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Originally Posted by dgavin
Well, there are a few things I don't like about it, but over all it's an OK definition.
Double Planets. Obital Dominace would not classify them as planets at all, even if they were the only two bodies in a particular orbit. The issue posted with the Bary Center didn't use the inverse square law of gravity but a simple proportion. It would actually take a body almost two times the moons mass at it's distance to have a bary center at earths surface, to get it out of the atmosphere, its almost 3.8 times the moons mass.
I think the Bary center for defining double planets is still a valid aproach, when you account gravity work on an inverse square.
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Fair enough. I wasn't sure if it worked like that for barycenters; I got my "simple proportion" information from another website and a second that corroborated it. I'll follow up on this again and make any necessary corrections to the original post. Thanks for bringing this up!
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The other issue.
I'm not fond of anything the declassifies the Pluto/Charon system as a planet(double). For this reason the Size classaifaction makes more sense then redefining.
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While I appreciate the "coolness" factor of double-planet systems, let's not lose sight of the fact that Pluto makes up 90% of the mass of the Pluto-Charon system. I don't know how low that number should be before we call something a "double planet", but let's not pretend that the size difference between Pluto and Charon is negligible.
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Although it's not likely it's entirely -posiible- to have a asteroid like belt of mars to earth sized objects in the habitable zone of a star, if they had a Gas Giant close enough on each size of the plantary cloud to keep the cloud stablized. In this case there could be easily a dozen or more habitable earths all sharing the same orbit.
And they would still be planets.
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I dunno. I think that the size of an orbital region and the maximum size of multiple large bodies in that region will be related. Do you really think that a handful of Earth-sized bodies could co-orbit in an orbital region the size of our asteroid belt? They would have to be in a Trojan-like resonance at their L3/L4 Lagrange points.
While this is theoretically possible, it seems impossible given our current understanding of planetary formation. That may be why we see only collections of very small bodies (i.e. glorified rocks) at the Lagrange points of large bodies like Jupiter. Hektor is the largest, and it is far too small to be gravitationally round.
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I think orbitable domince, and size should both make a case for planet hood, not one over the over.
In the case of Pluto/Charon, a classification as Double Planet fits, I see no valid reson to demote them, scietifically or other wise from that status.
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I believe that Pluto-Charon are still officially considered a planet-moon system, but someone correct me if that is wrong!
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Not pointing fingers at any -one- person, but lately I've come to the conclusion that for some reason classifing 2003 UB313 as a planets seems to actually offend a few people. I'm not exactly sure why that is.
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I think the primary objection is that 2003 UB313 is just one member of thousands of belt objects, much like Ceres is just one member of the asteroid belt. That's the most common explanation and that is what excludes it from planetary status in this scheme.
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I think any classification that excludes both Pluto/Charon and UB313 from a planatary classification is a mistake. 90% of the folk out there would be irritated to say the least if suddenly they had to back petal on calling Pluto a planet. How would science be precieved if suddenly they came out and said 'well, it's not really a planet' especially when that perception could be carried over into other issues where it's far more import for science to make a stand on.
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Perception is important, but a classification scheme still needs to be consistent. It is just as easy to claim that perceptions would be bad if the IAU decided on a system that swelled the Solar System to 25 planets over the next decade.
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Over all were stuck with pluto as a Planet, and hence, UB313 as one too.
But this is all moot until the IAU nails it down officially, one way or the other.
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Whether UB313 becomes a planet does depend on how the IAU resolves this issue. But, in an orbital dominance scheme, neither Pluto nor UB313 would be considered planets.