Quote Originally Posted by Khedrac View Post
That's what I grew up being taught, but it's not what they are now!
I was taught that Mercury, Venus, Earth, and Mars were "Inner Planets," and the others were "Outer Planets," with the distinction being that the asteroid belt separates the two. I think the intention was these terms applied just to our solar system, but thinking about it, it's not wrong to refer to planets outside our solar system as Outer Planets, either. I don't think they taught us any significance behind why we have the distinction, though, so it might have just been fill-in-the-blank fodder.

Quote Originally Posted by halfeye View Post
I don't see that. My understanding is that Pluto and the other Kuiper belt objects are largely snowballs that would become smaller iceballs or even comets nearer the sun. Haumea would presumably break up due to it's rotation rate if it melted.

The surface is infinitesimal, the main body of the Earth is rock allegedly solid most of the way down, but somehow flowing too.
Ice is a solid (okay, fine, most of the time), so if Pluto is mostly ice, then Pluto is mostly solid. Furthermore, any planet will become a liquid if you move it sufficiently close enough to its respective sun.

And now the core does matter, but only for Earth?

There's another issue that if we stop calling Jupiter, Saturn, Uranus, and Neptune planets (or plane-ettes, or wandering stars, or rudisplorks), we have another question to ask ourselves, "What are they?" I have lots of ideas, but I think the only category that is board appropriate is to call them Space Balls.