moons would be considered such regardless of their size. I’d do away with the “classical Planets “notion beyond its use for the original unaided eye “wanderers.” (Mercury, Venus, Mars, Jupiter, Saturn & maybe Earth too!)

    Our solar system would then consist of 2 gas giants, 2 ice giants, 4 Terrestrial, maybe up to 10 or 20 odd ice dwarfs, 1 to 4 asteroidean planets and the total would be about 30~40 or so. Children (& adults) could then be taught the types of planets and why they are those types and would learn the names of the major planets but only the largest and most significant ice & rock dwarfs (eg Eris, Pluto, Sedna, Ceres, Vesta)

    It seems, to me anyway, that this classification system has the virtues of being reasonable, useful, descriptive, widely applicable and simple. 8)

    I do wonder why the IAU did not come up with something close to this idea for the Pluto debate choosing instead what seems a fairly unclear and problematic system that has, as noted earlier, some pretty big holes! ;-P

    Oh & once again : GO Alan Stern go! Pluto most definitely is a planet & anyone saying otherwise can take a long flight into a deep black hole! ;-)

    ——————–

    (& folks can forward this on too if they so wish! Not that I could stop anyone anyway, but a-n-y-h-o-w .. ;-) )

  17. Maybe, in the end, planets are like pornography: you know ‘em when you see ‘em.

    I think it’s pretty obvious that most people (including many scientists) approach the debate like this. They test out trial definitions by comparing known and hypothetical bodies to them, and asking themselves if the result feels like a planet.

    And then they wonder why nobody agrees.

    I think more progress would be made by coming at it from the opposite direction. First think about scientifically meaningful categorisations of bodies, and then decide if the word ‘planet’ has any use in relation to any of these categories. And be prepared for the outcome that no, it doesn’t any more.

    So I don’t think the problem is that things get limited by being put in a box, in this case. The problem is that the box exists, and people insist that therefore something must be put in it.

    Meanwhile, the IAU doesn’t need a definition. It just needs to know how to name new discoveries, a relatively trivial exercise once you detach it from the emotional baggage of planethood.

  18. ShaneC

    “Principle Investigator”? What principles does he investigate?
    :P :D

  19. StevoR

    TMB Said on August 13th, 2008 at 9:55 am

    “StevoR, It looks to me like essentially all of your arguments are against the specific IAU adoption, and your conclusion is “the IAU adoption was bad and it excluded Pluto, therefore Pluto should not be excluded”, which is logically flawed. The IAU adoption also excluded my cat, but that doesn’t mean my cat is a planet (he is kind of getting spherical, but I don’t think it’s from his own self-gravity).”

    Afraid I’m not following you there.

    Yes, I’m saying the IAU got this definition wrong & should include Pluto as a planet. That’s my logical case in this issue. How is that illogical or logically flawed?

    My own cat (& my dog & chickens) clearly have nothing at all to do with this issue & I’ll thank you not to bring them into it! Insult me fine, (I’m sure a lot here do! ;-) ) but leave my pets out of it please! 8)

    ____

    PS. Yes, I know you were using an analogy but, no, I’m sorry I’m still searching in vain for your point there .. There’s no logic flaw in anything there I can spot.
    a planet

  20. Cheyenne

    Why don’t we just keep this simple and use mass as the defining factor? If it’s above a certain threshold, but not a star (doesn’t have fusion) then it’s a planet. Done. Next?

  21. This is the sort of problem we run into all the time in the information dciences – software engineering and such. When designing an object-oriented application, we want to classify all the potential objects you might use to get some sort of task done. So, to pick a canonical example, an order processing system will have classes of Customer, Product, Order, etc. Seems pretty basic, but what happens when you want to keep track of samples given out at a trade show? Are those really “orders”? Is a product really a product without a price? What about tracking prospective customers – do we classify them as Customers, or create a new entity called ProspectiveCustomer? Does ProspectiveCustomer inherit properties off of Customer, or is it its own thing? How does ProspectiveCustomer fit in with existing business rules we have in place for Customer?

    Clearly, even in the narrow artificial ecosystem of business, there’s gray areas when we try to classify stuff. Almost always, the right way to deal with these things is to merely tolerate the computer’s need for strict classification, but never lose the awareness that it’s at least slightly flawed. Stuff the data into the neat little boxes for the sake of storage and unified process, but maintain a healthy skepticism of the data’s true fidelity.

    Not sure what my point is, though, if I have one. Maybe that you oughtn’t draw such a clear line in the sand between planet and non-planet. Can’t we just say that, e.g., Pluto is sometimes a planet and sometimes a non-planet, depending on context? Like in certain scenarios, it’s useful to use the word “planet”, but in others, not so much. Maybe definitively picking one or the other is the real problem.

  22. StevoR

    Oh & TMB : While I did most definitely conclude that “the IAU adoption was bad and it excluded Pluto, …” I didn’t just mean it was bad only because it excluded Pluto!

    If you’ll read through what I’ve said, you’ll find I noted the IAU definition – if adhered to strictly – actually excludes EARTH as a planet
    – along with every other planet of the solar system!

    Remember comets can cross the orbit of all planets from Pluto or even Sedna right through to Mercury. Even really large comets do so on rare occassion – thus NO planet has cleared its neighbourhood! That last “Orbital clearing” criterion really is just plain dumb. :-(

    Oh & it excludes exoplanets too by definition!

    So according to a strict interpretation of the silly IAU definition there CANNOT be any such thing as a planet! Not logically.

    So its right back to the drawing board for that def! ;-)

    Fortunately, there’s already the sensible first Prague proposed definition (w/o the”orbital clearing” nonsense) to fall back on. :-)

  23. StevoR

    Cheyenne said on August 13th, 2008 at 10:19 am :

    “Why don’t we just keep this simple and use mass as the defining factor? If it’s above a certain threshold, but not a star (doesn’t have fusion) then it’s a planet. Done. Next?”

    (Emphasis added.)

    Good idea except we then have the question of where the mass threshold is separating Planet from Planetoid. (Or whatever equivalent word we choose to use.)

    It reminds me of the debate over when does a hill become a mountain … There is a particular height line for that but some hills are named as being mountains – esp. in my land of Australia where our landscape ispretty flat! ;-)

    I’d be happy with this if we set the cut-off mass as being, say the mass of Pluto or, better, Ceres. This would keep Pluto as a planet – which I think it deserves to be called. (Oh you noticed did you? ;-) This allows room for finding new ice dwarf and asteroidean type planets which I also think is cool.* If the mass is around the limit for self-gravity shaping something round well that makes most sense to me. 8)

    —————–

    * Plus it’ll annoy that arrogant and mean git, Neil DeGrasse Tyson, which I’m also good with! ;-) :-P

    (Okay folks, I know the dude has his fans but thats my opinion of him based on what I’ve seen of him so far. Maybe I’m biased but then who isn’t? ;-) )

  24. sandswipe

    I was going to suggest basing the difference between planets and planetoids on the average strength of gravity at the surface (some arbitrary number like 2m/s^2), but I like StevoR’s idea.

  25. Kesstra

    Thanks Phil for another fine article that makes me think about things a little differently. I for one couldn’t care less whether they classify Pluto as a planet, planetoid, ice ball or basketball. I let the more educated astronomers and cosmologists figure those things out and then try and understand how they reached that conclusion. What I do like about this debate is that it has interested the uninteresting public. People and mostly children are asking “why” and I delight in this. At work they call me “Space Girl” because my whole wall next to my desk is decorated in pictures from the Hubblesite. On my lunch break I read wonderful websites like yours, for me it’s all about the Cosmos. People find me droll at work and can’t understand my interest. I find them uncaring and ill-informed. However when the debate came up about Pluto who did everyone at work coming running to for information? !! My children’s friends started asking me questions and I got a whole new respect. That is not my goal but it helps me get to a means to an end…their interest in Science. Thank you Pluto for whatever you are, because you are the seeker of Knowledge as far as I am concerned. Knowledge is what helps us all become grow and become better human beings.

  26. I love Neil Degrasse Tyson, but I disagree with him when he argues Pluto is not a planet! (Insert fingers in ears) LALALA I can’t hear you! Pluto is a planet! It was when I was a kid and that’s good enough for me! LALALALALA!

  27. PG

    I think we need to go back and consider the original, ancient definition of a planet to sort of re-calibrate our sensibilities to this issue. The Greeks called all the bright points of light that seemed to move through several constellations over the course of a year (or years) a “planet” (Greek for “wanderer”).

    I think the whole Pluto/”what is a planet” debate is just politics. As far as I understand it, some people merely wanted to cross-list Pluto in a catalog of Kuiper Belt or Trans-Neptunian Objects (which are much more closely reminiscent of Pluto than anything else we’ve ever called “a planet”). The people whose careers are closely tied to Pluto “the planet” (e.g., Alan Stern) were upset by this apparent “downgrade”. Hence the debate.

    Either there are billions of planets, or there are around 8 or so. They won’t care what we call them, and their value to science will not change as a result of their name. There is a certain value in cataloging (this was pretty much all of “Astronomy” = “naming the stars” in the 19th century), but even stellar spectral types are not precise at the boundaries. E.g., what’s the difference between an F9 and G0 star? not much, but we can always change the definition to nudge a certain star either way. Even the boundary line between a planet and a star is fuzzy. Something with a mass of 5-10 Jupiters can be a “brown dwarf star” (with no nuclear fusion in its core even).

  28. StevoR:

    Sentimental, cultural and historical reasons – noting Pluto’s long-established and culturally scientific place as a recognised planet from its discovery in 1930 until its (hopefully temporary) demotion in 2006.

    Er, so? The first few asteroids that were discovered were originally classified as planets and then demoted several decades later. No one seems to have raised a stink like is being raised now. Why does “well, we did it this way before” matter? The point of classifications is ease of understanding patterns, not to hold to tradition. If the patterns have clearly shifted (and they have), shouldn’t the nomenclature keep up? Otherwise, aren’t you dangerously close to logic akin to Creationism?

    10) The decision to demote Pluto has had a generally negative reception from the general public and on public perceptions of astronomers bringing them into disrepute and making astronomy fans and science writers lives harder!

    So you’re saying we shouldn’t change it back now, then. I mean, re-opening the debate would just make this problem worse, not better.

    The undemocratic manner in which the IAU ruling was made.

    People chose not to attend or to leave early (including Alan Stern). If you fail to show up to vote, you don’t get to bitch about how undemocratic the system is, period. (I have the same lack of compassion for people who don’t vote in governmental elections and then complain about the results anyway, incidentally.)

    Pluto is a complex world with the key aspects of planets – it dominates its own satellite system of three moons (Charon, Hydra & Nix), has its own atmosphere, has a complex geology and weather system (of nitrogen frosting based on HST images and theory)

    Er, you’re using a definition of of “dominates” that I do not believe is standard. Charon definitely throws Pluto around. I mean, the two are mutually tidally locked after all and their barycenter is outside Pluto’s surface. Pluto is not dominant in that system, it’s a double system. Also, you can’t base a definition on what you theoretically speculate is on the surface. I can theoretically speculate that there’s no geology and no real weather, so where does that leave us?

    The “dwarf planet – dwarf” star analogy – just as dwarf stars are still stars so surely are dwarf planets still planets.

    Methinks you need to look up the term “minor planet”, which Pluto (along with all of the asteroids and comets) definitely is.

  29. StevoR

    D’oh! Italics error! The italics were meant to end after the line in brackets there ie. :

    *** CORRECTION :

    Good idea except we then have the question of where the mass threshold line is drawn separating Planet from Planetoid. (Or Plutoid or KBO, TNO, Small Solar System Big Mouthful or whatever equivalent word we choose to use.)

    It reminds me of the debate over when does a hill become a mountain … There is a particular height line for that but some hills are named as being mountains – esp. in my land of Australia where our landscape ispretty flat!

    I’d be happy with this if we set the cut-off mass as being, say the mass of Pluto or, better, Ceres. This would keep Pluto as a planet – which I think it deserves to be called. (Ah, you’ve noticed Isee! ;-) )

    ***

    If only we could EDIT here! Sigh. :-(

    Please Almight Dr Phil Plait, Bad Astronomer ,sir pretty please with a bright comet and supernova on top can we please, please, please get the ability to edit here! Oh the humanity! :-O

    Failing that, (& surely, surely, it can’t be that hard to arrange can it?) could we even simply get a better set-up just for doing bold, italics, et al .rather than these $#@#$%! annoying so-easy-to-mess up html tags? Please?

    That’d make life – well posting here at least – much better.

  30. PG

    I don’t really care what you call Pluto, but I am all for grandfathering it in. Or, since it can’t seem to make the playoffs the regular way, letting be our “wildcard planet”. ;)

  31. Wildride

    When the rules were being discussed, I thought the first proposed one was an effort to find a definition that included Pluto. But to do that they had to include other bodies, so they decided they didn’t like it. So then they decided to come up with a rule to exclude Pluto. Personally, I don’t like this kind of reasoning. Determine what you think a planet is and then there are however many of them as there are that fit said definition. To much is made of people just wanting to know how many of them there are, rather than what they are.

    One of the things that gets discussed is that Pluto appears to be a different kind of thing than, let’s say, Earth. Well, Jupiter is no more similar to Earth than Pluto is, so if Earth and Jupiter can co-exist in a category, then so can Earth and Pluto. And that’s even assuming that it’s useful to include any of those three types of things in the same grouping.

  32. TMB:

    Here’s the reason your argument is illogical. It basically went like this: here is the reason Pluto should be a planet: because the IAU definition sucks.

    That’s not an argument for Pluto being a planet, that’s an argument for why the IAU definition is piss poor.

    As for the rest of your arguments, the definition of a planet is a scientific tool. It should have real meaning, and be useful in the process of analyzing objects in the Universe. As such, it shouldn’t be based on sentimentality, politics, tradition, or any other such BS (which is what your other arguments essentially boil down to).

  33. Err… that last comment should be addressed to StevoR… not TMB. :)

  34. StevoR

    @ John Weiss (whose quotes are in italics or at least are meant to
    be assuming it works!)

    JW : “Er, so? [To sentiment, history, culture] The first few asteroids that were discovered were originally classified as planets and then demoted several decades later. No one seems to have raised a stink like is being raised now. Why does “well, we did it this way before” matter? The point of classifications is ease of understanding patterns, not to hold to tradition. If the patterns have clearly shifted (and they have), shouldn’t the nomenclature keep up? Otherwise, aren’t you dangerously close to logic akin to Creationism?

    SR (me) : Not at all. Nothing like the Creationism issue – a low blow false insult & please lets not go into an Off-topic debate there.

    Besides the asteroids were’nt considered planets anywhere near as long
    as Pluto. 1930-2006 is 76 years not just a few decades and Pluto is quite
    different in nature & much larger. Besides I’d be happy to welcome Ceres and even Pallas, Juno and Vesta to planetary ranks – again. 8)

    JW : 10) … So you’re saying we shouldn’t change it back now, [because the public view the IAU decison negatively & this reflects badly on astronomers] then. I mean, re-opening the debate would just make this problem worse, not better.

    SR : No, admitting a mistake and correcting it would make the IAU seem more respectable and reasonable and make things better! Esp. if the public thinks
    they’ve stuffed up in this matter – which, of course, they have! ;-) :-P

    JW : People chose not to attend or to leave early (including Alan Stern). If you fail to show up to vote, you don’t get to bitch about how undemocratic the system is, period. (I have the same lack of compassion for people who don’t vote in governmental elections and then complain about the results anyway, incidentally.)

    SR (me) : Is that really the case? I don’t know that it is. The meeting was a last-minute restricted access thing as I understand it – a manipulated undemocratic fixed “vote” not too dissimilar to how the Leninists took over the party in the bolshevik revolution! I stand by my view here until you can show otherwise and even still the numbers here don’t lie – only a handful of the people – NOT including some of the key experts and not fairly
    representating one side – were able to vote on the issue. That’s undemocratic.

    Pluto is a complex world with the key aspects of planets – it dominates its own satellite system of three moons (Charon, Hydra & Nix), has its own atmosphere, has a complex geology and weather system (of nitrogen frosting based on HST images and theory)

    JW : Er, you’re using a definition of of “dominates” that I do not believe is standard. Charon definitely throws Pluto around. I mean, the two are mutually tidally locked after all and their barycenter is outside Pluto’s surface. Pluto is not dominant in that system, it’s a double system. Also, you can’t base a definition on what you theoretically speculate is on the surface. I can theoretically speculate that there’s no geology and no real weather, so where does that leave us?

    SR : Well it leaves you wrong & me right that’s where! ;-)

    Pluto’s atmosphere has been measured by occultation among other things and it has been studied and calculated carefully. It ain’t mere speculation.

    As for my use of “dominates”, come on – the barycentre is much closer to
    Pluto, it contains far more mass than Charon (about twice Charon’s mass
    off the top of my head, maybe more.) Plus while some argue its “a double planet” which is kind of cool – & note the use of that word planet!
    almost everybody describes Charon as Pluto’s moon!

    So I think saying Pluto dominates its local space – and controls the
    orbits of three moons – is more than fair and accurate.

    JW : “The “dwarf planet – dwarf” star analogy – just as dwarf stars are still stars so surely are dwarf planets still planets. (Me)

    (JW) “Methinks you need to look up the term “minor planet”, which Pluto (along with all of the asteroids and comets) definitely is.”

    SR : NOT! Minor planet? Mate, “minor planet” means asteroid. And one thing
    Pluto definitely isn’t is an asteroid! ;-)

    Call Pluto a “dwarf planet” if you must – like folks call our Sun a
    dwarf star which was my original point here – or just call it a planet which it really is, IAU or no IAU!

    Anyway, John Wiess what you got so badly against Pluto? What did it
    ever do to you to deserve such antipathy? Well? ;-)

  35. tacitus

    StevoR:

    12 REASONS WHY PLUTO _IS_ A PLANET

    Your article is misleadingly titled. All I see is mostly a list of objections to the reasoning and process the IAU used to define what a planet is. At best, if your arguments hold water, that gets us back to square one, still trying to decide whether Pluto should be classified as a planet.

    Ho hum.

  36. tacitus

    However flawed the IAU process was, they arrived at the correct decision. We only arrived at the point where Pluto was called a planet because Pluto just happened to be an extreme outlier in a population of bodies that could be hundreds if not thousands in size. In another hundred years or two our descendants will be looking back and chuckling at the Pluto controversy as they look over the list of 567 Plutoids they have found so far.

    Getting used to the new reality of Pluto not being a planet is kind of like how things were in the UK when the British Meteorological Office decided that all weather forecasts would now be in Celsius rather than Fahrenheit. There was quite a public outcry at first… 32 degrees was supposed to be freezing, not melt-your-ice-cream hot, and it did take a lot of getting used to.

    Now, a couple of decades later, nobody frets over it at all. Anyone younger than 40 doesn’t even know what the Fahrenheit scale is (unless they come over to Disney World for their summer hols). Why the heck would the boiling point of water be 212F anyway?

    The point is, the fuss will did down. Pluto will be joined by dozens of other similar bodies in the outer solar system and all will be right with this little part of the Universe. :)

  37. StevoR

    Brett :

    Here’s the reason your argument is illogical. It basically went like this: here is the reason Pluto should be a planet: because the IAU definition sucks.That’s not an argument for Pluto being a planet, that’s an argument for why the IAU definition is p-ss poor. As for the rest of your arguments, the definition of a planet is a scientific tool. It should have real meaning, and be useful in the process of analyzing objects in the Universe. As such, it shouldn’t be based on sentimentality, politics, tradition, or any other such BS (which is what your other arguments essentially boil down to).

    I suggest you re-read what I’ve posted.

    Yes, I think the IAU got it very wrong & their definition sucks – as I’ve pointed out it fails the reductio ad absurdum test and if strictly adhered to would eliminate all planets. Comets cross all orbits – thus they
    aren’t strictly cleared!

    What is meant by orbital clearing anyway & who needs such nonsense to be any
    part of the criteria for what is a planet?

    Why over-complicate things with such nonsense that would make double planets impossible, colliding planets impossible, proto-planets impossible, exoplanets impossible, even current planets impossible??!

    The IAU definition is rubbish. So why was it adopted? Beats me. I
    can only conclude the reason was solely because some people for whatever warped and dubious reasons of their own hated Pluto … & wanted to wipe it off the charts. That’s whats BS – NOT my logical arguments against their BS. ;-)

    You saying “That’s not an argument for Pluto being a planet, that’s an argument for why the IAU definition is p*ss poor .. is right in that the IAU decison is p*ss poor – & its p*ss-poor because it was expressly designed to exclude Pluto. Y’see?

    Anyway you’re right to say : “the definition of a planet is a scientific tool. It should have real meaning, and be useful in the process of analyzing objects in the Universe.”

    I think it should be clear that all such definitions that are reasonable &
    scientifically useful do indeed include Pluto – & a
    few have already been suggested here.

    Hence its a planet if its of a certain mass (say about Ceres mass) = planet or an exclusive form of one decribed earlier – If an object meets these three basic criteria of :

    1) Not fusing so not a star or brown dwarf
    2) Not a moon but orbiting its star directly
    3) Not too small to become round through its own gravity

    then its a planet! Simple easy and useful ain’t it? Mind you the next
    big question has to be what type of planet an object is
    – Jovian, terrestrial, icedwarf etc ..

    Anyway its 3 am in Adelaide now so on that note I’m definitely going to bed! ;-)
    _______________________________

    ‘Night hides the world but reveals a universe.’
    Russian proverb.

    “The Ramans do everything in threes.”
    – Arthur C. Clarke, ‘Rendezvous with Rama’, Final page (252), Pan
    Books Ltd, 1973.

    Indeed, indeed do we indeed need ‘indeed’?

    As for

  38. I think this thread is a perfect demonstration of the necessity of carrying out my plan to destroy Pluto. Too much human energy has been expended arguing about an arbitrary designation: we must act to crash Pluto into Mars before it is too late.

  39. Daffy

    Honestly, my thought is who cares how they classify Pluto? A planet, plutoid, dwarf planet, cosmic marshmallow, or frozen dingus…it still remains what it is and has been for billions of years: a fascinating object that I, for one, look forward to learning more about.

    So call it whatever you like, as far as I am concerned.

  40. StevoR

    Well almost! ;-)

    After just adding by notfusing above I’m obviously meaning not fusing elements to shine as a star does, of course! ;-)

  41. tacitus

    Too much human energy has been expended arguing about an arbitrary designation: we must act to crash Pluto into Mars before it is too late.

    It wouldn’t work. There would be another furious argument over whether Pluto should be called a planet “In Memoriam”.

    BTW: Blockquote highlighting (or lack thereof) needs to be fixed stat!

  42. Thanny

    It’s obscenity, not pornography, that was the object of the famous SC justic quote (“I know it when I see it.”).

    As for Pluto, any time you create a definition that includes or excludes certain objects, you’re going to have arbitrary rules. However, any set of arbitrary rules that makes Earth a planet and Pluto not a planet must be torturously nonsensical. The newly-created ones used to oust Pluto are just a wad of nonsense that, as pointed out by someone else, actually exclude every object in the entire solar system from being a planet.

    Uranus and Neptune are excluded from any definition based on “classical” planets, unless you simply adopt the original meaning of “wanderers”, in which case every object in the sky that moves perceptibly from the surface of the Earth is a planet. That’d be quite a long list – much longer than the one generated by rules based on that least offensive of arbitrary distinctions: gravitational rounding.

  43. I don’t know about that, tacitus, I think most people would be focussed on how wicked cool Pluto crashing into Mars would be.

    “Hey, have you heard they’re crashing Pluto into Mars?”

    “Yeah. Hey, isn’t Pluto a planet? I think the IAU got it wrong on that count.”

    “Who the heck cares? They’re crashing PLUTO into MARS!”

  44. Fan-of-Who

    K9 is never wrong.

    K9 clearly stated in “The Sun Makers” episode (with Tom Baker as the 4th Doctor & Leela & K9 ascompanions) that Pluto is the 9th planet.

    K9 said it.
    K9 is always right.
    That settles it. ;-)

  45. Fan-of-Who

    Oh & no I did NOT say “Your comment is awaiting moderation” somebody else said that! ;-)

  46. madge

    Sorry guys I’m with Quiet Desperation on this one. Pluto ain’t a planet. Never was and never will be. And NOT just coz the IAU says so! If you count Pluto as a planet the list is going to go on and on and on as we find more (and bigger) objects out there. We have to draw the line somewhere and Pluto had just never “fitted in”. I don’t like the new classification of Plutoid though. Sounds like a medical condition. Why can’t we have eight planets and a growing number of Dwarf Planets? I will be listening to the debate with interest :)

  47. Todd W.

    I think crashing Pluto into Mars would be a bit too difficult. What we need to do is move the LHC to Pluto, flip the switch, and watch as all the other Kuiper Belt Objects are pulled into Pluto, thus increasing both its mass and size to the point where it will fit the definition of planet.

  48. Joe Meils

    Ah, yes… “What constitutes a “planet.” Easily the “how many angels can dance on the head of a pin” argument of our time.

    Call me silly, but I feel that:

    1. If it has enough mass to pull itself into a sphere…
    2. If it circles the sun…
    3. If it has even the vaugest whisper of an atmosphere… (even if that atmo lies frozen on it’s surface)
    4. (BONUS) if it has satillite(s)…

    Then it’s a planet. (If it has laser beams attached to it’s friggin’ head, that’s just icing on the cake.)

  49. @Joe:

    ‘Easily the “how many angels can dance on the head of a pin” argument of our time. ‘

    Don’t be silly. By the same token, as has been mentioned elsewhere, then there’s no point in trying to define “life”, or “species”, or “mountain”, or any number of other things that we, as human beings, love to classify. Classification is part of the way the human brain works, and it’s been a key component of science since… well… forever. Classifications give us the ability to group objects by like properties, and then to examine what makes them unique, and how they related to other objects. It’s an important discussion, and while from the outside it may seem silly, I can’t imagine any true scientist scoffing at the idea of trying to nail down a decent definition for the term “planet”.

    The real problem is the process. People are allowing their fondness for Pluto as a planet to cloud the issue. The definition for what should be labeled “planet” shouldn’t be shaped specifically so as to address the (frankly ridiculous) affection for the object Pluto. It should be shaped independently, based on ideas of planetary formation, etc, and if Pluto happens to fall into that definition, then so be it, and if it doesn’t, tough. It’s still an incredibly interesting object, in any case, and the passion with which people fight for Pluto retain it’s planet status never ceases to baffle me. It’s completely illogical, and deeply unscientific.

  50. Re: “playpuses”

    And it’s also Principal Investigator, not Principle Investigator – we want the one that refers to primary or most important of multiple investigators on the project, not the one that would mean we’re investigating laws of the universe (since every investigator on the project does that).

  51. tacitus

    t’s still an incredibly interesting object, in any case, and the passion with which people fight for Pluto retain it’s planet status never ceases to baffle me. It’s completely illogical, and deeply unscientific.

    You are baffled by this? Are you from Pluto or something?

    :)

  52. And what if I am??

  53. Paul Clapham

    John Weiss: “Biology may work to produce separate species, but in reality, that just doesn’t happen. There’s a huge problem trying to define “species” in a way that divides things as researchers want, very similar to defining “planet”.”

    Yes, there’s a species problem in biology. Sometimes. But species do exist even though we can’t always accurately say what they are. Most of the time. And there are biological mechanisms which have the effect of preserving species, even though they don’t work all the time. And there are mechanisms which can produce new species. Those mechanisms work based on interactions between individual organisms and by differences between ancestors and descendants. There isn’t any parallel to those mechanisms in astronomy.

    Sure, there are difficulties in demarcation in both fields. I was just trying to make the point that the difficulties don’t arise from the same source. In astronomy, planet-like objects are basically what they were when they were created. So the classification of planet-like objects, I think, is ultimately going to group objects based on how they were created. You won’t find a hierarchical, tree-like, structure of relationships like you do in biology.

  54. Jaison Lee

    Supreme Court Justice: “I don’t know how to define a planet, but I know it when I see it.”

  55. Torbjörn Larsson, OM

    how would know the difference between mammals and reptiles if you didn’t label them? And note that you recognize both as being subsets of animals. You may find weird things (platypuses) but they can help you understand the classifications better, divide up the different kinds of animals. What works for biology works for astronomy, too.

    Well then, in that case you don’t need to define a “planet” conception!

    First and famously, there are a huge number or species definitions in biology, each with its own coverage and problems. (John Wilkins, a specialist, counts to 26.) The biological species definition, that individuals from different species observably can’t or won’t breed, works on populations of most multicellular life. But it won’t work on fossils (that will have to use phylogenies by homologies instead, rather like what are actually used for biological species), nor for unicellular life (that can use, say, ecological definitions instead).

    Second, it is the evolutionary process of speciation and the characteristic phylogenetic tree (bush, whatever) it results in that interests biologist. And as noted, they don’t use a specific species conception to keep track on that. Actually, the current IAU definition of planets (or perhaps then: “planetification”) is rather analogous in AFAIU being based on the planetary formation process, as the bodies will start out embedded in a planetary disk so not planets and later perhaps clear their orbits and stop accumulating mass. And just as in biology, what will count as a planet (species) may depend on its environment inclusive other bodies (species).

    But if biology is such a good analogy I don’t see then why one wouldn’t want to define a convenient series of planet conceptions, say “classical planets” to define pre-IAU planets, “hydrostatic planets” to define round bodies, “exoplanets” as today, and “astronomical planets” (IAU) to define those that adheres best to the planet formation process in analogy with “biological species”.

    Btw, seeing the discussion with StevoR, neither biology’s “species concepts” (rather, conceptions) nor its actual definitions of each specie is made by a “democratic manner”. It’s research, so it is meritocratic. AFAIU those who first name a putative species (and I assume, point out the similarities and differences that makes it a separate species), and later gets it accepted by a very formal process, will get the name to stick.

    The analogy here would be to discover the putative planet and make it likely that it has cleared its orbit path.

    But plutoids!? Please! Who ordered that?

  56. Michelle

    Pluto’s a planet to me. Why? Simply because of the history. Being a planet isn’t explainable really, it’s more sentimental and cultural. And Pluto was a great discovery, it rocked the world for a good while, so it’s a planet to me.

    Stop wasting time trying to define a planet. Anyway, I have a hard time thinking we’re on the same level as Mercury.

  57. @Michelle:

    “Pluto’s a planet to me. Why? Simply because of the history.”

    Yeah! That’s the same reason why I refuse to believe in protons and neutrons. Dalton told me atoms are indivisible, and that’s just the way it is, science be damned!

    “Anyway, I have a hard time thinking we’re on the same level as Mercury.”

    Yes, that kind of anthropocentric thinking has served us so very well in the past…

  58. Tod

    @StevoR, who said: “Good idea except we then have the question of where the mass threshold is separating Planet from Planetoid.”

    As if that will help when CNN doesn’t know how to separate facts from factoids. :-)

  59. guestwork

    Our definition of “planet” is, or has been up to the silly debate, one of perspective, rooted in history. Etymologically, planets were the “wandering stars” in the sky and their name was established to distinguish them from the more or less fixed stars. As our quality of observation improved with technological progress, we discovered more of them that previously were invisible to the naked eye. It doesn’t make particularly much sense to mess with several thousand years of astronomy and the impact on our culture that they had (see as evidence how people flat out refuse to accept the “demotion” of Pluto and simply continue to regard it as a planet). Anything newly discovered might well be put into some category that’s a better fit and reflects our improved understanding of celestial bodies, and then given some nice name; planetoid, plutoid, cometoid, asteroid, small planet, large pluto, iceworld, rocky dwarf, metal body, companion moon, kuiper object, oortoid – it really doesn’t matter. Pluto is what it’s always been: Pluto, one of the original nine planets of our solar system.

  60. BMcP

    @Joe Melis says

    Ah, yes… “What constitutes a “planet.” Easily the “how many angels can dance on the head of a pin” argument of our time.

    Call me silly, but I feel that:

    1. If it has enough mass to pull itself into a sphere…
    2. If it circles the sun…
    3. If it has even the vaugest whisper of an atmosphere… (even if that atmo lies frozen on it’s surface)
    4. (BONUS) if it has satillite(s)…

    Then it’s a planet. (If it has laser beams attached to it’s friggin’ head, that’s just icing on the cake

    Heck Mercury got in, and it only has three of those qualifications, and barely the third one. It is just a glorified rock in space and little more. Plus what a cheat on the whole clearing the orbit of debris bit, it has such a small orbit to clear!

    I don’t want to kick Mercury out of the club, but let Pluto back in. Oh and Eris and MakeMake too. :D I love routing for the small guy.

  61. dkary

    I promised myself I wouldn’t do this. I really did. Ah, hell, here I go anyways.

    No, Pluto is not a planet.

    There are 2 basic approaches to defining a planet: base it on the size and shape of the object and ignore what’s around it, or not.

    If you want to base it mainly on the size and shape, you are left with two options: either you pick some arbitrary number with no physical meaning and say, “Here, this is it, anything bigger than this is a planet.” Of course, you then get stuck with the damn thing that’s just one measly km smaller than that doesn’t count. (Or worse, that is 1 km smaller, with an uncertainty of 50 km, what are you going to call it? This is not far from the current state of the observations).
    Or, you can insist on a physically meaningful criterion, like gravity modifying the surface (e.g. making the body round). This one has two problems, first is that you get dozens of planets (even if you exclude the moons, though the reason for doing so becomes itself pretty arbitrary). Some people have no problem with this, while others find it starts to lose the meaning of what makes something a planet.
    Second, how much modification is enough? Is Ceres round enough? How about Vesta? There are 1km asteroids that appear to have electrostatically charged dust that flows to the “low” points in the gravity field: does that count as a surface modified by gravity? At some point, you’re back to making a judgement about how much is enough to count.

    On the other hand, looking at whether or not something is dominates it’s neighborhood turns out to be far less arbitrary than this. You can compare the masses of bodies in a given part of the solar system, and show that in fact there is a clear distinction between the ones that push everything else around (the big 8 planets) and everything else (I think it was Steve Soter who did this analysis a few years ago). While we can imagine situations that might be more “in the middle” between these two groups, that simply hasn’t happened here, and there are some reasons to suspect it’s not very likely from the way we thing solar systems form.

    So, Pluto, as simply the second largest known member of the Kuiper belt, is really much better described as not a planet (just like Ceres and Vesta are no longer considered planets).

    Now, if astronomers really wanted to piss people off, how about this name change (which I saw on a lab blackboard back around 1990): 1930 P/Tombaugh.

    DK

  62. Irishman

    Paul Clapham, it’s called an analogy. Some parts of the analogy are valid, but if you stretch it or study it too closely, you find parts that are not valid. That’s inherent to every analogy.

    There is no clear definition of “species”. There are several definitions of “species” that work reasonably well depending upon the context. “Species” is a label we humans put on the classifications we make, just like “planet” is a label for classification that is fuzzy at the edges. Sometimes we learn more from the fuzzy edges than the bulk within the lines.

    I don’t really care if Pluto is a planet or not, but I think the IAU definition is flawed and confusing. If we still want “planet” to have a meaningful scientific use, we should approach the definition scientifically.

    That’s why I am annoyed by arguments that “making Pluto a planet means we’ll have to include a whole bunch of other bodies, and suddenly the number of “planets” will be in flux and some huge number, meaning I’ll have to learn a whole bunch more names.” No other field of science defines the categories by how many names school children will have to memorize.

    I also don’t have much sympathy for “I learned Pluto as a planet, so it must be a planet”. Science doesn’t limit itself to satisfy your whining.

    The IAU definition seems designed to purposefully excluse Pluto. Not because Pluto doesn’t fit with Earth and Saturn, but because there will be more bodies we haven’t discovered that will have to be planets.

    The alternative is for the science community to use “planet” in a very generic sense, an astronomical body smaller than a star/not fusing and bigger than dust (some sensible lower limit), and then discuss all the categories of “planets” – gas giants, terrestrial, icy, minor (i.e. asteroids), etc. If schools want to have elementary students memorize the “major” planets, they can choose to include Pluto for historical reasons if they want, the same way we call the color spectrum ROYGBIV, even though the only reason indigo is in there is because Newton was a numerologist had didn’t want an evil 6 number of colors, so made a 7th color.

    Scientifically, Pluto would still be a “planet”, but so would Ceres, and scientists would worry about more important distinctions (like Pluto is icy and in the Kuiper Belt, while Ceres is rocky and in the Asteroid Belt). The IAU could track it in whatever catalogue they want and nobody would care.

  63. one question I’ve had about the new “official” definition that I haven’t been able to find an answer for is: just what IS the “neighborhood” a planet (or dwarf-planet or whatever), without falling back to some variation on the long since discarded/debunked Titius-Bode “rule”, or some other set of lines in the sand, how is that neighborhood scientifically defined and how can/does/would it be applied to other solar systems?

  64. Hayden Planetarium? She’s so great in Heroes.

  65. amphiox

    I can think of no logical and self-consistent definition for planet that would include Pluto and also not include Ceres, not even the historical one, since Ceres was named a planet when it was first discovered, and stayed a planet for 8 years.

    I like the idea of having multiple planet classes. It is the form of definition that would be most useful. And frankly, usefulness is the only value of arbitrary definitions like these, because they help us organize our knowledge and ideas.

    (One day we’re going to discover a moon around an extrasolar jovian as big as or bigger than the earth, with an atmosphere, and geologically active, full blown plate tectonics, even, maybe even its own satellite. Will we still be justified in calling that a moon? What about a Jupiter-Saturn type arrangement? Would we call the Saturn-sized object a moon?)

  66. kkozoriz

    Star Trek had it right. That’s a class M and over there is a class D. The one we passed on the way in was class K and so on. There’s many different kinds of planets. Trying to come up with one definition is just silly.

  67. Dogran

    I still think of Pluto as a planet, purely out of habit. I suppose a lot of people do and will continue to do so. Maybe we’re our generation’s version of the older folks who just refuse to use metric measurements because they’re simply not how they’ve always done things.

    But, there again, I can’t help thinking that, really, ‘planet’ doesn’t actually mean that much any more. Like Phil said, we all know what a planet is – we know them when we see them. And I bet if we found aliens on Pluto (hypothetically, all right?), we’d say we’d found life on another planet.

    Can I suggest, though, for the sake of argument, that maybe as well as the existing definitions we’ve got, we add one more (‘guestwork’ got pretty close to it; sorry if anyone else has already said this):

    A ‘planet’ is not only a spherical or near-spherical body that orbits the system’s centre of gravity, and so on and so forth, but it’s also one that can be seen from Earth with the naked eye.

    Sure, that probably rules out one or two more than poor old Pluto – but since ‘planet’ is largely a historical term for stars seen to ‘wander’ across the sky, it seems fair to return to the historical basis of the word. It’s obviously of no more scientific use – so why not let it settle into a new role of historical interest?

  68. Radwaste

    Well, dang! The link is to a blog item called “Astronomy 101″, which compares the size of Earth to the other planets and then to a few choice stars. Ah. Try this one.

    ‘way cool.

  69. Phil, welcome to the right side of this debate!… ;)

    Other than this, I’d just like to say the following:

    1. Any argument that limits planethood to the Solar System is shortsighted and will be very quickly shattered by reality.

    2. Any argument that begins by stating how many objects we want to call “planet” and then tries to taylor a definition to end up with (more or less) that number is completely and absolutely laughable. In no other serious science are there definitions made to restrict the number of members in a category. You don’t hear biologists saying that “more than 10 mammals starts to lose the meaning of what makes a mammal”, you don’t see physicists first determining how many subatomic particles should there be, you don’t see chemists trying to make up a classification scheme wherein the proteins are eight, and the other thousands have to be called dwarf proteins (not proteins) or nothing at all. You also don’t hear astronomers saying that “more than 10 stars starts to lose the meaning of what makes a star”. So did I say loughable? Make it idiotic. That’s more to the point.

    3. Don’t believe in anyone that tries to tell you that the orbital clearing thing is less arbitrary than the alternative. It depends not on one arbitrary point, but on two: how clean is “clean” and where lie the borders of a “neighborhood”. Also, don’t believe in anyone that tries to tell you that there are non-arbitrary definitions. There aren’t. You’ll have to swallow some arbitraryness whatever the definition ends up being. But there are some that are more arbitrary, and others that are less. The orbital clearing this lies squarely on the highly arbitrary side.

    4. Pluto is not the point here. This discussion’s (usually very high) levels of sillyness would go way down if everybody just forgot all about Pluto. The point is trying to work out a good definition of planet. Where Pluto fits should follow what the definition ends up being and not the other way around. In fact, tayloring a definition because one wants Pluto to remain a planet is just as dumb as #2 above.

    5. A good definition of planet has to be simple. Has to be universal. Has to be solid in the sense that in the current state of understanding of these issues it’s not expectable that future discoveries will force it to change radically, only to be somewhat adapted and perfected.

    6. Planets should be divided in classes. In fact, that has always happened, which is why we talk about super-earths, gas giants, hot jupiters or terrestrials. Many of the criteria that are not adecuate to define what a planet is are adecuate to define classes and subclasses of planets. This is how things are done in other sciences too, and it’s absolutely scientific to do so.

    7. To define what planets are, roundness caused by self-gravity is the only real way to go.

    8. ‘Nuff said.

  70. If you accept the current IAU definition of a planet, then Mars was a non-planet for most of its lifetime. If you calculate the time needed for Mars to clear its orbit, then depending on the initial starting conditions, it can be anything from 5% to 75% of the age of the solar system.

    So using the worst-case numbers, just about the same time that lungfish first started crawling out of Earth’s primordial seas, Mars magically transformed from a non-planet into a planet.

    Similarly, when the solar system was about 1/8 of its current age, resonance between Jupiter and Saturn pushed Uranus and Neptune from their original, closer orbits out into their current ones. So Neptune became a planet, stopped being a planet when its orbit changed, and then regained planethood after clearing its new patch.

    This is why a physical definition makes more sense.

  71. Maybe, in the end, planets are like pornography (you were wondering when I’d get to that, weren’t you?)

    Actually, when I first saw the title in my feed reader, I thought this was a Greta Christina post :-) (No link, since a lot of what you find may be NSFW. But she’s worth googling.)

    making a definition for what a planet is will always be arbitrary, and the boundaries forever blurry.

    A good definition is one that “carves nature at the joints” (I wish I could remember who coined that phrase), i.e., the boundary lies along the smallest gray areas possible. Given the amount of discussion in the matter of Pluto, I gather that celestial objects are too smooth a continuum, with no good places to draw the line between planets and non-planets.

    So I’m fairly happy with the “dwarf planet” category, even if it means that poor old Pluto is no longer a full-fledged planet (snif!). It forms a useful category for smaller objects, but the word “planet” in the name is a reminder that this isn’t a natural distinction, but just one area of a continuum.

  72. It’s time to ditch the whole “planet” concept.

    It’s old, it implies stars wandering about, it gives us problems with Pluto, it does hardly tell us anything about neither the objects we apply it to, nor those we don’t.

    Originally, Earth wasn’t a planet (because it couldn’t be observed from Earth).

    In older, times, when people had mental problems, we called them “crazy”. And I’m willing to bet that we spent some time arguing who were “crazy” or not, and what “crazy” really meant.

    Today, we have ditched “crazy” in favour of some words that actually tell us a bit about what’s wrong. We call’em “schizophrenic”, “sosiopathic” or “depressed”, and it works much better. If you call a schizo (or an emo) “crazy” today, you’ll not only not make sense, you’ll make yourself look bad.

    It should be the same for walking around calling things “planets”.

  73. Dunc

    What works for biology works for astronomy, too.

    Indeed – time to dump the whole classification system and start from scratch.

  74. andy

    If geology (planetology) is really the most overriding criterion, you might as well drop the “orbits a star” requirement – no reason to ignore worlds like Titan or Ganymede just because they happen to orbit a planet, nor should an object lose planethood if gravitational encounters throw it out of the system into interstellar space.

    Alternatively “planet” refers to the most “important” set of objects in a solar system, in which case the current definition which excludes Pluto is good, if badly worded: instead of this whole orbit clearing thing, which is ambiguous – particularly if the object’s Lagrange points are stable (Trojan asteroids, etc.). Personally, I feel that if something is in the middle of a belt of similar-mass objects then it doesn’t qualify as a planet.

  75. Spaceman Spiff

    Read Steven Soter’s paper, which appeared in the Astronomical Journal: http://arxiv.org/abs/astro-ph/0608359. See especially Figures 1 and 2 and the discussions thereof. In particular the “mu” parameter, which compares a planet’s mass to that of all objects within its dynamical zone does not have the problem of “location”. And the fact that comets and asteroids cross “planetary” orbits is a red-herring argument. Those who drafted the language were aware of both Soter’s work and of a paper Alan Stern wrote with Harold Levison in 2002 in advising NASA on how to define “planets” in which was said:

    “we define an überplanet as a planetary body in orbit around a star that is dynamically important enough to have cleared its neighboring planetesimals …And we define an unterplanet as one that has not been able to do so,…”
    and a little further
    “our Solar System clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are Pluto and Ceres.” (kinda makes one wonder…)

    Soter’s arguments don’t solve all problems. But in my opinion, something physically profound separates the inner 8 larger bodies from those in the asteroid belt or Kuiper belt. In particular, Figure 2 shows clear separation between the “isolation zone” of fully accreted “planets” and the “swarm zones” of the other condensed objects.

  76. Irishman

    In other star systems, the “important” objects have masses 10 times that of Jupiter. Anything the size of Earth is just a dust mote. Hardly worth notice.

    “Dwarf planet” and “minor planet” only make sense if you have a working definition of “planet”. Otherwise, you are saying “dwarf scrimex”.

    I don’t mind Pluto being a “dwarf planet”, but that doesn’t stop it from being a planet, and to act like it does is stupid. Now if you want to throw a modifier on Mercury through Neptune as “Major Planets” and then have the category for “Dwarf Planets” that includes Pluto, Ceres, Sedna, MakeMake, etc, I could stand by that, but shortening “Major Planet” to “Planet” is a bit confusing, and writing the definition so that a “dwarf planet” is not actually a planet is ludicrous.

    Don’t define “planet” in order to include or exclude Pluto, define planet and then determine where Pluto fits.

  77. Dark Jaguar

    With “fuzzy” definitions, my personal rule is to use a “scale” or “slider” method of defining a planet. Namely, rather than a 1/0 defiinition, either it is a planet or it isn’t, it’s instead that “this object is very planetlike” versus “this object is not very planetlike”.

    This makes fuzzy edges part of the definition itself, and no one has to commit to saying pluto is or isn’t a planet, just that it’s mildly planet-like compaired to, say, Saturn.

  78. dkary

    Thank you Spaceman Spiff for finding that link to Soter’s paper. That’s the one I had in mind.

    And in response to Jorge, the “roundness” issue is at least as arbitrary, if not more so. Every planet has some component that is not defined by gravity (or to be more complete, gravity, rotation, and tides), whether it is mountains on the Earth and Mars or high and low pressure zones on Jupiter and Saturn (the shapes of those worlds have to be defined by a given air pressure, there being a lack of solid surface to work with). In the cases of the planets, this component is small, but it always exists (even stars have it as some level).

    There isn’t some magical transition from worlds that are round due to gravity into ones that are are not shaped by gravity. Instead, there is a continuous spectrum from very gravity-dominated objects (e.g. Earth, Jupiter, and yes, Pluto) to objects that are dominated by internal strength (like any typical small asteroid). Most of the largest hundred asteroids along with a fair fraction of the known KBO’s are in the size range where deciding which group they belong to is going to involve drawing an arbitrary line and saying, “here and no further”.

    Maybe Dark Jaguar is on the right track here, and we should instead talk about how “planet-like” different objects in. Of course, my kid’s class will still want to know which ones are planets and which ones aren’t.

    DK

  79. Torbjörn Larsson, OM

    So Neptune became a planet, stopped being a planet when its orbit changed, and then regained planethood after clearing its new patch.

    This is why a physical definition makes more sense.

    Hmm. Again, if biology is a good analogy (which may or may not be the case), species aren’t fixed either. Populations of animals or, especially, plants that have at some time lived as separate species may later hybridize.

    So I wouldn’t worry about the characteristic not only being fuzzy but temporarily.

  80. dkary, I know. That’s why I wrote: “Also, don’t believe in anyone that tries to tell you that there are non-arbitrary definitions. There aren’t. You’ll have to swallow some arbitraryness whatever the definition ends up being.”

    (But no, it’s not true that most of the largest hundred asteroids are in the border size range. For rocky objects, only those larger than about 400 km diameter are massive enough to round up under their own weight, which leaves 1 definite dwarf planet – Ceres – and 3 candidates – Vesta, Pallas and Hygea. We’ll have a lot more TNOs in that area, perhaps with a slightly lower lower limit)

    The point, however, is that the hydrostatic equilibrium criterion only demands the relatively small arbitrary decision about how much hydrostatic equilibrium is enough, wereas the orbital clearing criterion has not one, not even two as I wrongly wrote above, but three arbitrary points: how clear is clear enough, where does the neighborhood starts and ends and how long must we wait until checking if the neighborhood is clear or not. 3-1. In the Low Arbitrary Games, hydrostatic equilibrium wins a clear victory.

  81. Spaceman Spiff

    I’ve given a presentation, “Pluto – What’s in a Name?”, a few times to local astronomy clubs, and at the end I drew some conclusions based on what I discovered in my reading of the literature (I work on quasars, not planets). Here they are as I presented them:

    An important function of scientific nomenclature is to reflect natural relationships, not to obscure them.
    (I stole this from Steven Soter – because it was spot on.)

    We shouldn’t stay with a nomenclature that is as old as the hills and means “wandering lights in the sky”.

    It has become clear that the 8 “planets” (or how ever we are to distinguish them) are the dynamically dominant end results of gaseous disk and planetesimal accretion and differ radically in history, as well as in present day properties, from the vast swarms of KBOs, asteroids, etc.

    The IAU definition(s) should be improved (precision, flexibility), and probably cannot be described in a couple of sound bites (in 2009).

    The debate should be conducted with the public in mind (and more honestly) to demonstrate how scientific concepts are never written in stone and that understanding improves as new data are discovered.

    Nature needn’t conform to our inclination to put things into tidy boxes.

  82. Spaceman Spiff

    I’m not sure I can understand why “roundness” (whatever that means) constitutes whether something gets to be called a “planet” or a whatchamacallit. While it may be true that anything we end up designating with the word “planet” within it might look “round”, this doesn’t seem to me to be fundamental, but rather just a “fellow traveler”.

    Based on the criterion of what is “round” that I think everyone has in mind, if you set the crushing pressure K (plasticity limit) of the material to greater than the gravitational pressure (that from hydrostatic equilibrium) at the center of a constant density (rho) “spherical” object, one finds that the object’s diameter must exceed some value:

    D > 170 km * (K/MPa)^1/2 * (rho/g cm^-3)^-1, where K is measured in mega-Pascals and the density rho is measured in units of water’s density.

    Both K and rho depend on the type of material (ice, rock, iron, …), and here are some further comments….

    1) K ~ 40 MPa, ρ ≈ 2.5 g cm-3 for rocky compositions: D > 430 km

    2) icy compositions are less dense, but also have much smaller K, and so reach ’roundness’ at smaller radii (D > 340 km)

    3) ice/rock, rock/iron mixes give slightly different results

    4) depends also on melting history of object, which depends on mass and composition (and potentially even external tidal forces)

    5) depends on rotation, as well as collision history

    6) measuring shape difficult to do in practice (“shape” can be guestimated indirectly from measurements of object’s albedo and some idea of its composition based on same).

    7) And how round is “nearly round”?

    I am not suggesting a solution, but I am suggesting that one doesn’t lie in simply using a “roundness” criterion.

  83. Jya-Ja Binks Killer Spectroscoper

    Paul Clapham asked a long time ago but still in this Galaxy well asfar as I know ..) :

    August 13th, 2008 at 9:20 am – “What works for biology works for astronomy, too.” I’m not so sure about that. Biology works to produce species that are separate (at least with animals and plants, maybe not with bacteria so much.) But astronomy? Where’s the borderline between F and G stars?”

    The borderline is small & subtle but its in the stars spectrum and a matter of which lines are where in the “barcode” of its elements.

    As I understand it, (& correct me if I’m wrong, please) astronomers take a spectrograph of a stars’ light captured on a special machine (a rainbow is a naturally occurring solar spectrograph -I think ..) This spectrum is then studied and the position of various lines and bands reveals certain elements in the star which tells us what type it is.

    The temperature and mass of a star are revealed in the abundance of elemnets – which ones have the strongest lines.

    These types are for normal hydrogen fusing main-sequence stars and
    from hottest, bluest and most massive through to coolest, reddest and least massive : O, B, A, F, G (our Sun), K, M.

    But there’s a complication in that abnormal helium & other element fusing
    giant and supergiant stars also exist and tehse also fall into all these spectral classes plus a few extra ones too like W (Wolf-Rayet, super-hot, highly evolved stars with super strong stellar winds) R, N, S, C (rarely used classes for cooler giant stars with unusual chemical ratios.)

    So astronomers also use leters to distinguish whether a star of any spectral
    type is normal or strange in that second way. Normal hydrogen fusing main
    sequence stars get V, giants get II & III, sub-giants (stars turning into giants get IV, supergiants get I & hypergiants like Eta Carinae get 0.

    Oh and each spectral class is also sub-divided into 10 classes from 0 to 9 too based on how its precise spectrum looks which depends on exactly how massive and hot it is.

    Hence our Sun is G2 V but Capella a yellow giant gets G5 III.

    Now to finally answer the question :

    a star of class F is a smidgin hotter, a trifle more massive and has a spectrum that a teensy bit different in exactly how its spectral lines look.

    The borderline is between F9 to G0. Its subtle alright but its there.

    As far as Pluto goes, well I guess its a good thing Lucas set Star Wars in a galaxy far, far away so there’s no issue there vs what K9 said with
    Dr Who! ;-)
    _______________________________
    Bit of a long post I’m afraid & sorry if anyone’s already answered it better. :-(

    If folks want more info. I suggest Jim Kaler’s website or
    Wikipedia …

  84. Spelling nazi

    Wa-aay up thread too Gutav Nyström asked back on August 13th, 2008 at 9:39 am

    ” .. “playpuses” : Is that “platypuses” you mean, or am I missing something?”

    Yes I’m afraid you are. To be horrendously pedantic, the plural form of platypus is actually platypi! ;-)

  85. Spiff, don’t go Calvin on me ;), but:

    What you are saying is basically that there’s a murky area between diameters of 340 and 430 km. Agreed. In this size range, objects will have to be classified one by one, after further examination their properties, and some are likely to remain planet candidates for a long time.

    However, this also means that all the other objects, which are the vastest of the vast majority, will be classifiable immediately, most on detection (most detections of new objects have implicit a lower and an upper mass limit, whatever the method of detection might be), others after the crudest estimates of mass.

    Adding to this, it mustn’t be forgotten that exactly the same mass estimates needed to determine planethood through the hydrostatic equilibrium criterion will also be needed to determine planethood through the orbital clearing criterion. If anything, this one is even more dependent on fragile mass estimates, because it relies on estimating the masses of objects that are smaller than the object under consideration.

    It leaves objects in murky areas for much, much longer, because, given the chaotic nature or gravitational interactions, planetary systems are not deterministic, i.e., you can’t know for sure which young planets will survive and which will be tossed out of the system, into the star or smash head on with other young planets. The best we can do is come up with probabilities, and those… you may know that simulation studies of our own, neatly ordered and middle aged solar system have shown that there is a low, but significant, probability that Mercury’s orbit becomes unstable, which would mean that it might magically cease to be a planet overnight by the dynamical criterion, which in fact already happened with Neptune and Uranus. And this is a complete nonsense.

    And gets worse when we look away from the Solar System, into the stuff that rotates around nearby stars. By the physical criterion, we are able to say with absolute certainty that every single planetary object we’ve found so far is indeed a planet. Even PSR 1257+12 A (provided a pulsar is considered worthy of hosting planets), whose mass is similar to that of Ganymede. By the dinamic criterion, however, we can’t, because we don’t know practically anything about their “neighborhoods”, and won’t for centuries, especially as the mass of the new extrasolar planets becomes smaller.

    You can hardly murk things more than this, quite frankly.

    But don’t go Calvin on me ;). It’s just an opinion (hopefully an informed one).

  86. StevoR

    Spaceman Spiff said on August 14th, 2008 at 11:11 am :

    “Read Steven Soter’s paper, which appeared in the Astronomical Journal: http://arxiv.org/abs/astro-ph/0608359. See especially Figures 1 and 2 and the discussions thereof. In particular the “mu” parameter, which compares a planet’s mass to that of all objects within its dynamical zone does not have the problem of “location”.”

    SR (me) : Umm . Can we get a trabnslation to plain english please?

    SS (Spaceman Spiff – & should be quotes in italics too):
    And the fact that comets and asteroids cross “planetary” orbits is a red-herring argument.

    What!! The criteria is suposed to be “orbital clearing” & yet somehow you’re saying it doesn’t matter that orbits aren’t clear? That comets, asteroids even potentially other stars and planets can come past and make an orbit NOT clear? That orbits didn’t use to be clear in the past and might NOT be again at some stage in future history.

    Our solar system will encounter a star called Gliese 710 sometime in the distant future which may affect planetary orbits as will the death of the Sun as a red giant when it loses mass. Suppose in the latter case Jupiter’s orbit suddenly crosses Saturns – does that stop either of them being planets? According to the IAU baloney criteria it would – & how silly is that!

    No sorry the fact that orbits are crossed – by anthing other than radiation and solar wind means that they’re NOT clear to anyone spekaing the language of reasonable.

    Otherwise then what exactly is meant by “clear” and for how far around the orbit it needs to be ‘cleared’ needs to be well ..cleared up! ;-)
    Which it really just can’t be. Its a fatal flaw in a stupid and unnecessary third criterion. This “orbital clearance” nonsense just doesn’t make sense. Period.

    SS : Those who drafted the language were aware of both Soter’s work and of a paper Alan Stern wrote with Harold Levison in 2002 in advising NASA on how to define “planets” in which was said:

    “we define an überplanet as a planetary body in orbit around a star that is dynamically important enough to have cleared its neighboring planetesimals …And we define an unterplanet as one that has not been able to do so,…”
    and a little further
    “our Solar System clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are Pluto and Ceres.” (kinda makes one wonder…)

    SR : Uberplanets and unterplanets? Mein Gott! Vot ist happenink here are ve liffink in Germany!? ;-)

    Whats this Soter saying here – that Pluto is a gypsy sub-planet & Neptune from the superior planetary master race or some such tripe?! Come on!

    This seems to be just a pluto-haters rant, nothing much else is clear about Soter -whoever he is. Give me Alan Stern’s words instead anyday! ;-)

    SS : Soter’s arguments don’t solve all problems. But in my opinion, something physically profound separates the inner 8 larger bodies from those in the asteroid belt or Kuiper belt. In particular, Figure 2 shows clear separation between the “isolation zone” of fully accreted “planets” and the “swarm zones” of the other condensed objects.
    (Emphasis added.)

    SR : Well in my opinion your opinion is wrong. In my opinion a planet is aplanet regardless of whether it skims the surface of a star like a HotJupiter or orbits in the far reaches of the solar system where there;’s alot more roomfor otherobjevcts like Plutodoes. I see NO reason why you
    can’t have planets inside asteroid belts or cometary “swarms” or whatever if they fit the other criteria – roundness, non-nuclear fusing for energy, etc ..

    If you imagine otherwise I reckon your imagination is
    lacking. (& Einstein no less rated imagination over a lot else.)

    I think the problem arose because orbital dynamicists had aproblem with a planet – Pluto – they disliked because it seemed a misfit. Ironically once they’ve found a few bodies that are similiar (& lets remember only 1,
    Eris, is larger & that not by much at all.)
    now its nolonger amisfit by the leading member of its planetary sub-class they want to scrap it. For no good reason that reasonable folk (incl. many in the general public)
    can see. :-(

    Sorry but if that reflects badly on orbital dynamicists then the fault lies with them and the only way for them to correct it & regain our respect
    is for them to come to their senses and accept that : Yes Pluto really is a planet. :-P

    To paraphrase Dr Seus : A planets a planet no matter where it is & no matter how how small! ;-)

    Incidentally, its not just Pluto, I’d be happy to term Eris, Ceres , Sedna & others planets as well. I see no reason why we should limit the number of planets just so there are less names to remember. I mean for pity’s sake! If there are twenty or even a hundred planets in our solar system then so be it – thats how many there are. And Pluto is one of them.

  87. StevoR & Isaac Asimov

    Isaac Asimov, to name one, would agree with me given he termed Ceres a planet too :


    “… 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.”

    Source : Page 63, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by Isaac Asimov, Mercury Press, 1973.

  88. StevoR

    BMcP said on August 13th, 2008 at 3:41 pm :

    “… SNIP .. Call me silly, but I feel that:

    1. If it has enough mass to pull itself into a sphere…
    2. If it circles the sun…
    3. If it has even the vaugest whisper of an atmosphere… (even if that atmo lies frozen on it’s surface)
    4. (BONUS) if it has satillite(s)…

    Then it’s a planet. (If it has laser beams attached to it’s friggin’ head, that’s just icing on the cake

    Heck Mercury got in, and it only has three of those qualifications, and barely the third one. It is just a glorified rock in space and little more. Plus what a cheat on the whole clearing the orbit of debris bit, it has such a small orbit to clear!

    I don’t want to kick Mercury out of the club, but let Pluto back in. Oh and Eris and MakeMake too. I love routing for the small guy.”

    Me too. 8)

    Well said. I couldn’t agree more. (emphasis mine.)

    If there’s a sliding scale on any planetary continuum then Pluto and Mercury are at least equal if not Pluto being ahead when it comes to planetary status. & not meaning to diss Mercury at all but Pluto is a much more interesting and dynamic planet too.

    If Mercury was where Pluto is then it wouldn’t be counted as a planet by the IAU definition fools either. Which shows again how silly they are – to use an analogy its like saying that a person is not a classed as a real person just because they live in another country.

    —–

    Hmm.. My earlier post from a few minutes ago seems to have disappeared? Whats going on there I don’t think I swore anywhere in it or was too offensive? Was I? :-(

  89. StevoR

    Okay & now its back – & I’m seeing all the typos I’d like to be able to edit .. SIGH. :-(

    Okay folks I’m human I’m tired, I make mistakes, I hope y’all get the gist
    anyhow.

    Jorge said on August 15th, 2008 at 8:55 am :

    “dkary, I know. That’s why I wrote: “Also, don’t believe in anyone that tries to tell you that there are non-arbitrary definitions. There aren’t. You’ll have to swallow some arbitraryness whatever the definition ends up being.”

    (But no, it’s not true that most of the largest hundred asteroids are in the border size range. For rocky objects, only those larger than about 400 km diameter are massive enough to round up under their own weight, which leaves 1 definite dwarf planet – Ceres – and 3 candidates – Vesta, Pallas and Hygea. We’ll have a lot more TNOs in that area, perhaps with a slightly lower lower limit)

    The point, however, is that the hydrostatic equilibrium criterion only demands the relatively small arbitrary decision about how much hydrostatic equilibrium is enough, wereas the orbital clearing criterion has not one, not even two as I wrongly wrote above, but three arbitrary points: how clear is clear enough, where does the neighborhood starts and ends and how long must we wait until checking if the neighborhood is clear or not. 3-1. In the Low Arbitrary Games, hydrostatic equilibrium wins a clear victory.”

    (Emphasis added.)

    Sorry, call me thick if you like, but can we get that last bit in ordinary non-jargonese english please? ;-)

    You mean if its round right? Or close enough to? That some object being round makes more sense as a criteria and is less arbitrary and results in less contradictory and absurd results than the “orbital clearnace” stupidity? Would that be a fair distillation down into the language of Plain or have I misunderstood you? (If so then sorry, late in the night – or morn for me again now.)

    Oh & the three candidates bit I put in bold above –

    “… which leaves 1 definite dwarf planet – Ceres – and 3 candidates – Vesta, Pallas and Hygea.”

    What about Juno? Wasn’t that one of the original four asteroids found and one of the largest? Isn’t it another candidate? If so then as far as I’m concerned its “welcome to the planetary club Juno!” ;-)

    If not, oh well, good on you Juno for trying!

    Either way I’m curious – where does Juno (& for that matter Astraea?) fit in this? Anyone?

  90. StevoR

    tacitus said on August 13th, 2008 at 11:38 am :


    StevoR:

    12 REASONS WHY PLUTO _IS_ A PLANET

    Your article is misleadingly titled. All I see is mostly a list of objections to the reasoning and process the IAU used to define what a planet is. At best, if your arguments hold water, that gets us back to square one, still trying to decide whether Pluto should be classified as a planet. Ho hum.

    Well the thing is that the IAU definition was specifically drawn up to exclude Pluto.

    So showing that the IAU definition is ridiculous and downright wrong -which I think it was – shows they shouldn’t have adopted it but rather a beter definition that does include Pluto as a planet.

    So I don’t think my post was “misleading” because I was clearly (well I thought clearly) putting forward the pro-Pluto side of the debate.

    Now the big question is :

    Do you think my arguments hold water?

    I do – obviously or I wouldn’t have raised them! ;-)

    But do you agree with me?

    If not, why not & where am I wrong?

    Honestly, if you’ve got an opinion on that, let me know. ;-)

    (Please tell me even if you think I’m wrong. I’m open to civilised debate anytime. Well except when I’m going to bed like ..er ..now.)

    If my arguments hold water as I think, then I reckon we’re a lot further on than just square one. ;-)

    I think we’re close, at least, to saying : “Yes whatever definition we do have for “planet” has to include Pluto. Because the only one that doesn’t -that was specifically formulated deny Pluto’s planetary status – was downright dumb.

  91. Yes, StevoR, that’s what I meant, sorta.

    Regarding the emphasised bit, Juno, despite having been the 3rd discovered, is in fact only the 13th largest (or thereabouts) asteroid. At 236 km diameter it’s way too small to be rounded due to self-gravity, so it’s clearly in the asteroid class. Astraea is even (much) smaller, not even 150 km diameter. They are both unequivocally asteroids.

  92. Asimov Quoter

    Isaac Asimov stated that he considered Ceres (& by extension obviously Pluto too) to be “respectable” planets :

    “”… 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.”

    Source : Page 63, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by Isaac Asimov, Mercury Press, 1973.

    Emphasis added -apologies if this turns up here twice. Web issues? Server issues? Use of & issues? – StevoR (Hoping this works this time.)

    Looking at the HST image of Ceres, I don’t think toomany coild argue oit looks like planet too.
    _____________________

    Planets? The more the merrier ..

    There are, after all, now over 300 found found orbiting other stars outside our solar system.

    So why not add a few more to ours! ;-)

  93. StevoR

    Okay Jorge Thanks for that – that’s my question answered. :-)

    I’m also glad I understood you mostly right – afraid I’m geting quite zombiefied at this hour! ;-)

    Correction :

    “Looking at the HST image of Ceres, I don’t think toomany coild argue oit looks like planet too.

    is meant to read :

    Looking at the HST image of Ceres, I don’t think too many folks could argue that it does look like planet too.

    Argh! Typos – if only we could edit here! Sigh.

  94. Torbjörn Larsson, OM

    @ Jorge:

    The point, however, is that the hydrostatic equilibrium criterion only demands the relatively small arbitrary decision about how much hydrostatic equilibrium is enough, wereas the orbital clearing criterion has not one, not even two as I wrongly wrote above, but three arbitrary points:

    Well, if you had actually read the paper you would have found that dkary is correct, the clearing criterion is a lot less arbitrary, as there is a huge mass ratio difference between objects that have cleared their orbits or not.

    [More specifically, as you brought it up: IIRC the mass ratio doesn’t matter within broad limits (“how clear is clear enough”), the neighborhood is dynamically decided (by the clearing process) and so is the checking time.]

    IAU’s definition is a lot closer to looking at the actual formation process, and so more analogous to how biologists have solved their scientific problem of species recognition. But in fact they aren’t fully analogous – what IAU is trying to do is to establish a specific population of “planets”, a specific ‘species’. By looking at the process I would assume that would analogously mean incorporating all originally inner system bodies; planets, asteroids and most moons.

  95. Larsson, guess what: I did read the paper. And no, the clearing criterion is not a lot less arbitrary, quite the contrary.

    Oh, I know that the index shows a big gap, blah, blah, blah. You can create indexes showing big gaps in just about anything; you just have to taylor carefully what goes into them. If you wanted to exclude Mercury from planetary status, all you had to do was creating an index of atmospheric pressure versus body size, and there, you would’ve kicked Mercury from the group of planets. I’ll grant you that this one is a bit more significant than that, but is just as biased towards inner system bodies as the atmospheric index would be towards outer system bodies. It is, however, and as I’ve said numerous times, useful to separate two distinct groups of planets in the current state of our system, however arbitrary it might be. But does not work for any other and there are billions and billions of planets around other stars.

    And the mass ratio is everything. It’s at the very base of the index. Did you read the paper? Apparently not.

    The neighborhood being dinamically decided is yet another absolute nonsense. Orbits shift, and they shift to the point that we just can’t know for sure at present where the planets orbited in the past. If the neighborhood was indeed dinamically decided, throughout the whole of the system’s evolution, then neither Uranus nor Neptune would be planets, because it seems they’ve swapped positions somewhere in the past. If that theory is correct. We don’t know. But if it is, then a truly dynamically decided neighborhood would have to include both… unless it’s constantly shifting together with the body in question, in which case you may well get stuck in situations where, due to variations in eccentricity (which happen in a much larger scale than variations in semimajor axis) a given body is a planet during a few hundreds or thousands of years, stops being a planet for another few hundreds or thousands of years, becomes a planet again for a few hundreds or thousands of years, etc. Sheer nonsense.

    That’s probably why the paper doesn’t say that. Go read. What it says is that the neighborhood is what goes from perihelion to aphelion. Current apsis, nothing dynamical about it. Why? Because they said so. And the checking time is the age of the Solar System, which is why the index details the current state of affairs, not what happened in the past, nor any possible future state. Why? Because they said so.

    Not arbitrary, you say? Yeah, right.

  96. Tony

    Can someone re-post a link to the discussion? The one in the post gives a “Page not Found” error.

    Thanks,
    Tony

  97. Spaceman Spiff

    to StevoR:

    “we define an überplanet as a planetary body in orbit around a star that is dynamically important enough to have cleared its neighboring planetesimals …And we define an unterplanet as one that has not been able to do so,…”
    and a little further…
    “our Solar System clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are Pluto and Ceres.”

    Those were Alan Stern’s words! (in the 2002 paper with Harold Levison, with the purpose of defining what is meant by “planet”). That’s the point I was trying to make!

    StevoR said:
    “Whats this Soter saying here – that Pluto is a gypsy sub-planet & Neptune from the superior planetary master race or some such tripe?! Come on!

    This seems to be just a pluto-haters rant, nothing much else is clear about Soter -whoever he is. Give me Alan Stern’s words instead anyday! ;-)

    Sigh – these aren’t a scientifically useful arguments.

  98. Spaceman Spiff

    StevoR said:
    “What!! The criteria is suposed to be “orbital clearing” & yet somehow you’re saying it doesn’t matter that orbits aren’t clear? That comets, asteroids even potentially other stars and planets can come past and make an orbit NOT clear? That orbits didn’t use to be clear in the past and might NOT be again at some stage in future history.”

    No, it simply means you don’t understand what was meant by ‘orbital clearing’. As I said, the people who wrote this imprecise (for the lay public) language knew precisely what was meant because both Soter and Stern & Levison (and others) had defined it in at least two ways: as a characteristic time scale and as a mass ratio. Read Soter’s paper that I linked above. He also provides the caveat that his suggested characteristics would not be useful in an immature solar system.

    And I am not taking definite sides (because nature doesn’t care a wit about “sides”), except to suggest strongly that our choice of labels on our boxes should “reflect natural relationships, not to obscure them.”

    I have no difficulty using the word “planet” as long as it has a qualifier to reflect the fact that some objects belong to separate populations (by origin, by history, by dynamical importance, by…) than others. And no matter how this matter is settled, nature simply does not care (and will happily provide exceptions) — what matters is that our choice of language minimizes any obscuration and maximizes our understanding of our understanding of the phenomenon.

  99. amphiox

    Personally, I like the idea of using mass and mass alone to define planet. The upper limit we already have (enough mass for deuterium fusion = brown dwarf).

    For the lower limit it should be a fixed, specific number, based on parameters that the majority can agree are important. It will be arbitrary, but not excessively so if the parameters are chosen carefully, and all definitions are arbitrary at their extremes, and we accept that exceptions may occur (and in fact provide some of the most interesting science).

    I would humbly propose using the roundness by self-gravity as a basis, but remove the uncertainties associated with composition and how round is round enough by basing the definition on a hypothetical planet with fixed properties. For example, we could use the minimum mass required for a body of 100% pure hydrogen, with no rotation, and no outside forces of any kind acting upon it, to shape itself into a perfect sphere. This would make the definition of planet fall in line with the definitions for stars, and would reflect the reality that objects in the universe fall along a continuum of size and mass.

    (I chose hydrogen because it is the simplest and most common element, but one could choose other elements, such as iron, (endpoint of exothermic fusion), as well)

    Anything greater than this minimum mass, regardless of actual shape, size, or location, can then be called a planet. We could avoid the potentially embarassing circumstance of one day discovering an object that should be big enough to be spherical, but isn’t. (say a Jupiter sized gas giant that is oblong because of rapid spin, or distorted because it is orbiting a neutron star, or an earth-sized object that isn’t spherical because it was involved in a big collision recently and hadn’t had time to become spherical again, but we don’t know these details yet because we’ve just observed the object and don’t know anything beyond shape and mass)

    We could have separate subclasses for large moons and ejected planets.

  100. Sorry, Quiet Desperation, but “letting it go” when the IAU definition makes no linguistic sense and was made by a tiny minority of its members, most of whom are not planetary scientists, in a process motivated by political sentiment that did not even follow the recommendations of the IAU’s own committee is just plain wrong. What is this bias toward letting things go? If something is wrong, it’s still wrong a year or two years or ten years later and needs to be fixed.

    Thank you, SteveoR, for your very comprehensive list of why the IAU vote was a travesty and why Pluto, as well as Ceres, Eris, MakeMake, and other dwarf planets should be considered a subclass of planets. With the IAU having made such a mess, I hardly trust them as the “experts” in this field. In fact, most IAU members are not planetary scientists, and most planetary scientists are not members of the IAU. That leads to the inevitable conclusion that planetary scientists should form their own organization to deal with defining the objects they study.

    I was at the Great Planet Debate, and Stern makes a compelling case for hydrostatic equilibrium as the one criteria that unites both “major” or “classical” planets and dwarf planets and separates them from asteroids or small solar system bodies. Objects at the mass where they can attain hydrostatic equilibrium are fundamentally different than inert asteroids–they are differentiated geologically, the same way the terrestrial planets are. Yes, they are dynamically different, in that being smaller, they don’t dominate their neighborhoods. That’s why they are classified as dwarf planets, but it is not reason to state that they are not planets at all.

    Phil, Pluto DOES fit the bill for being a planet. You argue that science shouldn’t be driven by public opinion but don’t mention that neither should it be driven by fiat or decree of a small insular body whose members have their own personal agendas. I believe that when New Horizons gets to Pluto, we will have sufficient data to illustrate Pluto’s planetary features that there will be no need to rely on a decree because “someone says so.” The same is true for Ceres regarding Dawn. In the meantime, the public is fully capable of understanding that the issue is a subject of ongoing debate with two differing schools of thought and that there is no one “right” answer, at least until 2015.

  101. Torbjörn Larsson, OM

    @ Jorge:

    You can create indexes showing big gaps in just about anything; you just have to taylor carefully what goes into them. […] And the mass ratio is everything.

    That isn’t what the paper says. It establishes a natural gap. (Yes, using mass ratio as I said, to establish clearing/mass dominance. And a time scale, again naturally characteristic.)

    The neighborhood being dinamically decided is yet another absolute nonsense. Orbits shift, and they shift to the point that we just can’t know for sure at present where the planets orbited in the past.

    That wasn’t the process I (and, IIRC, the paper; yes, it was a while since I read it) described. My very first comment mentioned the planetary formation process from the planetary disk.

    As for your observation that some bodies may change status, it is exactly what I mentioned earlier in the analogy with biology. There it makes sense, and if we regard the planetary formation process as basis for defining planetary bodies it may make sense here too.

    [But if we try to make the analogy exact, IAU is actually trying to establish an analogue to a ‘species’, in which case they will have to look for a closely related population. That, I assume would include all inner system originated bodies; planets, most asteroids and most moons.]

    And the checking time is the age of the Solar System, which is why the index details the current state of affairs, not what happened in the past, nor any possible future state.

    As the current state is contingent on the historical path, I don’t see how one can argue that it isn’t a result of a dynamic process. [Btw, again this is very analogous to biology, because species are historically contingent on the dynamic process of evolution. But I’m not arguing that is necessarily a good thing.]

  102. Nails67

    Why do we need a definition of planet? Pluto is what it is, whether it’s a planet or not. Same with Earth, Jupiter, and Ceres. No one really gets bent out of shape about the definition of “continent”, and I defy you to establish a definition of continent that would uncontroversially yield the seven (or six or five, depending on what part of the world you are from) continents that we recognize today. Looking at a Dymaxion map makes it hard to argue that there is more than one continent.

  103. And this is why people ought to have classes in logic and rhetoric in undergrad schools, even if they are planning to be scientists.

    You’re running into the problem of the sorites: a bunch of things that add up, little by little, so that a very small difference pushes something from one (somewhat arbitrary) category to another. The canonical example is the category of “bald men.” I have a pretty full head of hair, at lest compared to most men my age; I’m not bald. If I were to lose one hair, I still wouldn’t be bal; lose two, and, well, that’s only a different oce one hair. Flash forward until I’ve alost all but one of my remaining hairs, and sure enough I’ll be bald. But you’ll be hard-pressed to find the time in between at which removing one hair moved me from “not bald” to “bald.”

    I talked with Dan about this myself, a couple of years ago, with the idea that there were inflections or discontinuities in the distributions of sizes that might give us a hint. The problem there is that, at least at that time and based on an over-lunch conversation, there really aren’t; the distribution of object size versus number of objects seems pretty continuous.

    Of course, if that’s true, it’s unlikely that any definition will be able to crisply determine whether an object is, or isn’t, a planet. No matter what, it’s likely that we’ll find there are objects that seem “planet-like” but which are just outside our definition.

  104. Dammit, I wish you had a preview. I hope you can close that link.

  105. Okay if this works folks can click my name to get back to the ‘Still Here’ thread where this debate is (sort of) still going as a sub-theme!

    Isn’t it funny how things keep ending upon Pluto! ;-)

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