Alien landscape. Artwork of the view from the surface of an extrasolar planet orbiting in the habitable zone of its parent star (upper left). A moon is at upper centre, and liquid water is on the surface of the planet. A planet's orbit and its size have to fit within a narrow range for it to have a surface temperature suitable for liquid water, and for it to retain an atmosphere. As of 2010, hundreds of planets have been detected around other stars.