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Extraterrestrial life

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   Extraterrestrial life is life that may exist and originate outside the
   planet Earth, the only place in the universe currently known by humans
   to support life. Its existence is currently purely hypothetical as
   there is yet no evidence of any planets that can support life, or
   actual extraterrestrial life that has been widely accepted by the
   scientific community.

   Most scientists hold that if extraterrestrial life exists, its
   evolution would have occurred independently in different places in the
   universe. An alternative hypothesis, held by a minority, is panspermia,
   which suggests that life in the universe could have stemmed from a
   smaller number of points of origin, and then spread across the
   universe, from habitable planet to habitable planet. These two
   hypotheses are not mutually exclusive.

   Speculative forms of extraterrestrial life range from sapient beings
   seen in works of science fiction to life at the much smaller scale of
   bacteria.

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

Possible basis of extraterrestrial life

Biochemistry

   All life on Earth is based on the building block element carbon with
   water as the solvent in which bio-chemical reactions take place.
     * Water is useful because it has a neutral pH, and due to its
       continued dissociation between hydroxide and hydronium ions.

                2H[2]O → H[3]O^+ + OH^-

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

     * Carbon is fundamental to conventional 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

                6CO[2] + 6H[2]O + sun energy → C[6]H[12]O[6] + 6O[2]
                C[6]H[12]O[6] + 6O[2] → 6CO[2] + 6H[2]O + biochemical
                energy

          The ability to form organic acids (-COOH) and amine bases
          (NH[2]-) gives it the possibility of neutralisation dehydrating
          reactions to build long polymer peptides and catalytic proteins
          from monomer amino acids, and with phosphates to build not only
          DNA, the information storing molecule of inheritance, but also
          adenosine triphosphate (ATP) the principle energy "currency" of
          cellular life.

   Given their relative abundance and usefulness in sustaining life it has
   long been assumed that life forms elsewhere in the universe will also
   utilize these basic components. However, other elements and solvents
   might be capable of providing a basis for life. Silicon is usually
   considered the most likely alternative to carbon, though this remains
   improbable. Silicon life forms are proposed to have a crystalline
   morphology, and are theorized to be able to exist in high temperatures,
   such as planets closer to the sun. Life forms based in ammonia rather
   than water are also considered, 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, since plasma is
   essentially the fourth state of matter, where electrons are not bound
   in their orbits around atomic nuclei.

Evolution and Morphology

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

   A useful 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 with differing materialization. There are a huge variety of
   eyes, for example, many of which have radically different working
   schematics as well as different visual foci: the visual spectrum,
   infrared, polarity and echolocation. Parochials, by contrast, are
   essentially arbitrary evolutionary forms which often serve little
   utility (or at least have a function which can be equally served by
   dissimilar morphology) and probably will not be replicated. Parochials
   include the five digits of mammals and the curious and often fatal
   conjunction of the feeding and breathing passages found within many
   animals.

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

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

Beliefs in extraterrestrial life

Ancient and Early Modern ideas

   Belief in extraterrestrial life may have been present in ancient Egypt,
   Babylon, and Sumer, although in these societies, cosmology was
   fundamentally supernatural and the notion of aliens is difficult to
   distinguish from that of gods, demons, and such. The first important
   Western thinkers to argue systematically for a universe full of other
   planets and, therefore, possible extraterrestrial life were the ancient
   Greek writers Thales and his student Anaximander in the 7th and 6th
   centuries B.C. The atomists of Greece took up the idea, arguing that an
   infinite universe ought to have an infinity of populated worlds.
   Ancient Greek cosmology worked against the idea of extraterrestrial
   life in one critical respect, however: the geocentric universe,
   championed by Aristotle and codified by Ptolemy, privileged the Earth
   and Earth-life (Aristotle denied there could be a plurality of worlds)
   and seemingly rendered extraterrestrial life impossible. Lucian in his
   novels described inhabitants of the Moon and other celestial bodies as
   humanoids, but with sufficient differences from human.
   Giordano Bruno, De l'Infinito, Universo e Mondi, 1584
   Enlarge
   Giordano Bruno, De l'Infinito, Universo e Mondi, 1584

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

   Hindu beliefs of endlessly repeated cycles of life have lead to
   descriptions of multiple worlds in existence and their mutual contacts
   ( Sanskrit word Sampark means 'contact' as in Mahasamparka = the great
   contact). However the relevance of such descriptions have to be
   evaluated in the context of 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.

   When Christianity spread throughout the West, the Ptolemaic system
   became 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.

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

Extraterrestrials and the Modern era

   This enthusiasm towards the possibility of alien life continued well
   into the 20th century. Indeed, the roughly three centuries from the
   Scientific Revolution through the beginning of the modern era of solar
   system probes were essentially the zenith for belief in
   extraterrestrials in the West: many astronomers and other secular
   thinkers, at least some religious thinkers, and much of the general
   public were largely satisfied that aliens were a reality. This trend
   was finally tempered as actual probes visited potential alien abodes in
   the solar system. The moon was decisively ruled out as a possibility,
   while Venus and Mars, long the two main candidates for
   extraterrestrials, showed no obvious evidence of current life. The
   other large moons of our system which have been visited appear
   similarly lifeless, though 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 should also be considered, 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.

   At present, some enthusiasts in the topic believe that extraterrestrial
   beings regularly visit or have visited the Earth. Some think that
   unidentified flying objects observed in the skies are in fact sightings
   of the spacecraft of intelligent extraterrestrials, and even claim to
   have met such beings. Crop circle patterns have also been attributed to
   the actions of extraterrestrials by some believers, although many were
   later found to be hoaxes. While one recent scientific paper published
   in the peer-reviewed Journal of the British Interplanetary Society has
   urged a re-evaluation of the UFO phenomenon, as of this time mainstream
   scientific opinion holds that such claims are unsupportable by the
   evidence currently available and unlikely to be true.

   The possible existence of primitive (microbial) life outside of Earth
   is much less controversial to mainstream scientists although at present
   no direct evidence of such life has been found. Indirect evidence has
   been offered for the current existence of primitive life on the planet
   Mars; however, the conclusions that should be drawn from such evidence
   remain in debate.

Scientific search for extraterrestrial life

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

Direct search

   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.
   Enlarge
   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.

   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. Independently in 1996 structures
   resembling bacteria were reportedly discovered in a meteorite,
   ALH84001, thought to be formed of rock ejected from Mars. This report
   is also controversial and scientific debate continues. (See Viking
   biological experiments.)

   In February 2005, NASA scientists reported that they had found strong
   evidence of present life on Mars. The two scientists, Carol Stoker and
   Larry Lemke of NASA's Ames Research Centre, 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.

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.
   Terrestrial Planet Finder - A planned Infrared interferometer for
   finding Earth-like extrasolar planets
   Enlarge
   Terrestrial Planet Finder - A planned Infrared interferometer for
   finding Earth-like extrasolar planets

Extrasolar planets

   Astronomers also search for extrasolar planets that would be conducive
   to life, especially those like OGLE-2005-BLG-390Lb which have been
   found to have Earth-like qualities. Current radiodetection methods have
   been inadequate for such a search, as the resolution afforded by recent
   technology is inadequate for detailed study of extrasolar planetary
   objects. Future telescopes should be able to image planets around
   nearby stars, which may reveal the presence of life (either directly or
   through spectrography which would reveal key information such as the
   presence of free oxygen in a planet's atmosphere). Darwin is an ESA
   mission designed to find Earth-like planets, and analyse their
   atmosphere. It has been argued that one of the best candidates for the
   discovery of life-supporting planets may be Alpha Centauri, the closest
   star system to Earth, given that two of the three stars in the system
   are broadly sun-like.

Extraterrestrial life in the Solar System

   Many bodies in the Solar System have been suggested as being likely to
   contain conventional organic life. The most commonly suggested ones are
   listed below; of these, three of the five are moons, and are thought to
   have large bodies of underground liquid, where life may have evolved in
   a similar fashion to deep sea vents.
     * Mars - Liquid water is widely thought to have existed on Mars in
       the past and there may still be liquid water beneath the surface.
       Recently, methane was found in the atmosphere of Mars. Main
       article: Life on Mars
     * Europa- Europa contains liquid water beneath it's 100-mile ice
       layer, vents on the bottom of the ocean warm the ice so that 60
       miles of Liquid exist beneath the ice layer, perhaps capable of
       supporting Microbes and Simple plants
     * Jupiter- Possible supporter of floating animals, as hypothisized by
       Carl Sagan. this point of view is somewhat controversial due to the
       fact that these creatures would be non-carbon-based
     * Ganymede- Possible underground ocean-see Europa
     * Callisto- Possible underground ocean-see Europa
     * Saturn- Possible floating creatures-see Jupiter
     * 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
       almost surely present on surface.
     * Enceladus - Saturn's sixth largest moon - Geologically active, with
       liquid water and geysers known to exist beneath its south pole.

   Numerous other bodies have been suggested as potential hosts for
   microbial life. 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.

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