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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. It is often also referred to as alien life, or simply aliens (or space aliens, to differentiate from other definitions of alien or aliens). These hypothetical forms of life range from simplebacteria-like organisms to beings far more complex than humans. HP Pavilion G61-336NR CPU Fan 

The possibility that viruses might also exist extraterrestrially has been proposed.[1]

The development and testing of hypotheses on extraterrestrial life is known as exobiology or astrobiology; the term astrobiology, however, includes the study of life on Earth viewed in its astronomical context. Scientists at the National Institutes of Health reported studies that life in the universe may have begun "9.7±2.5 billion years ago", HP Pavilion dv7-6184ca CPU Fan

billions of years before the Earth was formed, based on extrapolating the "genetic complexity of organisms" [from "major phylogenetic lineages"] to earlier times.[2][3] Many scientists consider extraterrestrial life to be plausible, but there is no direct evidence of its existence.[4] Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial life, from radios used to detect possible extraterrestrial signals, to telescopes used to search for potentially habitable extrasolar planets.HP Pavilion dv7-3125eo CPU Fan

It has also played a major role in works of science fiction.

Alien life, such as bacteria, has been hypothesized to exist in the Solar System and throughout the universe. This hypothesis relies on the vast size and consistent physical laws of the observable universe. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.[5][6HP Pavilion dv7-2185dx CPU Fan

] This argument is embodied in the Copernican principle, which states that the Earth does not occupy a unique position in the Universe, and the mediocrity principle, which holds that there is nothing special about life on Earth.[7] 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.[8IBM MCF-C10AM05 CPU Fan

] In any case, complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth based on computer model studies.[9] According to these studies, this same process may also occur around other stars that acquireplanets.[9] (Also see Extraterrestrial organic molecules.) SONY Vaio VGN-SZ2XP/C CPU Fan

Suggested locations at which life might have developed include the planets Venus[10] and Mars,Jupiter's moon Europa,[11] and Saturn's moons Titan and Enceladus.[12] 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".[13]

Since the 1950s, scientists have promoted the idea that "habitable zones" are the most likely places for life to be found. DELL Latitude E4300 CPU Fan

Numerous discoveries in this zone since 2007 have stimulated estimations of frequencies of Earth-like habitats numbering in the many billions[14][15] though as of 2013, only a small number of planets have been discovered in these zones.[16] Nonetheless, on November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way,[17][18] 11 billion of which may be orbiting Sun-like stars.[19] HP Pavilion dv7-2025es CPU Fan

The nearest such planet may be 12 light-years away, according to the scientists.[17][18] Astrobiologists have also considered a "follow the energy" view of potential habitats.[20][21]

No widely accepted evidence of extraterrestrial life has been found; however, various controversial claims have been made.[22] Beliefs that some unidentified flying objects are of extraterrestrial origin,[23] along with claims of alien abduction,[24] are dismissed by most scientists. FUJITSU LifeBook A6030 CPU Fan

Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes.[25]

In November 2011, the White House released an official response to two petitions asking the U.S. government to acknowledge formally that aliens have visited Earth and to disclose any intentional withholding of government interactions with extraterrestrial beings. SONY Vaio VGN-FS115Z CPU Fan

According to the response, "The U.S. government has no evidence that any life exists outside our planet, or that an extraterrestrial presence has contacted or engaged any member of the human race."[26][27] Also, according to the response, there is "no credible information to suggest that any evidence is being hidden from the public's eye."[26][27] HP 532617-001 CPU Fan

The response further noted that efforts, like SETI, the Kepler space telescope and the NASA Mars rover, continue looking for signs of life. The response noted "odds are pretty high" that there may be life on other planets but "the odds of us making contact with any of them—especially any intelligent ones—are extremely small, given the distances involved." ACER Aspire 5730Z CPU Fan

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. 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.[28] HP G56-126NR CPU Fan

Life on Earth requires water as the solvent in which biochemical reactions take place. Sufficient quantities of carbon and the other 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.[29] Terrestrial planetssuch as Earth are formed in a process that allows for the possibility of having compositions similar to Earth's.[30] HP Pavilion dv7-6166nr CPU Fan

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 viaphotosynthesis. FUJITSU Lifebook A3210 CPU Fan

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 tophosphates in genetic material and in energy transfer. DELL Precision M6400 CPU Fan

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. Compaq Presario CQ60-615DX CPU Fan

Additionally, thehydrogen 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. HP Pavilion dv2660se CPU Fan

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 and starches, such as glucose. The oxidation of glucose releases biochemical energy needed to fuel all other biochemical reactions. Toshiba Satellite L500-1XC CPU Fan

The ability to form organic acids (–COOH) and amine bases (–NH2) gives rise to the possibility of neutralization dehydrating reactions to build long polymerpeptides 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. DELL GM0503PEB1-8 (L2.M) CPU Fan

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. Life forms based in ammonia (rather than water) have been suggested, though this solution appears less optimal than water.[31] HP Pavilion dv7-1098eo CPU Fan

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.[32] Toshiba Satellite A205-S5804 CPU Fan

Life on the surface of a neutron star, based on nuclear reactions, has also been suggested. However, communicating with such creatures would be difficult because the time scales involved are much faster.[33]Several pre-conceived ideas about the characteristics of life outside of Earth have been questioned. For example, a NASA scientist suggested that the color ofphotosynthesizing pigments of hypothetical life on extrasolar planets might not be green. HP Pavilion dv7-1240us CPU Fan

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, insects, blobs, etc., have occurred in fictional representations of aliens.  DELL yy529 CPU Fan

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, Toshiba Qosmio G40-10E CPU Fan

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. SONY Vaio VGN-CR407E CPU Fan

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, by sounds created from structures unrelated to breathing, which happens on Earth when, for instance, HP Pavilion dv6-3131tx Laptop CPU Cooling Fan

cicadas vibrate their wings or crickets stridulate their wings, or visually throughbioluminescence or chromatophore-like structures.

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. HP 622029-001 Laptop CPU Cooling Fan

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 foreshadows an even greater variety in outer space, others point out that convergent evolution may dictate substantial similarities between Earth and extraterrestrial life. Toshiba Satellite L675D-S7047 Laptop CPU Cooling Fan

These two schools of thought are called "divergionism" and "convergionism" respectively.

Some bodies in the Solar System have been suggested as having the potential for an environment that could host extraterrestrial life, particularly those with possible subsurface oceans. Though due to the lack of habitable environments beyond Earth, HP Pavilion dv7-4170ez Laptop CPU Cooling Fan

should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophile microorganisms.

The planets Venus and Mars, along with several natural satellites orbiting Jupiter and Saturn, and even comets, are suspected to possess niche environments in which life might exist. HP Pavilion dv4-1506tx Laptop CPU Cooling Fan

A subsurface marine environment on Jupiter's moon Europa might be the most suitable habitat in the Solar System, outside of Earth, formulticellular organisms.

Panspermia suggests that life elsewhere in the Solar System may have a common origin. If extraterrestrial life was found on another body in the Solar System, it could have originated from Earth just as life on Earth may have been seeded from elsewhere (exogenesis). Compaq Presario CQ61-411WM Laptop CPU Cooling Fan

The Living Interplanetary Flight Experiment, developed by the Planetary Society launched in 2011 was designed to test some aspect of these hypotheses, but it was destroyed along with the carrier Fobos-Gruntmission.[35] The first known mention of the term Panspermia was in the writings of the 5th century BC Greek philosopher Anaxagoras.[36IBM FRU 46L4753 Laptop CPU Cooling Fan

] In the nineteenth century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834),[37] Kelvin (1871),[38] Hermann von Helmholtz(1879)[citation needed] and, somewhat later, by Svante Arrhenius (1903).[39] Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) were important proponents of the hypothesis who further contended that lifeforms continue to enter the Earth's atmosphere, HP Pavilion dv6-3190sf Laptop CPU Cooling Fan

and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution.[40]

Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent from Earth to seed new stellar systems with life. The Nobel prize winner Francis Crick, along with Leslie Orgel proposed that seeds of life may have been purposely spread by an advanced extraterrestrial civilization,[41] SONY Vaio VGN-FZ21E Laptop CPU Cooling Fan

but considering an early "RNA world". Crick noted later that life may have originated on Earth.[42]

In a virtual presentation on Tuesday, April 7, 2009, Stephen Hawking discussed the possibility of building a human base on another planet and gave reasons why alien life might not be contacting the human race, during his conclusion of the Origins Symposium at Arizona State University. HP G42-415DX Laptop CPU Cooling Fan

Hawking also talked about what humans may find when venturing into space, such as the possibility of alien life through the theory of panspermia.

Carl Sagan, David Grinspoon, Geoffrey A. Landis and Dirk Schulze-Makuch have put forward a hypothesis that microbes could exist in the stable cloud layers 50 km (31 mi) above the surface of Venus; the hypothesis is based on the premises of hospitable climates and chemical disequilibrium. HP 646578-001 Laptop CPU Cooling Fan

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. It may also be present as thin films of salty brine in the first centimeter or so of the soil for part of the year in some locations.[47][48] The origin of the potential biosignature of methane in Mars atmosphere is unexplained, although abiotic hypotheses have also been proposed.[49HP KSB06105HA CPU Fan

] 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 vapours produced by the heating of samples. Photographs from the Mars Global Surveyor from 2006 showed evidence of recent (i.e. within 10 years) flows of a liquid on Mars's frigid surface.[50] Dell Latitude E6500 Laptop CPU Cooling Fan

There is evidence that Mars 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. Nonetheless, present conditions on Mars may support life since lichens were found to successfully surviveMartian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).[51][52] SONY Vaio VGN-NR120Q Laptop CPU Cooling Fan

In June 2012, scientists reported that measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars.[53][54] According to the scientists, "...low H2/CH4ratios (less than approximately 40) indicate that life is likely present and active."[53] Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.[55][56] Toshiba Satellite A205-S5879 Laptop CPU Cooling Fan

On December 9, 2013, NASA reported that, based on evidence from Curiosity studying Aeolis Palus, Gale Cratercontained an ancient freshwater lake that could have been a hospitable environment for microbial life.[57][58]

On January 24, 2014, NASA reported that current studies on the planet Mars by the Curiosity and Opportunity rovers will now be searching for evidence of ancient life, including a biosphere based on autotrophic, HP KSB06105HA Laptop CPU Cooling Fan

chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability,taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective. Toshiba Satellite L645D-S4056 Laptop CPU Cooling Fan

Ceres, a dwarf planet, has recently been confirmed by the Herschel Space Observatory to have water vapor in its atmosphere.[63][64] Frost on the surface may also have been detected.[65] The presence of water, and the temperatures on Ceres has led to speculation that life may be possible there.[66][67][68] The Dawn space probe is scheduled to enter orbit around Ceres in spring 2015. HP Pavilion dv6-2016ax Laptop CPU Cooling Fan

Carl Sagan and others[70] in the 1960s and 1970s 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 thought necessary for molecular biochemistry, so life is thought to be unlikely.[71] SONY Vaio VGN-CR11S/P Laptop CPU Cooling Fan

However, some of Jupiter's moons may have habitats capable of sustaining life. Scientists have suggested that heated subsurface oceans of water may exist deep under the crusts of the three outer Galilean moons—Europa, Ganymede, and Callisto. The EJSM/Laplace mission is planned to determine the habitability of these environments. However, Europa is seen as the main target for the discovery of life. HP Pavilion dv7-6b55dx Laptop CPU Cooling Fan

Jupiter's moon Europa has been subject to speculation about the existence of life due to the strong possibility of liquid water beneath an ice layer. Hydrothermal vents on the bottom of the ocean, if they exist, may warm the ice and could be capable of supporting multicellular microorganisms.[11] It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice.[73] APPLE MacBook Pro 15 inch MA610LL Laptop CPU Cooling Fan

The case for life on Europa was greatly enhanced in 2011 when it was discovered that vast lakes exist within Europa's thick, icy shell. Scientists found that ice shelves surrounding the lakes appear to be collapsing into them, thereby providing a mechanism through which life-forming chemicals created in sunlit areas on Europa's surface could be transferred to its interior.[74][75] HP G71-340US Laptop CPU Cooling Fan

On December 11, 2013, NASA reported the detection of "clay-like minerals" (specifically, phyllosilicates), often associated with organic materials, on the icy crust of Europa, a moon of Jupiter.[76] The presence of the minerals may have been the result of a collision with an asteroid or comet according to the scientists.

Titan, the largest moon of Saturn, is the only known moon with a significant atmosphere. HP Pavilion dv7-6166nr Laptop CPU Cooling Fan

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 stable bodies of surface liquid discovered outside of Earth.[77][78][79] Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface which are consistent with—HP Pavilion dv6-3050et Laptop CPU Cooling Fan

but do not prove—the hypothesis that organisms there are consuming hydrogen, acetylene and ethane, and producing methane.[80][81][82]

An alternate explanation for the hypothetical existence of microbial life on Titan has already been formally proposed[83][84]—hypothesizing that microorganisms could have left Earth when it suffered a massive asteroid or comet impact Dell Vostro 3500 Laptop CPU Cooling Fan

(such as the impact that created Chicxulub crater only 66 mya), and survived a journey through space to land on Titan.

Enceladus, a moon of Saturn, has some of the conditions for life including geothermal activity and water vapor as well as possible under-ice oceans heated by tidal effects. The Cassini probe detected carbon, hydrogen, nitrogen and oxygen—HP KSB06105HA Laptop CPU Cooling Fan

all key elements for supporting living organisms—during a fly-by through one of Enceladus's geysers spewing ice and gas in 2005. The temperature and density of the plumes indicate a warmer, watery source beneath the surface. However, no life has been confirmed.

Small Solar System bodies have also been suggested as habitats for extremophiles. HP Pavilion dv5-2135dx Laptop CPU Cooling Fan

Fred Hoyle has proposed that microbial life might exist on comets.[85] Live bacteria were found on the camera of the Surveyor 3 probe that had stayed on the surface of the Moon for two and a half years.[citation needed] This finding was later considered doubtful as sterile procedures may not have been fully followed.

Scientists are directly searching for biosignatures within the Solar System, carrying out studies on the surface of Mars and examining meteors which have fallen to Earth. HP Pavilion dv7t-6100 CTO Laptop CPU Cooling Fan

At the moment, no concrete plan exists for exploration of Europa for life. In 2008, a joint mission by NASA and the European Space Agency was announced that would have included studies of Europa.[86] However, in 2011 NASA was forced to deprioritize the mission due to a lack of funding, and it is possible that the ESA will take on the mission by itself.[87] Compaq Presario CQ61-435SA Laptop CPU Cooling Fan

There is some limited evidence that microbial life might possibly exist (or have existed) on Mars.[88] An experiment on theViking 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.  ASUS M51SE Laptop CPU Cooling Fan

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.

In February 2005, NASA scientists reported that they may have found some evidence of present life on Mars.[89] HP G42-415DX Laptop CPU Cooling Fan

The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claim 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 distanced NASA from the scientists' claims, and Stoker herself backed off from her initial assertions.[90] ACER Aspire 4736ZG Laptop CPU Cooling Fan

Though such methane findings are still very much in debate, support among some scientists for the existence of life on Mars seems to be growing: an informal survey conducted at the conference at which the European Space Agency presented its findings on methane in Mars' atmosphere, indicated that 75% of the people present agreed that bacteria once lived on Mars. Roughly 25% agreed that bacteria inhabit the planet today.[91]  HP Pavilion dv4-1133tx Laptop CPU Cooling Fan

In November 2011, NASA launched the Mars Science Laboratory (MSL) rover which is designed to search for past or present habitability on Mars using a variety of scientific instruments. The MSL landed on Mars at Gale Crater in August 2012.[92][93][94]

The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium,  HP Pavilion dv6-3043ee Laptop CPU Cooling Fan

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.

In March 2011, Richard B. Hoover, an astrobiologist with the Marshall Space Flight Center, speculated on the finding of alleged microfossils similar tocyanobacteria in CI1 carbonaceous meteorites.[95][96 Compaq Presario CQ61-421SG Laptop CPU Cooling Fan

] However, NASA formally distanced itself from Hoover's claim.[97][98][99] 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.[100][101][10 HP Pavilion dv6-1101au Laptop CPU Cooling Fan

2] In October 2011, scientists reported thatcosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.[103][104][105] One of the scientists suggested that these compounds may have been related to the development of life on Earth and said that, HP Pavilion dv6-3125er Laptop CPU Cooling Fan

"If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."[103]

In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth.[106][ HP 606575-001 Laptop CPU Cooling Fan

107] Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.[108]

In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs),  HP Pavilion dv7-6185us Laptop CPU Cooling Fan

subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics - "a step along the path toward amino acids and nucleotides, the raw materials ofproteins and DNA, respectively".[109][110] Further, as a result of these transformations, DELL DFS491105MH0T Laptop CPU Cooling Fan

the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers ofprotoplanetary disks."[109][110]

In January 2013, Chandra Wickramasinghe reported in the fringe science Journal of Cosmology, HP 532613-001 Laptop CPU Cooling Fan

of shapes resembling fossil diatom frustules in a new carbonaceous meteorite that landed in the North Central Province of Sri Lanka on 29 December 2012.

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.[ Compaq Presario CQ60-417DX Laptop CPU Cooling Fan

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.[citation needed]

Projects such as SETI are conducting an astronomical search for radio activity which would confirm the presence of intelligent life. HP Pavilion dv7-3060us Laptop CPU Cooling Fan

A related suggestion is that aliens might broadcast pulsed and continuous laser signals in the optical, as well as infrared, spectrum;[112] 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,  HP KSB06105HA Laptop CPU Cooling Fan

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.

Some have hypothesized that very advanced civilizations may create artificial black holes as an energy source or method of waste disposal. Dell Latitude E6510 Laptop CPU Cooling Fan

Thus, they suggest that the observation of a black hole with a mass of less than 3.5 solar masses, the theoretical lower mass limit for a naturally occurring black hole, would be evidence of an alien civilization.

Astronomers search for extrasolar planets that may be conducive to life, narrowing the search to terrestrial planets within the habitable zone.[114][115] HP Pavilion dv6-2019er Laptop CPU Cooling Fan

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. HP Pavilion dv6-3126ea Laptop CPU Cooling Fan

It has been argued that Alpha Centauri, the closest star system to Earth, may contain planets which could be capable of sustaining life.[116]

On April 24, 2007, scientists at the European Southern Observatory in La Silla, Chile said they had found the first Earth-like planet. HP 532613-001 Laptop CPU Cooling Fan

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 HP Pavilion dv6-3300ss Laptop CPU Cooling Fan

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 degreesFahrenheit), 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.[117] IBM Lenovo Thinkpad W500 Series Laptop CPU Cooling Fan

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.[118]

In May 2011, researchers predicted that Gliese 581 d, not only exists in the "Goldilocks zone" where water can be present in liquid form, HP 646578-001 Laptop CPU Cooling Fan

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.[119]

In December 2011, NASA confirmed that 600-light-year distant Kepler-22b, at 2.4 times the radius of Earth, is potentially the closest match to Earth in terms of both size and temperature.[120][121] HP Pavilion dv5-1020et Laptop CPU Cooling Fan

Since 1992, hundreds of planets around other stars ("extrasolar planets" or "exoplanets") in the Milky Way Galaxy have been discovered. As of February 18, 2014, the Extrasolar Planets Encyclopaedia identified 1077 extrasolar planets (in 814 planetary systems and 179 multiple planetary systems); the extrasolar planets range in size from that of terrestrial planets similar to Earth to that of gas giants larger than Jupiter.[122HP Pavilion dv5030 Laptop CPU Cooling Fan

] The number of observed exoplanets is expected to increase greatly in the coming years. Because the Kepler spacecraft must view three stellar transits by exoplanets before it identifies them as candidate planets, it has so far only been able to identify planets that orbit their star at a relatively quick rate. The mission is expected to continue until at least 2016, in which time many more exoplanet candidates are expected to be found.[123] Compaq Presario CQ61-416EG Laptop CPU Cooling Fan

Despite these successes, the transit method employed by the Kepler spacecraft requires that planetary orbits be at a small inclination to the line of sight of the observer. Due to this constraint, the probability of detecting a planet of Earth's size and orbital radius around a distant star is just 0.47%. Thus, the number of planets we are currently able to detect is only a small fraction of the total number of planets present within the galaxy. HP 532614-001 Laptop CPU Cooling Fan

Criticism of the Drake equation follows mostly from the observation that the terms in the equation are entirely based on conjecture. Thus the equation cannot be used to draw firm conclusions of any kind.[125] Although the Drake equation currently involves speculation about unmeasured parameters, it was not meant to be science, but intended as a way to stimulate dialogue on these topics. HP Pavilion dv5-1020et Laptop CPU Cooling Fan

Then the focus becomes how to proceed experimentally. Indeed, Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference.[126]

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.[127]  HP 646578-001 Laptop CPU Cooling Fan

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,[128] 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, HP Pavilion dv5-1020et Laptop CPU Cooling Fan

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there would be some 6.25×109 (billion) life-supporting planetary systems in the observable Universe.

The apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations is known as the Fermi paradox.

In antiquity, it was common to assume a cosmos consisting of "many worlds" inhabited by intelligent, Compaq Presario CQ61-416EG Laptop CPU Cooling Fan

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.[129] An example would be the fourteen Loka of Hindu cosmology, or theNine 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. HP 532614-001 Laptop CPU Cooling Fan

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.

Buddhist and Hindu beliefs of endlessly repeated cycles of life called Samsara 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"). HP MCF-W13BM05 Laptop CPU Cooling Fan

According to Buddhist and Hindu scriptures, there are numerous universes to facilitate the fulfillment of the separated desires of the numerous living entities. However, the purpose of such creations is to bring back the deluded souls to correct understanding about the purpose of life and achieve Nirvana. Aside from the numerous universes which are material, there is an unlimited number spirit worlds, HP Pavilion dv5-1174ca Laptop CPU Cooling Fan

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.[citation needed]

The Jewish Talmud states that there are at least 18,000 other worlds, but provides little elaboration on the nature of those worlds, HP Pavilion dv7-3050ec Laptop CPU Cooling Fan

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.[130][131] HP KSB06105HA Laptop CPU Cooling Fan

According to Ahmadiyya 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..."; HP 646578-001 Laptop CPU Cooling Fan

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. HP 489126-001 Laptop CPU Cooling Fan

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.[132]

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. DELL Inspiron E1405 Laptop CPU Cooling Fan

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