Extraterrestrial life is defined as life that does not originate from Earth. Possible forms of extraterrestrial life range from simple bacteria-like organisms to sapient beings far more advanced than humans. It is unknown whether any such forms of life exist or ever existed.
The development and testing of theories about extraterrestrial life is known as exobiology, xenobiology or astrobiology; the term astrobiology however also covers the study of life on Earth, viewed in its astronomical context.
Several theories have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological viewpoint.
Alien life, such as bacteria, has been theorized by scientists such as Carl Sagan to exist in the Solar System and quite possibly throughout the Universe. No samples have been found, although there is some controversy about possible traces of life in Martian material (see life on Mars), of which the most famous are on the Allan Hills 84001 meteorite.
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