[QUOTE=Denethorn]My old physics teacher made a point about 'planets'. We arbitrarily name orbitting debris and rocks planets
. Pluto is effectively an asteroid/meteorite (not sure of the precise technical term
) in orbit of the sun - like many other mundane rocks floating around, only we decided to call it a planet.
I don't think there is a very firm definition of planet, and as such - well I'll call my little rock Bob the newest addition to our solar family
[/QUOTE]
The way we decide what a planet is, is the same as the way we decide what a continent is - complete arbitrariness. Why is Europe considered a continent in its own right while it's neither on a different tectonic plate than Asia, nor separated by a major mass of water?
I'm certain that if Pluto was discovered today, it'd be considered a planetoid instead of a planet. Two 'possible planets' were discovered last year as well - both the size of Pluto more or less. One was named Sedna, the other's name escapes me. However, conjectures were made that they were just a part of a distant asteroid belt and thus not a planet but an asteroid (edit: FWIW, an asteroid is a large chunk of rock [using the term rock loosely - it might be metallic, or icy. Or gaseous, in the case of planets {more brackets added for extra confusion}] flying in space. A planet is a large chunk of rock that kind of reminds us of Earth. A meteorite is a small rock that has fallen on earth. A meteoroid is a small rock that has not yet fallen on earth. A come. A meteor is a small rock which is in the process of falling on the ground and burning up in the atmosphere. A planetoid is pretty much synonymous to asteroid, possibly a bit bigger. A comet is none of the above. I hope I got the 'technical terms' cleared up
). Then more conjectures were made saying that Pluto was also a part of that asteroid belt and not a planet, but no one felt like re-writing textbooks.
So, your teacher was right