Experts meet to decide Pluto fate, Finally we'll know what a 'planet' is... |
Experts meet to decide Pluto fate, Finally we'll know what a 'planet' is... |
Aug 14 2006, 06:06 AM
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#1
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Member Group: Members Posts: 295 Joined: 2-March 04 From: Central California Member No.: 45 |
-------------------- Eric P / MizarKey
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Aug 16 2006, 02:33 PM
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#2
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Senior Member Group: Members Posts: 2517 Joined: 13-September 05 Member No.: 497 |
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. -------------------- Disclaimer: This post is based on public information only. Any opinions are my own.
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Aug 16 2006, 04:25 PM
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#3
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Member Group: Members Posts: 809 Joined: 11-March 04 Member No.: 56 |
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. |
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Aug 16 2006, 04:29 PM
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#4
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Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
[...]
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Aug 16 2006, 05:14 PM
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#5
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Member Group: Members Posts: 809 Joined: 11-March 04 Member No.: 56 |
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. |
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Guest_AlexBlackwell_* |
Aug 16 2006, 05:31 PM
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#6
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Guests |
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:
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