Extraterrestrial life (from the Latin words: extra [“beyond”, or “not of”] and terrestris [“of or belonging to Earth”]) is defined as life that does not originate from Earth. It is often also referred to as alien life, or simply aliens (or space aliens, to differentiate from other definitions of alien or aliens). These hypothetical forms of life range from simple bacteria-like organisms to beings far more complex than humans.

The development and testing of hypotheses on extraterrestrial life is known as exobiology or astrobiology; the term astrobiology, however, includes the study of life on Earth viewed in its astronomical context. Many scientists consider extraterrestrial life to be plausible, but there is no conclusive evidence for 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.

Alien life, such as bacteria, has been hypothesized to exist in the Solar System and throughout the universe. This hypothesis relies on the vast size and consistent physical laws of the observable universe. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.[1][2] This argument is embodied in the Copernican principle, which states that the Earth does not occupy a unique position in the Universe, and the mediocrity principle, which holds that there is nothing special about life on Earth.[3] Life may have emerged independently at many places throughout the Universe. Alternatively life may form less frequently, then spread between habitable planets through panspermia or exogenesis.[4] In any case, complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth based on computer model studies.[5] According to these studies, this same process may also occur around other stars that acquire planets.[5] (Also see Extraterrestrial organic molecules.) Suggested locations at which life might have developed include the planets Venus[6] and Mars, Jupiter’s moon Europa,[7] and Saturn’s moons Titan and Enceladus.[8] In May 2011, NASA scientists reported that Enceladus “is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it”.[9][10] Life may appear on 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.[11] In December 2011, scientists working with NASA’s Kepler space telescope announced the discovery of Kepler-22b, an exoplanet that appears to be orbiting a sun-like star within the habitable zone.[12]

No widely accepted evidence of extraterrestrial life has been found; however, various controversial claims have been made.[13] Beliefs that some unidentified flying objects are of extraterrestrial origin (see Extraterrestrial hypothesis),[14] along with claims of alien abduction,[15] are dismissed by most scientists. Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes.[16]

In November 2011, the White House released an official response to two petitions asking the U.S. government to acknowledge formally that aliens have visited Earth and to disclose any intentional withholding of government interactions with extraterrestrial beings. According to the response, “The U.S. government has no evidence that any life exists outside our planet, or that an extraterrestrial presence has contacted or engaged any member of the human race.”[17][18] Also, according to the response, there is “no credible information to suggest that any evidence is being hidden from the public’s eye.”[17][18] The response further noted that efforts, like SETI, the Kepler space telescope and the NASA Mars rover, continue looking for signs of life. The response noted “odds are pretty high” that there may be life on other planets but “the odds of us making contact with any of them—especially any intelligent ones—are extremely small, given the distances involved.”