Alien sky at sunset. Computer artwork of a sky seen from a hypothetical alien (extrasolar) planet, showing another sun (orange, centre left), a spiral galaxy (upper centre) and an orbiting moon (right). 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. 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.