Planets are the primary habitation of all races in the galaxy. Planets orbit stars, and habitable planets are classified by their planet quality. Up to five planets may be in orbit of a particular star. The number of planets, and habitable planets, may be modified by the user in the galaxy setup. The inhabitants of a given planet are represented by the planet's population. The amount of food produced determines the current maximum population of a planet, while the planet quality enforces an absolute maximum (as well as the number of usable tiles for the planet). (NOTE: the planet quality related maximum is for growth, you can build in a farm and ship people in via transport if you really want to, and they won't die off).
Planets have tiles in which planetary improvements can be built. The tiles are also determined by the planet quality. Some of the tiles may contain special tile resources that can make the improvements build in that tile more effective.
Planets also have special bonus that come as a result of having a moon or a ring. The moon gives 10% manufacturing bonus and the ring 10% research bonus.