Pluto still may be the biggest dwarf planet

By Phil Plait | November 16, 2010 7:00 am

Is Pluto a planet?Mike Brown is an astronomer, and in my opinion is mainly responsible for kick-starting the demotion of Pluto as a planet — he and his team found Eris, an object in the outer solar system that was apparently bigger than Pluto. It was this discovery which set in motion the events that led to the foofooraw about Pluto, and the vote that turned it (and Eris and many others) into "dwarf planets".

Mike continues to observe Eris and other dwarf planets (as well as search for new ones). These objects are small and far away — did you know our own Moon is substantially larger than Pluto? — and therefore hard to analyze. Even with huge telescopes, these objects are hardly more than dots.

However, a fortuitous event landed in the laps of astronomers recently: Eris passed directly in front of a faint star. To us on the ground, it appeared as if the star winked out as the dwarf planet passed in front of it. By carefully timing the duration of this mini-eclipse, the size of Eris could be estimated.

And, to everyone’s shock, Eris looks to be roughly the same size as Pluto. Mike describes all this on his blog.

What does this mean for Pluto?

Nothing. Seriously. Well, I should be careful: it has nothing to do with the argument about Pluto being a planet or not. Whether Eris is slightly bigger or smaller makes no difference. And it’s still possible there are lots of other objects out there roughly the same size or even bigger than Pluto. Whether you hate the decision to kick Pluto out of the planet club or not, this new finding doesn’t really swing the pendulum either way. [Update: After I wrote this, I found Mike and I agree here as well.]

But it does actually have some interesting scientific ramifications. We know Eris is substantially more massive than Pluto — both have moons which can be used to measure the masses of the objects using math known since the time of Kepler — which means that Eris must be denser than Pluto. That’s weird! Both objects should be pretty similar, but density is a clear indication of different composition. Building materials in the outer solar system aren’t exactly varied, so this new result means Eris must have a lot more rock inside than Pluto does (both have lots of ice, which is less dense than rock).

Why would Eris have such a different composition than Pluto? Ah, that’s where the real science is. Mike has thoughts about this on his blog as well.

One overarching thing this is telling us is that the solar system, our very own local neighborhood, is still full of surprises. There’s a lot left to see and explore, which means there’s a lot left to understand. And that may be my very favorite thing about science.


Related posts:

- Ten Things You Don’t Know About Pluto
- Xena, warrior… dwarf planet?
- Pluto wanders into a Messier situation
- Planet pr0n


CATEGORIZED UNDER: Astronomy, Piece of mind, Science

Comments (82)

  1. Rick

    When it comes to Pluto being a planet, I feel like the “Hubble Gotchu” guy about the Hubble telescope.

  2. Messier Tidy Upper

    @ ^ Rick : Me too. :-)

    I can’t stand the IAU’s “planet” definition which is, I think, utterly ridiculous and wrong.

    To say dwarf stars are proper stars (& the most numerous at that making up 90% of them all incl. our Sun) but dwarf planets somehow don’t count as proper planets is inconsistent and silly. Frankly, it would make as much sense – perhaps even more so – to say that the *gas giants* aren’t planets as it does saying ice dwarf worlds aren’t! ;-)

    To say that a gravitationally self-rounded world with not only three moons – one of them extremely large esp. relative to Pluto’s size – possible rings and an atmosphere with complex seasonal weather as well as complex gelogy and internal differentiation is not a planet really makes NO sense at all.

    Jupiter and Earth couldn’t “clear” Pluto’s orbit if they were located in that orbit rather than their own. Would we stop calling them planets inthat case? I very much doubt that. Therefore we should be consistent and say that the IAU ‘s “orbital clearence” criterion is nonsense. It is illogical, it is inconsistent with language use elsewhere (incl. with exoplanets in Pluto-like situations), it just doesn’t add up and should never have been adopted. The sooner theIAU come to their senses and correct their embarrassing and rather shameful mistake the better for them and astronomers generally.

    - In My Humble Opinion Naturally.

  3. Gary Ansorge

    If we ever succeed in transporting people to other star systems, I bet it will be thru using “stepping stones”, like Eris and other such bodies as refueling stops for mobile, massive space colonies. As I recall, the space between stars is littered with debris, billions of km apart but with lots of resources to support a hydrogen fusion economy. Even if it takes centuries to get to Alpha Centauri it’s no big deal if your craft is your home.

    We’re still a “planet centric” species but that will change as we become accustomed to living in space colonies. Eventually, we may even become averse to large bodies such as planets due to their energy expensive G fields.

    Gary 7

  4. notovny

    Well, we know Triton’s larger than Pluto, Triton’s almost certainly a captured object, and capture’s pretty blasted hard to pull off, so I’d say odds are good that a fair few somethings larger than Pluto are out there.

  5. Messier Tidy Upper

    Nor are Rick and I alone on this – see also :

    http://laurele.livejournal.com/

    Laurel’s excellent Pluto blog.

    Plus Ken Croswell’s articles here :

    http://kencroswell.com/HD45364.html

    Noting the existence of a pair of superjovian exoplanets in a similar orbital relationship to Neptune and Pluto around the fairly sun-like orange dwarf star HD 45364 located 110 light years away in Canis Major.

    As well as on Pluto & its planethood directly here :

    http://kencroswell.com/NinthRockFromTheSun.html

    where Croswell, an astronomer and one of my favourite astronomy authors, argues that :

    …a better definition exists: a planet of our solar system is an object that orbits the Sun and has a diameter that equals or exceeds Pluto’s.

    My own preferred definition of planet differs somewhat but would also include Pluto as well as the larger ice dwarfs.

    Although I’d be fine with accepting Croswell’s one as a huge improvement over the IAU one anyday! ;-)

  6. Messier Tidy Upper

    This is my preferred definition of planet (& BTW. I’m NOT claiming to have originated this at all – its actually pretty much the original more sensible definition the IAU proposed in Prague before the Pluto-bashers staged their mean little coup.) :

    ***

    How to define a “Planet”

    A good – simple, clear, logical and easy to determine – definition for planet is that a planet is an object which:

    a) Has enough gravity to be in hydrostatic equilibrium (i.e. is round or if rapidly spinning a rotationally flattened spheroid.)
    Thus a planet is not an asteroid or comet.

    b) Is incapable of ever being self-luminous through shining by nuclear fusion. Thus is not a star or brown dwarf.

    &

    c) Is not directly orbiting another planet. Thus is not a moon.

    ***

    Once an object has been classified as belonging tothe broad, inclusive and now much easier to tell category of “planet” such we can then assign it to a particlar *class* of planet given its nature in terms of composition, internal structure, size, orbit, etc ..

    Examples :

    Rocky and Earth-sized = terrrestrial planet,
    Gaseous Hydrogen & Helium & orbiting at Jupiter’s distance = gas giant,
    Gaseous H & Helium & orbiting at Mercury’s distance = Hot Jupiter,
    Icy & Pluto-sized orbiting in at Pluto’s distance = Ice dwarf,
    Has a composition of hot high-pressure ices = Hot Ice (eg.Gliese 436),
    Has a mass between 10 & 2 Earths = Super Earth / Supervenus / Gas dwarf,
    Orbits a pulsar = Pulsar planet,

    Et cetera..

    Too many planets? Hardly.

    Yes, this definition would mean our solar system would probably consist of well over thirty planets with Pluto being above average in size – just as our Sun is above average in size stellar~wise with most stars being red dwarfs. Analogously most planets would turn out to be new ice dwarfs, small, cold and yet often quite fascinating and full of potential suprises in their own right.

    Remember, we already don’t ask kids to memorise all the planets now – after all, there’s now hundreds that are known (about 500 last I heard) mostly around other stars.

    What we do here (in a i> is tell our kids about the different types – therocky ones like earth, thegassy one’s like Jupiter and theicy one’s like Pluto – and learn something about the most significant prototype ones in our solar system.

    Can anyone explain a better more logical approach than this?
    and there’s acase of gas giants in the

  7. noen

    Isn’t Pluto not one body but two in orbit around each other? Would that account for the lower density?

  8. gopher65

    I don’t care whether or not Pluto is a planet, but two things about the IAU’s decision bothered me a great deal:

    1) The process was very shady. They waited until the last day of the meeting, when many pro-Pluto astronomers had gone home. That’s downright fraudulent behaviour, and is unbefitting of a scientist. If they’d held a fair vote and came to the conclusion to remove Pluto’s planet status, I wouldn’t have cared.

    But the fact that they *cheated* during the voting process has soured me against the entire IAU. I will take a grain of salt with anything coming out of that body in the future, now that their lack of intellectual integrity has been revealed. They have lost my respect not for “demoting” Pluto (they did no such thing), but rather for engaging in reprehensible behaviour.

    2) The new definition of “planet” is… stupid. Earth wouldn’t be a planet in the far reaches of the outer solar system under the new definition. That makes no sense to me.

    What they should have done is created a spectrum definition based on the physical characteristics of the body in question (rather than arbitrarily drawn lines in the sand). IE, a continuum of objects starting at a single grain of dust, and building all the way up to an object that’s not quite big enough for deuterium fusion (brown dwarf).

    The continuum of objects would have branches all along it based on the characteristics of the objects being observed. Pluto is an icy dwarf planet. Mercury is a rocky dwarf planet. Saturn is a gas giant, and Jupiter is a gas supergiant (a gas giant that has reached the maximum radius of such objects).

  9. Keith (the first one)

    Why didn’t they just call Eris the 10th planet when they discovered it? Same for the other large objects out there.

  10. Oli

    A planet:

    - Orbits a star directly (it may also orbit a barycenter that it has in common with another object but which is outside both, like Pluto-Charon)

    - Is rounded by its own gravity

    - Has a mass greater than Ganymede but low enough for spontaneaous nuclear fusion to be impossible.

    Dwarf planets are those objects that orbit the sun directly and are rounded, but have a mass which is lower than Ganymede’s. Naturally, they are incapable of nuclear fusion by themselves because of this low mass. Now the current 8 planets are still planets, while the others will be named as dwarf planets with proper logic.

  11. I’m not going to argue whether or not Pluto is a planet. The fact that Eris is much denser intrigues me. It must be because the cheese that makes up Eris doesn’t have as many holes in it as Pluto.

  12. Messier Tidy Upper

    Oops. Apologies for my italics & typos stuff up at the end of comment # 6 :

    What we do here is teach our kids about the different *types* of planet – the rocky ones like Earth, the gassy one’s like Jupiter and the icy one’s like Pluto – and learn something about the most significant prototype ones in our solar system.

    Can anyone explain a better more logical & reasonable approach than this?

    Does anyone really prefer the illogical, inconsistent IAU definition which violates Occam’s Razor with its superflous “orbital clearence” criterion raising more questions* than it answers & thereby adding unnecessary complications. Where a planet ceases to be a proper planet once it has to cover more space getting unfairly penalised and discriminated against just for having wider, more distant orbit?

    Which definition makes most logical and scientific sense really?

    Also a couple more linked items to ponder here :

    http://kencroswell.com/PlutoQuestion.html

    &

    http://kencroswell.com/PlutoQuestion2.html

    &

    http://kencroswell.com/NitrogenInPlutosAtmosphere.html

    Weirdly, in some ways Pluto’s atmosphere arguably resembles that of Earth’s more closely than any other planet in our solar sytem.

    Equally, Pluto-Charon and Earth-Moon are the closest thing we get to seeing binary or double planets. (Which, incidentally, the current IAU’s diktat would reduce to “dwarf planets” no matter how large they might be, again contrary to logic and common sense!) So as noted earlier, Earth is induitably a planet and Pluto is much more like Earth in many ways than Jupiter is & the other gas giants are – yet, unsuprisingly if hypocritcially, no one calls for the gas giants to stripped of planethood! ;-)

    —-

    * Such as :

    1. Just how just clear does an orbit have to be anyhow?

    2. How long does it have to be clear for?

    3. What about comets or asteroids that cross planetary orbits? What about hypothetical planetary systems – incl. our own in past and future times – where we may have wandering rogue planets that drift in & cross other worlds orbits or planets that get onto chaotic orbits and collide like in the Big Splash collision that formed our moon?

    The IAU followers will argue they can answer (& add more metaphorical epicycles and deferents to their awkward definition) defining what they mean in weirdly germanic terms like unterplanet & uberplanet if memory serves me right. I would dispute that but in any case the fact that such questions can and will be asked shows their definition is needlessly over-complicated and problematic especially compared to better definitions like Croswell’s one and the earlier IAU proposal.

  13. Hevach

    “Thus a planet is not an asteroid or comet.”

    Ceres. And what about Vesta, which from observations seems to have achieved equilibrium in the past but been deformed by a relatively recent impact with a very large body.

    The problem isn’t just “too many planets,” but the fact that with such a definition, which is even LESS restrictive than the old one, most of them are not notable. At very least, it gives us 13. 14 if you count Vesta, which did achieve hydrostatic equilibrium at some point.

    Plus many more with varying liklihood – there still hasn’t been a good enough decision on how round an object has to be to be a dwarf planet, and observations out that far are quite hard (Last I knew, Sedna was still a dwarf planet candidate and not officially recognized yet) so there’s a bit of a waiting list built up to join the five that were defined initially.

    Assuming Wikipeida is up to date, there are SIXTY ONE dwarf planet candidates in the Solar System known to date. Pending more accurate measurements of their roundness, that gives us 74 or 75 planets by your definition – and counting, three of those were only discovered this year. These are only candidates and not dwarf planets because a decision is still pending on the bottom limit of dwarf planets – they all fit into the old definition of a planet as long as you use the same labored exceptions Pluto was given, and they fit even better into your definition.

    The fact that we know about hundreds in other solar systems is irrelevant, we know of no other system with as many as ours, and that’s without allowing a definition so inclusive that it’s not even notable when we discover twelve more in our own solar system.

    This is not a point of too many or too few planets, but the fact that the existing definition now included numerous objects that had no business being planets, which are not of note, and which share only a single aspect with planets, in that they’re in hydrostatic equilibrium. That has been said about Pluto for decades – that it had no business being a planet to begin with, as it lacked several shared characteristics of planetary orbits, but it’s uniqueness earned it forgiveness.

    “Once an object has been classified as belonging tothe broad, inclusive and now much easier to tell category of “planet” such we can then assign it to a particlar *class* of planet given its nature in terms of composition, internal structure, size, orbit, etc .. “

    Calling these objects planets does not make it any easier to classify them, any more than calling a crab a lobster makes it easier to tell what class of crab it is. It’s still a crab, and still classified in the same way, it’s just a subclassification of lobster, and it’s only there because crab fans feel that crabs are being disenfranchised by their reclassification as crabs while lobsters remain lobsters and command higher prices at restaurants.

    “Jupiter and Earth couldn’t “clear” Pluto’s orbit if they were located in that orbit rather than their own. Would we stop calling them planets inthat case? I very much doubt that.”

    If Jupiter was there, it probably would, actually, it’s big enough to eject or absorb pluto. But that’s not the point. If either Earth or Jupiter had formed in that orbit, they would have. Pluto would have never formed and survived past the very early days of the solar system with an Earth sized body in it’s orbit. And THAT is the criterion, Planets don’t have to be dropped into an arbitrary orbit to see if they can clear it now, they’ve already done that during their formation, and Pluto has not.

    Why do normally rational people get so emotional about keeping Pluto a planet? The dwarf planet definition wasn’t a witch hunt to exclude Pluto. But the term “Planet” is one of some note – few objects qualify. When it became clear that our existing rules would include dozens of objects that would never have been considered to begin with, it became necessary to shut the flood gates. As I said repeatedly, there’s at least 75 out there and with new ones every year that number is going to be in the hundreds eventually. When it became clear that our classification of planet was including an entire class of objects that didn’t behave in any way like planets, that classification had to be changed. Pluto wasn’t singled out for elimination, but there is was no rational way to preserve the intended exclusivity of planethood and still keep Pluto in that group.

    There was nothing shady about the proceedings. If you actually read even a publication as far from the scientific pulse as Astronomy Magazine, you’d have known the discussions were happening in the 80′s and quite common in the 90′s.

    Why didn’t they just call Eris the 10th planet when they discovered it? Same for the other large objects out there.

    Initially, that was the talk. Sedna was the first of many, however, if we did that, we’d be at 73, possibly 74 planets (still debating Vesta’s damaged state) and discussing making 2010 KZ39 the 74th or 75th.

    @10. Oli: Pluto is smaller than Ganymede.

  14. Hmm… not to distract from the fascinating re-opening of silly, very pointless human semantic arguments, but…

    How Did Eris Get That Rich In Silicates?!? (Or, if you don’t like that scenario, what’s it doing with a significantly higher metal:silicate ratio than everything else we’ve found out there?). It now seems likely that that *single* object as a silicate content that rivals the entire cumulative asteroid belt, yet is amazingly poor in ices for something in that location…

    …and the best we can do is argue about if we can make up an artificial system of rules to separate what is actually a continuum of bodies into two discrete classes?

    This opens up some really interesting new questions… yet we’re debating the old argument of how to define a planet (like the solar system cares)? This casts some significant doubts on even the composition of the pre-solar nebula (Pluto/Triton being role models… until we realized that as representative samples, they might be really poor examples). And even ejection from the inner system seems unlikely for Eris (both because of its satellite, and its orbit). The compositional examples on Mike’s blog are really informative (and even they aren’t completely representative… keeping mind that these are two-D slices of 3-D objects, the apparent rock:ice ratio is actually even *greater* than it appears in the image).

    To requote something… what the heck else have we missed?

  15. Stephen P

    You can tell that an issue is trivial when the arguments on both sides are silly. Yes, I thought the arguments for the change were pretty weak. But to claim it was “fraudulent behaviour” to hold a vote on the last day of a meeting is really plumbing the depths.

  16. They waited until the last day of the meeting, when many pro-Pluto astronomers had gone home. That’s downright fraudulent behaviour

    That’s a bit unfair on the IAU: it was on the last day, but that was timetabled in advance, so everyone knew about it. It’s not as if they suddenly said “today, we’re going to vote on planet definition”. I don’t see any fraud there.

    Where they did mess up somewhat was in springing the proposed definition almost at the last minute, at the beginning of the (just-over-a-week long) conference. However, the reason they did that was that planetary scientists had been wrangling for years and getting nowhere (read Alan Boss’s Crowded Universe for a hilarious account), so I think that whatever the IAU did, the result would have been just as messy.

  17. Messier Tidy Upper

    @11. John Says:

    I’m not going to argue whether or not Pluto is a planet. The fact that Eris is much denser intrigues me. It must be because the cheese that makes up Eris doesn’t have as many holes in it as Pluto.

    Ah but when we finally get around to collecting samples from each of these planets I think we’ll find the Plutonian cheese is much tastier and more delicious! ;-)

  18. It al started to turn bad for Pluto well before Erin was discovered. Basically, the start of it was when the first TNO (1991 QB1) was discovered by Luu and Jewitt, and more followed. This made clear Pluto was just a larger and one of the most near of the Kuiper Belt objects. Right at that time people like Marsden started to argue, quite rightly, that Pluto should no longer be considered a planet.

    I am growing very tired about people who keep insisting the IAU vote was “rigged”. This is nonsense. It was made well-known when the vote would be: those who weren’t there have solely themselves to blame. The international community has spoken, and you Americans simply have to accept that. Because, and I have said it before and will repeat it here: the only “problem” with the Pluto demotion is that Pluto was the only “planet” discovered by an American. All of the continuing grumble about the IAU and Pluto is almost solely coming from America. Get over it!

  19. gopher65

    Hevach said: “there is was no rational way to preserve the intended exclusivity of planethood”

    As I’ve said before, I don’t care whether or not Pluto is called “dwarf planet” or “planet” (it’s irrelevant to me), but your attitude as stated above puzzles me. What puzzles me even more is that it is apparently a common belief system among otherwise rational people.

    It seems to me that many people (both astronomers and non-vampires) have attached a mystical meaning to the word planet.

    For some people this means that Pluto was “demoted” because it is no longer defined as a planet. For others it means that the “integrity” of the word planet must be maintained at all cost. (I mean really, we’re not talking about the integrity of the definition of the word ‘marriage’ here, are we?) Was it really necessary to amend the constitution of the IAU (so to speak) just to keep an arbitrary definition of a word acceptable to people who aren’t open to new ideas (ie, hundreds of planets in a single system)? And why does anyone even care that Pluto isn’t a planet anymore? It’s not like its physical characteristics have changed.

    What has led people to attach such religious significance to a word?

  20. gopher65

    Marco Langbroek: I’m not American. Way to make unfounded assumptions.

  21. ruidh

    Isn’t this really a just lover limit on Eris’ diameter? One transit does not a diameter make. The star could have passed above or below Eris’ equator so that the duration of the transit could underestimate the diameter by some amount.

    I think this only tells us that Eris is *at a minimum* only slightly smaller then Pluto. We need more transits.

  22. ruidh: read Mike’s actual blog entry. They have more than one measurement, so if you assume Eris is spherical, you have a good estimate of its size (not just the minimum due to a single chord. It’s a great example of how to figure out what you need from what you have.

  23. @ #20 gopher65:

    You are the exception to the rule, and also ignored the word “almost” in my post.

  24. gopher65

    Marco Langbroek: Oops, didn’t see the “almost” in there. Sorry.

  25. Jamey

    I think the 9 planets we grew up with should be tagged the “Classical Planets” – aka, the ones known before we started detecting them in huge numbers. After all, Eris, Sedna, and the rest are all significantly more difficult to see than Pluto. This doesn’t get us away from the fact that there are likely several hundred planets – but then again, the “moons of Saturn” are getting pretty ridiculous, and that’s not considering that in fact, each particle of the rings counts as a moon, perhaps. Or do we have a definition on when something quits being “dust” and becomes a “body”? For that matter, when does it quit being a molecular cluster, and become a dust particle?

  26. gopher65

    Jamey: That’s why spectrum definitions are superior. They don’t have those grey areas in between arbitrarily drawn lines in the sand like classical definitions do.

  27. KC

    Messier Tidy Upper: the current definition of a planet may not be the greatest, but your simple definition is about as bad right from the get-go:

    “a) Has enough gravity to be in hydrostatic equilibrium (i.e. is round or if rapidly spinning a rotationally flattened spheroid.) Thus a planet is not an asteroid or comet.”

    There is often no way to measure the roundness of a newly discovered body. Sure you might be able to make some educated guesses but that’s no better. For solar system bodies, if you’re lucky you might get several well observed occultations. For exoplanets you’re pretty much screwed until far large telescopes and better techniques are invented.

  28. Messier Tidy Upper

    @15. Stephen P :

    You can tell that an issue is trivial when the arguments on both sides are silly.

    Why exactly are my arguments favouring a better definition of planet replacing the IAU’s current – hopefully temporary – hash-up “silly” in your view please?

    Yes, I thought the arguments for the change were pretty weak. But to claim it was “fraudulent behaviour” to hold a vote on the last day of a meeting is really plumbing the depths.

    Well I’d agree that saying “fraudulent” may be somewhat over-stating it but, come on, you’d have to admit the IAU “planet” decree really was done in a pretty dodgy and unsatisfactory manner :

    , of the 10,000 IAU members only 2,500 attended the 2006 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 as quoted on page 28, ‘Astronomy Now’ magazine, October, 2006.

    @13. Hevach :

    Ceres. And what about Vesta, which from observations seems to have achieved equilibrium in the past but been deformed by a relatively recent impact with a very large body.

    I’d class Ceres as an ice dwarf type planet too based on its thick mantle of water ice.

    Vesta is borderline but I’d certainly be prepared to accept it as a planet myself. There is a full spectrum of objects starting at Superjovians that overlap with Brown Dwarf Stars and extending down to the largest asteroids. Yeah, I guess we need to draw a line somewhere and hydrostatic equilibrium – being gravitationally rounded – seems to be the logical place to draw it. Pluto passes, Ceres passes, Vesta is a ‘maybe’, while Lutetia, Eros and Comet Hartley 2 are a definite ‘no’s! ;-)

    there are SIXTY ONE dwarf planet candidates in the Solar System known to date. Pending more accurate measurements of their roundness, that gives us 74 or 75 planets by your definition – and counting, three of those were only discovered this year.

    So? There are hundreds of billions of stars and galaxies out there but their sheer number doesn’t mean we redfine the word so there’s only ten or so left! ;-)

    The number of insects or animal species or breeds of dog doesn’t make us say, “Wait! Too large a number, can’t cope! Must redefine ‘dog’ so only Alsatians and Dobermens and a few other very similar breeds count!” Ditto insects so only flies count as insects etc .. So it should be the same here.

    However many planets there are is just however many planets there are NOT a reason to absurdly restrict the words definition. 70 planets? 100+ planets? Cool! The more the merrier far as I’m concerned. ;-)

    …included numerous objects that had no business being planets, which are not of note, and which share only a single aspect with planets, in that they’re in hydrostatic equilibrium. That has been said about Pluto for decades – that it had no business being a planet to begin with, as it lacked several shared characteristics of planetary orbits, but it’s uniqueness earned it forgiveness.

    Really? “No business being a planet?” Wow, Hevach, you sure do hate Pluto don’t you? :roll:

    Lets’ see now Pluto has more moons than Mercury, Venus and Earth combined, it triples their score on that count! Pluto has an atmosphere, seasonal weather, possibly rings, a complex internal and surface geology incl. possible cyrovolcanoes etc .. Pluto’s the largest in diameter (probably now beating Eris), the brightest and most prominent & historically important example of its type and its planetary type may be the most numerous, average planet out there.

    So then why not call it a planet? Just because its relatively small? Just because its distant? But we now know its NOT really that small compared to other similar types of world and is much closer than others. I think the case for Pluto being in the “planet business” is actually very strong indeed.

    Calling these objects planets does not make it any easier to classify them, any more than calling a crab a lobster makes it easier to tell what class of crab it is.

    Thing about that metaphor there is that you are really saying there that we shouldn’t call crabs and lobsters “crustaceans” or perhaps even animals versus plants, fungi and bacteria. “Planet” is a broad type equivalent to the biological kingdom or metaphorical animal mineral vegetable. Planet isn’t even close to the “species” level. So just as we’d say the crab and lobster are different species of crustaceans which are a type of animal we’d say that Pluto and Earth and Jupiter are different types of planet which are then split into their gas giant, rock dwraf and ice dwarf phyla / genus respectively.

    If Jupiter was there, it probably would, actually, it’s big enough to eject or absorb pluto.

    Actually I dont think that statement is true given the size ofthe orbit and the number of objects in it.

    Even *if* your assertion there was true, what about Neptune or Earth or Mars or Mercury? If they count as planets in the inner solar system region then it seems highly illogical and inconsistent to argue that they shouldn’t count as planets if they’re moved outwards a bit!

    But that’s not the point. If either Earth or Jupiter had formed in that orbit, they would have. Pluto would have never formed and survived past the very early days of the solar system with an Earth sized body in it’s orbit. And THAT is the criterion, Planets don’t have to be dropped into an arbitrary orbit to see if they can clear it now, they’ve already done that during their formation, and Pluto has not.

    But if Pluto had formed where Mars is and was there today then it would be counted as a planet so ..what? I’m not sure I see what you’re getting at here at all. [Puzzled.]

    Formation of planets was a messy chaotic and still not fully understood process. The inner solar systems was most likely crowded early on with a great many Mars and Moon sized planets and a few larger ones whirling and crashing around. Some of these probably got ejected – possibly explaining Eris’s density. Jupiter most likely formed further out and migrated inwards. Saturn, Ouranos and Neptune shifted outwards and were likely in chaotic orbits at one point thus explaining the Late Heavy Bombardment period. Planetary formation shouldn’t in my veiw have much bearing on how we define planet – being most useful perhaps at the other end of the size scale with perhaps telling brown dwarfs and superjovians apart.

    Why do normally rational people get so emotional about keeping Pluto a planet? The dwarf planet definition wasn’t a witch hunt to exclude Pluto.

    Wasn’t it eh? :roll:

    Many folks incl. myself would differ with that subjective opinion of yours and would, indeed, see a large element of exessive Pluto-bashing and, at least to some extent, anti-American prejudice at work in demoting Pluto and Eris.

    Why do normally rational people get so emotional about *rejecting* Pluto a planet? :-(

    But the term “Planet” is one of some note – few objects qualify.

    About 500 exoplanets now qualify for planethood and many more will follow. You don’t seem to have a problem with that – nor should you or anybody else. Why then say it is a different story for ice dwarfs – the third most recently discovered realm in our solar system? However many planets there are, surely we need a definition that make sense and the IAU one doesn’t.

    What do you have against Pluto and why do you hate it & its kind so much may I ask, Hevach?

  29. KC

    “I think the 9 planets we grew up with should be tagged the “Classical Planets” – aka, the ones known before we started detecting them in huge numbers.”

    What’s wrong with the current set of 8? – aka the pre-1930 “Classical Planets”?

    I’m sure my great-great-grandfather was pissed when Ceres et al were demoted to “asteroids” in the 1860s. (Not!)

  30. I agree that the “orbital clearance” clause in the definition of planet is meaningless and inconsistent. I would add this very simple and consistent piece to the definition:

    The center of gravity of the candidate planet and its moons must be inside the candidate planet.

    Pluto, Charon, Hydra, and Nix orbit a center of gravity that is outside of Pluto itself, therefore Pluto is not a planet. The center of gravity in the Earth-Moon system is within Earth itself, so Earth is a planet. I’m not sure what the situation is with Eris and Sedna, but the definition would be easy to apply to them, given the necessary data.

  31. Michel

    I still think we have to reconquer Pluto.

  32. Messier Tidy Upper

    @23. Marco Langbroek Says:

    @ #20 gopher65: You are the exception to the rule, and also ignored the word “almost” in my post.

    I’m an Australian originating from a British background (Aussie Mum, English Dad) and thus another exception for you there, Marco Langbroek.

    I’m not alone either so maybe you could stop or at least reconsider what looks rather unpleasantly like reflexive anti-Americanism there please?

    Just a thought and suggestion. ;-)

    @25. Jamey Says:

    I think the 9 planets we grew up with should be tagged the “Classical Planets”

    I’d rather see that term applied to the five pre-telescopic era “wandering stars” or planetes known to the classical Greeks and other earliest civilisations – Mercury, Venus, Mars, Jupiter, Saturn – personally. If that ‘classical’ term is retained, it seems best used in this sense as a historical and descriptive sense.

    Does anyone anywhere ever use “Classical Planets” in the IAU sense of the big eight? Or for that matter, “small solar system bodies” and, worst of all, yuck, “plutoid” as the IAU decree?

    ***

    PS. I’ll have to reply to some other folks here tomorrow – afraid, I really must get some sleep now – its well after 3 am in my timezone.

  33. Michel

    (Aussie Mum, English Dad)
    Read: convict, jailer

    ;^)

  34. RyShe

    Great Pluto arguing everyone, but any ideas why Eris is so dense?

  35. Blizzzzzaaaarrg!

    The thing to remember with any man-defined measurements or classifications is that they don’t necessarily have any relation to reality…they’re just categories and descriptions we make up for our own convenience in talking about such stuff.

    The universe doesn’t care whether or not we think Pluto is a “planet” or an overgrown ice cube. And the whole of the argument is really purely emotional – no science is going to be affected by how you classify something.

    Think about temperature, or speed. Someone says “oh, it’s going to be 100 degrees today! OMG :( ” – and then consider the arbitrary nature of “100 degrees” – it’s a meaningless number that we made up, but because it’s suddenly 3 digits long instead of 2 digits long, it’s like some important threshold has been crossed. Or the idea of driving 100 MPH. It’s not that different than 99 MPH, or 101 MPH – and it’s not a really interesting number at all when you look at it as 160.93 KPH. And 100 KPH is kind of interesting, but underwhelming once you think of it as 62 MPH.

    Anyway, my point is that the whole argument is more or less pointless. Measurements and classifications are all made up by man, and have no importance to the actual universe we live in anyway.

  36. Bigfoot

    Eris is roughly the same size, but more dense than Pluto? This reveals that the IAU is inarguably applying the wrong naming method here!

    Clearly, Eris should have been named “Goofy”. Case closed.

  37. Jeff

    yes, Mike’s research did lead to this demotion of Pluto, and rightfully so. I actually have visited Mt. Palomar which is far from my Florida home, but was fun.

    I myself think the 2006 conference was more political than scientific though, and I say that becase to throw all small bodies like Pluto, Eris, and Ceres into the same category makes no scientific sense in the view of how these bodies evolved. Clearly, the KBO’s like Pluto Eris, etc. all evolved from a common source in a location in orbit of Pluto, but Ceres has a different history altogether.

  38. TheBlackCat

    Even *if* your assertion there was true, what about Neptune or Earth or Mars or Mercury? If they count as planets in the inner solar system region then it seems highly illogical and inconsistent to argue that they shouldn’t count as planets if they’re moved outwards a bit!

    The whole point is that, at the present time, it looks like certain classes of body form in certain parts of the solar system. Imaginary scenarios which we have no reason to think could ever occur are not useful. The definition of “planet” is meant to reflect what we know about how solar systems are formed.

    Yes, Earth would be a dwarf planet if it formed in Pluto’s orbit. But we don’t have any reason to think that Earth could form in Pluto’s orbit. In my opinion, the definition shouldn’t be defined to accommodate all sorts of improbable scenarios, that would make it useless, it should reflect our best knowledge of how solar systems actually work.

    If we find some evidence that, in a solar system otherwise like ours, Earth-like planets can form in the Kuiper belt then yes, I think a change in the definition would be warranted. But I don’t think random “what-ifs” should be used in determining the definition without any reason to believe those scenarios are plausible.

    All indications are that the sort of object that forms in the Kuiper belt is fundamentally different than the sort of object that forms in the inner solar system. Therefore, I think a different definition is warranted.

    Even if you didn’t think that, the fact that Pluto’s orbit intersects that of a much larger body and it is locked in orbital resonance with that body would be sufficient to demote it in my opinion, but that is a different matter.

    The number of insects or animal species or breeds of dog doesn’t make us say, “Wait! Too large a number, can’t cope! Must redefine ‘dog’ so only Alsatians and Dobermens and a few other very similar breeds count!” Ditto insects so only flies count as insects etc .. So it should be the same here.

    It is relatively easy for animals, since we can check whether they can interbreed or not. But even with them, there are corner cases where there is debate about whether two species are the same or not.

    How do you define whether two bacteria are the same species? It is not a trivial manner, and some of the criteria used are far more arbitrary than the definition the IAU uses.

  39. amphiox

    Why didn’t they just call Eris the 10th planet when they discovered it? Same for the other large objects out there.

    One word. Ceres. There is no self-consistent, rational, or reasonable definition of planet that would include Pluto which would not also include Ceres. Even the historical circumstances that led to Ceres being declassified as a planet (it was considered a planet when it was first discovered) is almost exactly parallel to the circumstances that resulted in Pluto’s “demotion”. It just took less time (from discovery to reclassification). If you want to restore Pluto to planetary status you have to retroactively restore Ceres as well.

    The center of gravity of the candidate planet and its moons must be inside the candidate planet.

    But that would exclude Jovian-Saturnian double exo-planet pairs which would make no sense at all.

    Messier’s definition with the inclusion of subcategories is the one I personally prefer. However I do not share his ire over the current one. All classification schemes are arbitrary human constructs, which will change inevitably as we learn more. The current definition will be temporary and it will change, and all that would be required to turn the current definition into a prototype of Messier’s preferred system would be to drop the distinction between planet and dwarf planet one heirarchy level down to a subclass distinction. To me this change is trivial and simply not worth raising one’s blood pressure over.

    I also think that while the current “clear the orbit” criteria is problematic, the spirit of the criteria is one that is important. The difference between an object with sufficient mass and gravity to almagate (or remove) the majority of the material in its orbit during its formation and an object that cannot and ends up as a result a member of a debris ring is of major importance, and will have massive consequences to the subsequent evolutionary history of that object, and so must be addressed. If we deny the importance of this, then for consistency we would need to reclassify every single discrete object in Saturn’s rings as a moon, and provide a distinct name for each one.

  40. Jeff

    yes, Mike’s research did lead to this demotion of Pluto, and rightfully so. I actually have visited Mt. Palomar which is far from my Florida home, but was fun.

    I myself think the 2006 conference was more political than scientific though, and I say that becase to throw all small bodies like Pluto, Eris, and Ceres into the same category makes no scientific sense in the view of how these bodies evolved. Clearly, the KBO’s like Pluto Eris, etc. all evolved from a common source in a location in orbit of Pluto, but Ceres has a different history altogether.

    A better classification would be something like this:

    planets: including my very eager mother just sold us nine (….)

    asteroids: the asteroid belt plus the special groups like Amors Apollos

    comets: long period and short period, although long period might be merged into the oort cloud owing to the fact that that is probably their ultimate origin

    KBOs: Eris, Pluto, etc. hundreds of like formed bodies

    that is a start, not to mention moons

  41. amphiox

    The whole point is that, at the present time, it looks like certain classes of body form in certain parts of the solar system. Imaginary scenarios which we have no reason to think could ever occur are not useful. The definition of “planet” is meant to reflect what we know about how solar systems are formed.

    You’re forgetting about planetary migration. It is easy to envision a scenario where objects the size of Earth to Neptune, after formation are ejected due to interactions with other Jovian sized planets in the system into the Kuiper Belt region. Whether the displaced planet will then go on to clear the new orbit is going to depend on multiple factors that could easily vary greatly from case to case, from the ellipticity of the new orbit (imagine a planet in a highly elliptical orbit, half of which ploughs through an uncleared Kuiper Belt, and half of it being wholly cleared – is that a planet, or a dwarf planet?), to the density of the debris belt, to the size of the planet, to the time elapsed since the ejection. If one day we discover a Jovian sized exoplanet smack in the middle of a dense, uncleared Kuiper-belt-like debris belt around a relatively young star (with a bigger super-Jovian in a cleared orbit closer in) I would not be surprised at all. And how would we define that object?

  42. amphiox

    All indications are that the sort of object that forms in the Kuiper belt is fundamentally different than the sort of object that forms in the inner solar system.

    This sort of object is the sort of object from which outer solar system planets are formed. The cores of Jupiter, Saturn, Neptune and Uranus are made of this stuff (the gaseous envelopes captured after the cores attain sufficient size and gravity). The moons of the outer planets are also made of this stuff.

    A similar relationship exists between the belt asteroids and the inner planets. The stuff that makes up Ceres is the stuff that almagated to make Mars, more or less. The stuff that made up Earth and Venus would have been slightly different due to formation closer to the sun, but not substantially so.

  43. amphiox

    anti-American prejudice at work in demoting Pluto and Eris

    Huh???!!!

  44. andy

    IAU definition is flawed but the low-mass boundary of planethood is getting at a sensible kind of idea. We get a distinction between the planets, which are dynamically dominant in their location, and the rest, such as objects in “asteroid” belts. This gives a concise description of the overall structure of the solar system. Unfortunately for Pluto it turned out that the region beyond Neptune contains several populations of small bodies (e.g. the Edgeworth-Kuiper Belt and the Scattered Disc), and in the light of this downgrading Pluto’s status makes sense and has historical precedent. Nevertheless, the IAU’s use of the term “clearing the orbit” is not a very good way of expressing this concept, as the Pluto-supporters never tire of pointing out.

    The upper boundary, being defined on intrinsic properties of the object (lack of nuclear fusion) is inconsistent with the dynamical definition of the lower boundary, but since the IAU have decided to restrict the scope of their definition to the solar system which as far as we know lacks objects in the 3 orders of magnitude mass gap between Jupiter and the Sun, is not particularly problematic at present.

    Unfortunately several extrasolar planetary systems show that the deuterium-fusion criterion is not a sensible definition for the planet/brown dwarf boundary. There are now quite a few systems which have objects that dynamically-speaking look very planet-like, but are above the deuterium-fusion threshold (Upsilon Andromedae is one example, where the middle planet is massive enough to have undergone deuterium fusion). There is also evidence for massive heavy-element cores in some transiting objects which are above the deuterium fusion limit. This strongly suggests that the process that builds giant planets does not care about deuterium fusion.

    Similarly there are very wide separation objects in some systems (e.g. 2M1207b) which are below the deuterium fusion limit, but appear to be drawn from the distribution of low-mass stellar binaries. This strongly suggests that the process that builds stars does not care about deuterium fusion either (not particularly surprising, as it does not appear to care much about the ability of an object to undergo lithium or protium fusion either).

    On the other hand, if orbital dynamics should be ignored from the planet definition, so as to provide a definition of “planet” that includes Pluto, why should such a definition specifically exclude the case where the object is in orbit around something that is itself a planet? If it doesn’t matter that Pluto is a member of the Edgeworth-Kuiper Belt, why should it matter that the larger, but otherwise very similar object Triton is orbiting Neptune?

  45. Jeff

    Andy,

    I guess I don’t understand every subtlety of your argument, but I think one thing you are saying, which I agree with, is that Pluto-supporters are full of it in insisting for a particular orbital dynamics argument that lets Pluto slip in under “planet” status.

    It seems to me , and you might be agreeing with this, correct me if I’m wrong, but Pluto is clearly a KBO because it was formed by the same process, from the same materials in the same basic orbit, as all the others like Eris?

    Therefore, it makes no logical sense to include Pluto as a planet. Size isn’t as important as evolutionary history, and orbital dynamics is only important insofar as it is one indicator helping us to understand this history of formation.

  46. TheBlackCat

    If one day we discover a Jovian sized exoplanet smack in the middle of a dense, uncleared Kuiper-belt-like debris belt around a relatively young star (with a bigger super-Jovian in a cleared orbit closer in) I would not be surprised at all. And how would we define that object?

    We can worry about that when it happens. Just because you can imagine such a scenario does not mean it actually happens in practice.

    This sort of object is the sort of object from which outer solar system planets are formed. The cores of Jupiter, Saturn, Neptune and Uranus are made of this stuff (the gaseous envelopes captured after the cores attain sufficient size and gravity). The moons of the outer planets are also made of this stuff.

    They may be made of similar materials, but the end result is radically different.

    A similar relationship exists between the belt asteroids and the inner planets. The stuff that makes up Ceres is the stuff that almagated to make Mars, more or less. The stuff that made up Earth and Venus would have been slightly different due to formation closer to the sun, but not substantially so.

    Once again, all this means is that similar starting materials can result in very different outcomes.

  47. amphiox

    We can worry about that when it happens. Just because you can imagine such a scenario does not mean it actually happens in practice.

    Everything we currently know about planetary formation suggests that not only can such a set circumstances arise, but it should arise with reasonable frequency. To “worry about that when it happens” is just plain silly. A current classification system should encompass current understanding, including current theoretical knowledge, and should be flexible enough to accommodate reasonable theoretical predictions that have not yet been confirmed. If it cannot do this, it is basically useless for the purposes of doing science.

    Once again, all this means is that similar starting materials can result in very different outcomes.

    Agreed. Which is exactly my original point.

  48. Nick

    How do we categorize planets that have been ejected from their system? They are likely out there simply due to the size and dynamics of our universe.

    What do we do if we discover a planet that has suffered a severe blow…and thus is no longer round, but still manages to fulfill the other criteria…still a planet?

    How do we classify objects that may be in the process of fulfilling the criteria for “planet” definition? Even if it takes millions of years?

    Gas Giants and Terrestrial Worlds have little in common, save a few basic criteria. They don’t seem to occupy the same general orbit, and have some distinct characteristics that make them more like themselves than one another (gas giants are more similar to each other than they are to rocky worlds.) I don’t understand why this disqualifies these larger KBOs from taking on the status of “planet.” It seems far simpler to classify them as a category of planet than to give them the diminuitive “dwarf planet” moniker, and then insist that by “dwarf planet”, we mean they are NOT planets. Perhaps if their current moniker weren’t so similar to “planet” this debate would not be so heated, but it IS a little silly. I really don’t see why we can’t just call them planets and subclassify them as “terrestrial worlds”, “gas giants” and “icy worlds” or even “KBOs”.

    After all, wasn’t the original definition of planet, “wanderer”?

  49. amphiox

    The ideal classification system for planets needs to do at least all of the following:

    1. It must distinguish objects like Pluto from objects like Earth.

    2. It must distinguish objects like Earth from objects like Jupiter.

    3. It must differentiate planets from large moons.

    4. It must differentiate large planets from small brown dwarfs.

    5. It must be broadly applicable to exo-planets.

    6. The method of differentiation should be based on intrinsic properties of the objects in question, and need to be logical and consistent.

    7. There should be broad consensus among the people who use the system for useful scientific work (I suppose this will probably be the hardest criteria of all the manage!).

    Additional helpful features would include distinguishing between objects like Pluto from objects like Ceres, and between objects like Saturn from objects like Neptune.

    The specific details, however, do not matter. Whether you include everything as “planet” and have subcategories, or exclude the outliers into separate “not planet” classification, or even if you throw out the idea of “planet” entirely and classify each separate category with its own new name, it simply. does. not. matter.

    In the absence of such an “ideal” classification system, an imperfect one will do on an interim basis, so long as it is helpful for the purpose of doing useful science.

    The system needs to be useful. That is all. Nothing else matters.

  50. Oli

    @49. amphiox:

    Like these categories?

    Planets: gas giants (jup-sat), ice giants (ura-nep), rock dwarves (terrestrials, maybe splitting in rock dwarves and rock biggies or something? Mars and mercury are smaaaall)

    Dwarf planets: plutoids, large asteroids (ceres-vesta)

    Stuff: Everything else

  51. Matt B.

    Just because something isn’t a planet, doesn’t mean it isn’t a world.

    I would use the orbit-clearing criterion without the shape criterion and separate star orbiters into three classes:
    1. Planet – it and its moons have the majority of mass of all things that cross its orbit (fits MVEMJSUN)
    2. Sub-planet – anything under the majority mass when there is one (fits Trojan asteroids, comets and KPOs)
    3. Co-planet – anything in a situation where there is no majority mass (fits asteroids in the asteroid belt and probably the Oort Cloud)

    I think that covers everything, and would be usable almost upon discovery of the body in question. Status can be redetermined upon discovery of other bodies. We’ll worry about size later if it becomes problematic. The term “dwarf” would not be used because the definition is relative; there could be a belt (around some star) of gas giant-massed Co-Planets next to Earth-massed Planets.

  52. Inertially Guided

    Do you think Pluto CARES what we call it?

  53. Matt B.

    I should clarify: I meant that “sub-planet” fits KPOs such as Pluto, whose orbits cross Neptune’s. There should be plenty of other KPOs that are co-planets.

  54. John F

    “One word. Ceres. There is no self-consistent, rational, or reasonable definition of planet that would include Pluto which would not also include Ceres.”

    Back in the 80s in HS, my science teacher, as an aside said something like this, “ok 9 planets right? Wrong, there maybe 8 or there maybe 10, but certainly not 9, if Pluto is a planet then so is Ceres, if Ceres is not a planet, then neither is Pluto”

    sound of crickets chirping

    “Ok, has anyone ever heard of Ceres?”

    “…ummm it’s when you have a TV show on one week, and next week the same show, same actors and stuff but a different plot?”

  55. Jamey

    Heck, I’m all for promoting Ceres and Vesta to full planet status. I like the hydrostatic equilibrium basis for a lower bound because it’s something that can be reasonably measured, rather than something vague like “cleared its orbital neighborhood” – which considering how many asteroids are in the Jupiter L-4 and L-5 points, I’d have to say Jupiter really fails. Or are we going to designate them as moons of Jupiter? For that matter,is there anything in as close orbit of Pluto as, say, Mercury to Mars co-orbit each other? I seem to remember that the closest Pluto can possibly come to Neptune in its current orbital situation is quite a few AU, and considering Mars’ orbit is only about 1.3-1.4 AU of Mercury’s – how well has Mars performed clearing *ITS* orbital territory? Or Earth, for that matter?

  56. Keith Bowden

    When asked for a comment, Pluto barked and wagged its moons…

  57. amphiox

    @50 Oli:

    Something like that. What’s interesting to note is that this is of course the direction in which planet classification is informally going, irrespective of so-called “official” definitions. The working definitions evolve automatically as we learn more, and the official definitions will either eventually change to reflect this, or get completely ignored so that the informal working system becomes, de facto, the accepted one.

    Future generations are going to look back at the heated pretentions of this debate and laugh at the triviality or it. Or they’re going to take note of the number of astronomers whose productive careers were cut short by coronaries triggered by this debate and shake their heads sadly….

  58. amphiox

    @55:

    First one needs a consistent manner of defining “orbital territory”. It goes without saying that a big problem you either accept as a given or find some way to deal with is that “orbital territory” will automatically increase as the orbits go further and further out.

    But I do remember coming across some calculations relating to this back when the “Pluto is(not) a planet” debate was at its peak: Of all the objects that cross earth’s orbit, including all the Lagrange points, etc, the Earth-moon system accounts for >99% of the total mass. The same is true for Mercury, Venus, Jupiter (of course this includes all the Trojans – remember even in aggregate they mass less than Earth’s moon, I think), Saturn, Uranus, and Neptune. Mars was a bit of an outlier, only having some 90-95% of the mass in its orbit.

    But Pluto and Ceres, less than 1% each. (And for Pluto this is if you don’t count Neptune. That was the clear distinction that was used to justify the “clearing the orbit” criteria.

    Also, for the Trojans (and anything at all really in an L-point), one must remember that Lagrange points are defined by, and would not exist without, the gravitational interaction of two larger bodies. Anything that gets shepherded into a Lagrange point, by defintion, is being gravitationally dominated (and they will remain stably in the Lagrange point only if their mass is significantly less than that of the two dominant bodies that define the Lagrange point). Shunting debris into Lagrange points, for all intents and purposes, should functionally count as “cleared”, as gravitationally it is not really any different from ejecting that object from the orbital vicinity entirely.

    The hydrostatic equilibrium criteria is nice because it is solely based on the mass of the object in question, and makes planet definitions consistent with star definitions (which are also wholly based on physical properties, ie fusion capability, determined entirely by mass). However, as I and others had pointed out before, if you want to throw out orbital dynamics entirely and use just mass criteria, to be consistent you have to accept that all hydrostatic equilibrated moons are also planets, and you have to accept that absolutely anything, regardless of size, that orbits a planet, is a moon. Every manmade satellite is a moon of earth. And every single particle in Saturn’s (or Jupiter’s, Neptune’s, and Uranus’) rings is a moon. And every one will need a name.

  59. amphiox

    As in, if a small red dwarf star is in a binary system with a larger main sequence star, it is still considered a star, and not a planet, even if the center of gravity of that system lies within the larger star, wholly on the criteria that the red dwarf star is massive enough to fuse hydrogen in its core.

    Thus, using the hydrostatic equilibrium criteria in a consistent fashion, all round moons should count as planets, too.

  60. Mike Saunders

    You people arguing about this are stupider than those ninnies who argue about whether airbus or boeing passenger jets are better.

  61. Monkey

    Im happier now that Pluto has left.

    Where did they take it?
    :)

  62. Monkey

    Airbus, by the way.

  63. robbak

    i think many of us should read Mike Browns blog entry http://www.mikebrownsplanets.com/2010/11/so-is-pluto-planet-after-all.html , the second entry after the one linked. It makes the Planetary debate clear.

    his response to the ‘if earth was out there, it would not clear its orbit’ argument is interesting: Basically, KBOs are there because no planet formed. If Neptune had been small enough not to kick the Kupiter belt into chaos, then there would have been a ninth planet, in a near-circular orbit, out beyond Neptune.

    When you put pluto in the box marked ‘planets’, it doesn’t fit. When you put it in the box marked KBOs, it fits perfectly. Its a small object in a highly eliptical, inclined orbit, in a cloud of other such bodies. It happened to have slotted into a resonant orbit that keeps it away from Neptune, or it would have been tossed out of there, possibly to have become a very impressive comet. Might even happen to it one millennia.

  64. amphiox – those calculations come from Steven Soter’s ‘What is a Planet?’ paper, August 2006. It’s well worth a read.

    You’ll find it on the Arxiv, 0608359. Google will sort you out – I won’t provide a link, to save Phil digging this comment out of the spam filter.

  65. MaDeR

    Oh no, not this crap again. More impossible scenarios supposedly invaliding deplaneting pluto included this time, I see.

    My favorite hilarious idiocies:
    - “Jovian sized exoplanet smack in the middle of a dense, uncleared Kuiper-belt-like debris belt”: classic. Made up situation that somehow justify Pluto being planet, even if this is, you know, made up. And by the way, in these circumstances this uncleared deribs belt will be very, very soon… cleared.
    - “half of which [planet] ploughs through an uncleared Kuiper Belt, and half of it being wholly cleared”: taking aside further made upping, this mental image is hilarious and rather physically improbable.
    - all examples from early period of solar system forming are invalid, because, well, forming is key word. Also, protoplanet. Call me when mess and chaos of first few milion years will end and most exciting thing happening (if anything happens at all) is two planets exchanging places in bilion years or two.

    Also usual retarded arguments – especially those stupid non sequiturs (like having a moon(s) somehow contributing to being planet, good grief).

    Planet Pluto is dead. Long live dwarf planet Pluto. Get over it, immature kids.

    (and yes, dwarf planets are like planets, only less important – in same sense as small river is less important than large river)

  66. CB

    I don’t really care about the word “planet” per se, and whether or not Pluto gets to be called one or not. I do care about the clear and obvious distinction between the 8 bodies in the solar system with the unique property of having cleared their orbits of all significant debris, and those that have not.

    As Neil de Grasse said in an interview, ideally we’d just have a big catalogue of the bodies and then various meaningful criterion and you would just select whatever you were interested in whether that be “big enough to be round” or “has rings”. “Has cleared its orbit” would be one of these criterion, and Pluto would not show up when you selected it. Getting hung up on the name just shows that the problem is one of human classification and semantics.

    Oh and if Pluto and Earth somehow magically swapped places then yeah Pluto would be a planet and Earth would be debatable (it’d have a much higher mass ratio than any existing dwarf planet but it’d be much less than any of the other 7 planets). Excepting magic, it’s pretty hard to see how a Pluto-sized body could have formed in Earth’s orbit and successfully cleared out the orbit, yet not have in the process accumulated enough material to be closer to earth’s size. Similarly, it’s hard to see how an object as large as earth could have formed in the Kuiper Belt without also sweeping up a great deal of the belt in order to do so. If we’re assuming we’re keeping the total mass of the KB the same… well, actually it’d be impossible for earth to form there without more mass.

    And in any event, neither of those things happened. And that’s pretty much the point. The orbit-clearing abilities of objects in our solar system tells us something meaningful about their development.

    So if you want to call Pluto a planet and the big 8 “uberplanets” that’s fine with me. Or do as de Grasse suggests and abandon this whole planet-counting venture. Just recognize that there is a significant way in which Pluto is not like Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

  67. Jeff

    60. Mike Saunders,

    I do not think you are very thoughtful about this subject, saying that debate about it is stupid. Quite to the contrary, this debate about solar system classification is as old as the scientific revolution and invention of telescopes. It will go on as a debate, because it is that important, and it really gets to the core of some issues like the evolution of objects in our, and exo-, solar systems.

    I have no doubt these issues will continue to be debated at major IAU conferences.

    Mike, I do not know if you have studied science or not, but let me educate you here on something: I’ve taught science thirty years to college students, and one thing I know about science is , it is NEVER FINISHED! all hypotheses are tentative in the sense that they are (a) not carved in stone, (b) subject to revision when new evidence (new “data domains” ) are discovered.

    Imagine it is the year 1895 and scientists have it “all figured out” and rested on their laurels. We wouldn’t have these computers and internet then either

  68. Messier Tidy Upper

    @30. Carey Says:

    I agree that the “orbital clearance” clause in the definition of planet is meaningless and inconsistent. I would add this very simple and consistent piece to the definition:
    The center of gravity of the candidate planet and its moons must be inside the candidate planet.
    Pluto, Charon, Hydra, and Nix orbit a center of gravity that is outside of Pluto itself, therefore Pluto is not a planet. The center of gravity in the Earth-Moon system is within Earth itself, so Earth is a planet. I’m not sure what the situation is with Eris and Sedna, but the definition would be easy to apply to them, given the necessary data.

    But then what about double planets?

    Imagine a world that is made up of two Earth-sized bodies.* (or mars sied or Jupiter-sized for that matter.) Is it better described as a “dwarf planet” or two planets? I’d say the double / binary planet term makes far more sense there.

    While, yes, the existence of double planets is speculative our own solar system comes close to having a few examples – Earth & Moon, Pluto & Charon & 1998 WW 31 (see wikipedia link currently awaiting moderation below) in the Edgeworth-Kuiper cometary belt – among the more notable ones.

    Such double worlds do seem very likely to exist elsewhere & it seems reasonable to call them planets too.

    ———-
    * Or read Robert L. Forward’s novel ‘Rocheworld’ otherwise titled ‘Flight of the Dragonfly speculating on such a planet pair -Roche (rock) & Eau (water) orbiting Barnard’s Star. A greta read btw. withsome excellent

  69. Messier Tidy Upper

    See :

    http://en.wikipedia.org/wiki/Rocheworld

    For more – or your local bookstore or library. I would highly recomedn it as its one of my fave SF novels with some very cleverly considered aliens, robots and planets. :-)

    See :

    http://en.wikipedia.org/wiki/1998_WW31

    For the specific double planetoid in the Edgeworth-Kuiper belt referred to above & see :

    http://en.wikipedia.org/wiki/Double_planet

    for Wikipedia’s page ondouble planets generally. The IAU definition would bar them from planetary status, unreadsonably I think, whereas my preferred definition would recognise them as planets.

  70. Messier Tidy Upper

    @33. Michel Says:

    (Aussie Mum, English Dad) Read: convict, jailer.

    Actually, my Mum was a nurse and Dad was a teacher. Now would you care to tell me about your parents so I can insult them in return? ;-) :-P

    @63.robbak

    Basically, KBOs are there because no planet formed.

    But that presupposes the very idea you claim to use to prove your idea. That there, ladies & gentlemen, is a textbook case of the “Begging the question” logical fallacy. ;-)

    No planet formed? Bzzt. Wrong! The planet Pluto formed there as did the planets Eris, Makemake, Haumea, Sedna, “Buffy” (2004 XR190) and others. In fact, you could say that most of the planets actually formed in the cometary belt beyond Neptune and only a handful formed inside it. The “Big 8″ worlds are really the planetary oddities – the ice dwarf types are the average, typical class of planet! ;-)

    @65. MaDeR Says:

    Oh no, not this crap again.

    Who says its “crap?” Oh yeah, you stomp over here and imperiously assert so. Just because you don’t agree with or don’t enjoy a particular topic or debate doesn’t make it “crap”, MaDeR. If you are not interested and can’t be bothered thinking or reading about something you can always NOT comment and skip over it. No one will force you to take a stand or say anything here unless you want to. Belittling questions and insulting areas that other people are doing OTOH, well, that’s a pretty crappy thing for you to do. (Same applies to #60 Mike Saunders too.) :-(

    More impossible scenarios supposedly invaliding deplaneting Pluto included this time, I see. My favorite hilarious idiocies:
    - “Jovian sized exoplanet smack in the middle of a dense, uncleared Kuiper-belt-like debris belt”: classic. Made up situation that somehow justify Pluto being planet, even if this is, you know, made up. And by the way, in these circumstances this uncleared deribs belt will be very, very soon… cleared.

    Made up = hilarious idiocy? Sheesh. :roll:

    Yeah that example *was* hypothetical and “made up” – in the same way that neutrinos were made up before they were detected.

    In the same way that neutron stars and Black Holes were “made up” before they were detected.

    In the same way the Higgs boson is made up & why we’re currently looking for it using the LHC.

    Et cetera, etc ..

    You do realise that science starts with “making up” a hypothesis, right, MaDeR? You do remember that the first step of the scientific method is coming up with something that you then try to confirm experimentally – or in this case observationally – right? :roll:

    Also we know already that Fomalhaut b, 1RXS J160929.1-210524 b and the trio of exoplanets orbiting HR8799, to cite just a few examples, *are* located at distances equivalent to far out in our Edgeworth-Kuiper cometary zone ie. 100′s of Astronomical units. It seems likely that more such worlds are orbiting there – and these are superjovian planets. Nor I gather can even such massive gas giant worlds clear their orbital zones making them, effectively, superjovian “dwarf” planets!

    Ridiculous? Yes, absolutely – that’s my point! ;-)

    Planets around other stars have turned out to be much stranger and mind-bogglingly bizarre than we’d imagined beforehand. Jupiters going around their stars closer to Mercury and producing comet-like tails? We’d never have guessed they existed beforehand – yet look at “Osiris” or HD 209458 b which fits that description perfectly. Look at the exotic unpredicted planet we’ve found orbiting pulsars, around a triple star systems and in a globular cluster and so on. :-)

    Ruling out exoplanetary possibilities without having *very* good reason to do so – now *that* could be aptly described as “hilariously idiotic.” ;-)

    Or perhaps more accurately just sad and small-minded idiocy. :-(

    Just like trying to deny Pluto its planetary status deserves the same appellation.

  71. Messier Tidy Upper

    @55. Jamey Says:

    Heck, I’m all for promoting Ceres and Vesta to full planet status. I like the hydrostatic equilibrium basis for a lower bound because it’s something that can be reasonably measured, rather than something vague like “cleared its orbital neighborhood” – which considering how many asteroids are in the Jupiter L-4 and L-5 points, I’d have to say Jupiter really fails. Or are we going to designate them as moons of Jupiter? For that matter,is there anything in as close orbit of Pluto as, say, Mercury to Mars co-orbit each other? I seem to remember that the closest Pluto can possibly come to Neptune in its current orbital situation is quite a few AU, and considering Mars’ orbit is only about 1.3-1.4 AU of Mercury’s – how well has Mars performed clearing *ITS* orbital territory? Or Earth, for that matter?

    Well said & very good point there – I love those last few lines and agree with you completely. :-)

  72. IMO, the IAU definition is fine, except that the “cleared its orbit” criterion is a bit silly and ambiguous. If you choose to interpret it that way, then it could indeed exclude Earth and Jupiter.
    It would make much more sense, if this was simply changed to “gravitationally dominates its environment”.
    Earth gravitationally dominates its environment, because the Moon is in orbit around it, with the barycentre being inside the Earth. Ditto for Mars, Jupiter, Saturn, Uranus and Neptune with their moons. The Trojans don’t invalidate this criterion for Jupiter, as they are held in their orbital positions by the interaction of Jupiter’s gravity and that of the Sun.
    Earth and Jupiter gravitationally dominate their environment. Ceres, Pluto and Eris don’t.

  73. MTU @ 70:

    You do realise that science starts with “making up” a hypothesis, right, MaDeR? You do remember that the first step of the scientific method is coming up with something that you then try to confirm experimentally – or in this case observationally – right?

    I think you’ve hit the nail on the head there, actually. The point is, though, that our best hypothesis at the moment is that we won’t in fact find large objects stably in an uncleared belt. That could turn out to be wrong, in which case the IAU definition will have lost its value, but it’s our best science at the moment.

    Compared to that, random what-if scenarios are just that, random what-if scenarios that, on current understanding, are unphysical. The neutrino and other predictions were made on the basis of theory, not in defiance of it.

  74. CB

    But that presupposes the very idea you claim to use to prove your idea. That there, ladies & gentlemen, is a textbook case of the “Begging the question” logical fallacy. ;-)

    Only with regards to the semantic argument of what should be called a planet.

    If you take “planet” in that statement to mean the IAU definition of planet (or any definition which includes gravitational dominance of the orbit as a criterion) as I believe was intended, then the statement is simply saying that the KBOs are there because no object formed which was large enough to dominate the orbit and clear or absorb most of the other objects. Which is completely true and a salient observation about our solar system.

    No planet formed? Bzzt. Wrong! The planet Pluto formed there as did the planets Eris, Makemake, Haumea, Sedna, “Buffy” (2004 XR190) and others. In fact, you could say that most of the planets actually formed in the cometary belt beyond Neptune and only a handful formed inside it. The “Big 8″ worlds are really the planetary oddities – the ice dwarf types are the average, typical class of planet!

    Yes, if you define planet to include those objects.

    And yes, it is an accurate observation to say that the majority of bodies in our solar system did not accrete enough mass to become gravitationally dominant and clear their orbit.

    Which is exactly why the 8 bodies that have cleared their orbits stand out so clearly and obviously as distinct from the huge number of bodies that haven’t.

    So whether you call those “planets” and the smaller round but non-dominant objects “dwarf planets”, or call everything round a “planet” and the 8 dominating planets “uberplanets”, or whatever, I don’t care.

    But there is a salient distinction between them which it only makes sense to recognize and attempts to group Pluto and Earth in the same class of objects without distinguishing ignores this.

    I like the hydrostatic equilibrium basis for a lower bound because it’s something that can be reasonably measured, rather than something vague like “cleared its orbital neighborhood” – which considering how many asteroids are in the Jupiter L-4 and L-5 points, I’d have to say Jupiter really fails. … how well has Mars performed clearing *ITS* orbital territory? Or Earth, for that matter?

    Jupiter outmasses all of those asteroids by a ratio of over six hundred thousand to one. Mars has more than a hundred thousand times more mass than the other objects in its orbit. Earth has over one million times the mass of the rest of its orbit. Excepting the moon, which is only in earth’s orbit around the sun by virtue of being in orbit around earth.

    Pluto, on the other hand, accounts for less than 10% of the mass in its orbit — not counting Charon, of course, for the same reason we don’t count Luna. Pluto has cleared Charon from its orbit via capturing it as a direct satellite — setting aside discussions of whether it should be considered a binary system since that wouldn’t change anything. Look at the Pluto-Charon system vs the rest of the belt and it is still a small fraction of the total mass in its orbit.

    So yeah, “cleared it’s orbital neighborhood” is vague, and yet still perfectly sufficient to indicate the distinction between the 8 bodies who have obviously done it, and the bodies that obviously haven’t. There’s a huge difference between them, as in several orders of magnitude. There’s no point in being specific in that case, and in fact trying to draw an exact line through the gigantic empty space between things that have cleared their orbit and those that have not would be silly.

    Like I said, I’m not really concerned with exactly what word we use to describe these things. I do believe that the IAU definition touches on a significant and salient distinction between objects in our solar system that should be recognized in any classification system.

  75. CB

    Also we know already that Fomalhaut b, 1RXS J160929.1-210524 b and the trio of exoplanets orbiting HR8799, to cite just a few examples, *are* located at distances equivalent to far out in our Edgeworth-Kuiper cometary zone ie. 100’s of Astronomical units. It seems likely that more such worlds are orbiting there – and these are superjovian planets. Nor I gather can even such massive gas giant worlds clear their orbital zones making them, effectively, superjovian “dwarf” planets!

    Actually Fomalhaut b is orbiting inside — as in closer to its star than — a large belt of objects, while its own region appears clear,, and is thought to be responsible for maintaining this ring’s clean inner edge much like Jupiter shapes the inner asteroid belt. In other words, it seems likely that it has cleared its orbit despite its huge distance from the Fomalhaut.

  76. MaDeR

    “Yeah that example *was* hypothetical and “made up” – in the same way that neutrinos were made up before they were detected.”
    You seem do not distinguish between scientific hypothesis and coulda, woulda and tell tales about what maybe can be, by the way just accidentally biased to support planetness of Pluto. No surprise here.

    “Also we know already that Fomalhaut b, 1RXS J160929.1-210524 b and the trio of exoplanets orbiting HR8799, to cite just a few examples, *are* located at distances equivalent to far out in our Edgeworth-Kuiper cometary zone ie. 100’s of Astronomical units.”
    Already commented by CB.

    “It seems likely that more such worlds are orbiting there – and these are superjovian planets.”
    Ooch, so large. Great, then ratio of masses between these planets and mass of entire belt is zilion to one. Very strong argument for planetness of Pluto you have here. :)

    “Nor I gather can even such massive gas giant worlds clear their orbital zones making them, effectively, superjovian “dwarf” planets!”
    Taking aside “argument from incredulity” fallacy… do you suggest that “clearing orbit” means removing every little speck of dust? In this case no celestial body ever in universe is planet. So yes, it is ridiculous, but in bad (for you) way.

    “Ridiculous? Yes, absolutely – that’s my point! ;-)
    You make fool only from yourself. It seems concept of “clearing orbit” means for you various different things as is convienent for you. This is why I prefer “gravitational dominance”. With this term, is more obvious why moons or Trojans or sufficiently less massive space trash and dust are no problem for being planet.

    “Planets around other stars have turned out to be much stranger and mind-bogglingly bizarre than we’d imagined beforehand.”
    This is why IAU for time being constrained definition of planet to our solar system. We know them better and it will take long time to properly assess what is possible. This definition will be eventually changed.

    By the way, no, Pluto will not be back ever.

    “Ruling out exoplanetary possibilities without having *very* good reason to do so”
    Being contrived example prepared specifically to somehow support planetness of Pluto is sufficiently good reason.

    By the way, your lack of explanation why you still use retarded non sequitur arguments like “Pluto have moons, so Pluto is planet!” did not gone unnoticed.

  77. Bob Shepard

    As I recall, the IAU’s August 2006 definition of “planet” versus “dwarf planet” is valid only for our solar system. Thus, the whole thing is going to have to be revisited once we have a better idea of what kind of weird planetary objects exist out there in our galactic neighborhood.

    Personally, I prefer to think of “dwarf planets” as being merely small planets which don’t dominate their orbital zones. But, as someone else noted, it’s all a matter of semantics.

    I’m not inclined to number the dwarfs in among the eight biggest planets. Why? Simply because a term like “the fifteenth planet” is going to be subject to frequent change as more dwarfs are discovered and recognized officially. Remember, there could easily be hundreds of them. I’m not sure it’ll make sense to number them at all, for the foreseeable future.

    For memorization purposes, teachers could elect a “top ten” approach which would include Pluto and Eris and would resemble the solar system we grew up with. However, students would have to understand that Pluto and Eris have many, many smaller dwarf-planet siblings in the Kuiper Belt. And, we don’t want to forget about Ceres in the Asteroid Belt.

    Perhaps teachers could make it the “top ten plus Ceres” list.

    Or, of course, they could just focus on the “big eight” and ignore the dwarfs altogether. It would be their call.

    The most flexible proposed definition of “planet” I’ve encountered is on Jorge Candeias’ website http://thousandplanets.wordpress.com . He would classify anything anywhere as a “planet” if it has reached hydrostatic equilibrium, but is too small to be a “star”. The large moons would be “secondary planets”. I’ve also seen the terms “satellite planets” and “moon planets” proposed elsewhere.

    Under Jorge’s system, a “binary planet” would be, naturally, two planets orbiting a common center of gravity, no matter how similar or how different their relative sizes. This would include the Earth/Moon and Pluto/Charon systems.

    The gas giants in our solar system would represent multiple-planet systems.

    My prediction is that, once we have a better feeling for what’s out there in the cosmic neighborhood, a more flexible classification system will evolve on its own, with or without the IAU’s active participation. I think it’s already happening. And that’s fine with me.

  78. Nigel Depledge

    MTU (2) said:

    - In My Humble Opinion Naturally.

    This word you just used. I do not think it means what you think it means. ;-)

  79. Nigel Depledge

    MTU (5) said:

    . . .
    As well as on Pluto & its planethood directly here :

    [url omitted]

    where Croswell, an astronomer and one of my favourite astronomy authors, argues that :

    …a better definition exists: a planet of our solar system is an object that orbits the Sun and has a diameter that equals or exceeds Pluto’s.

    My own preferred definition of planet differs somewhat but would also include Pluto as well as the larger ice dwarfs.

    Although I’d be fine with accepting Croswell’s one as a huge improvement over the IAU one anyday!

    But Croswell’s (that you quote above) is just as arbitrary as the official IAU definition, so how is it any different, other than allowing Pluto to be called a “planet” again?

    Ultimately, I don’t believe any of us amateur astronomers has any right to tell professional astronomers what they should call the objects they study. Professional astronomers who care deeply about this terminology should be lobbying other professional astronomers – they should not be trying to involve the lay public.

    Being a professional biochemist, I’d be quite affronted if some amateurs tried to dictate to me and my colleagues how the professional organisation to which we belong should define and use (for example) the term “protein”.

  80. Nigel Depledge

    MTU (6) said:

    How to define a “Planet”

    A good – simple, clear, logical and easy to determine – definition for planet is that a planet is an object which:

    a) Has enough gravity to be in hydrostatic equilibrium (i.e. is round or if rapidly spinning a rotationally flattened spheroid.)
    Thus a planet is not an asteroid or comet.

    So, you’re quite happy for a handful of asteroids (those that have enough gravity to be round) to be renamed “planets” then, yes?

    (Actually, I’d be quite happy for us to have 30 or 40 planets instead of just 8, I am merely trying to point out that your preferred definition is not as clear and simple as you claim.)

    b) Is incapable of ever being self-luminous through shining by nuclear fusion. Thus is not a star or brown dwarf.

    &

    c) Is not directly orbiting another planet. Thus is not a moon.

    So, does this mean you’d be happy for Charon to be a “planet” and for Pluto to be its “moon”? This definition does not address this question at all, and since the system’s barycentre is within neither body (AFAIK) there is no self-evident choice for the dominant body, and no giudance embedded in your definition.

    What if you had a double planet system in which one were more massive but the other had a larger radius (say, if Neptune and Ouranos were a double-planet system instead of being separated by lots of empty space)? Then which one is the planet and which one the moon?

  81. Nigel Depledge

    Gopher65 (8) said:

    Earth wouldn’t be a planet in the far reaches of the outer solar system under the new definition. That makes no sense to me.

    Are you suggesting that Earth would not gravitationally dominate the Kuiper Belt if it had formed there instead of nearer the sun?

    If so, on what do you base this supposition?

    BTW, unless you are a professional astronomer, I do not think it is your place to whine about the way in which the IAU has arrived at a definition of the term “planet”.

  82. Nigel Depledge

    Gopher65 (19) said:

    For some people this means that Pluto was “demoted” because it is no longer defined as a planet. For others it means that the “integrity” of the word planet must be maintained at all cost. (I mean really, we’re not talking about the integrity of the definition of the word ‘marriage’ here, are we?) Was it really necessary to amend the constitution of the IAU (so to speak) just to keep an arbitrary definition of a word acceptable to people who aren’t open to new ideas (ie, hundreds of planets in a single system)? And why does anyone even care that Pluto isn’t a planet anymore? It’s not like its physical characteristics have changed.

    What has led people to attach such religious significance to a word?

    Simples:
    The word “planet” should have some meaning that recognises the qualitative differences between “planets” and “not planets”, as opposed to being merely “a member of this list of objects”.

    A lot of the objections, no matter how clearly or otherwise they may be phrased, seem to boil down to “Pluto was a planet when I learned about planets, so it’ll always be a planet to me,”.

    Others have pointed out that Pluto has been an anomaly for at least the last 15 or so years, because – as far as we can tell – it’s pretty much the same as dozens of other KBOs. Somewhere there has to be a line between “planet” and “not planet”, otherwise the word “planet” has no meaning. The IAU drew that line in such a way that Pluto is no longer a planet, and neither is any large KBO we may discover in the future.

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