Low gravity alien. Artwork of an alien life form adapted for life on a planet with low surface gravity. It closely resembles Earth-based plants and trees but has evolved to the low gravity and thin atmosphere by growing much taller than on Earth. A rocky planet that is smaller than the Earth would have lower surface gravity. Its thin atmosphere means it would also be much cooler. In order to sustain life, a smaller Earth-like planet must not have less than one-half of the surface gravity of Earth. Any less and the atmosphere would escape into space. This is the process that many believed occurred on the planet Mars.