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Originally Posted by Matt Hallock
I have a bit of trouble calling Pluto a planet. I've always defined a moon as something that orbits a planet. Pluto is odd because its moon, Charon, and itself orbit each other.
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The only reason that happens is because all planet/moon systems orbit around a center of gravity, not the planet itself (in fact, anything with mass acts this way). Both Pluto and Charon are close to each other in size (I think Charon is half the size. Proportionallly to its mother planet, it's the largest moon in the solar system). So the CoG of the system is about a third of the way out of Pluto towards Charon. That's what it orbits. It's the same with Earth and our moon (I believe we orbit around a CoG that's about at the surface of the Equator). And don't get me started about the sun and every rock going around it.