Extraterrestrial life is life originating outside of Earth. It is the subject of astrobiology, and its existence remains hypothetical. There is no evidence of extraterrestrial life that has been widely accepted by the scientific community.
There are several hypotheses regarding the origin of extraterrestrial life. One proposes that its emergence occurred independently, in different places in the universe. An alternative hypothesis is panspermia, which holds that life emerging in one location then spreads between habitable planets. These two hypotheses are not mutually exclusive. The study and theorization of extraterrestrial life is known as astrobiology, exobiology or xenobiology. Speculative forms of extraterrestrial life range from sapient beings to life at the scale of bacteria.
Suggested locations that might have once developed or continue to host life include the planets Venus and Mars, natural satellites of Jupiter and Saturn (e.g. Europa, europa/update_12142005.html"" target="_blank">http://www.planetary.org/programs/projects/explore_europa/update_12142005.html" _Enceladus and Titan). Gliese 581 c and d, recently discovered to be near Earth-mass extrasolar planets apparently located in their star's habitable zone, and having the potential to have liquid water.
