Extraterrestrial life - Misplaced Pages

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Extraterrestrial life refers to theoretical forms of life that may exist and originate outside of the planet Earth.

Speculative 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 hypothetical.

Extraterrestrial life forms, especially intelligent ones, are often referred to as aliens.

Possible forms of extraterrestrial life

All life on earth is based on carbon and water. A lot of people believe that carbon may not be the only element capable of accommodating life. Silicon is usually considered the most likely alternative. Ammonia-based lifeforms are also, though less frequently, considered.

The scientific study of extraterrestrial life is often called xenobiology.

Silicon-based life

Silicon-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, while 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 Life

Most 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, a lot of chemical reactions exist that take place in an ammonia solution: liquid ammonia has some chemical similarities with water, and as such it has been theorized that ammonia-based life forms might be possible. Ammonia can dissolve most organic molecules at least as well as water does, and in addition it is capable of dissolving many elemental metals.

Search for extraterrestrial life

Scientists are searching for extraterrestrial life in two very different ways, directly and indirectly.

Direct search

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.

Indirect search

It is theorised that any technological society in space will be transmitting information: man-made |UFO]]s may be 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.

Origins of extraterrestrial life

Numerous hypotheses have been developed to validate the possibility that extraterrestrial life could exist. (This is in contrast to most theories, which attempt to explain something that is commonly accepted as occurring.)

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.

Dealing with extraterrestrial life

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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