Planet

The Gudersnipe Foundation defines a "planet" as any spherical object in orbit around a star of sufficient gravity that an individual weighing 180lb in earth-normal gravity can stand on its surface without falling off into space.

The definition is somewhat relaxed to include gas giants, of course; Gudersnipe official definitions are made deliberately broad, to encompass a wide variety of objects and ease classification standards.

Moons

A moon is defined as any natural object found in orbit of a planet, thought to have entered orbit naturally. By this logic even small pieces of rock debris can be classified as moons.