[Ruth A.]

There is no firm definition of a planet, but the International Astronomical Union has ruled that Pluto meets the criteria. Generally, Pluto is considered a planet because it orbits the sun and is large enough that its own gravity gives it a circular shape.

This same reasoning, but not stated as elegantly.

[FreedomCalls]

I wondered because almost any criteria that can be chosen to make Pluto a planet also includes Ceres as well. There are even quite a few asteroids that have a moon orbiting them (but not Ceres).