Attack of the Pluto!

By Phil Plait | July 25, 2012 1:21 pm

I was interviewed once again on the G4 TV program "Attack of the Show!" – I guess they didn’t learn their lesson the first time – where my pal Matt Mira (from Nerdist!) and I talked about planets around other stars, and whether Pluto is a planet or not.

Here’s the segment (you may need to refresh this page to load it):

I pretty much restated my case that I’ve been making about Pluto for quite some time: you can’t really define what a planet is, so the argument over whether Pluto is one or not is the wrong thing to be talking about. The fact that a fifth moon was recently found is irrelevant; "planet" is more of a concept than a defined object. Trying to draw firm borders around a fuzzy thing like this only guarantees more arguing and less light shed on the topic.

Anyway, it was a fun interview, and it’s always a good day when I can get one good zinger in, especially at Matt’s expense.


Related Posts:

- Mars Attacks of the Show!
- In which we define what a planet is
- A fifth moon for Pluto!
- Shining shoes for NASA
- My Nerdist episode is online!

Comments (43)

  1. plutosdad

    Regardless, my dog, Pluto, is still mad about the demotion of his namesake. I think we can all agree you owe him some treats.

  2. Thanks, Phil. The more we learn about the Universe, the less we are able to plop all those objects into neat little boxes and label them. A good example is Greenland. It is considered the world’s largest island. But why? Why isn’t Australia the world’s largest island? Greenland and Australia are both landmasses. Why is one an island and the other a continent?

    Some people say Pluto should be reclassified as a planet. If so, then Eris should be a planet also. After all, it appears to be larger than Pluto, and it has at least one moon.

  3. I think your comment that folks need to get a new hobby nailed it. Pluto is exactly what it is regardless of what we call it.

  4. I’m about halfway through Mike Brown’s How I Killed Pluto, and am enjoying it immensely. Thanks to Phil for recommending it!

  5. Zero

    I am still struggling to understand your view on this planet issue.
    First of all (sorry to be nitpicking): Of course ‘planet’ is a concept. But if something can be defined at all, then it’s concepts. (‘Bachelor’ is also a concept. Arguably, it can be defined.)
    More importantly, we can often say whether something falls under a concept without having a definition for that concept. This clearly seems to be the case for ‘planet’, too (otherwise, what would be the use of applying that concept at all?): Jupiter is definitely a planet, while that table tennis ball you were holding in your hand is definitely not. So is your idea that Pluto is a bordeline case – it is neither definitely a planet nor definitely not?

  6. Liisa

    I love AOTS and I am glad to see you on it! Hope to see more frequent appearances :)

  7. Coda

    It’s appearances like these that prove that Phil isn’t on the payroll of Big-Pharma/Big-Climate. If he was, he’d have a better microphone! ;)

    But in all seriousness: great job, Phil!

  8. Dutch Railroader

    It’s actually simple to define what a planet is. The problem in doing so only arose when people had to live with the sociological implications of the definition. I was always happy with the simple definition of a planet as an object that orbits a star and has a equilibrium figure set by gravity. But this would require Ceres to be a planet, when everyone “knows” its an asteroid, and so the trouble started…

    The IAU definition is terrible, however. It 1) introduces a novel dynamical definition that had never been discussed previously as a criterion, 2) cannot be applied to exo-planets, 3) uses mass as does the “roundness” criterion, but in a highly complex relationship with the primary, the size of the orbit, and so on, but mainly 4) was selected for the desired outcome of excluding Pluto and its KBO kin.

    Does it matter? Yes, it very much does. Planetary astronomers now engage in an endless public debate that detracts from the real exploration being done. Astronomers over all look as foolish as we did when a handful of astronomers tried to rename the Big Bang years ago. But more important than that, it relegates fascinating new worlds to second class status. Kids will learn the now eight planets, but will never hear about the dwarfs. Planetary status to Eris and all the rest would have drawn greater attention to the frontier of the outer solar system, and the link to exoplanets, which like the KBOs come in forms never seen before…

  9. Doug Cornelius

    There are lots of arguments to be made, but I hope we can all agree that there are not 9 planets in our solar system.

    Using the IAU definition we have 8. Using other definitions we have more than 9.

  10. Keith Bowden

    Pluto – dwarf planet. It’s right there in the name, six little letters: p-l-a-n-e-t. :) Hahhahahahahaa!

    Next people will say a grapefruit isn’t a fruit…

  11. RAF

    Just glad Clyde Tombaugh wasn’t alive when his planet got demoted.

    …and with 5 objects orbiting it, what are supposed to call it…a KBO with satellites???

  12. Chris

    @2 Chuck
    Greenland and Australia are both landmasses. Why is one an island and the other a continent?>
    Greenland’s area = 840,000 square miles
    Australia’s area = 2,900,000 square miles = 3.5 x Greenland

    I still agree with you that the definition of a continent is arbitrary and if we one day find a planet with landmasses in water we’ll be having a similar debate. But this is just to point out that there is actually a big difference between the relative sizes that isn’t immediately obvious if you look at a 2D map of our 3D world.

  13. PsyberDave

    I like Dutch Railroader’s take. And I have no problem with Ceres being a planet. Didn’t it used to be called as such?

  14. Nyetwerke

    That initial face on the video before you hit the “play” buttom so screams “Hey you kids, get off my electrons!”
    Pluto as planetessimal?

  15. beerclark

    BA: You used to do video podcasts. I realize this is an interview so its a bad example, but would you ever start them up again since you are doing videos?

  16. Messier Tidy Upper

    The latest issue of New Scientist magazine has a page long article on the discovery of Pluto’s fifth moon which quotes Kevin Baines, a planetary scientist at JPL as saying :

    “If you’re important enough to have aquired five satellites you’re a planet.”
    - Page 11, NewScientist magazine, 21st July 2012.

    It is also worth noting that Isaac Asimov has referred to Ceres as the “four and halfth” planet* and another planetary scientist Jim Bell, professor of Astronomy and Planetary Science, University of Arizona recently said this :

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

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

    Perhaps then we should consider an object a planet if it isnt directly orbiting another planet (ie not a moon) and is between the mass of Ceres and the minimum mass for deuterium fusion of about eleven Jupiter’s allowing for metallicity?

    As some of you may already have an inkling from my past comments on this topic; I really think the current IAU definition clearly doesn’t work and is terribly flawed and actually downright silly.

    If a dwarf star is still a star as most stars are, then the same should surely apply to dwarf planets still being planets as most planets are! If we count dwarf planets as planets – as we should – then Pluto is actually far from the smallest and the largest of its class! :-)

    ++++++++++++++++++++

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

    Perhaps its time we stopped leaving out planets simply because they are small and may be in eccentric orbits!

  17. Messier Tidy Upper

    @MTU – July 25th, 2012 at 10:11 pm – Correction :

    Perhaps then we should consider an object a planet if it isnt directly orbiting another planet (ie not a moon) and is between the mass of Ceres Vesta and the minimum mass for deuterium fusion of about eleven Jupiter’s allowing for metallicity?

    Is what I was meaning even though I typed Ceres instead. D’oh!

    Although I guess Ceres mass or a value somewhere in between Ceres and Vesta would work too.

    That definition works a bit like the line between mountain and hill or continent and island.

    Nature, of course as in those cases creates objects along the whole continous range of sizes from the most massive Superjovians through to Earth-sized worlds to Pluto and Ceres and from Eurasia to Australia to Greenland from Mt Everest to the very low mountains we get here in the mostly rather flat nation of Australia.

    Arbitrary, sure but any definition is to some extent and it avoids the IAU definitions worst flaws such as ignoring exoplanets entirely, the inconsistent semantics of including dwarf stars but not dwarf planets and the reductio ad absurdum test failures of the, in my view utterly absurd, “orbital clearence”criterion.

    What ch’y'all think?

  18. Adrian Lopez

    What I get from the IAU’s classification scheme is that Pluto is a not-a-planet object of the dwarf planet kind.

    You don’t have to call Pluto a planet (even though it’s more like a planet than not), but if you’re not you should at least call it something other than a name with the word “planet” in it.

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

    Non-US readers probably won’t get the pun in the title of Asimov’s piece. (Asimov was a notorious punster.)

  20. Nigel Depledge

    Chuck Anziulewicz (2) said:

    Thanks, Phil. The more we learn about the Universe, the less we are able to plop all those objects into neat little boxes and label them. A good example is Greenland. It is considered the world’s largest island. But why? Why isn’t Australia the world’s largest island? Greenland and Australia are both landmasses. Why is one an island and the other a continent?

    I always wondered about that : why isn’t Africa + Eurasia the world’s largest island?

    BTW, Australia by itself isn’t a continent. The continent is Australasia, which includes the very large islands to the north of Australia.

    Some people say Pluto should be reclassified as a planet. If so, then Eris should be a planet also. After all, it appears to be larger than Pluto, and it has at least one moon.

    Not just Eris, but probably also a whole slew of as-yet-undiscovered KBOs.

  21. Nigel Depledge

    Zero (5) said:

    I am still struggling to understand your view on this planet issue.
    First of all (sorry to be nitpicking): Of course ‘planet’ is a concept. But if something can be defined at all, then it’s concepts. (‘Bachelor’ is also a concept. Arguably, it can be defined.)
    More importantly, we can often say whether something falls under a concept without having a definition for that concept. This clearly seems to be the case for ‘planet’, too (otherwise, what would be the use of applying that concept at all?): Jupiter is definitely a planet, while that table tennis ball you were holding in your hand is definitely not. So is your idea that Pluto is a bordeline case – it is neither definitely a planet nor definitely not?

    That’s a good question. I think this is what started the whole issue off.

    Prior to the discovery of Eris et al., there were questions about whether it was reasonable to include Pluto as a planet, because it seemed to be more like just a large KBO than like any of the other eight planets. When Eris was discovered it was thought to be larger than Pluto (IIUC, it is now considered to be a few km smaller in diameter).

    The IAU decided they needed a definition of what a planet was, and made one. A lot of people disagreed with their definition because it changed Pluto’s label from “planet” to “not a planet” (IIUC, it is considered a dwarf-planet). Pluto, quite obviously, does not care what we puny humans call it.

    However, ultimately the purpose of a definition is to serve as a useful category to facilitate discussion and / or research on the objects in question. IMO, it doesn’t matter to 99% of us how the IAU choose to define the term; we’ll carry on using the word “planet” as we always did. I grant them the prerogative to define it any way that makes it useful to them, in the same way that I, as a member of the Biochemical Society, would expect to have a say in defining any technical biochemical terms that we consider in need of a fresh definition. After all, the IAU definition only applies to technical discussions among professional astronomers.

    Some people (you know who you are) argue that the IAU definition is wrong (and I have never really seen any solid argument that it is either “wrong” or illogical, or any of the other less-than-complimentary adjectives that have been directed at the IAU), without recognising that the IAU members are entitled to call anything they study anything they collectively want to. And the rest of us will carry on using the words that work for us.

  22. Peter Davey

    As Humpty-Dumpty pointed out to Alice (in Wonderland): “When I use a word, it means what I want it to mean. The question is: Who’s to be master? That’s all.”

    Mind you, most people will remember what happened to Humpty-Dumpty.

    With regard to Mr Hellbig’s comment, I understand that Asimov’s title was actually a pun on “the World Series”, referring to baseball,which is a sort of inferior offshoot of cricket.

  23. Satan Claws

    @12 Chris:
    How large does an landmass have to be to be considered a continent? How large does it have to be so that we consider it an island instead?

    Suppose you have a large supply of sand and we pile it up. At which point does a sand mound gets to be called a mountain?

  24. Nigel Depledge

    Dutch Railroader (8) said:

    It’s actually simple to define what a planet is. The problem in doing so only arose when people had to live with the sociological implications of the definition. I was always happy with the simple definition of a planet as an object that orbits a star and has a equilibrium figure set by gravity. But this would require Ceres to be a planet, when everyone “knows” its an asteroid, and so the trouble started…

    Not really.

    Pluto was always the issue. Ceres was only classed as a planet for a short time after its discovery (when it was found that Ceres was just one of many main-belt asteroids, it was no longer considered to be a planet). As far as anyone could tell, Pluto resembled just another KBO far more than it resembled a planet. Also, its orbit is at the inner edge of the Edgeworth-Kuiper Belt, which makes it – like Ceres – just one of many similar objects.

    The issue appears to me to be twofold.

    First, the IAU definition really only applies to technical discussions among professional astronomers. Everybody else is free to use the words as loosely as they see fit. Despite this, some non-IAU members (no doubt stirred up by a vocal but small group of IAU members who disagreed with the definition) feel that they should have a say in the IAU definition.

    Second, because Pluto was considered to be a planet for about 70 years, most people alive now grew up with the idea of Pluto being counted as a planet. To hear the announcement that the IAU’s definition changed Pluto’s classification was perhaps a shock. But see my first point above.

    The IAU definition is terrible, however.

    This description depends entirely on two things: the intended purpose of the definition, i.e. how is the classification of solar-system objects to be used; and each individual’s opinion.

    It 1) introduces a novel dynamical definition that had never been discussed previously as a criterion,

    Not so. Ceres’s nomenclature was changed as soon as it was found to be but one of many similar objects. The “dynamical definition” merely formalises what has been recognised previously. If you consider the solar-system objects objectively, you will see that they naturally fall into categories. The four inner planets; the four giant planets; moons and trojans; the main-belt asteroids; KBOs; and comets and other small stuff with eccentric orbits.

    2) cannot be applied to exo-planets,

    Did you not think this might have been a deliberate choice?

    3) uses mass as does the “roundness” criterion, but in a highly complex relationship with the primary, the size of the orbit, and so on,

    If you consider it carefully, you will see that the “roundness” criterion is a minefield. Where, for instance, does one draw the line between round and not-round? There are very probably many objects orbiting our sun that fall into the grey area around the distinction. How does one measure hydrostatic equilibrium, other than by using the proxy of an object’s appearance? What about objects that resurface themselves? And so on.

    I’m not saying the criterion isn’t necessary or should not be used, but what I am saying is that a certain arbtrary pragmatism is involved in applying it. And we should acknowledge that pragmatism.

    but mainly 4) was selected for the desired outcome of excluding Pluto and its KBO kin.

    So, that the IAU had a specific purpose in mind when defining planet should be seen as a bad thing, should it?

    AFAICT, it is those people who are going to be using the definition (i.e. pretty much exclusively IAU members) who should decide the function that the definition needs to fulfill.

    In my own field of biochemistry, the guiding principle of defining a technical term is clarity. Does the term as defined add clarity to the field of investigation? I assume the same applies in astronomy. Thus, if the definition were to apply to exoplanets, you have the prospect of discovering objects in the future that are small enough to be in the grey area between round and not-round but have no way of deciding their classification. The answer? Exclude exoplanets from the definition. Similarly, if we were to take out the “gravitational clearance” criterion, then any KBO that happens to be large enough to be round will count, but these are such distant objects and we have such poor images of many of them that this is undecided for many of them, and will be the case for newly-discovered ones until we can send probes and / or train huge ‘scopes on them.

    So, which is clearer? A definition that excludes KBOs from being planets, or a definition that says “some KBOs are planets, but we won’t know exactly which ones (apart from the few for which we have good images or that are among the largest KBOs like Eris and Pluto) until we can get probes out there or train our biggest ‘scopes on them”?

    I must note that this is largely my own conjecture, but it makes sense to me. I have seen many arguments agianst the IAU definition, and they all seem to elide over the necessity that a definition be useful to the IAU members (i.e. those people who will use it) here and now, given our present state of knowledge.

  25. Nigel Depledge

    Satan Claws (23) said:

    Suppose you have a large supply of sand and we pile it up. At which point does a sand mound gets to be called a mountain?

    Interesting point.

    I have read of one definiton, that says (something along the lines of) a mountain is a peak whose summit is at least 1000 m above mean sea level. But that would mean that England has no mountains at all (its highest peak, Scafell Pike, is 981 m ASL), yet we all accept the historical description of the Cumbrian Mountains (i.e. those among which England’s highest four peaks are found).

    So there is some historical and perhaps aesthetic component to the definition. (IIUC, under the above definition, every mole-hill in Colorado would count as a mountain because the entire state is at a high enough elevation).

  26. Nigel Depledge

    Adrian Lopez (18) said:

    You don’t have to call Pluto a planet (even though it’s more like a planet than not), but if you’re not you should at least call it something other than a name with the word “planet” in it.

    Pluto has some characteristics in common with objects such as Earth.

    However, it also has many characteristics in common with comets and other KBOs.

    Hence the need for a precise definition in the first place – IIUC, it was the fact that Pluto resembled just another KBO more than it did the other eight planets that created the driving force to get a defintion that excluded Pluto. The discovery of Eris was merely the catalyst.

  27. amphiox

    If a dwarf star is still a star as most stars are, then the same should surely apply to dwarf planets still being planets as most planets are!

    I would go even further than this.

    If all binary stars are still stars, even puny red dwarfs partnered with hypergiants, then moons large enough to otherwise qualify as planets, should be planets too.

  28. amphiox

    …and with 5 objects orbiting it, what are supposed to call it…a KBO with satellites???

    Why not? That’s what it is. Irrespective of whether we end up calling it a planet or not, Pluto IS a KBO. And all its satellites are KBOs as well.

    And there is nothing in the definition of KBOs that says they can’t be in gravitationally bound systems with one another. So it is a multiple-KBO system. (A hextuple KBO system as it were….)

  29. amphiox

    Next people will say a grapefruit isn’t a fruit…

    Ah, but a ladybug ISN’T a bug (it’s a beetle), not by the specific scientific definition of the term “bug”, at any rate.

  30. Adrian Lopez

    IIUC, it was the fact that Pluto resembled just another KBO more than it did the other eight planets that created the driving force to get a defintion that excluded Pluto.

    I’m no astronomer, but it seems to me that Pluto is more like Earth than Earth is like Jupiter, and again more like Earth than like most other Kuiper belt objects.

  31. Adrian Lopez

    Ah, but a ladybug ISN’T a bug (it’s a beetle), not by the specific scientific definition of the term “bug”, at any rate.

    Scientists call them Coccinellidae.

    Koalas were once known as “koala bears”, but I don’t think any reputable zoologist would call them that today.

  32. Doctor Wheat

    Not to mention, Phil Plait is not actually a plate of any sort, but rather, a variety of astronomer.

  33. Georg

    Could somebody
    explain to me why the status of Pluto is such an
    issue in USA, whereas in Europe (and most other countries)
    nobody engages in that fruitless discussion?
    Georg

  34. realta fuar

    Dutch Railroader has a good take on this. Alan Stern’s geophysical definition of a planet is a good one as is his retort to Mike Brown about his (Brown’s) kid not being able to remember so many planets: “how many states are there?”!.
    This wouldn’t have been so bad except for the vindictiveness of the IAU and it’s idiotic definitions of planet and dwarf planet along with supreme self promotion of those who made themselves famous and much richer over this (notably, AFTER Clyde Tombaugh had died) of which the worse case is of course, Neil Tyson. Tyson’s not even a planetary astronomer and has published precious little research in ANY field of astronomy.
    I find a lot of irony in the fact that Mike Brown actually had a great take on this, during the short window in time when he THOUGHT he’d become famous for discovering the tenth planet! He said that the word “planet” has a cultural definition, much like the word “continent” (this is probably what the B.A. was stumbling awkwardly toward in his interview) and that this is perfectly fine. Of course Brown quickly started toeing the party line and never mentions this statement now.

  35. Infinite123Lifer

    Anybody know any super advanced alien civilization we might correspond with about our concept’s for defining things based upon clarity?

    jk . . . does anybody know which Universe we nail this?

    ok . . . but really I was with you MTU until you crossed out Ceres and replaced it with Vesta ;)

    I don’t have any other hobbies Phil! And yes I saw Prometheus and that initial image does look like your keyed on the possible abuse of some of your electrons

    but I did enjoy the link to “How to Settle, Once and for All, the Whole “What’s a Planet?” Debate” . . . nice to see its settled . . . once and for all that is ;)

    I am sorry IMHO this whole thing makes sense. We kept on and keep on discovering new things which causes us to question what we already know or how we perceive our observations or how we explain in words our observations . . . the whole thing seems like just another good process to focus in closer on the terms at hand.

    Nigel Depledge @24 said:

    “First, the IAU definition really only applies to technical discussions among professional astronomers. Everybody else is free to use the words as loosely as they see fit.”

    While I tend to agree with your statement and it is simple enough to digest I have this nagging sensation in my brain (coming from professional astronomy) telling me Pluto is not a planet . . . anymore. And for some reason me calling Pluto a planet just doesn’t seem satisfactory enough. I sense I am not alone :)
    _____

    @realta fuar

    I think the word planet does have a cultural definition and subsequent “moral booster” capabilities if you may. I don’t really understand why Brown would back away from such a distinction.

    _____

    Phil Plait said:

    “Try to lock something into too small a box and I guarantee nature will find an exception.”

    AND way, way, way, WAY out of context for Phil Plait if we are not talking about the reclassification of Pluto (or probably arguing with the spouse) ;) he said:

    “So I’m back to my original question: Why bother?”

    _____
    Hugs MTU. Pats brother on back. Consoles. Cracks Fosters. Looks at Pluto through the telescope anywho . . . damn IAU what the hell do they know anyway. Hi Pluto, my lovely planet :) cheers

  36. Messier Tidy Upper

    @ ^ 35. Infinite123Lifer : Cheers! :-)

    (By-the-by, Coopers Pale Ale is the go here! ;-) )

    @21. Nigel Depledge :

    Some people (you know who you are) argue that the IAU definition is wrong (and I have never really seen any solid argument that it is either “wrong” or illogical, or any of the other less-than-complimentary adjectives that have been directed at the IAU),

    You’ve seen ‘em, I’ve provided such solid arguments many times here. You just won’t admit that that’s what they are.

    .. without recognising that the IAU members are entitled to call anything they study anything they collectively want to. And the rest of us will carry on using the words that work for us.

    So basically we should totally ignore the IAU definition because it doesn’t work for us? ;-)

    As I put it in the other recent threat if the Royal Zoological Society decided that a duck was a type of fish – say because they bothlive on or in water and often move around in groups – would we accept it as scientific or just say they’re being silly and totallyreject that ruling?

    Plutio ‘s issues are that it is a small planet and that it is found in a zone of other similar small planets (Makemake, Eris, Quaoar, Haumea, Orcus, Varuna, etc ..) I don’t think thees are sufficent grounds to stop Pluto being a planet anymore than we disqualify the gas giants from planetary status

  37. Messier Tidy Upper

    Continued : .. because they’re very large and also confined to a “belt” in the middle of our solar system! ;-)

    (Yes, it’s not usually called a “belt” and only contains four worlds but from the perspective of the outer solar system – the Edgeworth-Kuiper cometary disk and Ooort cloud zone it arguably looks like one!)

    @ 33. Georg : I’m an Aussie – and I get asked my views on this issue quite a lot. So it ain’t just America.

    @22. Peter Davey :

    With regard to Mr Hellbig’s comment, I understand that Asimov’s title was actually a pun on “the World Series”, referring to baseball,which is a sort of inferior offshoot of cricket.

    Exactly my understanding too! ;-)

    @ 30. Adrian Lopez :

    “IIUC, it was the fact that Pluto resembled just another KBO more than it did the other eight planets that created the driving force to get a defintion that excluded Pluto.” [- Nigel Depledge #26 -ed.]
    I’m no astronomer, but it seems to me that Pluto is more like Earth than Earth is like Jupiter, and again more like Earth than like most other Kuiper belt objects.

    Same here, Adrian Lopez.

    Pluto unlike the cometary nuclei or asteroids elsewhere in the Edgeworth-Kuiper cometary disk has an atmosphere complete with seasonal snowfall, is geologically differentiated and quite possibly also rings, (cyro~)volcanoes and a sub-surface ocean. Its a lot more than just another lump of rock and ice.

    As the saying goes, “if it looks like a duck, and walks like a duck and quacks like duck, its probably a duck!” Well, Pluto looks like a planet, has moons and all these other features that you’d expect from a planet. I think to therefore say its not a planet requires a lot better reasoning than anything the IAU has offered.

  38. amphiox

    Sometimes I’m tempted to think that a proper classification system for planets actually should also include geological differentiation as one of its criteria or subcriteria (if we go on to subclasses).

  39. Nigel Depledge

    Adrian Lopez (30) said:

    I’m no astronomer, but it seems to me that Pluto is more like Earth than Earth is like Jupiter, and again more like Earth than like most other Kuiper belt objects.

    IIUC, current leading ideas have that all the giant planets have rocky cores. In Jupiter’s case, it’d be pretty big, perhaps 10 – 20 Earth masses, so there are some compositional similarities that Pluto probably does not share.

    Pluto has various frozen gases (such as ammonia ice) that are found only as traces on Jupiter and barely at all on Earth.

    But perhaps the biggest similarity between Earth and Jupiter, and the biggest difference between Pluto and the other planets, is in the orbital characteristics. Jupiter and Earth orbit in the same plane (within a degree or five), whereas Pluto’s orbital plane is significantly inclined relative to the ecliptic. Jupiter and Earth have nearly-circular orbits, whereas Pluto’s is significantly eccentric.

    Sure, Earth and Pluto both have plenty of water (though on Earth, it is mostly liquid and on Pluto it is entirely solid) and neither has much in the way of hydrogen gas, whereas Jupiter appears to be mostly hydrogen, but this just emphasises the need for a precise definition in the first place.

    We can play the “list of traits” game all day with all solar system objects, and end up concluding that they’re all unique in certain special ways, but where does that get us?

  40. Nigel Depledge

    Realta fuar (34) said:

    Dutch Railroader has a good take on this.

    Apart from the obvious flaws that I have pointed out (in #24).

    Alan Stern’s geophysical definition of a planet is a good one

    Why?

    as is his retort to Mike Brown about his (Brown’s) kid not being able to remember so many planets: “how many states are there?”!.

    Oooh!, Ooh, I know this. 46. And 4 commonwealths (Kentucky, Massachusetts, Pennsylvania, and Virginia).

    This wouldn’t have been so bad except for the vindictiveness of the IAU and it’s idiotic definitions of planet and dwarf planet

    Says you. Your hyperbole is mere rhetoric, as it is far from proven.

    Oh, also, citation needed re vindictiveness. When, where and how has the IAU been vindictive?

    along with supreme self promotion of those who made themselves famous and much richer over this (notably, AFTER Clyde Tombaugh had died) of which the worse case is of course, Neil Tyson. Tyson’s not even a planetary astronomer and has published precious little research in ANY field of astronomy.

    Even assuming you are right about Tyson, that does not provide any information about the validity or otherwise of the IAU definition. It is a mere ad hominem.

    I find a lot of irony in the fact that Mike Brown actually had a great take on this, during the short window in time when he THOUGHT he’d become famous for discovering the tenth planet! He said that the word “planet” has a cultural definition, much like the word “continent” (this is probably what the B.A. was stumbling awkwardly toward in his interview) and that this is perfectly fine. Of course Brown quickly started toeing the party line and never mentions this statement now.

    Planet still has a cultural definition. I’m pretty sure that the IAU has not the slightest interest in what you or I call Pluto. However, for technical discussions among professional astronomers, the IAU definition has a purpose.

    It seems to me that one could come up with no end of different possible definitions that would classify more or fewer solar-system objects as planets. However, whichever definition one settles upon will have an implication for objects such as Ceres, Eris, Pluto and Mercury, as well as implications about objects in the Edgeworth-Kuiper Belt that have either not been discovered or have been discovered but not yet adequately characterised.

    So, what definition one prefers will depend at least to some extent on what implications that definition has for known solar-system objects and for objects yet to be discovered.

    The IAU members have the collective prerogative to apply a definition that suits the purposes of the majority of their members. Alan Stern is part of a quite obvious and noisy minority, who appears to have refused to accept the vote of the majority of IAU members. No-one outside the professional astronomy field is under any obligation to use that definition. By way of parallel, as a biochemist I use a very specific definition of the term protein but most people use that word with only the vaguest notion of what a protein actually is. And that doesn’t matter.

    What I don’t understand is all the people who get hot under the collar about the IAU “forcing” a “ridiculous” definition on people, when the IAU is quite clearly doing no such thing, and the IAU definition has a fairly simple and obvious purpose – the removal of ambiguity to facilitate technical discussions. All of the arguments I have seen against the IAU definition either make certain tacit assumptions that are not really justified, or fail to recognise that any other definition would be equally arbitrary and require a similarly pragmatic application.

  41. Nigel Depledge

    MTU (36) said:

    You’ve seen ‘em, I’ve provided such solid arguments many times here. You just won’t admit that that’s what they are.

    They’re not solid, they’re built on sand – and this is what you refuse to recognise.

  42. Nigel Depledge

    MTU (36) said:

    So basically we should totally ignore the IAU definition because it doesn’t work for us?

    No, we should feel free to ignore it because it doesn’t apply to us.

    As I put it in the other recent threat if the Royal Zoological Society decided that a duck was a type of fish – say because they bothlive on or in water and often move around in groups – would we accept it as scientific or just say they’re being silly and totallyreject that ruling?

    Well, obviously the latter, but your analogy is not valid. The eight planets are individual objects, each unique in many ways, not entire species. Biological classification recognises the genealogical relationships between organisms, but no such nested hierarchies exist among objects that orbit the sun. It is impossible to apply the layers of classification found in biology (at its crudest level, this has seven layers of classifcation for every species, and many versions of it run to 10 or more layers) to solar-system objects, for two reasons: first, as alreay noted the genealogical relationships found in biology do not exist in asteroids, planets etc., and, second, because whereas biological organisms naturally form groups of similar organisms, any definition of planets or what-have-you necessarily contains arbitrary elements (such as the gravitational roundness criterion).

    The IAU definition recognises a natural and obvious distinction that exists among solar-system objects. The eight planets are each quite obviously the dominant objects within their orbital domains, whereas Pluto etc. are equally obviously the largest examples of groups of similar objects. It was for precisely this reason that Ceres was changed from being considered a planet upon its discovery to being considered an asteroid once other main-belt objects were discovered. The IAU is, from a certain point of view, simply following this precedent.

    Plutio ‘s issues are that it is a small planet and that it is found in a zone of other similar small planets (Makemake, Eris, Quaoar, Haumea, Orcus, Varuna, etc ..)

    Not exactly. The issue is that Pluto is simply the largest example of a group of similar objects that orbit the sun in the same zone, and that no individual object among them is large enough to dominate that zone through gravitational influence.

    I don’t think thees are sufficent grounds to stop Pluto being a planet

    You have made this abundantly clear.

    What is not clear, though, is why you consider a definition that is sure to include as-yet-undiscovered KBOs (i.e. the definition that you favour) to be a greater facilitator to technical discussions than a definition in which all ambiguity about what is and isn’t a planet has been removed.

    anymore than we disqualify the gas giants from planetary status

    But each gas giant is quite obviously the dominant object within the region in which it orbits. There is no dissonance here.

  43. Nigel Depledge

    MTU (37) said:

    Yes, it’s not usually called a “belt” and only contains four worlds but from the perspective of the outer solar system – the Edgeworth-Kuiper cometary disk and Ooort cloud zone it arguably looks like one!

    This is so contrived.

    Saturn is twice as far from the sun as Jupiter. As a proportion of their semi-major axis distances, they don’t ever get anywhere near each other. And while the ratios are not so dramatic for Ouranos or Neptune, their orbital characteristics are such that these two still don’t get near each other, even at opposition.

    As for the EKB, while we have only identified a small number of KBOs, our present ideas indicate the presence of a very numerous population of objects in the EKB. It seems likely that there are hundreds of thousands of KBOs, if not millions. The EKB is very probably a belt of objects with generally similar composition.

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