Extraterrestrial life

For fiction, see Extraterrestrials in fiction.

For other uses, see Alien (disambiguation).

A 1967 Soviet Union 16 kopeks postage stamp, depicting a satellite from an imagined extraterrestrial civilization

Extraterrestrial life (from the Latin words: extra ("beyond", or "not of") and‎ terrestris ("of or belonging to Earth")) is defined as life that does not originate from Earth. Referred to as alien life, or simply aliens, these hypothetical forms of life range from simple bacteria-like organisms to beings far more advanced than humans.

The development and testing of theories about extraterrestrial life is known as exobiology or astrobiology; the term astrobiology, however, includes the study of life on Earth, viewed in its astronomical context. Many prominent scientists consider extraterrestrial life to be plausible, but the scientific community does not currently recognize any verifiable evidence of such life.

Background

Alien life, such as bacteria, has been theorized to exist in the Solar System and quite possibly throughout the Universe. This theory relies on the vast size and consistent physical laws of the observable Universe. According to this argument, supported by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth. This argument is embodied in the Copernican principle, which states that the Earth does not occupy a favored position in the Universe, and the mediocrity principle, which holds that there is nothing special about life on Earth. Life may have emerged independently at many places throughout the Universe. Alternatively life may form less frequently, then spread between habitable planets through panspermia or exogenesis.

Suggested locations at which life might have developed, or which might continue to host life today, include the planets Venus and Mars, Jupiter's moon Europa, and Saturn's moons Titan and Enceladus. In May 2011, NASA scientists reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it". Life may appear on extrasolar planets, such as Gliese 581 c, g 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.

No samples of extraterrestrial life have been found. However, various controversial claims have been made for evidence of extraterrestrial life. Beliefs that some unidentified flying objects are of extraterrestrial origin (see Extraterrestrial hypothesis), along with claims of alien abduction, are dismissed by most scientists. Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes.

Possible basis

Several theories have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological viewpoint.

Biochemistry

Main articles: Biochemistry, Hypothetical types of biochemistry, and Water and life

All life on Earth is based upon 26 chemical elements. However, about 95% of this life is built upon only six of these elements : carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur; abbreviated CHNOPS by astrobiologists. These six elements form the basic building blocks of virtually all life on Earth, while most of the remaining elements are found in only trace amounts. In 2010, a possible exception to this rule was found in strain of bacteria called GFAJ-1 located in California's Mono Lake. This bacteria appears to use arsenic instead of phosphorus for creating its DNA and proteins.

Life on Earth 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 temperature range similar to that of Earth. Terrestrial planets, such as Earth, are formed from "stardust" in a process that allows for the possibility of other planets having formed with compositions similar to 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 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. Nitrogen is needed 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 (within specific temperature ranges) 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. When combined with phosphates, these acids can build the information-storing molecule of inheritance, DNA, and the principal energy transfer molecule of cellular life, ATP.

Due to their relative abundance and usefulness in sustaining life, many have hypothesized that life forms elsewhere in the universe would utilize these basic materials. However, other elements and solvents could provide a basis for life. Silicon is most often deemed to be the probable alternative to carbon. Silicon life forms 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 been suggested, though this solution appears less optimal than water.

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 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 occurred in fictional representations of aliens.

A division has been suggested between universal and parochial (narrowly restricted) 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. The most fundamental of these is probably bilateral symmetry, but more complex (though still basic) characteristics 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. 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 outer 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.

Beliefs

Ancient and medieval ideas

Main article: Cosmic pluralism

Anunciacion by Carpaccio

In antiquity, it was common to assume a cosmos consisting of "many worlds" inhabited by intelligent, non-human life-forms, but these "worlds" were mythological and not informed by the heliocentric understanding of the solar system, or the understanding of the Sun as one among countless stars. An example would be the fourteen loka of Hindu cosmology, or the Nine Worlds of Old Norse mythology, etc. The Sun and the Moon often appear as inhabited worlds in such contexts, or as vehicles (chariots or boats, etc.) driven by gods. The Japanese folk tale of The Tale of the Bamboo Cutter (10th century) is an example of a princess of the Moon people visiting Earth.

Such conceptions of a universe consisting of "many worlds" are found in classical Greek philosophy, and later in Christian and Jewish theology. (See exotheology.) 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.

The Jewish 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.

Hindu beliefs of endlessly repeated cycles of life have led to descriptions of multiple worlds in existence and their mutual contacts (Sanskrit word sampark (सम्पर्क) means "contact" as in Mahasamparka (महासम्पर्क) = "the great contact"). 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 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.

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.

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.

Early modern period

Giordano Bruno, De l'Infinito Universo et Mondi, 1584

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. 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 wrote that other worlds "have no less virtue nor a nature different to that of our earth" and, like Earth, "contain animals and inhabitants".

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".

In Baroque literature such as The Other World: The Societies and Governments of the Moon by Cyrano de Bergerac, extraterrestrial societies are presented as humoristic or ironic parodies of earthly society. 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). 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". 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.

19th century

In 1854, William Whewell, a fellow of Trinity College, Cambridge, who popularized the word scientist, theorized that Mars had seas, land and possibly life forms. Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparent Martian canals — which were however soon found to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilization. This idea led British writer H. G. Wells to write The War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet’s desiccation.

Spectroscopic analysis of Mars' atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen were present in the Martian atmosphere. By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal theory.

The science fiction genre, although not so named during the time, develops during the late 19th century. Jules Verne's Around the Moon (1870) features a discussion of the possibility of life on the Moon, but with the conclusion that it is barren. Stories involving extraterrestrials are found in e.g. Garrett P. Serviss's Edison's Conquest of Mars (1897). The War of the Worlds by H. G. Wells was published in 1898 and stands at the beginning of the popular idea of the "Martian invasion" of Earth prominent in 20th century pop culture.

20th century

The Arecibo message is a digital message sent to globular star cluster M13, and is a well-known symbol of human attempts to contact extraterrestrials.

A radio drama version of Wells' novel broadcast in 1938 over the CBS Radio Network led to outrage because it supposedly suggested to many listeners that an actual alien invasion by Martians was currently in progress. In the wake of this, conspiracy theories on the presence of extraterrestrials became a widespread phenomenon in the United States during the 1940s and the beginning Space Age during the 1950s, accompanied by a surge of UFO reports. 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 Majestic 12 documents published in 1982 suggest that there was genuine interest in UFO conspiracy theories in the US government during the 1940s.

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, and by the 1970s belief in UFOs had become part of the fringe beliefs associated with the paranormal, New Age, Earth mysteries, Forteana etc. A number of UFO religions developed during the surge in UFO belief during th 1950s to 1970s period, and some, such as Scientology (founded 1953) and Raëlism (founded 1974) remain active into the present. The idea of "paleocontact", supposing that extraterrestrials ("ancient astronauts") have visited the Earth in the remote past and left traces in ancient cultures, appears in early 20th century fiction such as The Call of Cthulhu (1926) and the idea came to be established as a notable aspect of the Ufology subculture in the wake of Erich von Däniken's Chariots of the Gods? (1968). Alien abduction claims were widespread during the 1960s and 1970s in the United States.

On the scientific side, the possibility of extraterrestrial life on the Moon was decisively ruled out by the 1960s, and during the 1970s it became clear that most of the other bodies of the Solar System do not harbour highly developed life, although the question of primitive life on bodies in the Solar System remains an open question. Carl Sagan, Bruce Murray, and Louis Friedman founded the U.S. Planetary Society, partly as a vehicle for SETI studies in 1980, and since the 1990s, systematic search for radio signals attributable to intelligent extraterrestrial life has been ongoing.

Recent history

The failure of the SETI program to announce an intelligent radio signal after 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 continues to be voiced in pseudoscience, conspiracy theories, and in popular folklore, notably "Area 51" and legends. It has become a pop culture trope given less-than-serious treatment in popular entertainment with e.g. the ALF TV series (1986–1990), The X-Files (1993–2002), etc.

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 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.

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.

In September 2010, it was reported that the U.N. General Assembly had appointed Mazlan Othman as their official extraterrestrial liaison by the UK paper The Sunday Times. This claim was later refuted.

Theoretical physicist Stephen Hawking in 2010 warned that humans should not try to contact alien life forms. He warned that aliens might pillage Earth for resources. "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said. Anthropologist Jared Diamond has expressed similar concerns. Scientists at NASA and Penn State University published a paper in April 2011 addressing the question "Would contact with extraterrestrials benefit or harm humanity?" The paper describes positive, negative and neutral scenarios.

Richard Hoover, an astrobiologist at the U.S. Space Flight Center in Alabama, stirred up quite a debate in March 2011 over life elsewhere on other planets. Tiny fossils of alien bugs in meteorites landed on earth. Hoover claimed that filaments and other structures in rare meteorites appear to be microscopic fossils of extraterrestrial beings that resemble algae known as cyanobacteria.

In May 2011, a planet in the Gliese system was found capable of sustaining life. Researchers believe Gliese 581 d, which orbits a red dwarf 20 light years away, not only exists in the "Goldilocks zone" where water can be present in liquid form, but is big enough to have a stable carbon dioxide atmosphere and "warm enough to have oceans, clouds, and rainfall," according to France's National Centre for Scientific Research.

Scientific search

The NASA Kepler Mission for the search of extrasolar planets.

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 proposed to Europa, one of Jupiter's moons with a possible liquid-water layer under its surface and 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 lander 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 controversial.

Electron micrograph of martian meteorite ALH84001 showing structures that some scientists think could be fossilized bacteria-like life forms.

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.

In November 2011, NASA plans to launch the Mars Science Laboratory (MSL) rover which is designed to search for past or present life on Mars using a variety of scientific instruments. The MSL is scheduled to land on Mars at Gale Crater in August 2012.

The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium, 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 their star are necessary before such spectroscopic methods can be used to analyze extrasolar planets.

On March 5, 2011, Richard B. Hoover, an astrobiologist with the Marshall Space Flight Center, speculated on the finding of alleged microfossils similar to cyanobacteria in CI1 carbonaceous meteorites. However, NASA formally distanced itself from Hoover's claim. See Hoover paper controversy for more details.

In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggests DNA and RNA components (adenine, guanine and related organic molecules), building blocks for life as we know it, may be formed extraterrestrially in outer space.

Indirect search

Terrestrial Planet Finder – A planned Infrared interferometer for finding Earth-like extrasolar planets (As of 2011, it has not received the funding from NASA which it needs—that funding is going towards the Kepler mission).

It is theorized that any technological society in space will be transmitting information. However, if there is an advanced extraterrestrial society, there is no guarantee that they are transmitting information in the direction of Earth or that this information could be interpreted as such by humans. 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. This results in radio transmission as the method of choice.

Extrasolar planets

See also: List of planetary systems

Astronomers search for extrasolar planets that they believe would be conducive to life, such as Gliese 581 c, Gliese 581 g, 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, since 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 – and would reveal key information, such as the presence of free oxygen in a planet's atmosphere:

Artist's Impression of Gliese 581 c, the first extrasolar planet discovered within its star's habitable zone.

• Darwin was a proposed ESA mission designed to find Earth-like planets and analyze their atmosphere.
• The COROT mission, initiated by the French Space Agency, was launched in 2006, and is currently looking for extrasolar planets; it is the first of its kind.
• The Terrestrial Planet Finder was supposed to have been launched by NASA, but as of 2011, budget cuts have caused it to be delayed indefinitely.
• The Kepler Mission, largely replacing the Terrestrial Planet Finder, was launched in March 2009.

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 581 c 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.

Since 1992, hundreds of planets around other stars ("extrasolar planets" or "exoplanets") in the Milky Way Galaxy have been discovered. As of 1 July 2024, 6,660 known extrasolar planets are listed in the Extrasolar Planets Encyclopaedia, ranging from the size of terrestrial planets somewhat larger than Earth to gas giants larger than Jupiter.

The Drake equation

Main article: 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:

• The rate of formation of suitable stars.
• The fraction of those stars which are orbited by planets.
• The number of Earth-like worlds per planetary system.
• The fraction of planets where intelligent life develops.
• The fraction of possible communicative planets.
• The "lifetime" of possible communicative civilizations.

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 observable 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 observable 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 observable Universe.

Candidate locations in the Solar System

Europa, due to the ocean under its icy crust, might host some form of microbial life.

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, where large bodies of underground liquid (oceans) are considered possible. Life there may have evolved in a way similar to biological communities surrounding deep sea vents.

• Venus — Recently, scientists have speculated on the existence of microbes in the stable cloud layers 50 km above the surface, evidenced by hospitable climates and chemical disequilibrium.
• Mars — Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and there may still be liquid water beneath the surface. Methane was found in the atmosphere of Mars and work is underway to discern its biological or abiotic origin. By July 2008, laboratory tests aboard NASA's Phoenix Mars Lander had identified water in a soil sample. The lander's robotic arm delivered the sample to an instrument which identifies vapors produced by the heating of samples. Recent photographs from the Mars Global Surveyor show evidence of recent (i.e. within 10 years) flows of a liquid on the Red Planet's frigid surface.

There is evidence that the Red Planet had a warmer and wetter past: dried-up river beds, polar ice caps, volcanoes and minerals that form in the presence of water have all been found. In 2008, the Phoenix Mars Lander sent back photos of ice chunks it had found after scooping up handfuls of soil, a huge discovery in the search for liquid water -- a key ingredient for life. Another key ingredient for life was found the following year: NASA scientists detected methane in the Martian atmosphere, indicating that the planet is still alive.

• Ceres (dwarf planet–asteroid) — There has been some speculation that life could have evolved early on Ceres and have survived up to the present.
• Jupiter — Carl Sagan and others in the 1960s and 70s computed conditions for hypothetical amino acid-based macroscopic life in the atmosphere of Jupiter, based on observed conditions of this atmosphere. However, the conditions do not appear to permit the type of encapsulation believed necessary for molecular biochemistry, so life is thought to be unlikely.
• Europa (Moon of Jupiter) — Europa contains liquid water beneath its thick ice layer. It is possible that vents on the bottom of the ocean warm the ice, so liquid exists beneath the ice layer, perhaps capable of supporting microbes and simple plants, just like in Earth's hydrothermal vents. It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice.
• Ganymede (Largest moon of Jupiter) — Possible underground ocean (see Europa).
• Callisto (Moon of Jupiter) — Possible underground ocean (see Europa).
• Titan (Largest moon of Saturn) — The only known moon with a significant atmosphere. Data from the Cassini–Huygens mission refuted the hypothesis of a global hydrocarbon ocean, but later demonstrated the existence of liquid hydrocarbon lakes in the polar regions—the first liquid lakes discovered outside of Earth. Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface which are consistent with —but do not prove— the hypothesis that organisms there are consuming hydrogen, acetylene and ethane, and producing methane.
• Enceladus (Moon of Saturn) — Geothermal activity, water vapor. Possible under-ice oceans heated by tidal effects. When Cassini did a fly-by through one of Enceladus's geysers spewing ice and gas in 2005, the probe detected carbon, hydrogen, nitrogen and oxygen -- all key elements for supporting living organisms. What's more, the temperature and density of the plumes could indicate a warmer, watery source beneath the surface. Still, no life has been confirmed.
• Exoplanets - Some estimates show that the Milky Way alone harbors around 400 billion stars and countless exoplanets, and that’s just within our own galaxy. So there could potentially be billions of habitable cosmic bodies out there.

An exoplanet is a planetary body that sits outside our solar system and orbits another star that is not our sun. We've only been exploring these outside worlds for the past decade (the first one, HD 209458, was discovered in 1999), with dozens more being discovered every year with many hosting organic compounds. HD 209458b, for example, was found with water, methane and carbon dioxide in its atmosphere, all key ingredients for life.

• Orion Nebula - A stellar nursery in the Milky Way has recently been investigated as a potential gold mine for finding life.

In May 2010, the European Space Agency's Herschel Space Observatory announced that the Orion Nebula, located about 1,500 light-years away from Earth just south of Orion's belt, showed signs of having life-enabling organic chemicals. Looking through the data collected by the telescope, astronomers were able to detect a pattern of spikes for various life-supporting molecules: water, carbon monoxide, formaldehyde, methanol, dimethyl ether, hydrogen cyanide, sulfur oxide and sulfur dioxide.

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 (later considered doubtful as sterile procedures may not have been fully followed). 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 was 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.

See also

Events and objects

• ALH84001 is a Mars meteorite with disputed seemingly microbial formations.
• Ceres (dwarf planet)
• Gliese 581 e, b, c, g and d.
• Wow! signal
• Close encounter
• First contact (science fiction)

Searches for extraterrestrial life

• Rejection of chemical evolution
• Communication with extraterrestrial intelligence
• Darwin – an ESA mission designed to find Earth-like planets and analyze their atmosphere for signs of life
• SETI (Search for Extra-Terrestrial Intelligence)
• Allen Telescope Array

Subjects

• Astrobiology
• Planetary protection
• Extraterrestrials in fiction
• Exotheology
• Habitable zone
• Planetary habitability
• Rare Earth hypothesis
• Extraterrestrial liquid water

Theories

• Aurelia and Blue Moon
• Back-contamination
• Drake equation
• Fermi paradox
• Mediocrity principle
• Panspermia
• Sentience quotient
• Kardashev scale
• Metalaw

Further reading

• Baird, John C. (1987). The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings. Hanover: University Press of New England. ISBN 0-87451-406-1.
• Cohen, Jack; Stewart, Ian (2002). Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. ISBN 0-09-187927-2.
• Crowe, Michael J. (1986). The Extraterrestrial Life Debate, 1750–1900. Cambridge. ISBN 0521263050.
• Crowe, Michael J. (2008). The extraterrestrial life debate Antiquity to 1915: A Source Book. University of Notre Dame Press. ISBN 0-268-02368-9.
• Dick, Steven J. (1984). Plurality of Worlds: The Extraterrestrial Life Debate from Democratis to Kant. Cambridge.
• Dick, Steven J. (1996). The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science. Cambridge. ISBN 0521343267.
• Dick, Steven J. (2001). Life on Other Worlds: The 20th Century Extraterrestrial Life Debate. Cambridge. ISBN 0521799120.
• Dick, Steven J. and James E. Strick (2004). The Living Universe: NASA And the Development of Astrobiology. Rutgers. ISBN 081353447X.
• Goldsmith, Donald (1997). The Hunt for Life on Mars. New York: A Dutton Book. ISBN 0-525-94336-6.
• Grinspoon, David (2003). Lonely Planets: The Natural Philosophy of Alien Life. HarperCollins. ISBN 0-06-018540-6.
• Lemnick, Michael T. (1998). Other Worlds: The Search for Life in the Universe. New York: A Touchstone Book.
• Pickover, Cliff (2003). The Science of Aliens. New York: Basic Books. ISBN 0-465-07315-8.
• Roth, Christopher F. (2005) "Ufology as Anthropology: Race, Extraterrestrials, and the Occult." In E.T. Culture: Anthropology in Outerspaces, ed. by Debbora Battaglia. Durham, NC: Duke University Press.
• Sagan, Carl; I.S. Shklovskii (1966). Intelligent Life in the Universe. Random House.
• Sagan, Carl (1973). Communication with Extraterrestrial Intelligence. MIT Press. ISBN 0262191067.
• Peter D. Ward: Life as we do not know it-the NASA search for (and synthesis of) alien life. Viking, New York 2005, ISBN 0-670-03458-4
• Michael Michaud: Contact with Alien Civilizations – Our Hopes and Fears about Encountering Extraterrestrials. Springer, Berlin 2006, ISBN 0-387-28598-9
• Ernst Fasan: Relations with alien intelligences – the scientific basis of metalaw. Berlin Verlag, Berlin 1970
• Diana G. Tumminia: Alien Worlds – Social and Religious Dimensions of Extraterrestrial Contact. Syracuse Univ. Press, Syracuse 2007, ISBN 978-0-8156-0858-5

External links

• Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox by Robert A. Freitas Jr.
• 'Is There Anybody Out There?' Freeview video by the Vega Science Trust and the BBC/OU.
• PBS: Life Beyond Earth a film by Timothy Ferris
• PBS: Exploring Space – The Quest for Life by Scott Pearson
• Xenopsychology" by Robert A. Freitas Jr.
• Really Unfriendly Aliens – slideshow by Life magazine
• "What Aliens Might Look Like" from National Geographic
• Top stars picked in alien search, from BBC News