A planet (from Ancient Greek ἀστὴρ πλανήτης (astēr planētēs), meaning "wandering star") is an astronomical object orbiting a star or stellar remnant that
*is massive enough to be rounded by its own gravity, *is not massive enough to cause thermonuclear fusion, and *has cleared its neighboring region of planetesimals. (from Wikipedia)
Planets are the primary locations for life to evolve and live, although some terraformed (or similar) moons and planetoids can all sustain life. The Twelve Colonies were each founded on a different planet in a single solar system.