A planet is an astronomical body in a hydrostatic equilibrium 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 neighbouring region of planetesimals and is not massive enough to undergo nuclear fusion. Typically they orbit a star in a Solar System, though they can be found in the interstellar medium.

They can be either rocky or gaseous in composition. The former are called terrestrial, or rocky planets; the latter are called jovian, or gaseous planets.

A civilization that uses power equal to its planet's input from solar radiation (roughly 1016 watts for Earth), or fully controls the available resources on a planet, is called a Type 1.0 Civilization.