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 as-yet-hypothetical life forms may range from simple bacteria-like organisms to beings with civilizations far more advanced than humanity. Although many scientists expect extraterrestrial life to exist, there is no unambiguous evidence for its existence so far. The science of extraterrestrial life is known as exobiology.
The science of astrobiology considers life on Earth as well, and in the broader astronomical context. 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. 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 have 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.
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