Re: Idiots Astronomical Union
Intepreting "has cleared its orbit" as "there's nothing, zip, nada sharing its orbit" is incorrect and specious, since that would mean nothing can ever be a planet.
Earth's mass is about 1.7 million times all the other matter "sharing" its orbit (excluding the moon, since it orbits the earth)
Neptune is about 50,000 times the mass of all the other matter "sharing" its orbit (and that includes Pluto) and is the least "planety" planet in the 3rd criterion.
Ceres is about 1/3rd of the mass of all the other matter "sharing" its orbit, so it isn't even close to "having cleared its orbit"
Pluto is at most 1/100,000th the mass of all the other matter "sharing" its orbit, so it is very much further away from matching criterion (3) than even Ceres is.
And planets around other stars are called "exoplanets", not "planets", so strictly speaking there's no problem there.
All objects orbiting another object "share a common Center of Gravity", in all cases (including Ganymede and Titan) that Center of Gravity is *inside* the main body, except in the case of Pluto & Charon where the Center of Gravity is outside of Pluto (Well, there are probably other examples if you look to small double asteroids, but Pluto-Charon is the only "big" couple where this is the case)
So while the definition of the IAU may be contentious in some circles, it is by no means idiotic...