Extraterrestrial life is defined as life that does not originate from Earth. It is unknown whether any such life exists or ever existed in the past, although many scientists think it likely that on Mars, for instance, life either exists or has existed. A less direct argument for the existence of extraterrestrial life relies on the vast size of the observable universe. According to this argument, endorsed by Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.

Suggested locations on which life might have developed, or which might continue to host life today, include the planets Venus and Mars; moons of Jupiter and Saturn such as Europa, Enceladus and Titan; and extrasolar planets such as Gliese 581 c and d, recently discovered to be near Earth mass and apparently located in their star's habitable zone, with the potential to have liquid water.

Several theories have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological viewpoint. Alien life, such as bacteria, has been theorized by scientists such as Carl Sagan to exist in the Solar System and quite possibly throughout the universe. No samples have been found, although there is some controversy about possible traces of life in Martian material, of which the most famous are on the Allan Hills 84001 meteorite.


All life on Earth requires carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus as well as numerous other elements in smaller amounts, notably minerals; it also requires water as the solvent in which biochemical reactions take place. Sufficient quantities of carbon and the other major life-forming elements, along with water, may enable the formation of living organisms on other planets with a chemical make-up and average temperature similar to that of Earth. Because Earth and other planets are made up of "stardust", i.e. relatively abundant chemical elements formed from stars which have ended their lives as supernovae, it is very probable that other planets may have been formed by elements of a similar composition to the Earth's. The combination of carbon, hydrogen and oxygen in the chemical form of carbohydrates (e.g. sugar) can be a source of chemical energy on which life depends, and can also provide structural elements for life (such as ribose, in the molecules DNA and RNA, and cellulose in plants). Plants derive energy through the conversion of light energy into chemical energy via photosynthesis. Life, as currently recognized, requires carbon in both reduced (methane derivatives) and partially-oxidized (carbon oxides) states. It also appears to require nitrogen as a reduced ammonia derivative in all proteins, sulfur as a derivative of hydrogen sulfide in some necessary proteins, and phosphorus oxidized to phosphates in genetic material and in energy transfer. Adequate water as a solvent supplies adequate oxygen as constituents of biochemical substances.

Pure water is useful because it has a neutral pH due to its continued dissociation between hydroxide and hydronium ions. 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 Earth's oceans from slowly freezing. Without this quality, the oceans could have frozen solid during the Snowball Earth episodes. Additionally, the hydrogen bonds between water molecules give it an ability to store energy with evaporation, which upon condensation is released. This helps to moderate the climate, cooling the tropics and warming the poles, helping to maintain the thermodynamic stability needed for life.

Carbon is fundamental to terrestrial life for its immense flexibility in creating covalent chemical bonds with a variety of non-metallic elements, principally nitrogen, oxygen and hydrogen. Carbon dioxide and water together enable the storage of solar energy in sugars, such as glucose. The oxidation of glucose releases biochemical energy needed to fuel all other biochemical reactions.

The ability to form organic acids (–COOH) and amine bases (–NH2) gives rise to the possibility of neutralization 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 ATP (the principal energy "currency" of cellular life).

Due to their relative abundance and usefulness in sustaining life, many have hypothesized that lifeforms elsewhere in the universe would also utilize these basic materials. However, other elements and solvents could also provide a basis for life. Silicon is most often deemed to be the probable alternative to carbon. Silicon lifeforms are proposed to have a crystalline morphology, and are theorized to be able to exist in high temperatures, such as on planets which are very close to their star. Life forms based in ammonia (rather than water) have also been suggested, though this solution appears less optimal than water.

When looked at from a chemical perspective, life is fundamentally a self-replicating reaction, but one which could arise under a great many conditions and with various possible ingredients, though carbon-oxygen within the liquid temperature range of water seems most conducive. 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.

Several pre-conceived ideas about the characteristics of life outside of Earth have been questioned. For example, NASA scientists believe that the color of photosynthesizing pigments on extrasolar planets might not be green.

Evolution and morphology

In addition to the biochemical basis of extraterrestrial life, many have also considered evolution and morphology. Science fiction has often depicted extraterrestrial life with humanoid and/or reptilian forms. Aliens have often been depicted as having light green or grey skin, with a large head, as well as four limbs—i.e. fundamentally humanoid. Other subjects, such as felines and insects, etc., have also occurred in fictional representations of aliens.

A division has been suggested between universal and parochial characteristics. Universals are features which are thought to have evolved independently more than once on Earth (and thus, presumably, are not too 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 are thought to have evolved several times here on Earth. There is a huge variety of eyes, for example, and many of these have radically different working schematics and different visual foci: the visual spectrum, infrared, polarity and echolocation. Parochials, however, are essentially arbitrary evolutionary forms. These often have little inherent utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. Intelligent aliens could communicate through gestures, as deaf humans do, or by sounds created from structures unrelated to breathing, which happens on Earth when, for instance, cicadas vibrate their wings, or crickets rub their legs.

Attempting to define parochial features challenges many taken-for-granted notions about morphological necessity. Skeletons, which are essential to large terrestrial organisms according to the experts of the field of gravitational biology, are almost assured to be replicated elsewhere in one form or another. Many also conjecture as to some type of egg-laying amongst extraterrestrial creatures, but mammalian mammary glands might 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 life on Earth foregrounds an even greater variety in space, others point out that convergent evolution may dictate substantial similarities between Earth and extraterrestrial life. These two schools of thought are called "divergionism" and "convergionism" respectively.[7]

Beliefs in extraterrestrial life

Ancient and medieval ideas

The first important Western thinkers to argue systematically for a universe full of other planets and, therefore, possible extraterrestrial life were the ancient Greek writer Thales and his student Anaximander in the 7th and 6th centuries B.C.[citation needed] The atomists of Greece like Epicurus 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, it favored the Earth and Earth-life (Aristotle denied that there could be a plurality of worlds) and seemingly rendered extraterrestrial life philosophically untenable. Lucian of Samosata, in his novels, described inhabitants of the Moon and other celestial bodies as humanoids, but significantly different from humans.

Authors of 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 those worlds, or on whether they are physical or spiritual. Based on this, however, the 18th century exposition "Sefer HaB'rit" posits that extraterrestrial creatures exist, and that some may well possess intelligence. It adds that humans should not expect creatures from another world to resemble earthly life any more than sea creatures resemble land animals.

According to Hindu scriptures, there are innumerable universes to facilitate the fulfillment of the separated desires of innumerable living entities. However, the purpose of such creations is to bring back the deluded souls to correct understanding about the purpose of life. Aside from the innumerable universes which are material, there is also the unlimited spiritual world, where the purified living entities live with perfect conception about life and ultimate reality. The spiritually aspiring saints and devotees, as well as thoughtful men of the material world, have been getting guidance and help from these purified living entities of the spiritual world from time immemorial. However, the relevance of such descriptions has to be evaluated in the context of a correct understanding of geography and science at those times.

Within Islam, the statement of the Qur'an "All praise belongs to God, Lord of all the worlds" indicates multiple universal bodies, and maybe even multiple universes, which may indicate extraterrestrial and even extradimensional life. Surat Al-Jinn also mentioned a statement from a Jinn regarding the current status and ability of his group in the heavens.

According to Ahmadiyya Islam a more direct reference from the Quran is presented by Mirza Tahir Ahmad as a proof that life on other planets may exist according to the Quran. In his book, Revelation, Rationality, Knowledge & Truth, he quotes verse 42:29 "And among His Signs is the creation of the heavens and the earth, and of whatever living creatures (da'bbah) He has spread forth in both..."; according to this verse there is life in heavens. According to the same verse "And He has the power to gather them together (jam-'i-him) when He will so please"; indicates the bringing together the life on Earth and the life elsewhere in the universe. The verse does not specify the time or the place of this meeting but rather states that this event will most certainly come to pass whenever God so desires. It should be pointed out that the Arabic term Jam-i-him used to express the gathering event can imply either a physical encounter or a contact through communication.

In Shia Islam the 6th Imam Ja'far al-Sadiq has been quoted as saying that there are living beings on other planets. He has also stated that they may be more intelligent or advanced than humans.

When Christianity spread throughout the West, the Ptolemaic system became very widely accepted, and although the Church never issued any formal pronouncement on the question of alien life, at least tacitly, the idea was aberrant. 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). Taking a further step, and arguing that aliens actually existed, remained rare. Notably, Cardinal Nicholas of Kues speculated about aliens on the moon and sun.

modern period

There was a 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 the Italian philosopher Giordano Bruno, who argued in the 16th century for an infinite universe in which every star is surrounded by its own planetary system. Bruno's thoughts about God and the universe, of which many contradicted essential dogmas of the Catholic Faith, led to his eventual condemnation as a heretic by a tribunal of the Roman Inquisition of the Roman Catholic Church. Contemporary civil authorities enforced the penal statute of Emperor Frederick II Inconsutilem Tunicam of 12 February 1220 mandating the burning of heretics, resulting in his being among the last of heretics to be burned alive at the stake in Rome in the year 1600.

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 [characteristics] of the two spheres". Dominican monk Tommaso Campanella wrote about a Solarian alien race in his Civitas Solis.[citation needed] The Catholic Church has not made a formal ruling on the existence of extraterrestrials. However, writing in the Vatican newspaper, the astronomer, Father José Gabriel Funes, director of the Vatican Observatory near Rome, said in 2008 that intelligent beings created by God could exist in outer space.

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 the 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 extraterrestrial inhabitants.

Extraterrestrials and the modern era

The trend to assume that celestial bodies were populated almost by default was tempered as actual probes visited potential alien abodes in the Solar System beginning in the second half of the 20th century. At the same time, the beginning Space Age was accompanied by a surge of UFO reports, particularly in the United States, during the 1950s. The term UFO itself was coined in 1952 in the context of the enormous popularity of the concept of "flying saucers" in the wake of the Kenneth Arnold UFO sighting in 1947.

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, to our knowledge, similarly lifeless, though the interesting geothermic forces observed (Io's volcanism, Europa's ocean, Titan's thick atmosphere and hydrocarbon lakes) have underscored how broad the range of potentially habitable environments may be.

Although the hypothesis of a deliberate cosmic silence of advanced extraterrestrials is also a possibility, the failure of the SETI program to announce an intelligent radio signal after four decades of effort has at least partially dimmed the prevailing optimism of the beginning of the space age. Notwithstanding, the unproven belief in extraterrestrial beings is voiced (not as a hypothesis) in pseudoscience, conspiracy theories in popular folklore like about 'Area 51' and legends. Critics view the search for extraterrestrials as unscientific, despite the fact that the SETI program is not the result of a continuous, dedicated search, but instead utilizes what resources and manpower it can, when it can. Furthermore, the SETI program only searches a limited range of frequencies at any one time.

In the words of SETI's Frank Drake, "All we know for sure is that the sky is not littered with powerful microwave transmitters". Drake has also noted that it is entirely possible that 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. The Wow! signal, from SETI, remains a speculative debate.

In 2000, geologist and paleontologist Peter Ward and astrobiologist Donald Brownlee published a book entitled Rare Earth: Why Complex Life is Uncommon in the Universe. In it, they discussed the Rare Earth hypothesis, in which they claim that Earth-like life is rare in the universe, while microbial life is common. Ward and Brownlee are open to the idea of evolution on other planets which is not based on essential Earth-like characteristics (such as DNA and carbon).

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 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 both 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 possible 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. Independently, in 1996, structures resembling nanobacteria 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.

In February 2005, NASA scientists reported that they had found strong evidence of present life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claims on methane signatures found in Mars' atmosphere resembling the methane production of some forms of primitive life on Earth, as well as on 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.

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 at which the European Space Agency presented its findings, 75 percent of the scientists in attendance were reported to believe that life once existed on Mars, and 25 percent reported a belief that life currently exists there.

The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere not in chemical equilibrium, which is relatively easy to determine from a distance by spectroscopy. However, significant advances in the ability to find and resolve light from smaller rocky worlds near to their star are necessary before this can be used to analyze extrasolar planets.

Indirect search

It is theorized that any technological society in space will be transmitting information, although this is arguable, as there are generally no human systems intentionally, randomly, transmitting information into deep space, so there is no guarantee that any other species would do so, either. Also, the length of time required for a signal to travel across the vastness of space means that any signal detected or not detected would come from the distant past.
Nevertheless, projects such as SETI are conducting an astronomical search for radio activity which 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. While other communication techniques, including laser transmission and interstellar spaceflight, have been discussed seriously and may well be feasible, the measure of effectiveness is the amount of information communicated per unit cost, resulting with radio as the method of choice.

Extrasolar planets

Astronomers also search for extrasolar planets that they believe would be conducive to life, such as Gliese 581 c, Gliese 581 d and 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 a 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 Alpha Centauri, the closest star system to Earth, may contain planets which could be capable of sustaining life.

On April 24, 2007, scientists at the European Southern Observatory in La Silla, Chile said they had found the first Earth-like planet. The planet, known as Gliese 581 c, orbits within the habitable zone of its star Gliese 581, a red dwarf star which is 20.5 light years (194 trillion km) from the Earth. It was initially thought that this planet could contain liquid water, but recent computer simulations of the climate on Gliese 581c by Werner von Bloh and his team at Germany's Institute for Climate Impact Research suggest that carbon dioxide and methane in the atmosphere would create a runaway greenhouse effect. This would warm the planet well above the boiling point of water (100 degrees Celsius/212 degrees Fahrenheit), thus dimming the hopes of finding life. As a result of greenhouse models, scientists are now turning their attention to Gliese 581 d, which lies just outside of the star's traditional habitable zone.

On May 29, 2007, the Associated Press released a report stating that scientists identified twenty-eight new extra-solar planetary bodies. One of these newly-discovered planets is said to have many similarities to Neptune.

As of March 4, 2010, 429 extrasolar planets have been discovered (with 48 multi-planet systems), and new discoveries occur monthly.

Drake equation

In 1961, University of California, Santa Cruz astronomer and astrophysicist Dr. Frank Drake devised the Drake equation. This controversial equation multiplied estimates of the following terms together:

Drake used the equation to estimate that there are approximately 10,000 planets in the Milky Way galaxy containing intelligent life with the possible capability of communicating with Earth.

Based on observations from the Hubble Space Telescope, there are at least 125 billion galaxies in the universe. It is estimated that at least ten percent of all sun-like stars have a system of planets, i.e. there are 6.25×1018 stars with planets orbiting them in the universe. Even if we assume that only one out of a billion of these stars have planets supporting life, there would be some 6.25×109 (billion) life-supporting planetary systems in the universe.

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

Numerous other bodies have been suggested as potential hosts for microbial life. 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 (i.e. animals and plants) could exist under these living conditions.

Even if microbial extraterrestrial life were found on another body in the Solar System, it would still need to be proven that such life did not originate from Earth in the recent or distant past. For example, an alternate explanation for the hypothetical existence of microbial life on Titan has already been formally proposed — theorizing that microorganisms could have left Earth when it suffered a massive asteroid or comet impact (such as the impact that created Chicxulub crater only 65 mya), and survived a journey through space to land on Titan 1 million years later. The Living Interplanetary Flight Experiment, developed by the Planetary Society and due to be launched in 2011, has been designed to test similar theories.



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