The IAU proposal is strange. I sort of like the idea of describing largish icy Kuiper belt objects as a separate category of planets.
But their proposal for the criterion for a planet, while admirably physically motivated, is amazingly expansive--I don't think anyone expected they'd decide that Ceres and Charon are planets! As Mike Brown (discoverer of "Xena", aka 2003 UB313) said, it's not really adding three planets, it's really adding more than 40. Those three are just the ones known for sure to fit the criteria today, and there are many more that probably do.
I'm also not sure I like the proposal that a system of hydrostatically shaped bodies whose barycenter is outside the primary is a double planet rather than a planet and a moon (which is why Charon is on the list). As Mark J. Musante pointed out on John Scalzi's blog comments, this means it's possible to have a Kuiper belt object that changes between being a planet and being a moon at various points in its orbit. None such are currently known, but I think I'd bet at least a dollar that such a system exists.
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Re: Minus 1 plus 3?
Date: 2006-08-16 10:44 pm (UTC)But their proposal for the criterion for a planet, while admirably physically motivated, is amazingly expansive--I don't think anyone expected they'd decide that Ceres and Charon are planets! As Mike Brown (discoverer of "Xena", aka 2003 UB313) said, it's not really adding three planets, it's really adding more than 40. Those three are just the ones known for sure to fit the criteria today, and there are many more that probably do.
I'm also not sure I like the proposal that a system of hydrostatically shaped bodies whose barycenter is outside the primary is a double planet rather than a planet and a moon (which is why Charon is on the list). As Mark J. Musante pointed out on John Scalzi's blog comments, this means it's possible to have a Kuiper belt object that changes between being a planet and being a moon at various points in its orbit. None such are currently known, but I think I'd bet at least a dollar that such a system exists.