Zillionth time: The proposal is poor, and so is any other because there exists no good definition. "Planet" is fundamentally cultural term, wanted we that or not.

Let's hope they vote for it so that we finally can stop that eternal dispute.

In the end, this may turn out positive for us planetary probe fanatics. If an object receives a planetary status, it gets more PR and is more likely visited by a probe (at least in the main asteroid belt). Ceres was badly outlooked because it was considered just another "rock" not a Real Planet™.

From Rob:

Hope you'll forgive me for this, Alan, but I can't resist playing devil's advocate: wouldn't that "technically" make Nix and Hydra numberable KBOs, rather than satellites of Pluto?


My comment:

The deflection of the Pluto/Charon barycenter by Nix and Hydra being less than the radius of either Pluto or Charon should secure these two bodies as satellites.

Exactly, so why bother? For PR purposes? Is that really what the IAU is there for? I'm sad and surprised to think that worlds like Ganymede or Titan might be considered less worthy of exploration because they each orbit a planet as well as the Sun. Why should that matter? It must be so because even the Cassini team seems to think so - the preamble to each Titan flyby page starts with a statement to that effect. I'm totally mystified by this.

According to Daniel Fischer from the HASTRO list, I quote:

There has just been the first open debate here at the IAU General
Assembly on the proposed resolution in which only planetary
scientists (planets of all sizes) took part: The proposal lost,
about 60:40, to an alternative put forward by a group of other
planetologists (which would have made "being by far the largest
object in the local population" plus roundness the criteria for
being a planet and thusly excluding Pluto). The term "plutons"
was rejected in a 2nd vote by an overwhelming majority, for
the linguistical confusion it may trigger (and has already done
so in places). None of these votes is binding in any way:
It's up to decide for the IAU Executive now to decide on
further action ...

Daniel


Daniel F. just posted another note with a link to the details, plus this:

More on today's events: http://www.astro.uni-bonn.de/~dfischer/mirror/300.html

I'm now being told that the vote was actually more like 75:25; the counting
was a bit chaotic ...

Daniel

Groan. Okay, let complete chaos ensue... cry havoc, and let slip the dogs of war.

I don't find the alternative proposal any clearer than the primary proposal. How does one measure a "by far"? What constitutes or does not constitute a "local population"? In what system of analysis do all TNOs constitute a single "local population"?

1 Ceres is three and a half times more massive than 4 Vesta. Is 3.5 too small for "by far"?

Sedna isn't even a KBO by the most generous definitions of the Kuiper Belt. Does that make it "by far the largest object in [its] local population"?

2003 UB313 also has an orbit that takes it well beyond the outer edge of the Kuiper belt. Strictly within the KB itself, Pluto is still the largest object discovered. But "by far"? Who knows what that means?

If the KB constitutes a single "local population", then perhaps the much smaller region inside Jupiter's orbit can also be considered a single "local population". The largest objects in this region (Earth, Venus) are close in size -- and both are much smaller than Jupiter -- so neither of them can be considered a planet!

[...]

What is a planet? by Steven Soter

http://arxiv.org/abs/astro-ph/0608359

The question of what a planet is has one important ramification: it affects what the name of 2003 UB313 can be. I think we're all tired of saying 2003 UB313, so it'd be nice if the IAU could resolve the question to that level.

I agree. However, there is an easy way for the IAU to resolve the question without making a decision on "what is a planet" -- let them abolish the rule providing for a different system of nomenclature for planets.

all sun-orbiting objects get numbers.

Welcome to 145675 Earth.

Hehe, good one! Don't forget to visit 145676 Luna (Earth's Moon) while you're at it! Later!

J P

I've just read this proposal. It completely ignores the actual physical attributes in favor of pure orbital population dominance. This position is pretty diametrically opposed to Alan Stern's. It does contain some questionable assumptions, like the 100:1 miniumum ratio for object-mass:mass-of-everything-else for planet qualification, and the fact that he says every single KBO can collide with Pluto.

Also, it uses accretion terminology in the definition. He seems to feel that intermediate cases when it comes to population dominance are impossible due to accretion mechanics.

I was amazed by Soter's comment myself:



My initial comment is that of Spock on Khan:



Here's another surprising one from Soter:


So he transitions from "more massive than the total mass of all other bodies" to comparing the mass of one object to the next most massive body, and likewise transitions from "similar orbit" to "cross[ing] its orbit". I think there may also be another body in the neighborhood of Earth with an orbit that frequently crosses Earth's that's just a bit bigger than 1036 Ganymed.

[...]

I think he overlooked that possibility when he excluded objects in resonance with each other from counting when it comes to crossing orbits (when discussing one of the extrasolar planet systems). He also exludes resonance with Neptune when discussing objects crossing Neptunes orbit, but ignores that this implies some KBO's have orbits synced with each other.

Personally, I think it might be interesting if some sort of hackish compromise between this and the IAU proposal could be hammered out that might be not too disgusting to either the 'type of object' or the 'population dominance' crowds, by making a more explicit distinction between major and dwarf planets.

Their orbits may not intersect with Pluto's now but orbits do precess. Eventually this potentially brings them all into orbits which could collide with Pluto. Using mass ratios will tend to break down eventually as border line case may be found in other sysstems. I guess he uses them because he needs a way to quantify "a planet is large enough to dominate its orbital region"

I think a better way to state this would be a "planet is large enough to remove, absorb or control other objects in its region during a the life of its system"

[...]

Yes, it's a term that's more cultural in origin than anything else, but I do I think SOME definition has to be agreed upon, because in addition to its use by astronomers when talking to the public about our solar system for years, modern astronomers have made it very clear to the public that they've found other PLANETS around other stars. That implies that there is some important, universal meaning of the word planet. It's time for "planet" to be defined.

Every field has its taxonomy and nomenclature and sometimes the divisions between kinds of objects are arbritary, and in areas as old as astronomy (and with as much amateur influence) a fair amount of terms won't be what one would want if he/she were working from scratch. We do have other choices besides these proposals, like simply going absurdly generic and totally unscientific and calling almost anything that goes around a star a planet, but that makes astronomy look even worse to the general public who are part of the reason why this debate is being held in the first place. (And public perception does matter when it comes to research and exploration.)

In any event, I'm probably the only one who thinks the "first" main proposal is about as good as we're ever going to get.

How about we avoid the whole mess and have a different category for every object? Thus Earth could be the only "Earth", Venus the only "Venus", and so forth.

When we run out of names, we can start on numbers. Those don't run out as quick.

[...]

I went back to some reference works from 1930 (the year Pluto was discovered) to try to figure out what people 76 years ago would have thought of the controversy.

The first thing I found was that back then the word "planet" was inclusive in meaning, not exclusive: any non-luminous body revolving around a star was a "planet". The question at the time was not how to define "planet", but how to define "minor planet" -- then the technical term for "asteroid".

Minor planets were distinguished from planets by two factors: first, being smaller in size; second, being numerous and (mostly) confined to the area between Mars and Jupiter. To some extent the latter distinction is haunted by "Bode's Law"; it was believed that there was room for one planet between Mars and Jupiter, but to find several implied that they were all something less than a planet.

The identification of Pluto as a planet follows directly from these ideas. It's true that Pluto was once thought to be about the size of Earth, but even if its true size had been known, I think it would still have been identified as a planet. The Kuiper Belt was quite unknown then; Pluto wasn't inside Jupiter's orbit; it wasn't a sungrazer like a comet; and it was out in that vasty unknown where "Planet X" had been postulated. Hence, it had to be a planet.

If a large population of KBOs had all been discovered at the same time, it's anyone's guess what would have been done. The classification of Pluto as a planet has been considered a "mistake", but in light of the absence of evidence that it was one of a swarm, there was really no other alternative.

Since there seems to be no consensus on what a "planet" is, or any agreement that "major planets" constitute a single group in anything but the negative, perhaps it is better to ask the reverse question: what is a "minor planet"? If both asteroids and KBOs belong to the latter group, what unites them? If we have a unified, positive answer to that question, then the definition of "major planet" can be made in purely negative terms; it is a planet that isn't a minor planet. There need be no question of demotions -- because every object but the Sun would be in some sense "a planet" -- but just a question of whether something fits the minor planet category or not.

Just one other thing: it occurs to me that the definition presented makes it impossible to define the planets in a system until it has completely finished formation and settled into a steady state, which may take hundreds of millions of years.

I haven't been following this debate because to me it's a dumb semantic exercise.

But if no one has suggested it yet, why don't we consider Bode's Law within a defined range? That way we get to keep Pluto (more or less), Ceres is in, but Neptune is out.

It's as valid and as arbitrary as any other of the many sets of rules tossed about this week.

Interestingly, I've tried to bring up the 'minor planet' issue before, and it seems that perhaps the name 'minor planet' is no longer in favour: the only reply I got insisted that 'minor planet' does not imply the object is a planet, just in the same way some astronomers seem to think 'dwarf planet' is not a real planet either. Every single definition or discussion of 'planet' I've seen so far ignores the term 'minor planet'. The IAU proposal explicitly drops the term.

Personally, I think that perhaps one could define planet the old way, and then divide them up into minor (non-spherical), major (the most important ones), and meso (everything else), similar to the Asimov proposal. The meso-planet category is pretty much exactly like the 'dwarf planet' one, except that the term acknowledges the term 'minor planet'

I'm not sure that's quite an apt comparison; individual names can be quite eccentric, but they're generally not good clues as to the class they belong to. New York City is a city, but Pentagon City is an unincorporated neighborhood. Charles Town, on the other hand, is a city. Collier Mountain, whatever its name, may in fact be a hill. Attempts have been made to define "hill" and "mountain" (the latter sometimes being said to be a rise of at least 2000 feet above surrounding terrain); but these definitions are necessarily arbitrary, as there is a pretty complete spectrum of peak heights.

Because of the relatively small number of large objects in the Solar system, there are necessarily going to be gaps in the sizes (and other characteristics) of those objects rather than a continuum; and there's a tendency to hope that such gaps are natural rather than adventitious. The continuiing ability of discovered extrasolar planets to break every rule of thumb we thought we knew about planetary distributions suggests to me that this is a forlorn hope, however.

Whatever happened to the word "planetoid"?

And if a planetoid hits the earth someday, will it be a planetite?

[...]

To add some needed humor to this debate, Stephen Colbert takes on Neil
deGrasse Tyson for his removal of Pluto from the Hayden Planetarium of
the main planet members.

http://www.youtube.com/watch?v=b-7l2G2a6js

And maybe this is coincidence - or not - a recent New York Times article
discusses the mathematical concept that anything can be a sphere so long
as it doesn't have a hole:

http://www.nytimes.com/2006/08/15/science/...amp;oref=slogin

Shudda just called everything either planetoids or Kuiperoids.

Tombaughoids?

And of course, Uranus must be reclassified as a Hemorrhoid...

I guess what unites asteroids and KBOs is that they're swarms of objects. While the largest members are noteworthy, for the most part they're large groupings of indistinct objects. The classical planets are unique enough to not be part of a larger swarm. There are subgroupings, but not large, uncountable swarms.

One can argue whether the largest asteroids, KBOs, and whatever else are distinct enough to be major planets. Is Pluto just a large KBO, or does it stand out enough to be classified as a relatively unique object? Ceres is possibly more distinct from other asteroids than Pluto is from other KBOs, so is it just an asteroid, or something more?

If it's at all important to have a formal definition of what a planet is, and I'm not convinced that it is (though it has generated a lot of discussion!), then I think it should be a definition that's only going to include a small number of the largest, most distinctive, most unique bodies in a system. I think we should wait until we have better knowledge of the characteristics of the TNO population before settling on a definition, though. The current proposal could allow for hundreds of planets, which seems too much like a swarm of objects to me.

An alternative proposal now means Pluto might be demoted after all, and the number of planets in our solar system not increased but reduced, to just eight...

This is starting to sound a bit like a Monty Python sketch, dontcha think? I can see the vote next Thursday... an astronomer staggers into the hall carrying an enormous spherical object. Another shouts over at him...

"What's that you've got there?"

"Pluto. It's a planet..."

"No it's not - it's a dwarf".

"A dwarf? Nonsense. You can clearly see it's a planet..."

"I can see no such thing. It's too small. Weird orbit, too. No, it's a dwarf, or a Pluton. Either way it's an ex-planet. A deceased planet. A planet that never was in the first place - "

"A Pluton? Are you out of your bleeding mind? LOOK at it!" (astronomer bangs planet on table to emphasise its mass) "What about all those innocent little kiddies! Are you going to be the one to tell them their rhyme is wrong? It's a planet!"

"Not according to my list here..."

"Right! If it's NOT a planet then what is it?"

"Calm down! I didn't expect the Spanish Inquisition - "

(Figures in red burst in from side door)

"Ha! NO-ONE expects the Spanish Inquisition...!!!"

"This *is* the IAU, is it not?"

"Yes, sir, it is."

"And... you really don't have any definitions of what's a planet, do you?"

"Ummm... no, sir. We don't. Sorry."

"I see. You do realize, I'm going to have to shoot you, now."



-the other Doug

Wow--backlash!

http://www.space.com/scienceastronomy/0608...w_proposal.html

Apparently Charon-as-planet crossed the line.

Perhaps Christine Lavin was prescient:

"Scorpios look up in dismay
because Pluto rules their sign;
Is now reading their daily Horoscope
just a futile waste of time?"

hmm, so what exactly does "by far the largest object in its local population" mean exactly? How is that defined? Without a precise understanding of what that means, you could expand or shrink the region to what ever fits your fancy. Titan is the largest object by far in its local population, is big enough to be roundish, isn't a star. Wow, it must be a planet. Yeah, this one is going to open up a whole new can of worms.

I would have thought "Titan's local population" would be Saturn and the rest of its moons... so Saturn would be the largest object in that population..?

Or, as I said, chaos.

Here's how they define "local population":

[1] The local population is the collection of objects that cross or close approach the orbit of the body in consideration.

I suspect that, historically, this notion of planetary uniqueness in a "locality" has something to do with the Ptolemaic conception of the crystalline spheres in which planets were embedded; these spheres had to be nested within each other because obviously they couldn't interpenetrate.

I don't know what a "close approach" is technically, but I suspect that Sedna doesn't "closely approach" any other object currently known. It isn't just the largest object in its local population; it is (as far as we know) currently the only body in its local population. As we detect more objects deeper into space, that is likely to change, but that points up a problem in the definition: it allows objects to be classified as planets on first discovery (if applied retroactively to Ceres in 1801, it would have unambiguously allowed Ceres to be classified as a planet -- as of course it was) but then allows the objects' planetary status to be yanked if it turns out there are more of the same kind.

In terms of general classification this is not a problem, but if planetary status is linked to nomenclature it is a problem, because it means that newly discovered objects may be stuck with names that are (retroactively) inappropriate. As may become the case with (UNNUMBERED) Pluto. It would be nice to have a way of fixing the identity of an object soon after discovery in a way that could not be altered by subsequent discoveries. It would also be nice to not upset current classifications too much, at least not ones that have been in place for decades or centuries. For this reason I would have supported a thoroughly arbitrary 2000km diameter or 10^22kg mass boundary, either of which would have retained current classifications untouched. But this kind of thinking has been dismissed as "unscientific".

But it depends on how you define "local population". I could define it as those objects that orbit in the outer part of the Saturn system. In which case, Titan is the largest object. the definition makes no stipulation that a "planet" must orbit a star. This would thus open the door for at least 16 objects (plus 1-3 borderline cases) currently classified as "moons".

I think it actually does.

"A Planet is a celestial body that (a) has sufficient mass for its self gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and ( b ) is in orbit around a star and is neither a star nor the satellite of a planet."

Doug

In the official proposal, yes it does stipulate that. But in the alternative definition linked to by vexgizmo, there is no such stipulation:



Thanks to that little omission, I would support this. I'd take the new planets Io, Titan, Enceladus, Europa (yes, even Europa), and Triton over Pluto ;)

Maybe they should use Justice Potter Stewart's definition "I know it when I see it"

Ironically, the satellites of our Solar system are (probably adventitiously) far more amenable to taxonomic classification than the planetary bodies. They can be split into three categories, which might be called Major Moons, Mesomoons, and Minor moons, or more whimsically Satellites, Selenoids, and Moonlets.

Category 1 is a set of objects that cluster around or just below the lower end of what are currently called "planets": the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton -- these are all large and, for the most part, interesting bodies.

Category 2 is a set including objects smaller than this (after a quite large gap) down to the minimum "round" sizes: Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, Miranda, Ariel, Umbriel, Titania, Oberon, Charon -- these are all icy satellites which -- with the notable exception of Enceladus -- are mostly interesting for their craters, though Iapetus and Miranda have pretty interesting topography.

Category 3: everything smaller.

I realize this is all after-the-fact justification on my part, and quite useless to inject into the controversy at this point, but what about "large enough to retain an atmosphere" (whether it actually has one or not) as a pseudo-natural basis for a dividing line? Since we seem to have established that Enceladus' atmosphere is really a localized mist due to the southern polar geysers, that pretty much lets off anything smaller than Pluto.

Poor Proteus; bigger than Mimas, but forgotten again.

Well, that's what it gets for being a slacker. Not having the common decency of pulling itself into a roundish shape. If it did, people wouldn't forget it.

Aren't a lot of the asteroids now believed to be "rubble piles"? Seems like there is a difference between a solid body that is sphereical, and a pile of boulders that might settle into a sphereical shape.

Just chiming in, I'm in the camp that says Pluto is a Kupier Belt object.
I saw it once suggested to create a separate class: Classic Planets. That would include the 9 we now know. Pluto would have a side note, in that it is likely a Kupier Belt object, but was mistaken as a planet due to the comparatively inferior technology at the time of its discovery.
But after that, well, that's about it. Anything beyond the orbit of Neptune seems like it should be classified as a Kupier Belt Object.

But who knows, in a few hundred years, when we discover an object made/coated entirely of whatever covers the dark side of Iapetus, is the size of Titan, and is maybe 75AU from the sun, and has a regular orbit, maybe then we'll have this same argument all over again.
It seems space just keeps getting weirder and weirder, constantly challenging our terminology and classification systems. Even here on Earth, new animals are still being discovered which defy classification.

Simply getting the public to know about the 9 classic planets seems enough. Maybe add in that there are other sizable chunks of matter way the heck out there, but if they could just know that Mercury, Venus, Earth, and Mars are the rocky ones, while Jupiter and Saturn are almost entirely gas, and Uranus and Neptune are icy rocks with thick, dense atmospheres, with little Pluto on the outskirts, well, that's probably more than most know right now. Save the 53 planets, or however many it may be, for the more intensive science/astronomy classes. Otherwise, people will hear "53 Planets" and just figure "Oh well, it's too complicated for me to bother with at all."


Interesting sidenote on the definition of a planet - the object must be "round". How about Saturn? From NASA's website:
"The planet's diameter is 13,000 kilometers miles (8,000 miles) larger at the equator than between the poles."
Earth's diameter is right around that much. How round is "round?"
(I'm not seriously debating Saturn's planetness, just pointing out how abstract wording might cause more problems.)

This is why I think it's premature to settle on a definition for planets. I think we need more time to find out what the distribution of TNOs is like.



I think arbitrary cutoffs are as good as anything at this point as well, though I'd set the limit higher, at a 4,000 km diameter, or a mass of 10^23 kg. That would put the cutoff at Mercury. Not that I dislike Pluto (I have my name going there and beyond on New Horizons), but I can't help seeing it as more of a KBO than a planet.

I do like the rationale of the alternative proposal that a planet should be an object that was a dominant body in an accretion zone. That satisfies my objection to definitions of planets that could include members of swarms of objects. Maybe one could base a cutoff on the magnitude of error in the ephemerides of nearby objects if the object in question is excluded.

The problem of getting planet status retroactively yanked seems to be a problem with any definition as long as the information used to classify an object is uncertain and incomplete. That's a fundamental problem with Pluto - it's much smaller than it was thought to be, and it's part of a class of objects that weren't known of at the time it was discovered and labeled a planet.

This will only work until we find a KBO larger than Mercury...then the real fun begins.

I'm actually in the "Save Pluto" camp, not just because it's big enough by some of the standards being ping-ponged about but simply because - and this makes me something of a Johnny No Mates, I know - of tradition and history. I know, they're "dirty words" to some people who live their lives in the sterilised, digital scientific world and have no time for such concepts, and my reasoning isn't exactly scientific I know, but we've all grown up with it as a planet, most of the men and women in the street think of it as a planet, and... oh, I don't know... in my gut it just feels wrong and hideously disrespectful to Clyde Tombaugh to demote Pluto just on a scientific principle, you know?

I mean, don't forget, 70 years ago, before the luxury of automated searches with computerised telescopes, in the days when people actually took those great fat photographic plates of the sky, the guy laboured for what must have been hundreds of hours, sweating over those plates in his clunky cyberpunk blink comparator, with absolute dedication. No lazy digital surveys or computer programs in those days... hunting was done with the good ol' Mark One Eyeball!. He found Pluto basically manually, extending the solar system by a magnitude, and we're going to sweep that away just because it's inconvenient for us and possibly confusing for the astronomers of the future? I just feel uneasy about that.

I'm absolutely sure there's a good scientific case - with many points - against Pluto retaining its planetary status. Many very knowledgeable people have made some great arguments for that here, but in my mind this issue is just as much about personal respect as it is about astronomical facts and figures. Just leaving Pluto alone, giving it unspoken honourary planet status, would hurt no-one, would it? I mean, we'd all know it's an anomaly, but we'd just acknowledge it sneaked into the planetary party without a genuine invite but now it's in just Let It Be. Would that really be so awful? In doing so would not be dismissing or disrespecting the work of a wonderful observer who gave the world a strange a distant planetary outpost on the frontier of the Deep Dark. And with Pluto sitting happily in the corner, content to just sip its drink and not make any trouble, we could draw up new entry rules for the planetary candidates we now all know the names of, unhindered by preconceptions about Pluto. ( But maybe that's just me being naive and romantic, and I'm sure some people here will have a little sneer at that. But hey, I can live with that. )

To many people "out there", the oft-quoted "men and women in the street" this seems very petty and pointless, astronomers tinkering with things just for the sake of it. I know this because I have had conversations with many of them about it, at work and literally in that street. Keeping Pluto a planet just because it's thought of as a planet isn't a good enough reason in itself, I know, but we should have more respect for that view because if we don't we're - that's the astronomical community, the people who "do" and care about this stuff every day - going to come across as rather pompous and uncaring, I fear.

If it was decided to keep Pluto as a planet, then admitting that part of the reason for that was honouring the planet's history and the common man's conception of it would go down very well, I think, make us seem more human at a time when scientists and those who have a passion for science are not viewed very generously "out there".

We really could just leave Pluto sat over there in the corner enjoying the music. Besides, for purely selfish reasons, I want Pluto to remain a planet because I haven't seen it yet, and when I catch my first glimpse of it I want to be looking at a planet, not a "Pluton" or an "icy dwarf" or whatever.

Nice argument. I would definitely consider myself in the "Keep Pluto as a Planet" crowd. When objects like Jupiter and Earth are lumped in the same category, the word "planet" has long sense left behind any real use for myself at the very least, so I don't see the harm in keeping Pluto as a planet.

Thanks! Now all we have to do is convince everyone else in the world...