text: Just size.  Where we make the cutoff is somewhat arbitrary.  Some people dispute the idea that Pluto is a planet because it is too small.  Asteroids are also known as "minor planets" (planet = wanderer in Greek, because they seemed to "wander" among the stars).  Not all planets have atmopheres.  Mercury's is essentially zip.  And some moons of other planets (such as Titan) have substantial atmopsheres.