Experts meet to decide Pluto fate, Finally we'll know what a 'planet' is... Options

[alan]

They can't change Pluto's clasification without getting hate mail from millions of children so they had to come up with another definition. Once Quaour's size has been confirmed by stellar occultion it will be added too. Then the kids will have to learn how to pronounce Quaoar, that'll fix them.

[David]

I'd have tried to make the minimum barycenter distance some function of the body radii so as to exclude Charon.

Let me suggest instead that the criterion should be in terms of ratio of the smaller distance to the barycentre / the total distance between the gravitational centers of the two objects -- regardless of the radius of either of the two bodies. That ratio would be the same as the ratio of the smaller of the two bodies to the combined mass of the system.

A "perfect" double planet would consist of two objects of the same mass with the barycentre halfway between them. Nobody would quarrel with that being called a double planet.

A possible criterion for a double planet system might be something like: a system in which the distance from the gravitational center of the smaller body to the barycentre is at least one third of the total distance between the gravitational centers of the two bodies -- in other words, the mass of the smaller body should be at least half the mass of the larger one.

[JRehling]

[...]

[jsheff]

Results are in:

IAU

[maycm]

Results are in:

IAU

....well not yet.... they vote Thursday 24th August

[Guest_AlexBlackwell_*]

I'm not sure if this has been mentioned, but here's Kevin Drum's take on the issue.

[vexgizmo]

"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...."

The "roundness" criterion seems to me to be the start of a good definition. And if Pluto is a planet so is Ceres. However, the qualificaitions of "dwarf planet," "pluton," and planet-if-beyond-the barycenter seem extraneous. But there is a problem here. Keep in mind that we have no idea if Callisto is truly in hydrostatic equilibrium, so surely we have no idea about 2003 UB313. In reality the definition can't really be of "hydrostatic equilibrium" without defining an error bar. This seems pedantic but becomes a real issue: does Vesta count? Before its large impact, its shape was probably hydrostatic (cf. Thomas et al., Science 277, 1492 - 1495, 1997). Is Vesta penalized just for being whacked with an impact large compared to its radius?

[David]

I don't see how, especially with distant KBOs and extrasolar objects being up for consideration, anyone can be happy with a criterion that depends upon precise measurement. This would mean that as new observations are made, we'll discover planets, then, with arbitrarily minute revisions, have to say in some cases, "We were mistaken -- that wasn't a planet."

Ultimately all taxonomic systems have "marginal cases" problems. That's not a flaw in the concept of taxonomy, it just comes with the territory. What's a little odd, however, is to see one taxonomic criterion denounced for having marginal cases, or depending upon precise measurement, while another taxonomic criterion -- that also has marginal cases problems -- is proposed as a replacement.

Anyway, while the IAU's proposal creates a framework for discussion, I don't expect it to be the last word; I imagine that, whatever the IAU decides on, it will be revisited many, many times in the years to come, as more and more data better defines the shape of the problem.

"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...."
This seems pedantic but becomes a real issue: does Vesta count? Before its large impact, its shape was probably hydrostatic (cf. Thomas et al., Science 277, 1492 - 1495, 1997). Is Vesta penalized just for being whacked with an impact large compared to its radius?

Alan Stern discussed this above (post #28 above, with my reply following). One of the reasons I remain focused on actual roundness as opposed to essential or intrinsic or original or probable roundness is that it has at least one practical application in terms of planetary cartography: in those terms a (really) round object is one that you can map using cartographic tools and techniques developed for mapping the earth without extensive error, and an irregular object is one that requires the use of exotic shape models and grids. I don't know exactly where that line would be drawn, but you could try asking Phil Stooke.

[hendric]

The correct answer for the marginal cases is simply making an edict from on high, after a suitable amount of time researching the size. For example, KBO 2010 XYZ112 is bright enough it's albedo could let it be greater than the cut off. Wait a decade (seems like a reasonable time limit), then if there still is uncertainty, pronounce an edict.

[Guest_AlexBlackwell_*]

Ultimately all taxonomic systems have "marginal cases" problems. That's not a flaw in the concept of taxonomy, it just comes with the territory.

That's very true, and very legalistic, too. In fact, my wife, who happens to be a lawyer, quoted Justice John Paul Stevens from the oral argument in Roper v. Simmons:

...but the purpose of a [constitutional] bright line test is to avoid litigation over the borderline cases...

[Guest_AlexBlackwell_*]

The editorial that appears in the August 17, 2006, issue of Nature:

Editorial

Nature 442, 719 (17 August 2006) | doi:10.1038/442719a; Published online 16 August 2006

Round objects

Planets are spherical, and the International Astronomical Union's attempt to make this part of their definition has merit.

There was once a prissy British civil servant who, when he came across a passage in a memo that displeased him, wrote "round objects" in the margin as a synonym for something ruder. This arch circumlocution was lost on the bluff minister he served, who fired back a query as to who this Round fellow was, and why he objected so much.

We can expect there to be plenty of members of the International Astronomical Union (IAU) who, reading the proposed new definition of a planet offered to them by their executive committee, will want to scrawl something equally rude and rather blunter in the margin — and will want to make their objections heard, possibly quite vociferously, at their general assembly in Prague this week (see page 724).

We understand and, to some extent, sympathize. But we would suggest that, instead, they acquiesce in the new definition, which will have the effect of increasing the number of planets in the Solar System to 12, and open the doors to more. They should do this for two reasons: it is not a bad definition; and it will at least stop the rumbling debate over the status of Pluto.

In the 1990s, it became clear that Pluto, the most newly discovered planet, was the most conspicuous of a crowd of icy 'trans-neptunian objects' (TNOs), some of which might well be larger. There was an obvious historical parallel to this situation with asteroids in the nineteenth century. When it was found that there were dozens of asteroids, Ceres, the largest and first discovered, was demoted from its position as a proper planet; it is now a 'minor planet' along with all the other asteroids. Pluto, it was argued by analogy, should be a minor planet with the rest of the TNOs on similar grounds.

This proposal sparked a degree of public debate that irritated many astronomers, who felt that the question of whether a particular body gets called a planet or not is of no scientific interest whatsoever. Still, the IAU decided that it should try and resolve the matter: planets loom large in the public imagination, and it seemed only reasonable for astronomers to be able to say whether a new discovery (or for that matter an old friend) was a planet or not.

The IAU's proposal is that the term 'planet' should apply to an object that has a sufficiently strong gravitational field to have pulled itself into a spherical shape, that is in orbit around a star, but that is not a star itself. This lets in Pluto and 2003 UB313, a TNO that is a touch bigger and not yet equipped with an IAU-approved name. It also readmits Ceres. And in the most peculiar aspect of the whole business, Charon, previously considered to be a moon of Pluto, will become a planet in its own right. Moons, however spherical, will remain satellites, not planets, in the IAU's eyes. But because the centre of mass of the Pluto–Charon system lies outside the body of Pluto, Charon, although tiny compared with, say, Neptune's moon Triton, qualifies as a planet.

Nine more TNOs, and three more asteroids, will become candidate planets, pending further investigation of how spherical they are. More planetary TNOs may follow, when discovered. To tidy things up, the minor planets will get renamed: those that don't have enough of a gravitational grip on themselves to be proper planets will now be 'small Solar System bodies'.

All this will doubtless lead to ructions. But it is at least a coherent approach, and it has a fairly clear basis in physical properties. It has been convenient to have a small and easily memorized number of planets in the Solar System, but convenience is not the only thing that counts. The effects of mass define (unofficially) the upper limits of the planetary realm; anything big enough for fusion is a star. It is fitting, then, that mass should define the lower limit too. This, we think, adds up to a case for IAU members to accept the proposal.

[JRehling]

[...]

[Alan Stern]

[
> DPS PRESS RELEASE (Released 16 August 2006)
>
> "Planetary Scientists Support Proposed Redefinition of a Planet"
>
> Recent discoveries of objects in the outer reaches of our Solar System
> have forced scientists to reconsider what it means to be a planet. The
> International Astronomical Union (IAU) has proposed a new definition of a
> planet as a celestial body whose gravity is strong enough for it to be
> nearly round in shape and which is in orbit around a star but is itself
> neither a star nor a satellite of a planet. According to this definition,
> the nine traditional planets in our Solar System would be joined by Ceres
> (the largest of the asteroids), by Charon (Pluto's largest moon), and by
> 2003 UB313 (the provisional name for a recently discovered object larger
> and more distant from the Sun than Pluto). Pluto and Charon would be
> regarded as a double planet, rather than as a planet and satellite,
> because their center of gravity lies outside of Pluto itself (the only
> such case known in our Solar System.) There is a candidate list of
> additional objects that may be large enough to qualify as planets, subject
> to confirmation by the IAU.
>
> The IAU resolution also recognizes Pluto as the prototype of a new class
> of planetary objects to be known as "plutons." In contrast to the
> classical planets, plutons typically have quite non-circular orbits and
> take more than 200 years to orbit the Sun. With increasingly sensitive and
> broad searches of the outer solar system well underway, it is quite likely
> that additional Pluto-like planets will be discovered.
>
> The Division for Planetary Sciences (DPS) of the American Astronomical
> Society is the world's largest international professional society of
> planetary scientists. The DPS Committee, elected by our membership,
> strongly supports the IAU resolution. It was proposed after two years of
> careful review by an international panel of expert planetary scientists,
> followed by a broadly representative international group of historians,
> writers, and scientists. The new definition is clear and compact, it is
> firmly based on the physical properties of celestial objects themselves,
> and it is applicable to planets found around other stars. It opens the
> possibility for many new Pluto-like planets to be discovered in our Solar
> System.
>
> The proposed definition will be brought to the IAU General Assembly for a
> vote on August 24, 2006. As representatives of an international community
> of planetary scienti

[vexgizmo]

Alan Stern discussed this above (post #28 above, with my reply following).

I's about whether its massive enough **Tto be rounded by gravity** in the absence of
the other effects.

-Alan

But Vesta likely was once "rounded" by gravity, and then later smashed. In fact, its post-impact shape still may prove to be well-approximated by a hydrostatic figure. Again, I like this definition, but I suspect we will be arguing Vesta and others, and arguably Xena doesn't seem to make the cut until we measure its triaxial shape.

-Bob P.

[David]

But Vesta likely was once "rounded" by gravity, and then later smashed. In fact, its post-impact shape still may prove to be well-approximated by a hydrostatic figure. Again, I like this definition, but I suspect we will be arguing Vesta and others, and Xena doesn't seem to make the cut until we measure its triaxial shape.

I have some thoughts on Vesta's shape, but having gotten myself unnecessarily exercised on this point in the past, I'm inclined to wait until either Dawn or the next generation of telescopes provides a much better image of Vesta's shape than we currently have. I don't mind waiting several years to have an argument.