Habitable alien planet. Computer artwork of land and oceans on an Earth-like alien planet. Its moon is at upper left. A planet's size and its orbit around its sun, have to fit within a narrow range in order to have a surface temperature suitable for liquid water, and to retain an atmosphere. It is thought that the emergence of life on Earth may have been helped by the presence of a relatively large moon, such as seen here, which caused large tides and so prompted the evolution of sea life into land life.