About Alien Extraterrestrial life
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 simple bacteria-like organisms to beings far more complex than humans.
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. Nonetheless, scientists at the National Institutes of Health reported studies that life in the universe may have begun "9.7±2.5 billion years ago", billions of years before the Earth was formed, based on extrapolating the "genetic complexity of organisms" (from "major phylogenetic lineages") to earlier times. Many scientists consider extraterrestrial life to be plausible, but there is no direct evidence of its existence. 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. It has also played a major role in works of science fiction.
Background
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. 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. 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. 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. According to these studies, this same process may also occur around other stars that acquire planets. (Also see Extraterrestrial organic molecules.) Suggested locations at which life might have developed include the planets Venus[8] and Mars, Jupiter's moon Europa, and Saturn's moons Titan and Enceladus. In May 2011, NASA scientists reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it". Since the 1950s, scientists have promoted the idea that "habitable zones" as the most likely places for life to be found. Numerous discoveries in this zone since 2007 have stimulated estimations of frequencies of Earth-like habitats numbering in the many billions though as of 2013, only a small number of planets have been discovered in these zones. More recently, astrobiologists have increasingly shifted toward a "follow the energy" view of potential habitats.
No widely accepted evidence of extraterrestrial life has been found; however, various controversial claims have been made. Beliefs that some unidentified flying objects are of extraterrestrial origin (see Extraterrestrial hypothesis), along with claims of alien abduction, are dismissed by most scientists. Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes.
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. According to the response, "The US 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." Also, according to the response, there is "no credible information to suggest that any evidence is being hidden from the public's eye." 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."Possible basis
Several hypotheses have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological viewpoint.
Biochemistry
Main articles: Biochemistry, Hypothetical types of biochemistry, and Water and life
All life on Earth is based upon 26 chemical elements. However, about 95% of this life is built upon only six of these elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur, abbreviated CHNOPS. 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.[23]
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.[24] Terrestrial planets such as Earth are formed in a process that allows for the possibility of having compositions similar to Earth's.[25] The combination of carbon, hydrogen and oxygen in the chemical form of carbohydrates (e.g. sugar) can be a source of chemical energy on which life depends, and can provide structural elements for life (such as ribose, in the molecules DNA and RNA, and cellulose in plants). Plants derive energy through the conversion of light energy into chemical energy via photosynthesis. Life, as currently recognized, requires carbon in both reduced (methane derivatives) and partially oxidized (carbon oxides) states. Nitrogen is needed as a reduced ammonia derivative in all proteins, sulfur as a derivative of hydrogen sulfide in some necessary proteins, and phosphorus oxidized to phosphates in genetic material and in energy transfer.
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. Additionally, the hydrogen bonds between water molecules give it an ability to store energy with evaporation, which upon condensation is released. This helps to moderate the climate, cooling the tropics and warming the poles, helping to maintain the thermodynamic stability needed for life.
Carbon is fundamental to terrestrial life for its immense flexibility in creating covalent chemical bonds with a variety of non-metallic elements, principally nitrogen, oxygen and hydrogen. Carbon dioxide and water together enable the storage of solar energy in sugars and starches, such as glucose. The oxidation of glucose releases biochemical energy needed to fuel all other biochemical reactions.
The ability to form organic acids (–COOH) and amine bases (–NH2) gives rise to the possibility of neutralization dehydrating reactions to build long polymer peptides and catalytic proteins from monomer amino acids. When combined with phosphates, these acids can build the information-storing molecule of inheritance, DNA, and the principal energy transfer molecule of cellular life, ATP.
Due to their relative abundance and usefulness in sustaining life, many have hypothesized that life forms elsewhere in the universe would utilize these basic materials. However, other elements and solvents could provide a basis for life. Life forms based in ammonia (rather than water) have been suggested, though this solution appears less optimal than water.[26]
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.[27] 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.[28]
Several pre-conceived ideas about the characteristics of life outside of Earth have been questioned. For example, a NASA scientist suggested that the color of photosynthesizing pigments of hypothetical life on extrasolar planets might not be green.[29]
Evolution and morphology
In addition to the biochemical basis of extraterrestrial life, many have considered evolution and morphology. Science fiction has often depicted extraterrestrial life with humanoid and/or reptilian forms. Aliens have often been depicted as having light green or grey skin, with a large head, as well as four limbs—i.e. fundamentally humanoid. Other subjects, such as felines, insects, blobs, etc., have occurred in fictional representations of aliens.
A division has been suggested between universal and parochial (narrowly restricted) characteristics. Universals are features which are thought to have evolved independently more than once on Earth (and thus, presumably, are not too difficult to develop) and are so intrinsically useful that species will inevitably tend towards them. The most fundamental of these is probably bilateral symmetry, but more complex (though still basic) characteristics include flight, sight, photosynthesis and limbs, all of which are thought to have evolved several times here on Earth. There is a huge variety of eyes, for example, and many of these have radically different working schematics and different visual foci: the visual spectrum, infrared, polarity and echolocation. Parochials, however, are essentially arbitrary evolutionary forms. These often have little inherent utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. Intelligent aliens could communicate through gestures, as deaf humans do, by sounds created from structures unrelated to breathing, which happens on Earth when, for instance, cicadas vibrate their wings or crickets stridulate their wings, or visually through bioluminescence 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. 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. These two schools of thought are called "divergionism" and "convergionism" respectively.[27]
Planetary habitability in the Solar System
See also: Planetary habitability and Natural satellite habitability
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, 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. A subsurface marine environment on Jupiter's moon Europa might be the most suitable habitat in the Solar System, outside of Earth, for multicellular 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). 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-Grunt mission.[30] The first known mention of the term Panspermia was in the writings of the 5th century BC Greek philosopher Anaxagoras.[31] In the nineteenth century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834),[32] Kelvin (1871),[33] Hermann von Helmholtz (1879)[citation needed] and, somewhat later, by Svante Arrhenius (1903).[34] 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, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution.
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,[36] but considering an early "RNA world". Crick noted later that life may have originated on Earth.[37]
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. Hawking also talked about what humans may find when venturing into space, such as the possibility of alien life through the theory of panspermia.[38]
Venus
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.[39] [40] [41]
Mars
Main article: Life on Mars (planet)
See also: Water on 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. 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.[42][43] The origin of the potential biosignature of methane in Mars atmosphere is unexplained, although abiotic hypotheses have also been proposed.[44] 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 produ 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 survive Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).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. According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active."[48] Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.Jupiter
Carl Sagan and others[52] 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.[53]
However, some of Jupiter's moons may have habitats to sustain 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 is planned to determine the habitability of these environments. However, Europa is seen as the main target for the discovery of life.
Europa
Subsurface oceans such as the one pictured of Europa could possibly harbour life.[54]
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.[9] It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice.[55]
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.[56][57]
Saturn
While Saturn is itself considered inhospitable to life, the planet's natural satellites Titan and Enceladus have been speculated to possess possible habitats for life.
Titan
See also: Life on Titan
Titan, the largest moon of Saturn, is the only known moon with a significant atmosphere. Data from the Cassini–Huygens mission refuted the hypothesis of a global hydrocarbon ocean, but later demonstrated the existence of liquid hydrocarbon lakes in the polar regions—the first stable bodies of liquid discovered outside of Earth.[58][59][60] Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface which are consistent with—but do not prove—the hypothesis that organisms there are consuming hydrogen, acetylene and ethane, and producing methane.[61][62][63]
An alternate explanation for the hypothetical existence of microbial life on Titan has already been formally proposed[64][65]—hypothesizing that microorganisms could have left Earth when it suffered a massive asteroid or comet impact (such as the impact that created Chicxulub crater only 65 mya), and survived a journey through space to land on Titan.
Enceladus
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—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
Small Solar System bodies have also been suggested as habitats for extremophiles. Fred Hoyle has proposed that microbial life might exist on comets. 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. This finding was later considered doubtful as sterile procedures may not have been fully followed.
Scientific search
The NASA Kepler for the search of extrasolar planets.
The scientific search for extraterrestrial life is being carried out both directly and indirectly.
Direct search
Scientists are directly searching for biosignatures within the Solar System, carrying out studies on the surface of Mars and examining meteors which have fallen to Earth. 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. 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.
There is some limited evidence that microbial life might possibly exist (or have existed) on Mars. An experiment on the Viking Mars lander reported gas emissions from heated Martian soil that some argue are consistent with the presence of microbes. However, the lack of corroborating evidence from other experiments on the Viking lander indicates that a non-biological reaction is a more likely hypothesis. Independently, in 1996, structures resembling nanobacteria were reportedly discovered in a meteorite, ALH84001, thought to be formed of rock ejected from Mars. This report is controversial.
Electron micrograph of martian meteorite ALH84001 showing structures that some scientists think could be fossilized bacteria-like life forms.
In February 2005, NASA scientists reported that they may have found some evidence of present life on Mars.[70] The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claim on methane signatures found in Mars's 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.
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.
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.
The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium, which is relatively easy to determine from a distance by spectroscopy. However, significant advances in the ability to find and resolve light from smaller rocky worlds near their star are necessary before such spectroscopic methods can be used to analyze extrasolar planets.
On March 5, 2011, Richard B. Hoover, an astrobiologist with the Marshall Space Flight Center, speculated on the finding of alleged microfossils similar to cyanobacteria in CI1 carbonaceous meteorites. However, NASA formally distanced itself from Hoover's claim. See Hoover paper controversy for more details.
In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggests DNA and RNA components (adenine, guanine and related organic molecules), building blocks for life as we know it, may be formed extraterrestrially in outer space. In October 2011, scientists reported that cosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. One of the scientists suggested that these compounds may have been related to the development of life on Earth and said that, "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."
On August 29, 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.[87][88] 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.
In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), 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 of proteins and DNA, respectively". Further, as a result of these transformations, 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 of protoplanetary disks."
On January 10, 2013, Chandra Wickramasinghe reported in the fringe science Journal of Cosmology, 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.
Indirect search
Terrestrial Planet Finder
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.[citation needed] 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. A related suggestion is that aliens might broadcast pulsed and continuous laser signals in the optical, as well as infrared, spectrum;laser signals have the advantage of not "smearing" in the interstellar medium, and may prove more conducive to communication between the stars. While other communication techniques, including laser transmission and interstellar spaceflight, have been discussed seriously and may well be feasible, the measure of effectiveness is the amount of information communicated per unit cost. This results in radio transmission as the method of choice.[citation needed]
Some have hypothesized that very advanced civilizations may create artificial black holes as an energy source or method of waste disposal. 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.
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