Planets other than the initial "home" planet can be destroyed at any time by the planets' owners. A player might do this if that planet doesn't contain a sufficient number of fields to properly develop it (around 200 fields is usually considered optimum). Also, sometimes colonies are established for specific purposes, and later destroyed to be relocated elsewhere. Lastly, before the version 1.0 redesign, there was a strict cap of 9 planets per player, so if an established player needed another one, he had to destroy one of the previous ones.
Under certain conditions, a moon may be added to a planet. If a planet is destroyed by its owner, the moon is also lost.