[Airehead]

Well, anyone can have a go at this. I suggest:

It should have some attributes which might make it capable of sustaining life
e.g. it should be in regular orbit round a star.
it should be larger in both mass and diameter than our Moon (because the concept of a planet is to some extent a tradition, and by the same tradition our Moon is a satellite not a planet)
its orbit round the star should not be shared by other bodies which do not share a mutual orbit (this precludes the asteroids but we do not want to preclude all our planets with satellites) (incidentally, the asteroids would be better named 'planetoids'. After all, they are not like small stars.)
it should be observably spheroid.

Regardless of the above, if it houses little green men it qualifies as a planet.