Extraterrestrial life

Extraterrestrial life (extra and‎ terrestris ("of or belonging to Earth")) is defined as life that does not originate from Earth. Hypothetical forms of extraterrestrial life range from simple bacteria-like organisms to sapient beings far more advanced than humans. It is currently unknown whether any such forms of life exist or ever existed.

The development and testing of theories about extraterrestrial life is known as exobiology or astrobiology; the term astrobiology however also covers the study of life on Earth, viewed in its astronomical context.

Background

Various controversial claims have been made for evidence of extraterrestrial life.A less direct argument for the existence of extraterrestrial life relies on the vast size of the observable Universe. According to this argument, supported by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.

One possibility is that life has emerged independently at many places throughout the Universe. Another possibility is panspermia or exogenesis, in which life would have spread between habitable planets. These two hypotheses are not necessarily mutually exclusive.

Suggested locations at which life might have developed, or which might continue to host life today, include the planets Venus and Mars; moons of Jupiter, such as Europa; moons of Saturn, such as Titan and Enceladus; and extrasolar planets, such as Gliese 581 c, g and d, recently discovered to be near Earth mass and apparently located in their star's habitable zone, with the potential to have liquid water.

Beliefs that some unidentified flying objects are of extraterrestrial origin (see Extraterrestrial hypothesis), along with claims of alien abduction, are considered bogus by most scientists. Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes.

Possible basis of extraterrestrial life

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.