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

[MizarKey]

One of many articles regarding the upcoming conference...

Experts meet to decide Pluto fate

[mcaplinger]

The barycenter rule is laughable, IMHO. You'd think they could have tried a little harder if the intent was to handle extrasolar double planets in the future, unless somebody had some political agenda to make Charon a planet. I'd have tried to make the minimum barycenter distance some function of the body radii so as to exclude Charon.

I also wonder how well the hydrostatic rule will work in practice around the low end, something we are likely to see either for KBOs or even for the larger asteroids.

Leave it to the IAU to overcomplicate what was a seemingly simple question.

[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]

[...]

[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.

[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...

[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