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Extraterrestrial life refers to theoretical forms of life that may exist and originate outside of the planet Earth.
Forms of extraterrestrial life, or "life on other planets", range from the humanoid and monstrous beings like those from science fiction works, to life at the level of microbes and bacteria. Since little potential evidence of life on other planets exists, and none which has been confirmed by science, the notion that extraterrestrial life exists is entirely theoretical.
Extraterrestrial life forms, especially intelligent ones, are often referred to as aliens.
Some theoreticists on the topic believe that life as we know it, namely carbon-based organisms, may not be the only structure upon which what we consider the concept of life (e.g. growth, consumption, reproduction, or even sentience) can be built.
The scientific study of extraterrestrial life is often called xenobiology.
Scientists are searching for extraterrestrial life in two very different ways, directly and indirectly.
They are directly searching for evidence of unicellular life within the solar system: searching Mars and examining meteors which have fallen to Earth, and a proposed mission to Europa, one of Jupiter's moons with a liquid water layer under its surface, which may contain life.
There is some suggestion of the existence of microbial life on Mars. An experiment on the Viking Mars lander reported gas emissions from heated Martian soil that some argue are consistent with the presence of microbes, though the lack of corroborating evidence from other experiments on the Viking indicates that a non-biological reaction is a more likely hypothesis. Independently, in 1996 structures resembling bacteria were reportedly discovered in a meteorite known to be formed of rock ejected from Mars. Again, this report is vigorously disputed.
It is theorised that any technological society in space will be transmitting information: man-made electromagnetic radiation is already detectable within an 80 light-year radius of Earth, and is constantly spreading. SETI, the Search for Extraterrestrial Intelligence, takes the data gathered by the world's largest radiotelescopes and analyses it for artificial patterns using supercomputers and one of the largest distributed computing projects in the world, SETI@home.
Some scientists believe that some UFOs are the spacecraft of intelligent extraterrestrials; however since these scientists are currently very much in the minority, work such as SETI continues in the hopes that a signal will be detected.
Astronomers also search for extrasolar planets that would be conducive to life. Current radiodetection methods have been inadequate for such a search, as the resolution afforded by recent technology is inadequate for detailed study of extrasolar planetary objects. Future telescopes should be able to image planets around nearby stars, which may reveal the presence of life (either directly or through spectrography revealing, for instance, the presence of free oxygen in a planet's atmosphere). It has been argued that one of the best candidates for the discovery of life-supporting planets may be Alpha Centauri, the closest star system to Earth.
Numerous hypotheses have been developed to validate the possibilty that extraterrestrial life could exist. (This is in contrast to most theories, which attempt to explain something that is commonly accepted as occuring.)
One such theory is panspermia, which holds that all life throughout the universe stems from one initial distribution of spores which consitute the seeds of life. If true, it would then follow that life is prevalent through space as these spores have traveled, and that life in various forms may exist throughout the universe.
Remote viewing is claimed to be a scientific method to gather data about extraterrestrial life. Dr. Courtney Brown, an Associate professor of political science at Emery University, wrote two controversial books about data that he gathered through remote viewing sessions.
If extraterrestrial life is found, some argue there will be a need to determine how we should interact with the life forms, and what policy should be taken by the governments of Earth in dealing with them. The development of policy guidelines for dealing with extraterrestrial beings and territory has been termed exopolitics by authors such as Michael Salla and Alfred Webre.
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