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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. Referred to as alien life, or simply aliens, these hypothetical forms of life range from simple bacteria-like organisms to beings far more advanced than humans.

The development and testing of theories about 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 prominent scientists consider extraterrestrial life to be plausible, but no direct evidence has yet been found.

Background

Alien life, such as bacteria, has been theorized to exist in the Solar System and quite possibly throughout the Universe. This theory relies on the vast size and consistent physical laws 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. This argument is embodied in the Copernican principle, which states that the Earth does not occupy a favored position in the Universe, and the mediocrity principle, which holds that there is nothing special about life on Earth. 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.

Suggested locations at which life might have developed, or which might continue to host life today, include the planets Venus and Mars, Jupiter‘s moon Europa, and Saturn‘s moons Titan and Enceladus. 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”. 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.

No samples of extraterrestrial life have been found. However, various controversial claims have been made for evidence of extraterrestrial life. Beliefs that some unidentified flying objects are of extraterrestrial origin , along with claims of alien abduction, are dismissed by most scientists. Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes.

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.” Also, according to the response, there is “no credible information to suggest that any evidence is being hidden from the public’s eye.”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.”

Recent history

The failure of the SETI program to announce an intelligent radio signal after decades of effort has at least partially dimmed the prevailing optimism of the beginning of the space age. Notwithstanding, the unproven belief in extraterrestrial beings continues to be voiced in pseudoscience, conspiracy theories, and in popular folklore, notably “Area 51” and legends. It has become a pop culture trope given less-than-serious treatment in popular entertainment with e.g. the ALF TV series (1986–1990), The X-Files (1993–2002), etc.

The SETI program is not the result of a continuous, dedicated search, but instead utilizes what resources and manpower it can, when it can. Furthermore, the SETI program only searches a limited range of frequencies at any one time.

The Catholic Church has not made a formal ruling on the existence of extraterrestrials. However, writing in the Vatican newspaper, the astronomer, Father José Gabriel Funes, director of the Vatican Observatory near Rome, said in 2008 that intelligent beings created by God could exist in outer space.

Theoretical physicist Stephen Hawking in 2010 warned that humans should not try to contact alien life forms. He warned that aliens might pillage Earth for resources. “If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans,” he said. Anthropologist Jared Diamond has expressed similar concerns. Scientists at NASA and Penn State University published a paper in April 2011 addressing the question “Would contact with extraterrestrials benefit or harm humanity?” The paper describes positive, negative and neutral scenarios.

In May 2011, a planet in the Gliese system was found capable of sustaining life. Researchers believe Gliese 581 d, which orbits a red dwarf 20 light years away, not only exists in the “Goldilocks zone” where water can be present in liquid form, but is big enough to have a stable carbon dioxide atmosphere and “warm enough to have oceans, clouds, and rainfall,” according to France’s National Centre for Scientific Research.