Pluto and other odd balls

There's a lot (well, a little) brouhaha about whether or not the new KBO that's larger than Pluto (affectionately known as 2003 UB313) should be called a planet. And if not, then what do we do with Pluto?

This all comes down to: "what is a planet?"

You can come up with a list-based definition ("it's one of these 9 things") but that doesn't tell you anything about those 9 things, and therefore you can never tell whether or not you should add something to the list - there's no test for inclusion.  I've heard some really, really lame arguments for keeping Pluto's planet definition but denying 2003 UB313 - like "it'll confuse people" or "we'll have to update text books." If that's the case, I shudder at explaining to Caspian that "Pluto is called a planet, but 2003 UB313 isn't, because old people are afraid of change". Besides, Caspian already has some recently published kids space books that list 8 major planets plus debris. Kids can adapt. And the old folks will all die off eventually.

Slightly less bad - "Pluto and 2003 UB313 are planets, but 2005 FY9 isn't... and we can't say why" - that's another KBO almost as big as Pluto. In that case you could define planet vs. non-planet in terms of some arbitrary size - anything smaller than Pluto doesn't count, anything as big or larger does.

That still sounds dumb to me.

The discoverer of 2003 UB313 and other KBOs - Mike Brown - has some great possible definitions on the Sedna (2003 VB12) page. It contains my favorite definition - population classification. A planet is something that's way bigger than anything else in the same orbit. So our system has 8 planets plus debris. Works for me.

But if that makes you sad, there's a fun alternative.

You see, way back when the asteroids were first being discovered, at first they were considered full blown planets. Only much later were they doomed to "minor planet" status. It took some digging, but James L. Hilton found some delicious details about when and how this happened:

When Did the asteroids Become Minor Planets?
So if you think more is better, consider one more possible definition: if it's massive enough to be round, it's a planet. That's cool - not only do we get to keep Pluto and add 2003 UB 313 and the other big KBOs, but we get to restore Ceres, Vesta, Pallas and Juno to planet status. Woo-hoo! The more the merrier!