Extraterrestrial life.

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Extraterrestrial life is life that may exist and originate outside the planet Earth, the only place in the universe currently known by humans to support life.

Its existence is currently purely hypothetical as there is yet no evidence of any planets that can support life, or actual extraterrestrial life that has been widely accepted by the scientific community.

Most scientists hold that if extraterrestrial life exists, its evolution would have occurred independently in different places in the universe.

An alternative hypothesis, held by a minority, is panspermia, which suggests that life in the universe could have stemmed from a smaller number of points of origin, and then spread across the universe, from habitable planet to habitable planet.

These two hypotheses are not mutually exclusive.

Note:   The above text is excerpted from the Wikipedia article "Extraterrestrial life", which has been released under the GNU Free Documentation License.