Extraterrestrial life, Where "extraterrestrial" is derived from the Latin extra ("beyond", "not of") and terrestris ("of Earth", "belonging to Earth").</ref> also called alien life (or, if it is a sentient or relatively complex individual, an "extraterrestrial" or "alien"), is life that does not originate from Earth. These hypothetical life forms may range from simple single-celled organisms to beings with civilizations far more advanced than humanity. Although many scientists expect extraterrestrial life to exist in some form, there is no evidence for its existence to date. The Drake equation speculates about the existence of intelligent life elsewhere in the universe. The science of extraterrestrial life in all its forms is known as exobiology.
- [[Paul Davies |Davies, Paul]] (18 November 2013). "Are We Alone in the Universe?". New York Times. https://www.nytimes.com/2013/11/19/opinion/are-we-alone-in-the-universe.html. Retrieved 20 November 2013.
- Pickrell, John (4 September 2006). "Top 10: Controversial pieces of evidence for extraterrestrial life". New Scientist. http://www.newscientist.com/article/dn9943-top-10-controversial-pieces-of-evidence-for-extraterrestrial-life.html. Retrieved 18 February 2011.