Alien night sky. Artwork of a large spiral galaxy, moons, planets (one at upper right), stars and a comet (upper centre) in the night sky of an alien (extrasolar) planet. This planet has oceans of liquid water and clouds of water vapour. If a planet is to have a surface temperature suitable for liquid water, and to retain an atmosphere, it must be a suitable size and distance from its star (on horizon, lower left). The number of 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.