Extraterrestrial life is life that does not originate from Earth. It is also called alien life, or, if it is a sentient and/or relatively complex individual, an “extraterrestrial” or “alien” (or, to avoid confusion with the legal sense of “alien,” a “space alien”). Hypothetical forms of life range from simple bacteria-like organisms to beings far more complex than humans. The possibility that viruses might exist extraterrestrially has also been proposed.
The development and testing of hypotheses on extraterrestrial life is known as “exobiology” or “astrobiology,” although astrobiology also considers Earth-based life in its astronomical context. Many scientists consider extraterrestrial life to be plausible, but there is no direct evidence of its existence. Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial life, from radios used to detect possible extraterrestrial signals, to telescopes used to search for potentially habitable extrasolar planets. It has also played a major role in works of science fiction.