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All life on Earth is made up of the principal elements, carbon, hydrogen, oxygen, nitrogen, and phosphorus; 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 Earth. Because Earth and other planets are made up of "star dust", relatively abundant chemical elements formed from stars which have ended their life as supernova, it is very probable that other planets may have been formed by elements of a similar composition as Earth. The combination of carbon and water in the chemical form of carbohydrates (e.g., sugar), can be a source of chemical energy on which life depends, and 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 requires carbon in both reduced (methane derivatives) and partially-oxidized (carbon oxides) states. It also requires 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 solid. Additionally, the Van der Waals forces between water molecules give 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.

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 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 principal energy "currency" of cellular life.

Due to their relative abundance and usefulness in sustaining life, many have hypothesized that life forms 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.

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 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 could be non-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., this depiction is fundamentally humanoid. Other subjects such as felines and insects have also occurred in fictional representations of aliens.

A division has been suggested between universal and parochial (narrowly restricted) 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. 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. A classic example of a parochial is the curious and often fatal conjunction of the feeding and breathing passages found within many animals, although it is possible this conjunction allowed for the evolution of human speech.

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 assuredly 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 Earth life foregrounds even greater variety in space, others point out that convergent evolution may dictate 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 India, Egypt, Arabia, China, Babylon, Assyria and Sumer, although in these societies, cosmology was fundamentally supernatural and the notion of alien life 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 writer 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, it favored the Earth and Earth-life (Aristotle denied there could be a plurality of worlds) and seemingly rendered extraterrestrial life philosophically untenable. Lucian in his novels described inhabitants of the Moon and other celestial bodies as humanoids, but with significant differences 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 the worlds and on whether they are physical or spiritual. Based on this, however, the 18th century 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.

Hindu beliefs of endlessly repeated cycles of life have led to descriptions of multiple worlds in existence and their mutual contacts ( Sanskrit word Sampark (समपर्क) means 'contact' as in Mahasamparka (मह‌समपर्क) = the great contact). According to Hindu scriptures there are innumerable universes created by the Supreme Personality of Godhead 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. Apart from the innumerable universes which are material, there is also the existence of unlimited spiritual world, where the purified living entities live with perfect conception about life and ultimate reality. The life of these purified beings is centered around loving devotional services to Supreme Personality of Godhead. The spiritually aspiring saints and devotees as well as thoughtful men of material world have been getting guidance and help from these purified living entities of 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 that 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. A more direct reference from Quran is presented by Mirza Tahir Ahmad as a proof that life on other planets may exist according to 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.

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) yet we know by revelation He only made one. Taking a further step and arguing that aliens actually existed remained rare. Notably, Cardinal Nicholas of Cusa speculated about aliens on the moon and sun.

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 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 by the Catholic church for his heretical ideas. The Catholic church under John Paul II apologized for this. 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." Dominican monk Tommaso Campanella wrote about a Solarian alien race in his Civitas Solis.

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 extraterrestrial inhabitants.

Extraterrestrials and the modern era

This enthusiasm toward 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 the interesting geothermic forces observed (Io's volcanism, Europa's ocean, Titan's thick atmosphere) 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 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. Emboldened critics 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. Still, 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 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.

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 in the universe.

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 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. Recently, Circadian rhythms have been allegedly discovered in Viking data. The interpretation is controversial, see Viking biological experiments. 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 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.

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.

The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere that is 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 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. And while other communication techniques including laser transmission and interstellar spaceflight have been discussed seriously and may not be infeasible, the measure of effectiveness is the amount of information communicated per unit cost, resulting with the radio as method of choice.

Extrasolar planets

Astronomers also search for extrasolar planets that they believe would be conducive to life, such as Gliese 581 c 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 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.
• 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 be launched by NASA, but as of 2007 , budget cuts have caused it to be delayed indefinitely
• The Kepler Mission, largely replacing the Terrestrial Planet Finder, to be launched in November 2008

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 a scant 20.5 light years (194 trillion km) from Earth. It was initially thought that this planet could contain liquid water. However, 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 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 have identified twenty-eight exo-solar planetary bodies. One of these newly discovered planets is said to have many similarities with Neptune.

Drake equation

In 1961, University of California, Santa Cruz astronomer and astrophysicist Dr. Frank Drake devised the Drake equation, which mathematically simplifies the rate of formation of suitable stars, the fraction of those stars which contain planets, the number of Earth-like worlds per planetary system, the fraction of planets where intelligent life develops, and the fraction of possible communicative planets, and the "lifetime" of possible communicative civilizations which scientifically stated there are an estimated 10,000 planets containing intelligent life with the possible capability of communicating with Earth in the Milky Way galaxy.

Extraterrestrial life in the Solar System

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 nine 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.

• 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. Recent photographs from Mars Global Surveyor show evidence of recent (within 10 years) flows of a liquid on the Red Planet's frigid surface.
• Europa - Europa may contain liquid water beneath its 100-mile (160 km) thick ice layer. Vents on the bottom of the ocean warm the ice so that 60 miles (97 km) of liquid could exist beneath the ice layer, perhaps capable of supporting microbes and simple plants.
• Jupiter - Possible supporter of floating animals, as hypothesized by Carl Sagan for gas giants in general. This point of view is somewhat controversial because these creatures would not be water-based, but ammonia-based.
• Ganymede - Possible underground ocean (see Europa).
• Callisto - Possible underground ocean (see Europa).
• Saturn - Possible floating creatures (see Jupiter).
• Enceladus - Geothermal activity, watervapour. Possible underice oceans heated by tidal effects.
• 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 present on the surface (the first liquid lakes discovered outside of Earth).
• Venus - Recently, scientists have speculated the existence of microbes in the stable cloud layers 50 km above the surface, evidenced by hospitable climates and chemical disequilibrium.

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 (animals, plants) could exist under these living conditions.

See also

Events and objects

• ALH84001 - a Mars meteorite with disputed microbial formations
• Gliese 581 c
• Gliese 581 d
• Red rain in Kerala
• Wow! signal

Searches for extraterrestrial life

• 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
• Cryptobiology
• Exobiology
• Extraterrestrial life in popular culture
• Habitable zone
• Planetary habitability
• Rare Earth hypothesis
• Scientific skepticism
• Extraterrestrial liquid water

Theories

• Zecharia Sitchin
• Aurelia and Blue Moon
• Back-contamination
• Drake equation
• Exopolitics
• Fermi paradox
• Panspermia
• Sentience Quotient
• Kardashev scale

References

1. "Ammonia based life". daviddarling.info.
2. NASA - NASA Predicts Non-Green Plants on Other Planets
3. "Variety of extraterrestrial life". daviddarling.info.
4. "Star Struck, a letter to a Rabbi". ohr.edu.
5. Kaplan, Rabbi Aryeh. "Extraterrestrial life". torah.org.
6. Revelation, Rationality, Knowledge & Truth, by Mirza Tahir Ahmad. Chapter; The Quran and Extraterrestrial Life
7. Wiker, Benjamin D. "Christianity and the Search for Extraterrestrial Life". crisismagazine.com.
8. "Rheita.htm". cosmovisions.com.
9. An intelligent design : Controlled hominization in cosmic apartheid
10. Crichton, Michael (January 17, 2003). "Aliens Cause Global Warming". crichton-official.com. {{cite web}}: Check date values in: |date= (help)
11. SETI: Search For Extra-Terrestial Intelligence
12. Amazon.com: Rare Earth: Why Complex Life is Uncommon in the Universe: Books: Peter Ward,Donald Brownlee
13. Spherix: Makers of Naturlose (tagatose), a natural, low-calorie sugar made from whey that may be useful as a treatment for Type 2 diabetes
14. Berger, Brian (2005). "Exclusive: NASA Researchers Claim Evidence of Present Life on Mars".
15. "NASA denies Mars life reports". spacetoday.net. 2005.
16. Spotts, Peter N. (2005-02-28). "Sea boosts hope of finding signs of life on Mars". The Christian Science Monitor. Retrieved 2006-12-18. {{cite news}}: Check date values in: |date= (help)
17. "The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum". The Columbus Optical SETI Observatory.
18. "http://planet.iap.fr/OB05390.news.html". {{cite journal}}: Cite journal requires |journal= (help); External link in |title= (help)
19. SPACE.com - Major Discovery: New Planet Could Harbor Water and Life
20. 1997AJ 113.1445W Page 1445
21. Hopes dim for life on distant planet - USATODAY.com
22. BBC NEWS | Science/Nature | Planet hunters spy distant haul
23. Boyd, Padi. "The Drake Equation". Imagine the Universe. NASA. Retrieved 2008-02-05. Frank Drake's own current estimate puts the number of communicating civilizations in the galaxy at 10,000
24. Possibility of Life on Europa
25. ^ http://www.planetary.org/programs/projects/explore_europa/update_12142005.html"
26. BBC NEWS | Science/Nature | Water 'flowed recently' on Mars
27. Jupiter, life on
28. SPACE.com - Scientists Reconsider Habitability of Saturn's Moon
29. SPACE.com - Lakes Found on Saturn's Moon Titan
30. "Lakes on Titan, Full-Res: PIA08630". July 24, 2006.
31. Venusian Cloud Colonies :: Astrobiology Magazine - earth science - evolution distribution Origin of life universe - life beyond :: Astrobiology is study of earth science evolution distribution Origin of life in universe terrestrial

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 and Ian Stewart (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.
• 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.
• Dick, Steven J. (2001). Life on Other Worlds: The 20th Century Extraterrestrial Life Debate. Cambridge.
• Dick, Steven J. and James E. Strick (2004). The Living Universe: Nasa And the Development of Astrobiology. Rutgers.
• 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, N.C.: Duke University Press.
• Sagan, Carl and I.S. Shklovskii (1966). Intelligent Life in the Universe. Random House.
• Sagan, Carl (1973). Communication with Extraterrestrial Intelligence. MIT Press.

External links

• 'The UFO Disclosure Project'A Demand for Congressional hearings on UFOs and related technology "Video"
• 'The truth is out there'UFOs Filmed 26 July 2007.
• '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
• ufoskeptic.org by Bernard Haisch
• Xenopsychology by Robert A. Freitas Jr.
• "What Aliens Might Look Like" from National Geographic
• Sylvia Engdahl, "Early Space Poetry" Part I: Didactic and other poetry concerning other inhabited worlds, well-known and obscure poets, 17th-18th centuries. "Part II": 19th century
• Top stars picked in alien search
• "Snaiad", a realistic world-design exercise by Nemo Ramjet

• Astrobiology
• Extraterrestrial life
• Life