Alien planetary system. Computer artwork of a ringed gas giant planet with an Earth-like moon, both of which are orbiting the star at right. An alien (or extrasolar) planet has to fit a narrow range of sizes and distances from its star, if it is to have a surface temperature suitable for liquid water, and to retain an atmosphere. The number of other habitable planets in the Milky Way galaxy is not known, but has been estimated at anything between 1 (just Earth) and as many as 10 billion. Many planets have already been discovered orbiting nearby stars.