Alien Imagination

Extraterrestrial life is defined as life that does not originate from Earth and sometimes described as extraterrestrial biological entities (EBE). It is unknown whether any such life exists or ever existed in the past, although many scientists think that life either exists or has existed, for instance, on Mars. Various claims have been made for evidence of extraterrestrial life, such as those listed in a 2006 New Scientist article, which the magazine describes as "hints" rather than proof. A less direct argument for the existence of extraterrestrial life relies on the vast size of the observable Universe. According to this argument, endorsed by Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.
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.
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. Possible forms of extraterrestrial life range from simple bacteria-like organisms to sapient beings far more advanced than humans.
Suggested locations on which life might have developed, or which might continue to host life today, include the planets Venus and Mars; moons of Jupiter and Saturn such as Europa, Enceladus and Titan; and extrasolar planets such as Gliese 581 c 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.