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Extraterrestrial life〔Where "extraterrestrial" is derived from the Latin ''extra'' ("beyond", "not of") and ''terrestris'' ("of Earth", "belonging to Earth").〕 is life that does not originate from Earth. It is also called alien life, or, if it is a sentient and/or relatively complex individual, an "extraterrestrial" or "alien" (or, to avoid confusion with the legal sense of "alien", a "space alien"). These as-yet-hypothetical life forms range from simple bacteria-like organisms to beings with civilizations far more advanced than humanity. Although many scientists expect extraterrestrial life to exist, so far no unambiguous evidence for its existence exists. The science of extraterrestrial life is known as exobiology. The science of astrobiology also considers life on Earth as well, and in the broader astronomical context. Meteorites that have fallen to Earth have sometimes been examined for signs of microscopic extraterrestrial life. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia, when the young Earth was about 400 million years old.〔 Early edition, published online before print.〕 According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe."〔 Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial intelligence, 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. Over the years, science fiction works, especially Hollywood's involvement, has increased the public's interest in the possibility of extraterrestrial life. Some encourage aggressive methods to try to get in contact with life in outer space, whereas others argue that it might be dangerous to actively call attention to Earth.〔(BBC News – Scientists in US are urged to seek contact with aliens )〕〔Baum, Seth; Haqq-Misra, Jacob; Domagal-Goldman, Shawn. ("Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis" ), ''Acta Astronautica, 2011, 68 (11–12):2014–2129'', April 22, 2011, accessed August 18, 2011.〕 ==Background== Alien life, such as microorganisms, 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. This argument is embodied in the Copernican principle, which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle, which states that there is nothing special about life on Earth. The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old. Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread—by meteoroids, for example—between habitable planets in a process called panspermia. In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth. According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars.〔 Since the 1950s, scientists have argued the idea that "habitable zones" around stars are the most likely places to find life. Numerous discoveries in these zones since 2007 have generated estimations of frequencies of Earth-like planets —in terms of composition— numbering in the many billions though as of 2013, only a small number of planets have been discovered in these zones. Nonetheless, on November 4, 2013, astronomers reported, based on ''Kepler'' space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way, 11 billion of which may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists.〔〔 Astrobiologists have also considered a "follow the energy" view of potential habitats. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Extraterrestrial life」の詳細全文を読む スポンサード リンク
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