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This is from a scientific point of view. For aliens in popular culture see extraterrestrial life (popular culture). For other uses, see Extraterrestrial (disambiguation).

Extraterrestrial life is life that may exist and originate outside the planet Earth, the only place in the Universe known to support life. Its existence is currently hypothetical; there is as yet no evidence of extraterrestrial life that has been widely accepted by scientists.

Most scientists hold that if extraterrestrial 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 smaller number of points of origin, and then spread across the universe, from habitable planet to habitable planet. These two hypotheses are not mutually exclusive.

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 .

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 astrobiology or xenobiology.

Possible basis of extraterrestrial life

Biochemistry

Main articles: Biochemistry and Alternative biochemistry

All life on Earth is based on the building block element carbon with water as the solvent in which bio-chemical reactions take place.

  • Water is useful because it has a neutral pH, and due to its continued dissociation between hydroxyl and hydrate ions.

2H2O --> H3O+ + OH-

As a result, it can dissolve both positive metallic ions and negative non metallic ions with equal ability. Furthermore, the fact that organic molecules can be either hydrophobic (repelled by water) or hydrophilic (soluble in water) creates the ability of organic compounds to orient themselves to form water enclosing membranes. The fact that solid water (ice) is less dense than liquid water also means that ice floats, thereby preventing the Earth's oceans from slowly freezing solid. Additionally, the Van der Waals forces between water molecules gives it an ability to store energy with evaporation, which upon condensation is released. This helps moderate climate, cooling the tropics and warming the poles, helping to maintain a thermodynamic stability needed for life.

6CO2 + 6H2O + sun energy --> C6H12O6 + 6O2
C6H12O6 + 6O2 --> 6CO2 + 6H2O + biochemical energy

The ability to form organic acids (-COOH) and amine bases (NH2-) gives it the possibility of neutralisation dehydrating reactions to build long polymer peptides and catalytic proteins from monomer amino acids, and with phosphates to build not only DNA, the information storing molecule of inheritance, but also adenosine triphosphate (ATP) the principle energy "currency" of cellular life.

Given their relative abundance and usefulness in sustaining life it has long been assumed that life forms elsewhere in the universe will also utilize these basic components. However, other elements and solvents might be capable of providing a basis for life. Silicon is usually considered the most likely alternative to carbon, though this remains improbable. Silicon life forms are proposed to have a crystalline morphology, and are theorized to be able to exist in high temperatures, such as planets closer to the sun. Life forms based on ammonia rather than water are also considered, though less frequently.

Indeed, technically life is little more than any self-replicating reaction, which could arise in a great many conditions and with various ingredients, though carbon-oxygen within the liquid temperature range of water seems most conductive. Suggestions have even been made that self-replicating reactions of some sort could occur within the plasma of a star, though it would be highly unconventional, since plasma is essentially the fourth state of matter, where electrons are not bound in their orbits around atomic nucliae.

Theoretical evolution and morphology

Along with the biochemical basis of extraterrestrial life, there remains a broader consideration of evolution and morphology. What might an alien look like? Science fiction has long shown a bias towards humanoid or (often in the case of villains) reptilian forms. The classical alien is light green or grey skinned, with a large head, and the typical four limb and two to five digit structure—i.e., it is fundamentally humanoid with a large brain to indicate great intelligence. Other subjects from animal mythos such as felines and insects have also featured strongly in fictional representations of aliens.

In considering the subject more seriously, a useful division has been suggested between universal and parochial characteristics. Universals are features which have evolved independently more than once on Earth (and thus presumably are not difficult to develop) and are so intrinsically useful that species will inevitably tend towards them. These include flight, sight, photosynthesis and limbs, all of which have evolved several times here on Earth with differing materialization. There are a huge variety of eyes, for example, many of which have radically different working schematics as well as different visual foci: the visual spectrum, infrared, polarity and echolocation. Parochials, by contrast, are essentially arbitrary evolutionary forms which often serve little utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. Parochials include the five digits of mammals, the genitalia and sexual mechanics of animals, as well as the curious and often fatal conjunction of the feeding and breathing passages found within many animals.

A consideration of which features are ultimately parochial challenges many taken for granted notions about morphological necessity. Skeletons, in some form, are likely to be replicated elsewhere, yet the vertebrate spine—while a profound development on Earth—is just as likely to be unique. Similarly, it is reasonable to expect some type of egg laying amongst off-Earth creatures but the mammary glands which set apart mammals may be a singular case.

The assumption of radical diversity amongst putative extraterrestrials is by no means settled. While many exobiologists do stress that the enormously heterogeneous nature of Earth life foregrounds even greater variety in space, others point out that convergent evolution dictates substantial similarities between Earth and off-Earth life. These two schools of thought are called "divergionism" and "convergionism", respectively *.

Beliefs in extraterrestrial life

Ancient and Early Modern ideas

Belief in extraterrestrial life may have been present in ancient Egypt, Babylon, and Sumer, although in these societies, cosmology was fundamentally supernatural and the notion of aliens is difficult to distinguish from that of gods, demons, and such. The first important Western thinkers to argue systematically for a universe full of other planets and, therefore, possible extraterrestrial life were the ancient Greek writers Thales and his student Anaximander in the 7th and 6th centuries B.C. The atomists of Greece took up the idea, arguing that an infinite universe ought to have an infinity of populated worlds. Ancient Greek cosmology worked against the idea of extraterrestrial life in one critical respect, however: the geocentric universe, championed by Aristotle and codified by Ptolemy, privileged the Earth and Earth-life (Aristotle denied there could be a plurality of worlds) and seemingly rendered extraterrestrial life impossible.

Ancient Jewish sources also considered extraterrestrial life. The Talmud states that there are at least 18,000 other worlds, but provides little elaboration on the nature of the worlds and on whether they are physical or spiritual. Based on this, however, the medieval exposition "Sefer HaB'rit" posits that extraterrestrial creatures exist but that they have no free will (and are thus equivalent to animal life). It adds that human beings should not expect creatures from another world to resemble earthly life, any more than sea creatures resemble land animals. [http://www.torah.org/features/secondlook/extraterrestrial.html

Within Islam, the statement of the Qur'an "All praise belongs to Allah, Lord of all the worlds" indicates multiple universal bodies that may indicate extraterrestrial life.

When Christianity spread throughout the West, the Ptolemaic system became dogma, and although the Church never issued any formal pronouncement on the question of alien life * at least tacitly the idea was heretical. In 1277 the Bishop of Paris, Étienne Tempier, did overturn Aristotle on one point: God could have created more than one world (given His omnipotence) yet we know by revelation He only made one. Taking a further step and arguing that aliens actually existed remained rare.

This situation changed, however, with the dramatic shift in thinking initiated by the invention of the telescope and the Copernican assault on geocentric cosmology. Once it became clear that the Earth was merely one planet amongst countless bodies in the universe the extraterrestrial idea moved towards the scientific mainstream. God's omnipotence, it could be argued, not only allowed for other worlds and other life, on some level it necessitated them. The best known early-modern proponent of such ideas was Giordano Bruno, who argued in the 16th century for an infinite universe in which every star is surrounded by its own solar system; he was eventually burned at the stake for heretical ideas. In the early 17th century the Czech astronomer Anton Maria Schyrleus of Rheita mused that "if Jupiter has…inhabitants…they must be larger and more beautiful than the inhabitants of the Earth, in proportion to the of the two spheres."[http://cosmovisions.com/Rheita.htm

Such comparisons also appeared in poetry of the era. In "The Creation: a Philosophical Poem in Seven Books" (1712) Sir Richard Blackmore observed: "We may pronounce each orb sustains a race / Of living things adapted to the place". The didactic poet Henry More took up the classical theme of the Greek Democritus in "Democritus Platonissans, or an Essay Upon the Infinity of Worlds" (1647). With the new relative viewpoint that the Copernican revolution had wrought, he suggested "our world's sunne / Becomes a starre elsewhere." Fontanelle's "Conversations on the Plurality of Worlds" (translated into English in 1686) offered similar excursions on the possibility of extraterrestrial life, expanding rather than denying the creative sphere of a Maker.

The possibility of Extraterrestrials remained a widespread speculation as scientific discovery accelerated. William Herschel, the discoverer of Uranus, was one of many 18th-19th century astronomers convinced that our Solar System, and perhaps others, would be well populated by alien life. Other luminaries of the period who championed "cosmic pluralism" included Immanuel Kant and Benjamin Franklin. At the height of the Enlightenment even the Sun and Moon were considered candidates for hosting aliens.

Extraterrestrials and the Modern era

This enthusiasm towards the possibility of alien life continued well into the 20th century. Indeed, the roughly three centuries from the Scientific Revolution through the beginning of the modern era of solar system probes were essentially the zenith for belief in extraterrestrials in the West: many astronomers and other secular thinkers, at least some religious thinkers, and much of the general public were largely satisfied that aliens were a reality. This trend was finally tempered as actual probes visited potential alien abodes in the solar system. The moon was decisively ruled out as a possibility, while Venus and Mars, long the two main candidates for extraterrestrials, showed no obvious evidence of current life. The other large moons of our system which have been visited appear similarly lifeless, though interesting geothermic forces observed (Io's volcanism, Europa's ocean, Titan's thick atmosphere) has underscored how broad the range of potentially habitable environments may be. Finally, the failure of the SETI program to detect anything resembling an intelligent radio signal after four decades of effort has partially dimmed the optimism that prevailed at the beginning of the space age and emboldened critics who view the search for extraterrestrials as unscientific, despite the fact the SETI program is not the result of a continuous, dedicated search but instead utilizes what resources and manpower it can, when it can. *

Thus, the three decades preceding the turn of the second millennium saw a crossroads reached in beliefs in alien life. The prospect of ubiquitous, intelligent, space-faring civilizations in our solar system appears increasingly dubious to many scientists ("All we know for sure is that the sky is not littered with powerful microwave transmitters" in the words of SETI's Frank Drake), who has also noted that it's entirely possible advanced technology results in communication being carried out in some way other than conventional radio transmission. At the same time, the data returned by space probes and giant strides in detection methods have allowed science to begin delineating habitability criteria on other worlds and to confirm that, at least, other planets are plentiful though aliens remain a question mark.

Amongst the general public belief and interest in extraterrestrials remains high and skepticism towards galaxy-exploring alien civilizations is not shared by many individuals. 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. Crop circle patterns have also been attributed to the actions of extraterrestrials, although many were later found to be hoaxes. While at least one recent scientific paper published in a respected, peer-reviewed journal has urged a re-evaluation 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.

The possible existence of primitive (microbial) life outside of Earth is much less controversial to mainstream scientists although at present no direct evidence of such 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 remain in debate.

Scientific search for extraterrestrial life

The scientific search for extraterrestrial life is being carried out in two 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 that 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. Recently, Circadian rhythms have been allegedly discovered in Viking data. The interpretation is controversial. Independently in 1996 structures resembling bacteria were reportedly discovered in a meteorite, ALH84001, thought to be formed of rock ejected from Mars. This report is also controversial and scientific debate continues. (See Viking biological experiments.)

In February 2005, 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 assertions (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 conducted at the conference in which the European Space Agency presented its findings, 75 percent of the scientists in attendance reported to believe that life once existed on Mars; 25 percent reported a belief that life currently exists there (Michelson, 2005).

Recently, scientists have discovered mysterious red cells in India unlike any cells on Earth; this may be the first confirmed case of alien life. * 2006

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. A related suggestion is that aliens might broadcast pulsed and continuous laser signals in the optical as well as infrared spectrum *; laser signals have the advantage of not "smearing" in the interstellar medium and may prove more conducive to communication between the stars.

Extrasolar planets

Astronomers also search for extrasolar planets that would be conducive to life, especially those like OGLE-2005-BLG-390Lb which have been found to have Earth-like qualities. 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). Darwin is an ESA mission designed to find Earth-like planets, and analyse their 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, given that two of the three stars in the system are broadly sun-like.

Extraterrestrial life in the Solar System

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

  • Venus - Carbonyl sulfide was recently discovered in Venus' atmosphere, which is suggestive of life.
  • Mars - Liquid water is known to have existed on Mars in the past and there may still be liquid water beneath the surface. Recently, methane was found in the atmosphere of Mars. Main article: Life on Mars
  • Titan - Saturn's largest moon - The only known moon with a significant atmosphere was recently visited by the Huygens probe. Latest discoveries indicate that there is no global or widespread ocean, but small and/or seasonal liquid hydrocarbon lakes are still possible.
  • Europa - Jupiter's fourth largest moon - Likely to have a salt ocean under a thick ice crust. If the moon is host to life, many expect to find it living in habitats similar to the hydrothermal vents of Earth. Moreover, astrobiologists are hopeful that we could find aerobic life, living off oxygen brought down into the ocean through the churning of the moon's icy surface.
  • Enceladus - Saturn's sixth largest moon - Geologically active, with liquid water and geysers known to exist beneath its south pole. *

Numerous other bodies have been suggested as potential hosts for microbial life. For example, atmospheric life has been hypothesised on Venus and the gas giants. Fred Hoyle has proposed that life might exist on comets, as some Earth microbes managed to survive on a lunar probe for many years. However, it is considered highly unlikely that complex multicellular organisms of the conventional chemistry of terrestrial life (animals, plants) could exist under these living conditions.

See also

References

Related books and media

External links

Astrobiology | Extraterrestrials

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