Habitable alien planet. Artwork of land and oceans on an Earth-like alien planet. A planet's orbit and its size have to fit within a narrow range in order to have a surface temperature suitable for liquid water, and to retain an atmosphere. A moon is seen at centre left. The Orion Nebula (red) is seen in the background. This is a region of star formation that is some 1500 light years from Earth in the constellation of Orion.