People were aware of the existence of satellites since 1610, and arguably since 1543 (when Copernicus postulated that the Moon was the one body in the Solar system not moving in a heliocentric orbit). The change in name had nothing to do with an increase in knowledge about their nature.

When the reclassification of asteroids began in 1851, only about 20 asteroids had been discovered; enough to make bookkeeping difficult, but too few to make conclusions about the existence of a "belt". And in the initial stages of the process (which took decades), Ceres was not "demoted"; for a while it enjoyed an intermediate existence, listed both among the "major planets" and also the asteroids. The key factor in its definite reclassification as an asteroid was the assignment of a number to it, replacing the formerly planetary symbol. This was, again, a bookkeeping exercise, not a scientific one.

I don't see why that should be in the least bit true.

I'll let Stu's comment stand for what I might have said in response to that.

I'm firmly grounded in reality thanks, just putting across my point of view after yours. That you chose to mock my post was disappointing but sadly not out of keeping with the tone of some of the input from astronomers involved in this process.

I wasn't suggesting planetary status should be based on emotions, but that they should be considered. Sometimes dry science has to be hydrated a bit with a sprinkling of common sense and humanity. Demoting Pluto won't actually achieve anything, except offending the memory of Clyde Tombaugh and confusing people even more. It simply wouldn't hurt to just leave Pluto the hell alone, with everyone knowing it's not, in the strictest scientific terms, a planet. We'd all know it's an honourary planet, but so what? make an exception for it! Yes, just do it! It's been thought of as a planet by everyone out there for seventy odd years, why mess with that? It would gain astronomers, and the astronomical community, so much goodwill to just be honest and say "Ok, well it's not really a planet if we're going to be pedantic about it, but we figure why cause trouble for the sake of it?" People would appreciate and support a bit of humility and good old common sense here.

And by "intellectually canibalistic" I meant that this debate has shown, horrendously, that the world of professional astronomy is just as rife with people trying to score points against each other, ruin careers and reputations and puff up their own chests as other sciences. I used to think it was different, nobler somehow, that because astronomers studied, and "got" the Big Picture it might give them some humility. Guess not. When all this is finished - this time, for I fear there will be a fudge tomorrow - the man and woman in the street is going to be left with the view that astronomers are nit-picky boffins who just like messing with things for the sake of it, and argue amongst themselves. And why? Because we've done a frankly crap job of communicating to people just why this debate is so important, why it has repercussions for the future of science, and why it should matter to Mr or Mrs Average. It's come over as self-indulgent and very badly organised.

When the dust settles, whatever the outcome, astronomy will be the loser, and that makes me sad.

Sure, why don't we promote a bunch of asteroids we LIKE to planethood due to emotional reasons. I am all in for Itokawa. I really like that ol' stony rubble pile.





There is nothing wrong in bringing human aspects and science together. They often mix well in science fiction art of all kinds which I really enjoy. But mixing them too much simply creates a lot of nosense. And denying the KBO status of Pluto as well as re-promoting Ceres is a nice example of such nonsense. The same applies to Charon - actually the baricenter of the Jupiter-Sun system also lies outside of the Sun's surface. But that does not make the Sun a double planet, does it? Unless you REALLY like Jupiter and would like to give it a nice promotion. Let's just wait for tomorrow's outcome. And if they make 9 planets 12 to even the number nicely I might as well eat a talking ;-) SNC meteorite. But I will also demand that the IAU be named International Astrological Union ;-)

volcanopele: May I ask why? These all seem to me to be things related to the original formation of the system.

Let me relate this back to my own field, pattern matching/machine learning, because I see it as a classification problem. (It's hard to argue that this isn't about classification.) :-)

If you look at the distribution of bodies in solar orbit by mass, inclination, and eccentricity, I suspect you see something that looks like two smooth distributions superimposed on each other, with a few spikes. The two distributions would be the Asteroid Belt and the Kuiper Belt, and the spikes would be the eight planets. Without using distance from the sun as a parameter, there would be a lot of overlap between the two belts (I believe KBOs have far more variation in inclination). If we include distance, though, the separation should be even more stark.

It would be fun to play with the data (is there a handy single source that has this info in it?) and see for sure, but I'll bet all eight planets are more than three sigmas away from the mean of either major distribution, and I'll bet no other body of nontrivial mass (except the sun) is. (More formally, I believe the simple statistical model will predict that no planet has any significant probability of being an asteroid or a KBO [except Pluto], but every other body of significant mass falls into one or ther other category.

Distributions like this tend to arise from natural processes; they're not arbitrary. Something happened during the formation and evolution of the solar system to produce these distributions and these eight exceptions to them.

The near-perfect power-law spacing of the ten entities suggests this as well. I get an r^2 fit of 0.9933 on the logs of the semi-major-axes of the 8 planets [using Ceres for the Asteroid Belt] and I find myself wondering if the center of the Kuiper Belt is going to end up at about 49.7 AU.

To me, the data strongly suggest that the Solar system consists of one sun, eight planets, two asteroid belts, and a small handful of leftovers. But to make the distinction between the distribtions sharp, I do think you need to include some of the orbital parameters.

As for extrasolar systems with different distributions, I suspect the super-Jupiter with an eccentric orbit will still stand alone.

I prefer to view Clyde Tombaugh's achievements as doing 1990s science in 1930s - discovering a KBO well ahead of the mass discoveries that followed years after that. And I really doubt that he would put any personal or emotional reasons above those of scientific merit. From what I read here he initially firmly regarded Pluto a planet but that was years before the discovery of another KBO which happened in 1992.



And it is exactly that humility and common sense that made astronomers demote Ceres in 1850s when miriads of other minor planets started to emerge in its vincinity ;-) And its discoverer, Piazzi died in 1826 but apparently then there were no claims that it might disgrace him in any way. And I do not really think he would feel disgraced by adjusting the classification of Solar System to the increasing understanding of the Solar System and the Universe we live in. And I do not think Tombaugh would mind either. I guess they were both all for science, not astrology.

The IAU didn't suggest plucking Ceres from asteroidal obscurity because they liked it, but because it was gravitationally pulled into a rounded shape.

My point, however, was that there's absolutely no reason to think that calling Ceres a planet or calling Pluto a planet will make people forget that they are members of the Main Belt or the Kuiper Belt, or forget that the Main Belt or the Kuiper Belt exist. Quite the reverse: Pluto, and 2003 UB313, draw attention to the existence of the Kuiper Belt which otherwise might be considered a forgettable pile of rubble at the edge of the Solar system.

And so were lots of other small bodies. Roundness alone is not a good criterion. It is like calling every 4-legged animal a horse. And even if it is a fly who lost 2 legs, it still would be a horse, right? ;-) Basing the classificiatin on roundness does help to preserve Pluto as a planet but also adds dozens more "planets". And also, they would need to determine the roundness of EACH AND EVERY KBO no matter how distant to actually say how many planets there are in the Solar System



But it would be hopellessly artificial. I guess we might distinguish large round bodies like Ceres, Pluto or Xena by calling them planet-oids but calling them planets is like promoting randomly picked flies to be horses :-) And calling KBOs planets actually draws the public's attention AWAY from the Kuiper Belt as they do not need to bother with the rest of small bodies there as they are happy remembering the existence of a handful of their nice round planets.

[...]

[...]

Why isn't roundness as good (and as bad) a criterion as any other? What's intrinsically wrong with more planets? What's not right about a closer telescopic (or UMSF) investigation of as many KBOs as possible?

It seems to me that saying "calling KBOs planets actually draws the public's attention AWAY from the Kuiper Belt" is like saying that calling certain administrators "cabinet members" draws the public's attention away from the Executive Branch. I do not think anyone really thinks like that.



Also note the smaller, but still quite large gaps: Mars-Venus: .562; Mercury-Mars: .717 . There's also Vesta-Ceres at .593, but there's a whole lot of KBOs that fit in that gap. If you count the satellite bodies Callisto, Io, the Moon, Europa, and Triton in between Mercury and 2003 UB313, the "Mercury-Xena" gap quite disappears; the biggest gap left is Callisto-Mercury at a not-too-significant .759. Pallas-Juno isn't even a legitimate gap; the fourth largest asteroid is Hygiea. Juno's about the 12th largest. In the smaller sizes I doubt there's a gap under .9.

The problem, Greg, is that while objects forming around stars may start out with relatively low inclinations and eccentricities, they don't need to end up that way. We have observed extrasolar planets with high eccentricities. See http://jilawww.colorado.edu/~pja/planets/extrasolar.html for a graph showing eccentricities of observed extrasolar planets. As you can see, most of the ones we've seen have higher eccentricities than the "classical" planets. Are all these objects no longer planets because of this fact. (I leave out inclination from this discussion simply because it is harder to observe inclination from the available data). So while such a definition could arguably work in our solar system, it would not work for the majority of extrasolar planets observed thus far.

Can we reliably say that, no matter how the IAU chooses to define "planet," we'll have official names for "Xena" and "Gabrielle" by the end of the week?

And is the approval of names for Cassini-discovered features of the Saturnian satellites on the agenda?

Michael

I still can't think of better terms than planet, minor planet, and a new designation, "major minor planet."

Michael

[...]

I wouldn't get your hopes up on either case. If "Xena" is given planet status, then it may take another committee to pick a name for it. If it isn't, then a new name may come faster since Mike Brown has already submitted a minor planet name for it.

As for names for features on Saturn's satellites, the Titan and Phoebe names should be formally approved. For the other satellites, I have no idea where the names are in the process. They weren't given provisional approval at the very least, and I have no idea if they need provisional approval by the WGPSN. I know they are usually provisionally approved by that working group, and then are formally approved by the IAU Division III during the General Assemblies. If the provisionally approved step is necessary (prior to the general assembly), don't expect new names for features on the other icy sats.

Good point. Actually the continents are in a sense defined by tectonic plates - Europe used to be a separate plate (the Ural mountains are where it was "stitched" to Asia) but so was India and we do not call it a continent (although the term sub-continent is sometimes used). But sub-continent makes about as much sense as "dwarf planet". And I think it is also a good point about throwing the ball away no matter what. I do not consider inviting non-astronomers to decide to be a "good" way of public involvement. It only shows that astronomers are helpless at it themselves. And the public is reported rather to laugh at the 12-planet proposal and rather than being a good PR it rather undermines astronomers' credibility:

Space.com - Public Laughs and Shrugs at 12-Planet Proposal

"Planet" has not grown far beyond its cultural meaning because we do not understand planets well - we only know ONE planetary system in detail - being our own. And hence eccentricity or inclination are not good criteria because of jupiters and neptunes in elliptical, inclined orbits - and presumably our own Uranus as well (in the past). We are too Sol-centered to decide upon the definition yet. We are not ready. But we do realise what the Kuiper Belt is. And thus we may at least clarify things with Pluto. Leaving it "as is" as proposed by Stu does not really disturb me that much as re-promoting Ceres and messing with Charon. If in a given zone there is no large dominant body formed, you are left with an asteroid belt. And Ceres came closest to becoming that dominant body but failed - otherwise the belt would have been cleared. THIS is what is wrong about calling Ceres a planet.

And there is nothing intrinsically wrong with having many planets as long as they are there. Promoting arbitrarily chosen asteroids and KBOs to "make" them planets IS wrong and clearly desperate. It is like inflation in economy - if you are paid in thousands, money becomes worthless. If anything can be a planet if we say so, then nothing is.

I think too, roundiness alone isn't a good criterion, consider as extreme example Saturn which shape is strongly deformed from centrifugal force (yes, we could transform "spherical" word in "equipotential surface" but this is a less intuitive concept).
However, I think you misunderstood: the real criterion in the committe remain the size (or, even better, mass)! We do not need to measure the shape of every object in solar system, this would be really crazy... they introduce the roundiness criteria only to justify an otherwise arbitrary size limit! This sounds reasonable.
Unfortunately, this "objective" size threshold do not fulfill the initial (untold) target to promote Pluto and take away smaller objects, historically considered planetesimal or asteroids or KBO...
And I suspect THIS is the main source of conflict. At this point, I think isn't worth to discuss again and again because there are historical, cultural and (perhaps) political issues related to the change. We already have enough real, stupid wars around (oops!) and the last considerations from David convinced me that best thing is to leave untouched historical planet designation and put all remaining objects in other cathegories (unless we discover a Earth sized KBO!).
Perhaps I'm slightly tired by discussion, but at this point I vote for the "status quo"!

Yeap, determining absolute roundness of each individual KBO would not only be tiresome and unproductive but actually next to impossible with the current resolution of even the largest scopes. And from astronomical point of view it is not that important. But MASS would not be good either - it can only be precisely determined if an object has a moon orbiting it.

Leaving the status quo would be far better than making the laughable "12-planets thing", but it would still leave that unpleasant feeling of ambiguity and "cheating" as Mike Brown called it

But leaving CERES as is, as well as Charon and Xena and demoting Pluto would make way for embarking on a search for a REAL 9TH PLANET - there might still be a jupiter or at least a neptune hiding in the outer reaches of Oort cloud (ejected from inner Sol system or even captured from another one!). Cleaning up the mess and embarking on a new search for the real stuff - I believe that would bring much more sanity to the issue.

Assuming status quo+"Xena", we could also begin searching for the 11th, 12th, etc. planet.

What disturbs me is not that people are laughing at this astronomical predicament - I myself find it a source of no small amusement - but that people are taking it seriously to the point of being angry. It is not a scientific issue, or one with any essential significance. It is, at best, an issue of bookkeeping convenience, but more probably one purely of aesthetics. There are a great many possible applications of the word "planet", some of which fit more or less well with historical usages, but none of them is right, and nobody is stupid or wicked for preferring one to the others. The comedians have more of a sense of proportion with regard to this question than some of the astronomers. I have enjoyed the debate so far, but some of the responses are beginning to acquire a tone of self-righteousness which I think is wholly inappropriate. I can live with 8, 10, 12, 20, 50, or an indefinite number of planets. But I would prefer not to be bullied into accepting one version or another on the spurious grounds that one and only one definition of "planet" is the True Scientific one.

Personally I'm taking this seriously because 1) I thought the attack on Dava Sobel was unfair and unjustified and based on a misunderstanding of her views, 2) this is threatening to make astronomers, and astronomy itself, a laughing stock, and 3) it is going to - in fact, it has already started to - impact on the Outreach work I do, and, I'm sure, many others do too.

Basically, based on conversations I've had with people before, during and now almost-after this "debate", I just know that I'm going to get hassle and, well, not ridicule, but definitely good natured sniggering about this. This past week wnenever I've been talking about something, maybe a new MER picture, a new Cassini pic, the first direct observation of dark matter etc, what have I got back? "Ha, how can I believe all that when you don't even know how many planets there are?"

So, I also just know that when I stand up at the front of a room to talk to a group about something - the search for ET signals, the hunt for life on Mars, the Return to The Moon etc - some *****ole is going to heckle me about "not knowing how many planets there are". Whenever I do my solar system talk, I'm going to get stick for astronomers "changing their minds" about Pluto... and it goes on and on...

I know science moves forwards, things change, ideas evolve and develop and mature. The old is tossed out, the new embraced with open arms... until it in turn becomes The Old and then out it goes into the snow... but Pluto doesn't need to be demoted just for the sake of it, which is what is basically happening I feel. What's wrong with just leaving it alone because everyone already thinks of it as a planet? Seriously, can't we just grant it honourary planet status and move on from that? Everyone Out There "knows" Pluto is a planet. Why? Because we've been TELLING them that! We have! Now we're going to tell them we've changed our minds, it's not a planet after all, never was... well, excuse me while I bang my head against the wall because that's exactly what I feel like doing. It's like someone suggesting renaming the pyramids, or suggesting whales really should have been called "fish" all along.

We're meddling here to make ourselves look clever and brainier than the man or woman in the street, and that smacks of arrogance to me. If this is nothing more than a "book keeping exercise" then shame on us, because that's not a good enough reason for unravelling seven decades of history and public perception.

The final text for tomorrow's votes is out:

Just read those resolutions...

Weak. I was rooting for the planet Ceres.

According to the source of this text, http://astro.cas.cz/nuncius/nsiii_09.pdf , the most important vote will be for Resolution 5b. If one were to vote for Resolution 5a, but not 5b, you would support the Fernandez et al. position that Pluto is just a dwarf planet, and a not a full-fledge planet. If one were to vote for Resolutions 5a and 5b, you support the 12 planets and counting model, with 8 "classical" planets as well as several "dwarf" planets. In that case, I would vote for resolution 5a and 5b (if I could).

volcanopele seems to be referring to the "for" and "against" text that accompanies the resolutions. Here it is from that pdf. The accompanying cartoon is the best part.

250 words for
Compromise. Achieving a planet definition has been all
about compromise. There are two equally valid descriptions
of what should be the principal criterion for defining a
planet. One is dynamical, an object that has “cleared out
its zone.” The other is based on the physical nature of the
body itself. The pendulum of argument has swung both
ways during the General Assembly discussions. But now
it has swung too far.
Resolution 5B is all about finding the middle ground.
Using qualifiers gives equal status to both points of
view and leaves open the possibility to define other
types of planets in our Universe. Resolution 5B restores
the “global and cultural points of view” that the Planet
Definition Committee had responsibility to achieve. The
public recognizes Mars, for example, as a “planet” not
because it has cleared out its zone, but because it is a
fascinating world.
To illustrate why Resolution 5B is cultural, and not silly
semantics, consider how you must answer two questions:
How many planets are there? Is Pluto a planet? A vote
in favor of 5B yields: “There are 8 classical planets and
many dwarf planets yet to be discovered” and “Pluto is a
planet, but in the dwarf planet category.” These answers
highlight and communicate the tremendous revolution
of new discoveries in our outer Solar System. Further, it
saves enormous public backlash by still being able to say
the words “Pluto is a planet, but”. Do not underestimate
the global cultural importance of these first four words.
The word “planet” deserves to be shared equally.

250 words against
Resolution 5B represents a small but significant change to
Resolution 5A.
The key issue is the definition of “planet”. Resolution 5A is
close to the version agreed by consensus on Tuesday evening
where it was made clear that three distinct categories of
objects orbiting the Sun were being defined: planets, dwarf planets,
and small bodies. The logical implications from the
rules of grammar cannot be ignored. By using the name
“planet” with two different adjectives “classical” and “dwarf”
a larger category of planets is implied. This contradicts
the first paragraph of both Resolutions 5A and 5B and
transforms three distinct categories into two (planets and
small bodies) and two sub-groups of planets.
To the question “is Pluto a planet?” the two resolutions
give different solutions – “Yes” for 5B and “No” for 5A. To
the question “How many planets are there?” Resolution 5A
gives 8, Resolution 5B currently gives 12 and soon at least
50.
The total number of planets may not matter to scientists,
it is critical for education and the dissemination of science.
For scientists, it is relevant that dynamical and cosmogonical
criteria, which are now the source for the definition of
planets, would in Resolution 5B be relegated to a secondary
role. In Resolution 5A the arguments from geophysics and
from dynamical astronomy are given equal weight. Such a
balanced solution had received very strong support in the
meeting of Division III (Planetary Systems Science) and the
Planet Definition Information Meeting.
Resolution 5B is misleading and should be rejected.

Click to view attachment

I guess we must no longer consider Jupiter a full fledged planet. One look at a map showing the distribution of the Jupiter trojan asteroids, hundreds and hundreds of them, will clearly show that Jupiter has not "cleared out its neighbourhood around its orbit".

1) Does anybody know what the IAU's Resolution 5A is referring to by "has cleared the neighbourhood around its orbit"?

Presumably they are referring to the situation seen in the asteroid belt. On the other hand a body surrounded by ring material and/or numerous satellites might also be said to have not fully "cleared the neighbourhood" around its orbit.

Then there is the issue of just how fully a body is supposed to have "cleared the neighbourhood around its orbit" before it is elevated to the status of "planet". To take an obvious example, Earth's orbit is approached or even criss-crossed by assorted asteroids. Some of them might be said to be more or less resident for the time being in Earth's general neighbourhood. Others are frequent visitors to that neighbourhood. Do the presence of such asteroids mean that the Earth has not yet "cleared the neighbourhood around its orbit"?

2) I note that the definition for "dwarf planet" does not define "satellite". Since technically speaking bodies like "Pluto"and Ceres are satellites of the Sun, that could be construed to mean that strictly speaking the Sun has no "dwarf planets"! Merely "Planets" and "Small Social System Bodies".

3) What is the betting that in normal everyday conversation the word "dwarf" is going to vanish from in front of the word "planet", and we are all going to go back to speaking (or start speaking as the case requires) of the "planet Pluto" or the "planet Ceres" rather than the "dwarf planet Pluto" and the "dwarf planet Ceres"? (The alternative would seem to be to start referring to them as the "dwarf Pluto", but that kind of makes them sound like escapees from Tolkien. )

4) Equally not even astronomers and planetologists are not going to be going around calling asteroids and KBOs "Small Solar System Bodies" except perhaps in their scientific papers. If there is to be an SSSB category they need to be come up a term that is less of a mouthful.

5) Overall, this latest debate by the experts on how-to-define-a-planet with its various attempts--and in particular comments like "under the new definition, there could soon be dozens of new '‘planets' in our solar system. That struck many astronomers as the wrong result"--merely illustrate what I said on a different thread of UMSF.com back in April: that behind this whole "what is a planet?" debate lies an element of terrestrial snobbery.

I cannot help feeling the only reason this debate has arisen at all with "planet" is because Earth just so happens to bear the label "planet"; and despite all that has come and gone there is still a subconscious wish, even amongst some astronomers, for Earth to be part of a group with a certain degree of...exclusivity.

Nobody seems to mind seeing the word "moon" used to describe objects ranging from giants like Ganymede and Titan to irregular midgets like Epimetheus and Amalthea yet the notion of having the word "planet" encompass objects the size Pluto as well as those the size of Earth and Jupiter seems to generate a strange unease in some parts of the astronomical and planetological communities.

As I went on to add in that previous post:

[I]t is all right for Jupiter to be called a "planet". That then puts Earth among the giants. We are even prepared to tolerate midgets like Ceres and Pluto being one--so long as there was only just one of each. Once there start to be too many of such small fry [though] the feeling seems to be that the term "planet" is losing its currency. Hence, while nobody seems to mind labelling Janus or Miranda "moons" were they orbiting the Sun rather than Saturn & Uranus nobody would be calling them "planets". Similarly with Pluto or (say) Ida. Were Pluto or Ida in orbit around a planet astronomers would be quite happy to label both of them "moons". Only when they start circling the Sun does size suddenly become an issue.

Which begged the question:

[I]f Earth had been a moon rather than a planet would we now be arguing over whether to admit Janus to the hallowed ranks whilst not giving two hoots about using "planet" for Pluto and Ceres?

These latest definitions merely illustrate that Occam's Razor is being cast aside in favour of contrivances of varying degrees of complicatedness whose sole purpose is to keep the word "planet" in some way "exclusive". They have no problem with there being dozens or even hundreds of moons yet they balk at there being dozens of planets in the solar system.

In that context the issue of Pluto is in some respects a furphy that has muddied the waters. Those who want Pluto to be numbered among the chosen few try to contrive a definition sufficiently wide to accommodate it. Those who don't try to contrive one narrow enough it winnow it out, without at the same time risking demotion for more favoured astronomical objects. This in turn leads to varying unease, either at demoting a loved one from the family circle or at the cost of allowing an interloper to stay (some of the riff-raff which got booted into the outer darkness a couple of hundred years would have to be admitted back into the family circle again).

Yet the bottomline seems to be that not even those who want Pluto as part of "Club Planet" want to see dozens of other objects joining it.

======
Stephen

(EDIT: some minor editing to fix a few too many grammatical blunders. )

Don't forget Neptune and its asteroid buddies.

Agreed that "cleared the neighborhood around its orbit" could have been more strictly defined.

For example, neighborhood could mean the region bounded on the inside by a circular (e=0) orbit with period equal to 1/2 of the period of the candidate object, and on the outside by a circular orbit with period equal to 2 times the period of the candidate object. This example does not work for Venus and Earth, but perhaps a case could be made for some other ratios of orbital resonance such as 2:3 inside and 3:2 outside.

As for the clearing part, I recall Michael Brown's earlier webpage on 2003UB313 had listed the "population in neighborhood" proposal with a discussion of the candidate object's mass. I don't recall if Brown had a specific definition of mass ratios, so an ad hoc example here could be that the candidate object mass must exceed the combined mass of all other objects within its neighborhood. Note that objects within the neighborhood should probably include objects with orbits that do not lie entirely in the neighborhood with some limits, e.g., the object's orbital period must be within some ratio of the candidate object's orbital period, with either the object's perihelion or it's aphelion being within the neighborhood.

The rationale for a neighborhood bounds would be that over cosmological timescales, objects within the bounds would either have coalesced or collided to form the candidate object.

I like the name of the link on Michael Brown's home page -- "Astronomers are revolting!"

http://www.gps.caltech.edu/~mbrown/

Taking a second look at the figures, I note that Pluto (and, tacitly, 2003 UB313) really do deserve some of the ink spilt on them. They really are liminal objects. It has been pointed out, ad nauseam, that they are extraordinarily small for "planets". What is much more rarely pointed out is that they are extreme outliers as KBOs, being many times larger than the vast majority of such objects. They may be "dwarf planets", but they are gigantic KBOs.

Has anyone ever done a back-of-the-envelope calculation to estimate what percent of mass of the entire Kuiper Belt is tied up in Pluto and 2003 UB313?

One might as well claim that objects like Metis and Amalthea are "extraordinarily small" for moons.

If the IAU is going to make Club Planet an exclusive preserve for a select few then by rights it ought to do the same for Club Moon as well. After all how many of the solar system's moons would fit the IAU's planet requirement that they have "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape"?

Maybe then we'll start hearing about "moons", "dwarf moons", and "Small Solar System Bodies (Moons)".

======
Stephen

Exactly nineteen. Eighteen if you dismiss Charon for one reason or another. S (2003 UB313)1 is a possible but marginal candidate.



Not an entirely absurd idea, though, as you imply, not one that astronomers have felt a great deal of need for.

Except that a better alternative to "dwarf moon" already exists: "moonlet".

The same might also be said for "dwarf planet": "planetoid" would be a better choice. (Or alternately "asteroid".)

======
Stephen

Oh, dear! That sounds like way too many members for Club Moon to have.

======
Stephen

...then the Main Belt will be fearing the IAU's wrath.

Andy

No mention of the dreaded barycentre now but presumably it's still lurking there in 5B. What will we call a really big trans-Neptunian when we find one? Not classical for sure, since that's presumably a historical term. Giant dwarf planet perhaps? A definition valid for the solar system only is surely a very short-sighted idea at a time when at last we are acquiring the means to explore other systems. For all these reasons and others I'm with JRehling. They really should cut their losses and simply agree that there is no consensus. Science would lose nothing by this. As to Stu's problem with the 'man in the street' I see no difficulty at all. Everyone is welcome to use the word planet as they wish, the man in the street included. As a man in the street myself I would much prefer this to being told what to think and how to speak by a committee. All of their proposals are scientifically pointless and culturally patronising, in a word pompous. Unfortunately they will probably go ahead anyway.

When exactly do they vote?

Well, you could always call the big ones "moons", and the small ones "satellites". Only thing is, this would make the recently coined term "moonlet" obsolete. "Satellitelet" don't have the same ring.

I tend to think of "moons" in terms of the "Big Seven": the Galileans, Luna, Titan and Triton -- the bodies that would arguably be planets were they circling the Sun instead of a planet. After that, there's the big jump down to Titania, Oberon and Rhea, which is a convenient place to draw the line.

However, as we're now seeing with the Pluto debate, defining things in terms of "convenient gaps" is a Very Bad Idea, because new discoveries tend to fill up those gaps. So, yeah, I could go for the "Big Nineteen" (plus whatever else turns out to be out there).

Here's the big problem

There is, for whatever reason, a need to classify what is and isn't a planet. There is a body of support to ensure that Pluto remains within that group. However - any scientific set of rules that dictate Pluto is a planet will innevitably lead to the number of planets reach double figures and far beyond over the next few years - and that would dilute the 'power' of the title 'planet' The body of public support which would like Pluto to be a planet is self defeating in that regard.

ANY definition requires, at some juncture, numerical limits.

If you say 'if it's round' - how round is round? A radius that varies by less than 10% from the average? At some point you'd have to have a cut off between round and not round, and that seems quite arbritrary to me.

If two bodys orbit around a point outside the surface of the parent body, then the moon becomes a planet. That's not helpefull as moons can move. Charon could be a moon were its orbit a little different, and our moon could be a planet in a few billion years. Again - that can not be right because a cow is a cow regardless of WHERE it is. It doesnt stop being a cow and turn into a goat if you put it in a barn.

There seems a reluctance to extend the title planet to things beyond Pluto, as if they don't 'belong' there. A cow is a cow if it's in a field or on a pavement...and that rule should apply to a planet.

I'm quickly reaching the conclusion that NO set of rules will EVER be acceptable to a majority...there is too much culture, too much fondness and too much historical influence to ever make a decision that sticks.

Doug

Just had this email, not sure if it's right or not...

"Leading astronomers have declared that Pluto is no longer a planet in approving
new guidelines that downsize the solar system from nine planets to eight, The
Associated Press reports."

It's always nice when you can draw clear lines or find "convenient gaps," but for lots of real-world problems you just can't -- even though human beings confidently say "I know one when I see one." Computer handwriting recognition (for example) has this same problem, but in a different space.

I really think the IAU has reached the right decision, but I still don't think it's possible to make a principled definition without using statistics. You need at least a 2-dimensional (mass and semi-major axis) and probably 4-dimensional (eccentricity and inclination) model to capture what we intuitively see: e.g. the asteroid belt is a "fuzzy torus." So is the Kuiper Belt. Something that's really far off on any of those 4 parameters is NOT a member. A statistical model just lets you pin down exactly what "really far off" means.

This also lets you give a crisp definition to "cleared out it's orbit." The reason the trojan asteroids can't compete with Jupiter is that they clearly don't belong to the same distribution. Let's say you plotted the distribution (just by mass) of all the objects in Jupiter's orbit. There will be some number of small objects, more smaller ones, even more tiny ones, etc. I haven't seen the data, but I'd guess it's a Zipf distribution. From this model, you can predict how likely a new object (just observed) is to belong to the distribution. Jupiter itself, however, does not belong -- it's way, way too big. The math will give it something like 10^-1000 (or less) probability of belonging.

Ceres, on the other hand, fits quite nicely into its distribution. The largest members are close in size, and for any given range of sizes, (e.g. +/- 1km), the smaller the size, the more members there are. From a classification standpoint, everything in the Asteroid Belt is an asteroid.

The reason to want to include eccentricity and inclination is that prevents a passing comet from "becoming" an asteroid while it's passing through the Belt.

The eight planets are the big pieces that are left over after you've classified all the Asteroids and KBOs. Each planet is unique. They don't fit any of the other distributions, and they don't belong to one themselves. Unlike what Volcanopele said earlier, even if (say) Earth had the same orbit as Hally's Comet, we'd still call it a planet, if our definition is "orbiting the sun, large enough to be round and belonging to no other distribution of objects."

A different way to look at it is to say that we can give a good definition for things that are NOT planets; planets are whatever is left over.

This really will work in any other solar system, I think. I'm surprised not to hear other arguments from statistics, though. I know that statistical classification has been used to automatically find galaxy images in digital pictures, so Astronomers do know the technique, and they're not afraid of math in general. Also, althogh it can be tedious to characterize a distribution, for things like Asteroids and KBO's, you only have to do it once (for each belt). Orbital elements, by contrast, have to be computed for each individual object.

In sum, it seems that the neatest way to characterize a solar system (not just ours) would be to say, "It contains these planets, with these orbital elements, and these asteroid belts, with these distributions, plus an assortment of irregular objects." Given the orbital elements of any unknown object, you can instantly assign it to one of those categories, with some probability. Given the "sharp edges" in the solar system, though, I expect the probabilites for real objects to be close to 100% (probability that Juno is an asteroid) or close to 0% (probability that Jupiter is an asteroid).

Resolutions 5A passed and resolution 5B not passed, meaning Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are planets and Pluto and other would-be planets are "dwarf planets".

Counting on Resolution 6A votes are under way, seems to be near-tie.

In addition to the AP, there is now "breaking news" on Space dot com that Pluto has been officially demoted.



The working definition was round due to the fact that any rigid body forces were overcome by gravitational ones. The only significate out of roundness of such a defined planet would be due entirely to rotational forces or tidal ones, its shape still determined by hydrostatics.



If the moon Titan were ejected somehow from the Saturn system, and continued to orbit the sun, we would no longer be calling it a moon, and would most likely be calling it a planet. Orbital dynamics determine what is and is not a moon, and as the dynamics change, there is no reason why the definitions can't change, at least in my humble opinion.

I knew I can count on astronomers! Exactly the resolution what I wanted (i.e. Pluto not a planet nor a minor planet, er, small solar system object).

Resolution 6A passed, Pluto and other large trans-Neptunians form a new class.

Read 5A carefully. Earth is not a planet now, thanks to NEOs. Ditto Jupiter owing to Trojans.
Such a farce.

"Cleared its path" = dominant object (why couldn't they keep that wording?) Therefore Earth and Jupiter are planets.

Define significant. One could argue that the depths of the Pacific trenches to the peaks of the Himalaya would classify the Earth as significantly non-round. At some point you have to put a measurable factor into when something is no longer round. Is Phobos round? Mars? Olympus Mons + Valles Marineris makes mars quite un-round. I understand that the shape is determined by it's own gravity - that's fine....but you have to put down a marker to say at what point that shape is round.




That's a good point. So Luna, Titan, the Galileo 4....should all be planets?

Doug

Clared its zone is simple english. It doesn't say dominate naything. It says cleared. Earth is a dwarf. So is
Jupiter.

Alan

Well, according to the current definition if they were ejected from their planetary system and if they, by some miracle, achieved an orbit significantly different than their parent planet (the clearing out rule), it would be logical to call them planets as well.