Alien (extraterrestrial)

Extraterrestrial life, 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 prokaryotes to beings with civilizations far more advanced than humanity. 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.

Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial life. The search encompasses a search for current and historic extraterrestrial life, and a narrower search for extraterrestrial intelligent life. Reflecting the category of search, methods range from the analysis of telescope and specimen data to radios used to detect and send signals of communication.

The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, chiefly including works of science fiction. Over the years, science fiction both communicated scientific ideas and influenced public interest and perspectives of extraterrestrial life. One shared space is the debate over the wisdom of attempting communication with possible extraterrestrial intelligence: Some encourage aggressive methods to try for contact with intelligent extraterrestrial life, while others argue that it may be dangerous to actively call attention to Earth.

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, as well as well-regarded thinkers, such as Winston Churchill, 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.

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