Henry Imler August 16th, 2006

And then there were Twelve: The new definition of a planet is as follows:

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, and (b) is in
orbit around a star, and is neither a star nor a satellite of a planet.

I like this because there is a formal definition that we can agree
on and go forward with. The sentimental side of me wants there to only
admit the “cool” planets, you know, ones with rings, ones that are
huge, ones that could harbor life, and so on. But, in reality, I think
that the self-gravitational and the non-satellite are good criteria.
So, that allows for Xena, Charon, Ceres.

Xena is the newest planet discovered and put pressure on the
community to come out with a better and universal definition of a
planet. Ceres is the largest asteroid and I guess was believed to be a
planet for a long time. Charon a planet? Isn’t it the moon of Pluto? I
thought the new definition discluded Charon. However, is now a planet
because the center of gravity of its revolution around Pluto does not
lie inside Pluto. Because of this, Charon is not really a satellite of
Pluto, they are a bi-planetary system.

Update: The Scientific American Blog as some suggestions for amending the planetary deffinition, adding geologic activity to the list.

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