[glatt]

Originally Posted by Scientific American

A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Bodies that meet criteria (a) and (b) but not (c) are "dwarf planets". The IAU turned down a proposal to name transneptunian objects "plutonian objects."