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Extraterrestrial life

The Allen Telescope Array (ATA), CaliforniaThe Allen Telescope Array (ATA), California Extraterrestrial life is any life that does not originate from Earth. None has so far been discovered. Often described as alien life, or aliens, extraterrestrial life might range from simple, bacteria-like organisms (living things) to intelligent, complex life-forms—perhaps even more advanced than humans. The search for other worlds that could host life ranges from our neighbouring planet Mars, to more distant moons such as Europa or Titan, or planets that have been discovered in other solar systems many light years away.

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Conditions for life

Most scientists assume that extraterrestrial life will probably have the same chemical composition as organisms (living things) on Earth. Carbon is the one element that can form the wide range of complex molecules necessary to create living cells. The presence of liquid water provides the essential solution in which such molecules form.

There also needs to be a source of energy, providing food for the organism. It was once thought that this energy must come direct from the Sun. Plants use sunlight to produce sugars from carbon dioxide and water—and plants, both on land and in water, are the vital link in any food chain. Now it is known that certain deep-sea organisms rely for their food on an energy source other than the Sun.

Six elements—carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur, together known as CHNOPS—form the basic chemical make-up of nearly all life on Earth. Life elsewhere in space would probably be based on these same elements.

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