Extraterrestrial life

This article is about the scientific study of extraterrestrial life, for treatment in popular culture, see Extraterrestrial life in popular culture.

Extraterrestrial life is life that may exist and originate outside our planet Earth. Its existence is currently hypothetical: there is as yet no evidence 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 bacteria and viruses.

Extraterrestrial life forms, especially intelligent ones, are often referred to in popular culture as aliens or ETs. The putative study and theorisation of ET life is known as exobiology.


Possible basis and origins of extraterrestrial life

All life on earth is based on carbon and water, and this could also be true of life forms elsewhere in the universe. However, other elements might be capable of providing a basis for life (See also: Non-carbon biology). 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 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, 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.

There are examples of silicon based life in science fiction, such as an episode of the original Star Trek series, which included a silicon life form called the Horta or the Chenjesu and Taalo in the science-fiction adventure game Star Control II.

Ammonia-based life

All 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.

On the other hand, ammonia does have some problems as a basis for life. The heat of vaporization of ammonia is half that of water and its surface tension three times smaller. This means that hydrogen bonds between ammonia molecules will always be much weaker than those in water, so ammonia is less able to concentrate non-polar molecules through a hydrophobic effect. For this reason, mainstream science questions how well ammonia could hold prebiotic molecules together in order to allow the emergence of a self-reproducing system.

A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual for terrestrial life. Terrestial life usually exists within the melting point and boiling point of water at normal pressure, between 0C (273 K) and 100C (373 K); at normal pressure ammonia's melting and boiling points are between −78C (195 K) and −33C (240 K). Problems with biospheres at extremely cooled temperatures are that biochemical reactions are slowed down tremendously as well as some biochemicals may precipitate out of solution due to high melting points. Ammonia could be a liquid at normal temperatures but at much higher pressures, for example at 60atm ammonia boils at 98C and melts at −77C.

Ammonia-water mixed solution could provide conditions for a water like biochemistry at lower temperatures then water normally supports. And such conditions could exist under the surface of the Saturn's largest moon Titan [1] (http://www.es.ucl.ac.uk/research/planetaryweb/postgraduate/olddomweb/titan.pdf).

Beliefs in extraterrestrial life

Belief in extraterrestrial life may have been present in ancient Egypt, Babylon and Sumer. The first important Western thinker to hit upon the idea of inhabited worlds was the ancient Greek writer Thales and his student Anaximander in the 7th century BC. The atomists of Greece took up the idea, arguing that an infinite universe ought to have an infinity of populated worlds. The cosmology of Aristotle (which placed the Earth at the center of the universe) seemed to work against the idea of extraterrestrial life and when Christianity spread through the West the idea became a heresy. The best known pre-modern proponent of extra-solar planets and widespread life off Earth was Giordano Bruno who was burned at the stake for this and other unorthodox ideas in 1600.

At present, some 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. Some also attribute crop circle patterns to the action of extraterrestrials.

While at least one recent scientific paper published in a respected, peer-reviewed journal has urged a reevaluation of the UFO phenomenon (Deardorff et al., 2005), as of this time mainstream scientific opinion holds that such claims are unsupportable by the evidence currently available and unlikely to be true.

However, the general existence of extraterrestrial life, outside of the UFO phenomenon, is considered less controversial. Many mainstream scientists believe that, given the nature of natural selection, and the enormous number stars in the universe, primitive extraterrestrial life is highly probable, and intelligent life on other planets is not at all unlikely, although at present no direct evidence of such extraterrestrial life has been found. Indirect evidence has been offered for the current existence of primitive life on the planet Mars; however, the conclusions that should be drawn from such evidence remains in debate.

Scientific search for extraterrestrial life

The scientific search for extraterrestrial life is being carried out in two very different ways, directly and indirectly.

Direct search

Scientists 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, ALH84001, known to be formed of rock ejected from Mars. Again, this report is vigorously disputed.

In February 2005, two NASA scientists reported that they had found strong evidence of present life on Mars (Berger, 2005). The two scientists, Carol Stoker and Larry Lemke of NASA’s Ames Research Center, based their claims on methane signatures found in Mars’ atmosphere that resemble the methane production of some forms of primitive life on Earth, as well as their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon denied the scientists’ claims, and Stoker herself backed off from her initial assertations (spacetoday.net, 2005). However, only a few days after Stoker and Lemke made their claims, scientists from the European Space Agency reported that their own measurements of methane on Mars suggested an organic origin (Michelson, 2005).

Though such findings are still very much in debate, support among scientists for the belief in the existence of life on Mars seems to be growing. In an informal survey of scientists attending the conference at which the European Space Agency presented its findings, 75 percent of the scientists at the conference reported to believe that life once existed on Mars; 25 percent reported a belief that life currently exists there (Michelson, 2005).

Extraterrestrial life in the Solar System

Many bodies in the Solar System have been suggested as being likely to contain life. The most commonly suggested ones are listed below; of these, four of the five are moons thought to have large bodies of underground liquid, and life may have evolved there in a similar fashion to deep sea vents.

  • Mars - The best known, and most earthlike of the other planets and moons in the Solar system.
  • Titan - Only known moon with an atmosphere. Recently visited by the Huygens probe. May have ocean.
  • Europa - May have ocean.
  • Ganymede
  • Enceladus - May have liquid water beneath surface. [2] (http://www.newscientist.com/article.ns?id=dn7159)

Numerous other bodies have been suggested as potentially life-bearing. For example, atmospheric life has been hypothesised on Venus and the gas giants. Fred Hoyle also proposed that microbial life may exist on comets. Some Earth microbes also managed to survive on a lunar probe for some years. It is considered highly unlikely that complex multicellular organisms exist in any of these places.

Indirect search

It 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 life

If intelligent extraterrestrial life is found and it is possible 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

External links


da:Liv i rummet de:Auerirdischer es:Extraterrestre ja:地球外生命 pl:Życie pozaziemskie


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