August 25, 2006 5:21 AM BST
It was not just the orbit and size - it was also about the geological makeup, but perhaps more importantly, the fact that it's just one object amongst many similar objects (about 800 at the last count) in the middle of the Kuiper Belt - and pluto wasn't even the biggest of them. More are being discovered all the time. It's orbit is less of an orbit, and more of a beaded bracelet around the solar system.
A key part of the definition is that a planet must have cleared it's area (meaning that if it's just another rock in a belt of rocks, it's not a planet.)
Well, that was a pretty pathetic first post in I don't know how long... Hi people!