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Extraterrestrial life refers to forms of life that may exist and originate outside of the planet Earth. Extraterrestrial life is currently a hypothetical notion - there is as yet no evidence of the existence of extraterrestrial life that has been widely accepted by scientists. Speculative forms of extraterrestrial life range from humanoid and monstrous beings seen in works of science fiction, to life at the much smaller scale of microbes and bacteria. Extraterrestrial life forms, especially intelligent ones, are often referred to in popular culture as aliens.
Possible basis and origins of extraterrestrial lifeAll life on earth is based on carbon and water, and this could also be true of other life forms elsewhere in the universe. However, many people believe that elements other than carbon might be capable of providing a basis for life. Silicon is usually considered the most likely alternative, though still improbable. Ammonia-based lifeforms are also considered, though less frequently. The scientific study of the possible biochemical basis for extraterrestrial life is often called xenobiology. Most scientists hold that if extraterrestial 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 single initial distribution of spores which provide the basis for living beings to develop. If true, this theory would suggest that life in various forms may exist throughout the universe. Silicon-based lifeSilicon-based life is regarded as improbable by most scientists. Superficially, the chemistries of carbon and silicon are similar; just as carbon can form methane (CH4), silicon can form silane (SiH4), and both elements can form long chains of polymers. But silicon's affinity for oxygen means that it cannot easily be used for respiration. Whereas CO2 is a gas that can easily be removed from the organism, SiO2 is a solid that will instantly organize itself into lattices, making it hard to dispose of. On top of that, silicon fails to give rise to many compounds that exhibit chirality, which is a common feature of carbon-based molecules that are essential to the proper functioning of enzymes. There is also astronomical evidence to suggest that silicon-based life is unlikely. Wherever astronomers have looked, they have failed to find the simplest precursors to silicon-based biochemistry. Complex carbon-based compounds are abundant in space, but in the case of silicon, most of what we have observed in space are simple oxides of silicon, with no record of more complex molecules such as silanes and silicones. Ammonia-based lifeMost life on Earth is based on water and its numerous chemical properties, and indeed a large portion of modern chemistry is devoted to the study of aqueous solutions. However, numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has some chemical similarities with water. Ammonia can dissolve most organic molecules at least as well as water does, and in addition it is capable of dissolving many elemental metals. Given this set of chemical properties it has been theorized that ammonia-based life forms might be possible. Beliefs in extraterrestrial lifeSome enthusiasts in the topic believe that extraterrestrial beings regularly visit or have visited the Earth. Some think that unidentified flying objects observed in the skies are in fact sightings of the spacecraft of intelligent extraterrestrials, and even claim to have met such beings. They may also attribute crop circle patterns to the action of extraterrestrials. However, mainstream scientific opinion finds all these hypotheses unsupported by strong evidence and extremely unlikely to be true. Scientific search for extraterrestrial lifeThe scientific search for extraterrestrial life is being carried out in two very different ways, directly and indirectly. Direct searchScientists are directly searching for evidence of unicellular life within the solar system, carrying out studies on the surface of Mars and examining meteors which have fallen to Earth. A mission is also proposed to Europa, one of Jupiter's moons with a liquid water layer under its surface, which might contain life. There is some limited evidence that microbial life might possibly exist or have existed 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. However, the lack of corroborating evidence from other experiments on the Viking indicates that a non-biological reaction is a more likely hypothesis. And 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. Indirect searchIt is theorised that any technological society in space will be transmitting information. Projects such as SETI are conducting an astronomical search for radio activity that would confirm the presence of intelligent life. 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 which would reveal key information such as 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. Dealing with extraterrestrial lifeIf intelligent extraterrestrial life is found and we are able to communicate with it, the people of the world and their governments will need to determine how to manage those interactions. The development of policy guidelines for dealing with extraterrestrial beings and territory has been considered by authors such as Michael Salla and Alfred Webre and termed exopolitics. See also
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