Extraterrestrial life, or alien life (colloquially, alien), is life which does not originate from Earth. No extraterrestrial life has yet been scientifically conclusively detected. Such life might range from simple forms such as prokaryotes to intelligent beings, possibly bringing forth civilizations that might be far more advanced than humans.[1][2][3] The Drake equation speculates about the existence of sapient life elsewhere in the universe. The science of extraterrestrial life is known as astrobiology.
Speculation about the possibility of inhabited worlds beyond Earth dates back to antiquity. Early Christian writers discussed the idea of a "plurality of worlds" as proposed by earlier thinkers such as Democritus; Augustine references Epicurus's idea of innumerable worlds "throughout the boundless immensity of space" in The City of God.[4]
Pre-modern writers typically assumed extraterrestrial "worlds" are inhabited by living beings. William Vorilong, in the 15th century, acknowledged the possibility Jesus could have visited extraterrestrial worlds to redeem their inhabitants.[5] Nicholas of Cusa wrote in 1440 that Earth is "a brilliant star" like other celestial objects visible in space; which would appear similar to the Sun, from an exterior perspective, due to a layer of "fiery brightness" in the outer layer of the atmosphere. He theorised all extraterrestrial bodies could be inhabited by men, plants, and animals, including the Sun.[6] Descartes wrote that there was no means to prove the stars were not inhabited by "intelligent creatures", but their existence was a matter of speculation.[7]
When considering the atmospheric composition and ecosystems hosted by extraterrestrial bodies, extraterrestrial life can seem more speculation than reality, due to the harsh conditions and disparate chemical composition of the atmospheres,[8] when compared to the life-abundant Earth. However, there are many extreme and chemically harsh ecosystems on Earth that do support forms of life and are often hypothesized to be the origin of life on Earth. Hydrothermal vents,[9] acidic hot springs,[10] and volcanic lakes[11] are examples of life forming under difficult circumstances, provide parallels to the extreme environments on other planets and support the possibility of extraterrestrial life.
Since the mid-20th century, active research has taken place to look for signs of extraterrestrial life, encompassing searches for current and historic extraterrestrial life, and a narrower search for extraterrestrial intelligent life. Depending on the category of search, methods range from analysis of telescope and specimen data[12] to radios used to detect and transmit communications.[13]
The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, especially extraterrestrials in fiction. Science fiction has communicated scientific ideas, imagined a range of possibilities, and influenced public interest in and perspectives on extraterrestrial life. One shared space is the debate over the wisdom of attempting communication with extraterrestrial intelligence. Some encourage aggressive methods to try to contact intelligent extraterrestrial life. Others – citing the tendency of technologically advanced human societies to enslave or destroy less advanced societies – argue it may be dangerous to actively draw attention to Earth.[14][15]