Planets in our solar system are the bodies larger than Pluto or about 1500 kilometers. Objects smaller than this MAY be satellites around some of the planets, but if they were orbiting by them selves, we might be forced to call some of the larger ones planets in their own right. Comets and asteroids are of the same size, but comets are icy bodies that are capable of producing tails.
Asteroids are rocky bodies.
Dr. Sten Odenwald Return to Ask the Astronomer .