Earth-like gas giant moon. Artwork of an Earth- like moon of an extrasolar gas giant planet. Another moon and the planet's parent star are also seen. 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.