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150-516: The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. Those affirming the paradox generally conclude that if the conditions required for life to arise from non-living matter are as permissive as the available evidence on Earth indicates, then extraterrestrial life would be sufficiently common such that it would be implausible for it not to have been detected yet. The quandary takes its name from
300-650: A G2 star due to Earth's television and telecommunication broadcasts. In the absence of an apparent natural cause, alien observers might infer the existence of a terrestrial civilization. Such signals could be either "accidental" by-products of a civilization, or deliberate attempts to communicate, such as the Arecibo message . It is unclear whether "leakage", as opposed to a deliberate beacon, could be detected by an extraterrestrial civilization. The most sensitive radio telescopes on Earth, as of 2019, would not be able to detect non-directional radio signals (such as broadband ) even at
450-474: A geological scale , let alone a cosmological one . Since there are many stars older than the Sun, and since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy has not been colonized already. Even if colonization is impractical or undesirable to all alien civilizations, large-scale exploration of the galaxy could be possible by probes . These might leave detectable artifacts in
600-416: A basic component of the development of a potential extraterrestrial technological civilization, as it is on Earth. Fossil fuels may likely be generated and used on such worlds as well. The abundance of chlorofluorocarbons in the atmosphere can also be a clear technosignature, considering their role in ozone depletion . Light pollution may be another technosignature, as multiple lights on the night side of
750-399: A better estimation of how common these potentially habitable worlds are. The Hart–Tipler conjecture is a form of contraposition which states that because no interstellar probes have been detected, there likely is no other intelligent life in the universe, as such life should be expected to eventually create and launch such probes. Self-replicating probes could exhaustively explore a galaxy
900-611: A catalog of microorganisms, with the way each one reacts to sunlight. The goal is to help with the search for similar organisms in exoplanets, as the starlight reflected by planets rich in such organisms would have a specific spectrum, unlike that of starlight reflected from lifeless planets. If Earth was studied from afar with this system, it would reveal a shade of green, as a result of the abundance of plants with photosynthesis. In August 2011, NASA studied meteorites found on Antarctica, finding adenine , guanine , hypoxanthine and xanthine . Adenine and guanine are components of DNA, and
1050-418: A civilization might try to harness a large part of the energy produced by a star. He proposed a hypothetical "Dyson sphere" as a possible means: a shell or cloud of objects enclosing a star to absorb and utilize as much radiant energy as possible. Such a feat of astroengineering would drastically alter the observed spectrum of the star involved, changing it at least partly from the normal emission lines of
1200-513: A fraction of a light-year away, but other civilizations could hypothetically have much better equipment. A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through SETI organizations such as the SETI Institute and Breakthrough Listen . Several decades of SETI analysis have not revealed any unusually bright or meaningfully repetitive radio emissions. Exoplanet detection and classification
1350-535: A host of fortuitous circumstances, such as a galactic habitable zone , a star and planet(s) having the requisite conditions, such as enough of a continuous habitable zone , the advantage of a giant guardian like Jupiter and a large moon , conditions needed to ensure the planet has a magnetosphere and plate tectonics , the chemistry of the lithosphere , atmosphere , and oceans, the role of "evolutionary pumps" such as massive glaciation and rare bolide impacts. Perhaps most importantly, advanced life needs whatever it
1500-478: A living entity must have the capacity to replicate itself, the capacity to avoid damage/decay, and the capacity to acquire and process resources in support of the first two capacities. Life on Earth started with an RNA world and later evolved to its current form, where some of the RNA tasks were transferred to DNA and proteins . Extraterrestrial life may still be stuck using RNA, or evolve into other configurations. It
1650-524: A math equation, none of its values were known at the time. Although some values may eventually be measured, others are based on social sciences and are not knowable by their very nature. This does not allow one to make noteworthy conclusions from the equation. Based on observations from the Hubble Space Telescope , there are nearly 2 trillion galaxies in the observable universe. It is estimated that at least ten per cent of all Sun-like stars have
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#17328589121891800-466: A mixed aromatic - aliphatic structure") that could be created naturally, and rapidly, by stars . It is still unclear if those compounds played a role in the creation of life on Earth, but Sun Kwok, of the University of Hong Kong, thinks so. "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." In August 2012, and in
1950-436: A narrower search for extraterrestrial intelligent life . Depending on the category of search, methods range from analysis of telescope and specimen data to radios used to detect and transmit communications. The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, especially extraterrestrials in fiction . Science fiction has communicated scientific ideas, imagined
2100-529: A natural stellar atmosphere to those of black-body radiation , probably with a peak in the infrared . Dyson speculated that advanced alien civilizations might be detected by examining the spectra of stars and searching for such an altered spectrum. There have been some attempts to find evidence of the existence of Dyson spheres that would alter the spectra of their core stars. Direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications. In October 2015, there
2250-494: A perfect universe for us to be in, because nobody would be able to boss us around, and we'd get to be the voyagers, the Star Trek people, the starship builders, the policemen, and so on." The rapid increase of scientific and technological progress seen in the 19th and 20th centuries, compared to earlier eras, led to the common assumption that such progresses will keep growing at exponential rates as time goes by, eventually leading to
2400-627: A planetary system formed by a star at the center, the Sun , and the objects that orbit it: other planets, moons, asteroids, and comets. The sun is part of the Milky Way , a galaxy . The Milky Way is part of the Local Group , a galaxy group that is in turn part of the Laniakea Supercluster . The universe is composed of all similar structures in existence. The immense distances between celestial objects
2550-462: A potential explanation to the Fermi paradox. If extraterrestrial life exists, it could range from simple microorganisms and multicellular organisms similar to animals or plants, to complex alien intelligences akin to humans . When scientists talk about extraterrestrial life, they consider all those types. Although it is possible that extraterrestrial life may have other configurations, scientists use
2700-507: A range of possibilities, and influenced public interest in and perspectives on extraterrestrial life. One shared space is the debate over the wisdom of attempting communication with extraterrestrial intelligence. Some encourage aggressive methods to try to contact intelligent extraterrestrial life. Others – citing the tendency of technologically advanced human societies to enslave or destroy less advanced societies – argue it may be dangerous to actively draw attention to Earth. Initially, after
2850-566: A reasonable scientific explanation for any gathered data. In 1961, astronomer and astrophysicist Frank Drake devised the Drake equation as a way to stimulate scientific dialogue at a meeting on the search for extraterrestrial intelligence (SETI). The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilisations in the Milky Way galaxy . The Drake equation is: where: and Drake's proposed estimates are as follows, but numbers on
3000-517: A research position at the University of Texas' Applied Research Laboratories. Mark retired from the faculty of the University of Texas at Austin on July 1, 2014. He was voted by the Regents of the University as a Professor Emeritus of Aerospace Engineering and Engineering Mechanics. Mark was a member of the National Academy of Engineering , the nation's highest honor for engineering professionals. He
3150-443: A result has been conceived, even if hypothetical. Evolution requires life to be divided into individual organisms, and no alternative organisation has been satisfactorily proposed either. At the basic level, membranes define the limit of a cell, between it and its environment, while remaining partially open to exchange energy and resources with it. The evolution from simple cells to eukaryotes, and from them to multicellular lifeforms,
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#17328589121893300-453: A rocky planet can be a sign of advanced technological development. However, modern telescopes are not strong enough to study exoplanets with the required level of detail to perceive it. Hans Mark Hans Michael Mark (June 17, 1929 – December 18, 2021) was a German-born American government official who served as Secretary of the Air Force and as a Deputy Administrator of NASA . He
3450-468: A scale. Life on Earth is quite ubiquitous across the planet and has adapted over time to almost all the available environments in it, extremophiles and the deep biosphere thrive at even the most hostile ones. As a result, it is inferred that life in other celestial bodies may be equally adaptive. However, the origin of life is unrelated to its ease of adaptation, and may have stricter requirements. A celestial body may not have any life on it, even if it
3600-406: A series of calculations on the probability of earthlike planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over." Teller remembers that not much came of this conversation "except perhaps a statement that
3750-470: A series of factors that range from the location in the galaxy and the configuration of the Solar System to local characteristics of the planet, and that it is unlikely that all such requirements are simultaneously met by another planet. The proponents of this hypothesis consider that very little evidence suggests the existence of extraterrestrial life, and that at this point it is just a desired result and not
3900-407: A significant fraction of the size of the observable universe . Even if such civilizations are rare, the scale argument indicates they should exist somewhere at some point during the history of the universe, and since they could be detected from far away over a considerable period of time, many more potential sites for their origin are within range of human observation. It is unknown whether the paradox
4050-575: A significant number of extant civilizations in the Milky Way. This assumes the mediocrity principle , by which Earth is a typical planet . The second aspect of the Fermi paradox is the argument of probability: given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats , it seems possible that at least some civilizations would be technologically advanced, seek out new resources in space, and colonize their star system and, subsequently, surrounding star systems. Since there
4200-497: A slow speed-of-light dialogue between vastly distant neighbors. Rather than contending with the long delays a radio dialogue would suffer, a probe housing an artificial intelligence would seek out an alien civilization to carry on a close-range communication with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilization at light speed, but an information-gathering dialogue could be conducted in real time. Direct exploration of
4350-563: A solvent other than water. The low temperatures required would add an extra problem, the difficulty to kickstart a process of abiogenesis to create life in the first place. Norman Horowitz , head of the Jet Propulsion Laboratory bioscience section for the Mariner and Viking missions from 1965 to 1976 considered that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to
4500-440: A star's light and heat, and so its lifeforms would not grow beyond a certain complexity. There is also research in assessing the capacity of life for developing intelligence. It has been suggested that this capacity arises with the number of potential niches a planet contains, and that the complexity of life itself is reflected in the information density of planetary environments, which in turn can be computed from its niches. It
4650-599: A system of planets, i.e. there are 6.25 × 10 stars with planets orbiting them in the observable universe. Even if it is assumed that only one out of a billion of these stars has planets supporting life, there would be some 6.25 billion life-supporting planetary systems in the observable universe. A 2013 study based on results from the Kepler spacecraft estimated that the Milky Way contains at least as many planets as it does stars, resulting in 100–400 billion exoplanets. The apparent contradiction between high estimates of
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4800-530: A water world. In other words, 32 percent continental mass may be high among water worlds..." Brin continues, "In which case, the evolution of creatures like us, with hands and fire and all that sort of thing, may be rare in the galaxy. In which case, when we do build starships and head out there, perhaps we'll find lots and lots of life worlds, but they're all like Polynesia. We'll find lots and lots of intelligent lifeforms out there, but they're all dolphins, whales, squids, who could never build their own starships. What
4950-461: 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. 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
5100-519: Is abiogenesis : a gradual process of increasing complexity of the first self-replicating molecules by a randomly occurring chemical process. Other proposed great filters are the emergence of eukaryotic cells or of meiosis or some of the steps involved in the evolution of a brain capable of complex logical deductions. Astrobiologists Dirk Schulze-Makuch and William Bains, reviewing the history of life on Earth, including convergent evolution , concluded that transitions such as oxygenic photosynthesis ,
5250-421: Is (nearly) impossible argue that the conditions needed for the evolution of life—or at least the evolution of biological complexity —are rare or even unique to Earth. Under this assumption, called the rare Earth hypothesis , a rejection of the mediocrity principle , complex multicellular life is regarded as exceedingly unusual. The rare Earth hypothesis argues that the evolution of biological complexity requires
5400-585: Is a difficulty for the study of extraterrestrial life. So far, humans have only set foot on the Moon and sent robotic probes to other planets and moons in the Solar System. Although probes can withstand conditions that may be lethal to humans, the distances cause time delays: the New Horizons took nine years after launch to reach Pluto . No probe has ever reached extrasolar planetary systems. The Voyager 2 has left
5550-507: Is a very active sub-discipline in astronomy; the first candidate terrestrial planet discovered within a star's habitable zone was found in 2007. New refinements in exoplanet detection methods , and use of existing methods from space (such as the Kepler and TESS missions) are starting to detect and characterize Earth-size planets, to determine whether they are within the habitable zones of their stars. Such observational refinements may allow for
5700-421: Is also an active field of scientific research. This includes both efforts to find any indication of life, and efforts specifically directed to finding intelligent life. These searches have been made since 1960, and several are ongoing. Although astronomers do not usually search for extraterrestrials, they have observed phenomena that they could not immediately explain without positing an intelligent civilization as
5850-501: Is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon , such as methane , ethane or propane . Another unknown aspect of potential extraterrestrial life would be the chemical elements that would compose it. Life on Earth is largely composed of carbon, but there could be other hypothetical types of biochemistry . A replacement for carbon would need to be able to create complex molecules, store information required for evolution, and be freely available in
6000-399: Is an ordered system that can sustain itself against the tendency to disorder , Stephen Hawking's "external transmission" or interstellar communicative phase, where knowledge production and knowledge management is more important than transmission of information via evolution , may be the point at which the system becomes unstable and self-destructs. Here, Hawking emphasizes self-design of
6150-416: Is being carried out both directly and indirectly. As of September 2017 , 3,667 exoplanets in 2,747 systems have been identified , and other planets and moons in the Solar System hold the potential for hosting primitive life such as microorganisms . As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms on Venus (via phosphine ) and Mars (via methane )
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6300-403: Is common knowledge that the conditions on other planets in the solar system, in addition to the many galaxies outside of the Milky Way galaxy , are very harsh and seem to be too extreme to harbor any life. The environmental conditions on these planets can have intense UV radiation paired with extreme temperatures, lack of water, and much more that can lead to conditions that don't seem to favor
6450-485: Is complex and defined by several factors. Being in the habitable zone is not enough for a planet to be habitable, not even to actually have such liquid water. Venus is located in the habitable zone of the Solar System but does not have liquid water because of the conditions of its atmosphere. Jovian planets or gas giants are not considered habitable even if they orbit close enough to their stars as hot Jupiters , due to crushing atmospheric pressures. The actual distances for
6600-509: Is embodied in the Copernican principle , which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle , which states that there is nothing special about life on Earth. Other authors consider instead that life in the cosmos, or at least multicellular life, may be actually rare. The Rare Earth hypothesis maintains that life on Earth is possible because of
6750-455: Is everybody?" (Teller's letter), or "Don't you ever wonder where everybody is?" (York's letter), or "But where is everybody?" (Konopinski's letter). Teller wrote, "The result of his question was general laughter because of the strange fact that, in spite of Fermi's question coming out of the blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life." In 1984 York wrote that Fermi "followed up with
6900-404: Is everybody?"—although the exact quote is uncertain . There have been many attempts to resolve the Fermi paradox, such as suggesting that intelligent extraterrestrial beings are extremely rare , that the lifetime of such civilizations is short, or that they exist but (for various reasons) humans see no evidence. The following are some of the facts and hypotheses that together serve to highlight
7050-414: Is explored both in fiction and in scientific hypothesizing. In 1966, Sagan and Shklovskii speculated that technological civilizations will either tend to destroy themselves within a century of developing interstellar communicative capability or master their self-destructive tendencies and survive for billion-year timescales. Self-annihilation may also be viewed in terms of thermodynamics : insofar as life
7200-439: Is likely devoid of life. However, Venus is still of interest to astrobiologists, as it is a terrestrial planet that was likely similar to Earth in its early stages and developed in a different way. There is a greenhouse effect , the surface is the hottest in the Solar System, sulfuric acid clouds, all surface liquid water is lost, and it has a thick carbon-dioxide atmosphere with huge pressure. Comparing both helps to understand
7350-444: Is no guarantee that it is transmitting radio communications in the direction of Earth. The length of time required for a signal to travel across space means that a potential answer may arrive decades or centuries after the initial message. The atmosphere of Earth is rich in nitrogen dioxide as a result of air pollution , which can be detectable. The natural abundance of carbon, which is also relatively reactive, makes it likely to be
7500-444: Is no significant evidence on Earth, or elsewhere in the known universe, of other intelligent life after 13.8 billion years of the universe's history, there is a conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than is thought, that assumptions about the general development or behavior of intelligent species are flawed, or, more radically, that current scientific understanding of
7650-657: Is not guaranteed. The Cambrian explosion took place thousands of millions of years after the origin of life, and its causes are not fully known yet. On the other hand, the jump to multicellularity took place several times, which suggests that it could be a case of convergent evolution , and so likely to take place on other planets as well. Palaeontologist Simon Conway Morris considers that convergent evolution would lead to kingdoms similar to our plants and animals, and that many features are likely to develop in alien animals as well, such as bilateral symmetry , limbs , digestive systems and heads with sensory organs . Scientists from
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#17328589121897800-500: Is relatively early in the potential timeline of intelligent life in the universe, as loud aliens would otherwise be observable by astronomers. Earlier in 2013, Anders Sandberg and Stuart Armstrong examined the potential for intelligent life to spread intergalactically throughout the universe and the implications for the Fermi Paradox. Their study suggests that with sufficient energy, intelligent civilizations could potentially colonize
7950-422: Is stronger for the Milky Way galaxy or for the universe as a whole. The theories and principles in the Drake equation are closely related to the Fermi paradox. The equation was formulated by Frank Drake in 1961 in an attempt to find a systematic means to evaluate the numerous probabilities involved in the existence of alien life. The equation is presented as follows: Where N {\displaystyle N}
8100-466: Is that the chemical elements that make up life, such as carbon and water, are ubiquitous in the universe. The third is that the physical laws are universal, which means that the forces that would facilitate or prevent the existence of life would be the same ones as on Earth. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking , it would be improbable for life not to exist somewhere else other than Earth. This argument
8250-421: Is that there are natural sources of such signals as well, such as gamma-ray bursts and supernovae, and the difference between a natural signal and an artificial one would be in its specific patterns. Astronomers intend to use artificial intelligence for this, as it can manage large amounts of data and is devoid of biases and preconceptions. Besides, even if there is an advanced extraterrestrial civilization, there
8400-660: Is the number of technologically advanced civilizations in the Milky Way galaxy, and N {\displaystyle N} is asserted to be the product of The fundamental problem is that the last four terms ( f l {\displaystyle f_{l}} , f i {\displaystyle f_{i}} , f c {\displaystyle f_{c}} , and L {\displaystyle L} ) are entirely unknown, rendering statistical estimates impossible. The Drake equation has been used by both optimists and pessimists, with wildly differing results. The first scientific meeting on
8550-447: Is unclear if our biochemistry is the most efficient one that could be generated, or which elements would follow a similar pattern. However, it is likely that, even if cells had a different composition to those from Earth, they would still have a cell membrane . Life on Earth jumped from prokaryotes to eukaryotes and from unicellular organisms to multicellular organisms through evolution . So far no alternative process to achieve such
8700-474: The Big Bang the universe was too hot to allow life. 15 million years later , it cooled to temperate levels, but the elements that make up living things did not exist yet. The only freely available elements at that point were hydrogen and helium . Carbon and oxygen (and later, water ) would not appear until 50 million years later, created through stellar fusion. At that point, the difficulty for life to appear
8850-666: The Massachusetts Institute of Technology in 1954. After receiving his doctorate, Mark stayed on at MIT as a research associate and acting head of the Neutron Physics Group Laboratory for Nuclear Science. He returned to UC Berkeley in 1955 and remained there until 1958 as a research physicist at the University's Lawrence Radiation Laboratory in Livermore, California . Mark then returned to MIT as an assistant professor of physics. In 1960, he again returned to
9000-750: The Rio Tinto river in Spain. NASA officials soon distanced NASA from the scientists' claims, and Stoker herself backed off from her initial assertions. In November 2011, NASA launched the Mars Science Laboratory that landed the Curiosity rover on Mars. It is designed to assess the past and present habitability on Mars using a variety of scientific instruments. The rover landed on Mars at Gale Crater in August 2012. A group of scientists at Cornell University started
9150-401: The Sun , from an exterior perspective, due to a layer of "fiery brightness" in the outer layer of the atmosphere. He theorised all extraterrestrial bodies could be inhabited by men, plants, and animals, including the Sun. Descartes wrote that there was no means to prove the stars were not inhabited by "intelligent creatures", but their existence was a matter of speculation. When considering
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#17328589121899300-609: The University of Texas at Austin . In July 1998, he began work at The Pentagon upon President Clinton 's nomination of him as Director of Defense Research and Engineering . In 2001, he returned to the University of Texas at Austin, where he held the John J. McKetta Centennial Energy Chair in Engineering as a professor in the Department of Aerospace Engineering and Engineering Mechanics and held
9450-403: The atmospheric composition and ecosystems hosted by extraterrestrial bodies, extraterrestrial life can seem more speculation than reality, due to the harsh conditions and disparate chemical composition of the atmospheres, when compared to the life-abundant Earth. However, there are many extreme and chemically harsh ecosystems on Earth that do support forms of life and are often hypothesized to be
9600-540: The eukaryotic cell , multicellularity , and tool -using intelligence are likely to occur on any Earth-like planet given enough time. They argue that the Great Filter may be abiogenesis, the rise of technological human-level intelligence, or an inability to settle other worlds because of self-destruction or a lack of resources. Paleobiologist Olev Vinn has suggested that the great filter may have universal biological roots related to evolutionary animal behavior. In 2021,
9750-564: The human genome ( transhumanism ) or enhancement via machines (e.g., brain–computer interface ) to enhance human intelligence and reduce aggression , without which he implies human civilization may be too stupid collectively to survive an increasingly unstable system. For instance, the development of technologies during the "external transmission" phase, such as weaponization of artificial general intelligence or antimatter , may not be met by concomitant increases in human ability to manage its own inventions. Consequently, disorder increases in
9900-459: The hydrothermal vents found on the ocean floor are known to support many chemosynthetic processes which allow organisms to utilize energy through reduced chemical compounds that fix carbon. In return, these reactions will allow for organisms to live in relatively low oxygenated environments while maintaining enough energy to support themselves. The early Earth environment was reducing and therefore, these carbon fixing compounds were necessary for
10050-437: The search for extraterrestrial intelligence (SETI), which had 10 attendees including Frank Drake and Carl Sagan , speculated that the number of civilizations was roughly between 1,000 and 100,000,000 civilizations in the Milky Way galaxy. Conversely, Frank Tipler and John D. Barrow used pessimistic numbers and speculated that the average number of civilizations in a galaxy is much less than one. Almost all arguments involving
10200-469: The Drake equation suffer from the overconfidence effect , a common error of probabilistic reasoning about low-probability events, by guessing specific numbers for likelihoods of events whose mechanism is not yet understood, such as the likelihood of abiogenesis on an Earth-like planet, with current likelihood estimates varying over many hundreds of orders of magnitude . An analysis that takes into account some of
10350-418: The Fermi paradox is a function of the scale or the large numbers involved: there are an estimated 200–400 billion stars in the Milky Way (2–4 × 10 ) and 70 sextillion (7×10) in the observable universe . Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might still be a great number of extant civilizations, and if the percentage were high enough it would produce
10500-454: The Fermi paradox that rely on empirical evidence—that there are many potentially habitable planets , and that humans see no evidence of life. The first point, that many suitable planets exist, was an assumption in Fermi's time but is now supported by the discovery that exoplanets are common. Current models predict billions of habitable worlds in the Milky Way. The second part of the paradox, that humans see no evidence of extraterrestrial life,
10650-547: The Great Silence, and silentium universi (Latin for "silence of the universe"), though these only refer to one portion of the Fermi paradox, that humans see no evidence of other civilizations. In the summer of 1950 at Los Alamos National Laboratory in New Mexico , Enrico Fermi and co-workers Emil Konopinski, Edward Teller, and Herbert York had one or several lunchtime conversations. In one, Fermi suddenly blurted out, "Where
10800-466: The Italian-American physicist Enrico Fermi : in the summer of 1950, Fermi was engaged in casual conversation about contemporary UFO reports and the possibility of faster-than-light travel with fellow physicists Edward Teller , Herbert York , and Emil Konopinski while the group was walking to lunch. The conversation moved on to other topics, until Fermi later blurted out during lunch, "But where
10950-471: The Solar System at a speed of 50,000 kilometers per hour, if it headed towards the Alpha Centauri system, the closest one to Earth at 4.4 light years, it would reach it in 100,000 years. Under current technology such systems can only be studied by telescopes, which have limitations. It is estimated that dark matter has a larger amount of combined matter than stars and gas clouds, but as it plays no role on
11100-601: The Solar System has yielded no evidence indicating a visit by aliens or their probes. Detailed exploration of areas of the Solar System where resources would be plentiful may yet produce evidence of alien exploration, though the entirety of the Solar System is vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded. In 1959, Freeman Dyson observed that every developing human civilization constantly increases its energy consumption, and he conjectured that
11250-464: The Solar System, such as old probes or evidence of mining activity, but none of these have been observed. The second form of the question is "Why are there no signs of intelligence elsewhere in the universe?". This version does not assume interstellar travel, but includes other galaxies as well. For distant galaxies, travel times may well explain the lack of alien visits to Earth, but a sufficiently advanced civilization could potentially be observable over
11400-702: The University of California's Livermore Radiation Laboratory's Experimental Physics Division. He remained there until 1964, when he became chairman of the university's Department of Nuclear Engineering and administrator of the Berkeley Research Reactor . Mark also taught undergraduate and graduate courses in physics, engineering and management at Boston University , the University of California, Davis and Stanford University . In February 1969, he became director of NASA's Ames Research Center , located in Mountain View, California . In this role, he managed
11550-616: The University of Oxford analysed it from the perspective of evolutionary theory and wrote in a study in the International Journal of Astrobiology that aliens may be similar to humans. The planetary context would also have an influence: a planet with higher gravity would have smaller animals, and other types of stars can lead to non-green photosynthesizers . The amount of energy available would also affect biodiversity , as an ecosystem sustained by black smokers or hydrothermal vents would have less energy available than those sustained by
11700-413: The ability to construct a radio telescope are presumed to be a natural advance for technological species, theoretically creating effects that might be detected over interstellar distances. The careful searching for non-natural radio emissions from space may lead to the detection of alien civilizations. Sensitive alien observers of the Solar System, for example, would note unusually intense radio waves for
11850-487: The apparent contradiction: Fermi was not the first to ask the question. An earlier implicit mention was by Konstantin Tsiolkovsky in an unpublished manuscript from 1933. He noted "people deny the presence of intelligent beings on the planets of the universe" because "(i) if such beings exist they would have visited Earth, and (ii) if such civilizations existed then they would have given us some sign of their existence". This
12000-410: The area surrounding these hydrothermal vents which can suggest that some form of life can be supported even in the harshest of environments like the other planets in the solar system. The aspects of these harsh environments that make them ideal for the origin of life on Earth, as well as the possibility of creation of life on other planets, is the chemical reactions forming spontaneously. For example,
12150-568: The atom speeds, either too fast or too slow, make it difficult for specific ones to meet and start chemical reactions. A liquid medium also allows the transport of nutrients and substances required for metabolism. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth. Life based on ammonia rather than water has been suggested as an alternative, though this solvent appears less suitable than water. It
12300-500: The boundless immensity of space" in The City of God . Pre-modern writers typically assumed extraterrestrial "worlds" are inhabited by living beings. William Vorilong , in the 15th century, acknowledged the possibility Jesus could have visited extraterrestrial worlds to redeem their inhabitants. Nicholas of Cusa wrote in 1440 that Earth is "a brilliant star" like other celestial objects visible in space; which would appear similar to
12450-493: The center's research and applications efforts in aeronautics , space science , life science , and space technology . He subsequently served as Under Secretary of the Air Force from 1977 until July 1979, when he was promoted to Secretary of the Air Force . Concurrently, he served as Director of the then-classified National Reconnaissance Office from August 1977 to October 1979. He remained at this position until 1981, when he
12600-402: The cold temperatures would make such chemistry take place at a very slow pace. Water is rock-solid on the surface, but Titan does have a subsurface water ocean like several other moons. However, it is of such a great depth that it would be very difficult to access it for study. The science that searches and studies life in the universe, both on Earth and elsewhere, is called astrobiology . With
12750-621: The collapse of France the Mark family moved to London. Mark's father, Herman Francis Mark , a prominent polymer chemist, secured a position with a Canadian paper company and left before the family could accompany him. Finally, in late 1939, his family joined him in Hawkesbury, Ontario . About a year later the family moved to the United States, settling in the Flatbush section of Brooklyn, New York after
12900-605: The concepts of quiet, loud, and grabby aliens were introduced by Hanson et al. The possible "loud" aliens expand rapidly in a highly detectable way throughout the universe and endure, while "quiet" aliens are hard or impossible to detect and eventually disappear. "Grabby" aliens prevent the emergence of other civilizations in their sphere of influence , which expands at a rate near the speed of light. The authors argue that if loud civilizations are rare, as they appear to be, then quiet civilizations are also rare. The paper suggests that humanity's current stage of technological development
13050-469: The creation or maintenance of extraterrestrial life. However, there has been much historical evidence that some of the earliest and most basic forms of life on Earth originated in some extreme environments that seem unlikely to have harbored life at least at one point in Earth's history. Fossil evidence as well as many historical theories backed up by years of research and studies have marked environments like hydrothermal vents or acidic hot springs as some of
13200-443: The distances to the next location of living beings may be very great and that, indeed, as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center." Fermi died of cancer in 1954. However, in letters to the three surviving men decades later in 1984, Dr. Eric Jones of Los Alamos was able to partially put the original conversation back together. He informed each of
13350-469: The elder Mark accepted a professorship at the Polytechnic Institute of Brooklyn . After becoming an American citizen in 1945, he graduated from New York's selective Stuyvesant High School in 1947. He went on to receive a bachelor's degree in physics from the University of California, Berkeley (where he was a member of Sigma Pi fraternity) in 1951. He then earned a Ph.D. in physics from
13500-458: The emergence of detectable technological civilizations. Another hypothesis in this category is the "Water World hypothesis". According to author and scientist David Brin : "it turns out that our Earth skates the very inner edge of our sun's continuously habitable—or ' Goldilocks '—zone. And Earth may be anomalous. It may be that because we are so close to our sun, we have an anomalously oxygen-rich atmosphere, and we have anomalously little ocean for
13650-413: The end, the star blows much of its content back into the stellar medium, where it would join clouds that would eventually become new generations of stars and planets. Many of those materials are the raw components of life on Earth. As this process takes place in all the universe, said materials are ubiquitous in the cosmos and not a rarity from the Solar System. Earth is a planet in the Solar System ,
13800-428: The entire Milky Way galaxy within a few million years, and spread to nearby galaxies in a timespan that is cosmologically brief. They conclude that intergalactic colonization appears possible with the resources of a single solar system and that intergalactic colonization is of comparable difficulty to interstellar colonization, and therefore the Fermi paradox is much sharper than commonly thought. There are two parts of
13950-416: The existence of sapient life elsewhere in the universe. The science of extraterrestrial life is known as astrobiology . Speculation about the possibility of inhabited worlds beyond Earth dates back to antiquity. Early Christian writers discussed the idea of a "plurality of worlds" as proposed by earlier thinkers such as Democritus ; Augustine references Epicurus 's idea of innumerable worlds "throughout
14100-403: The first hominids appeared 60 million years ago. Life on other planets may have started, evolved, given birth to extraterrestrial intelligences, and perhaps even faced a planetary extinction event millions or even billions of years ago. The brief times of existence of Earth's species, when considered from a cosmic perspective, may suggest that extraterrestrial life may be equally fleeting under such
14250-462: The first places that life could have originated on Earth. These environments can be considered extreme when compared to the typical ecosystems that the majority of life on Earth now inhabit, as hydrothermal vents are scorching hot due to the magma escaping from the Earth's mantle and meeting the much colder oceanic water. Even in today's world, there can be a diverse population of bacteria found inhabiting
14400-410: The formation of planets, eventually arriving on young planets early in their formation. In December 2023, astronomers reported the first time discovery, in the plumes of Enceladus , moon of the planet Saturn , of hydrogen cyanide , a possible chemical essential for life as we know it, as well as other organic molecules , some of which are yet to be better identified and understood. According to
14550-451: The giant planets themselves are highly unlikely to have life, there is much hope to find it on moons orbiting these planets. Europa , from the Jovian system, has a subsurface ocean below a thick layer of ice. Ganymede and Callisto also have subsurface oceans, but life is less likely in them because water is sandwiched between layers of solid ice. Europa would have contact between the ocean and
14700-404: The habitable zones vary according to the type of star, and even the solar activity of each specific star influences the local habitability. The type of star also defines the time the habitable zone will exist, as its presence and limits will change along with the stars stellar evolution. The Big Bang took place 14 billion years ago, the Solar System was formed 4 and a half billion years ago, and
14850-571: The hierarchy of lifeforms from Earth for simplicity, as it is the only one known to exist. The first basic requirement for life is an environment with non-equilibrium thermodynamics , which means that the thermodynamic equilibrium must be broken by a source of energy. The traditional sources of energy in the cosmos are the stars, such as for life on Earth, which depends on the energy of the sun. However, there are other alternative energy sources, such as volcanoes , plate tectonics , and hydrothermal vents . There are ecosystems on Earth in deep areas of
15000-472: The idea that technologically advanced civilizations are rare in the universe. Amedeo Balbi and Adam Frank propose the concept of an " oxygen bottleneck" for the emergence of technospheres. The "oxygen bottleneck" refers to the critical level of atmospheric oxygen necessary for fire and combustion . Earth's current atmospheric oxygen concentration is about 21%, but has been much lower in the past and may also be on many exoplanets. The authors argue that while
15150-454: The latter may lead to biological or mental degeneration. Possible means of annihilation via major global issues, where global interconnectedness actually makes humanity more vulnerable than resilient, are many, including war, accidental environmental contamination or damage, the development of biotechnology , synthetic life like mirror life , resource depletion , climate change , or poorly-designed artificial intelligence . This general theme
15300-464: The level of technological advancement necessary to communicate. Along with non-intelligent life, such civilizations would also be very difficult to detect. A trip using conventional rockets would take hundreds of thousands of years to reach the nearest stars. To skeptics, the fact that in the history of life on the Earth, only one species has developed a civilization to the point of being capable of spaceflight and radio technology, lends more credence to
15450-558: The long noses of animals such as aardvarks and elephants. As he points out, "dolphins have had ~20 million years to build a radio telescope and have not done so". In addition, Rebecca Boyle points out that of all the species that have ever evolved in the history of life on the planet Earth, only one—human beings and only in the beginning stages—has ever become space-faring. New life might commonly die out due to runaway heating or cooling on their fledgling planets. On Earth, there have been numerous major extinction events that destroyed
15600-450: The majority of complex species alive at the time; the extinction of the non-avian dinosaurs is the best known example. These are thought to have been caused by events such as impact from a large meteorite, massive volcanic eruptions, or astronomical events such as gamma-ray bursts . It may be the case that such extinction events are common throughout the universe and periodically destroy intelligent life, or at least its civilizations, before
15750-543: The medium. To create DNA , RNA , or a close analog, such an element should be able to bind its atoms with many others, creating complex and stable molecules. It should be able to create at least three covalent bonds: two for making long strings and at least a third to add new links and allow for diverse information. Only nine elements meet this requirement: boron , nitrogen , phosphorus , arsenic , antimony (three bonds), carbon , silicon , germanium and tin (four bonds). As for abundance, carbon, nitrogen, and silicon are
15900-583: The men that he wished to include a reasonably accurate version or composite in the written proceedings he was putting together for a previously held conference entitled "Interstellar Migration and the Human Experience". Jones first sent a letter to Edward Teller which included a secondhand account from Hans Mark . Teller responded, and then Jones sent Teller's letter to Herbert York. York responded, and finally, Jones sent both Teller's and York's letters to Emil Konopinski who also responded. Furthermore, Konopinski
16050-566: The most abundant ones in the universe, far more than the others. On Earth's crust the most abundant of those elements is silicon, in the Hydrosphere it is carbon and in the atmosphere, it is carbon and nitrogen. Silicon, however, has disadvantages over carbon. The molecules formed with silicon atoms are less stable, and more vulnerable to acids, oxygen, and light. An ecosystem of silicon-based lifeforms would require very low temperatures, high atmospheric pressure , an atmosphere devoid of oxygen, and
16200-441: The native planet that may not be caused by natural causes. There are three main types of techno-signatures considered: interstellar communications , effects on the atmosphere, and planetary-sized structures such as Dyson spheres . Organizations such as the SETI Institute search the cosmos for potential forms of communication. They started with radio waves , and now search for laser pulses as well. The challenge for this search
16350-416: The nature of the universe itself is quite incomplete. The Fermi paradox can be asked in two ways. The first is, "Why are no aliens or their artifacts found on Earth, or in the Solar System ?". If interstellar travel is possible, even the "slow" kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy. This is relatively brief on
16500-418: The ocean that do not receive sunlight, and take energy from black smokers instead. Magnetic fields and radioactivity have also been proposed as sources of energy, although they would be less efficient ones. Life on Earth requires water in a liquid state as a solvent in which biochemical reactions take place. It is highly unlikely that an abiogenesis process can start within a gaseous or solid medium:
16650-568: The origin of extraterrestrial life. The Solar System has a wide variety of planets, dwarf planets, and moons, and each one is studied for its potential to host life. Each one has its own specific conditions that may benefit or harm life. So far, the only lifeforms found are those from Earth. No extraterrestrial intelligence other than humans exists or has ever existed within the Solar System. Astrobiologist Mary Voytek points out that it would be unlikely to find large ecosystems, as they would have already been detected by now. The inner Solar System
16800-451: The origin of life on Earth. Hydrothermal vents , acidic hot springs, and volcanic lakes are examples of life forming under difficult circumstances, provide parallels to the extreme environments on other planets and support the possibility of extraterrestrial life. Since the mid-20th century, active research has taken place to look for signs of extraterrestrial life, encompassing searches for current and historic extraterrestrial life, and
16950-468: The others are used in other biological processes. The studies ruled out pollution of the meteorites on Earth, as those components would not be freely available the way they were found in the samples. This discovery suggests that several organic molecules that serve as building blocks of life may be generated within asteroids and comets. In October 2011, scientists reported that cosmic dust contains complex organic compounds ("amorphous organic solids with
17100-547: The planetary core ceased to generate a magnetic field, solar winds removed the atmosphere and the planet became vulnerable to solar radiation. Ancient life-forms may still have left fossilised remains, and microbes may still survive deep underground. As mentioned, the gas giants and ice giants are unlikely to contain life. The most distant solar system bodies, found in the Kuiper Belt and outwards, are locked in permanent deep-freeze, but cannot be ruled out completely. Although
17250-511: The precise differences that lead to beneficial or harmful conditions for life. And despite the conditions against life on Venus , there are suspicions that microbial life-forms may still survive in high-altitude clouds. Mars is a cold and almost airless desert, inhospitable to life. However, recent studies revealed that water on Mars used to be quite abundant, forming rivers, lakes, and perhaps even oceans. Mars may have been habitable back then, and life on Mars may have been possible. But when
17400-512: The premise that extraterrestrial civilizations exist. In 1975, Michael H. Hart published a detailed examination of the paradox, one of the first to do so. He argued that if intelligent extraterrestrials exist, and are capable of space travel, then the galaxy could have been colonized in a time much less than that of the age of the Earth. However, there is no observable evidence they have been here, which Hart called "Fact A". Other names closely related to Fermi's question ("Where are they?") include
17550-452: The probability of the existence of extraterrestrial civilisations and the lack of evidence for such civilisations is known as the Fermi paradox . Dennis W. Sciama claimed that life's existence in the universe depends on various fundamental constants. Zhi-Wei Wang and Samuel L. Braunstein suggest that a random universe capable of supporting life is likely to be just barely able to do so, giving
17700-476: The probe to study ocean water. Still, Cassini detected complex organic molecules, salts, evidence of hydrothermal activity, hydrogen, and methane. Titan is the only celestial body in the Solar System besides Earth that has liquid bodies on the surface. It has rivers, lakes, and rain of hydrocarbons, methane, and ethane, and even a cycle similar to Earth's water cycle . This special context encourages speculations about lifeforms with different biochemistry, but
17850-438: The problems of survival of life on other planets. However, he also considered that the conditions found on Mars were incompatible with carbon based life. Even if extraterrestrial life is based on carbon and uses water as a solvent, like Earth life, it may still have a radically different biochemistry . Life is generally considered to be a product of natural selection . It has been proposed that to undergo natural selection
18000-429: The progress level required for space exploration. The "universal limit to technological development" (ULTD) hypothesis proposes that there is a limit to the potential growth of a civilization, and that this limit may be placed well below the point required for space exploration. Such limits may be based on economic reasons , natural reasons (such as faster-than-light travel being impossible), and even limitations based on
18150-433: The researchers, "these [newly discovered] compounds could potentially support extant microbial communities or drive complex organic synthesis leading to the origin of life ." Although most searches are focused on the biology of extraterrestrial life, an extraterrestrial intelligence capable enough to develop a civilization may be detectable by other means as well. Technology may generate technosignatures , effects on
18300-426: The right conditions on Earth were met, life started by a chemical process known as abiogenesis . Alternatively, life may have formed less frequently, then spread – by meteoroids , for example – between habitable planets in a process called panspermia . During most of its stellar evolution stars combine hydrogen nuclei to make helium nuclei by stellar fusion, and the comparatively lighter weight of helium allows
18450-405: The right side of the equation are agreed as speculative and open to substitution: 10,000 = 5 ⋅ 0.5 ⋅ 2 ⋅ 1 ⋅ 0.2 ⋅ 1 ⋅ 10,000 {\displaystyle 10{,}000=5\cdot 0.5\cdot 2\cdot 1\cdot 0.2\cdot 1\cdot 10{,}000} The Drake equation has proved controversial since, although it is written as
18600-422: The rocky surface, which helps the chemical reactions. It may be difficult to dig so deep in order to study those oceans, though. Enceladus , a tiny moon of Saturn with another subsurface ocean, may not need to be dug, as it releases water to space in eruption columns . The space probe Cassini flew inside one of these, but could not make a full study because NASA did not expect this phenomenon and did not equip
18750-528: The same samples suggests that a non-biological reaction is a more likely hypothesis. In February 2005 NASA scientists reported they may have found some evidence of extraterrestrial life on Mars. 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
18900-649: The self-organizing properties of life. It is possible that even if complex life is common, intelligence (and consequently civilizations) is not. While there are remote sensing techniques that could perhaps detect life-bearing planets without relying on the signs of technology, none of them have any ability to tell if any detected life is intelligent. This is sometimes referred to as the "algae vs. alumnae" problem. Charles Lineweaver states that when considering any extreme trait in an animal, intermediate stages do not necessarily produce "inevitable" outcomes. For example, large brains are no more "inevitable", or convergent, than are
19050-574: The size of the Milky Way in as little as a million years. If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Another speculation for contact with an alien probe—one that would be trying to find human beings—is an alien Bracewell probe . Such a hypothetical device would be an autonomous space probe whose purpose is to seek out and communicate with alien civilizations (as opposed to von Neumann probes, which are usually described as purely exploratory). These were proposed as an alternative to carrying
19200-574: The society's annual meeting in Pasadena, California. In 2008, the Space Foundation awarded Mark its highest honor, the General James E. Hill Lifetime Space Achievement Award. It is presented annually to recognize outstanding individuals who have distinguished themselves through lifetime contributions to the welfare or betterment of humankind through the exploration, development and use of space, or
19350-583: The source. For example, pulsars , when first discovered in 1967, were called little green men (LGM) because of the precise repetition of their pulses. In all cases, explanations with no need for intelligent life have been found for such observations, but the possibility of discovery remains. Proposed examples include asteroid mining that would change the appearance of debris disks around stars, or spectral lines from nuclear waste disposal in stars. Explanations based on technosignatures , such as radio communications , have been presented. Radio technology and
19500-621: The species is able to develop the technology to communicate with other intelligent species. However, the chances of extinction by natural events may be very low on the scale of a civilization's lifetime. Based on an analysis of impact craters on Earth and the Moon, the average interval between impacts large enough to cause global consequences (like the Chicxulub impact ) is estimated to be around 100 million years. It may be that while alien species with intelligence exist, they are primitive or have not reached
19650-446: The species' own biology. This is the argument that technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or spaceflight technology. The astrophysicist Sebastian von Hoerner stated that the progress of science and technology on Earth was driven by two factors—the struggle for domination and the desire for an easy life. The former potentially leads to complete destruction, while
19800-399: The star to release the extra energy. The process continues until the star uses all of its available fuel, with the speed of consumption being related to the size of the star. During its last stages, stars start combining helium nuclei to form carbon nuclei. The higher-sized stars can further combine carbon nuclei to create oxygen and silicon, oxygen into neon and sulfur, and so on until iron. In
19950-482: The stellar evolution of stars and planets, it is usually not taken into account by astrobiology. There is an area around a star, the circumstellar habitable zone or "Goldilocks zone", where water may be at the right temperature to exist in liquid form at a planetary surface. This area is neither too close to the star, where water would become steam, nor too far away, where water would be frozen as ice. However, although useful as an approximation, planetary habitability
20100-508: The study of Earth's life, the only known form of life, astrobiology seeks to study how life starts and evolves and the requirements for its continuous existence. This helps to determine what to look for when searching for life in other celestial bodies. This is a complex area of study, and uses the combined perspectives of several scientific disciplines, such as astronomy , biology , chemistry , geology , oceanography , and atmospheric sciences . The scientific search for extraterrestrial life
20250-539: The survival and possible origin of life on Earth . With the little amount of information that scientists have found regarding the atmosphere on other planets in the Milky Way galaxy and beyond, the atmospheres are most likely reducing or with very low oxygen levels, especially when compared with Earth's atmosphere. If there were the necessary elements and ions on these planets, the same carbon fixing, reduced chemical compounds occurring around hydrothermal vents could also occur on these planets' surfaces and possibly result in
20400-661: The system: global governance may become increasingly destabilized, worsening humanity's ability to manage the possible means of annihilation listed above, resulting in global societal collapse . Extraterrestrial life Extraterrestrial life , or alien life (colloquially, alien ), is life that originates from another world rather than on Earth . No extraterrestrial life has yet been scientifically conclusively detected. Such life might range from simple forms such as prokaryotes to intelligent beings , possibly bringing forth civilizations that might be far more advanced than humans. The Drake equation speculates about
20550-413: The threshold of oxygen required for the existence of complex life and ecosystems is much lower, technological advancement, particularly that reliant on combustion, such as metal smelting and energy production , requires higher oxygen concentrations of around 18% or more. Thus, the presence of high levels of oxygen in a planet's atmosphere is not only a potential biosignature but also a critical factor in
20700-561: The uncertainty associated with this lack of understanding has been carried out by Anders Sandberg , Eric Drexler and Toby Ord , and suggests "a substantial ex ante probability of there being no other intelligent life in our observable universe". The Great Filter, a concept introduced by Robin Hanson in 1996, represents whatever natural phenomena that would make it unlikely for life to evolve from inanimate matter to an advanced civilization . The most commonly agreed-upon low probability event
20850-445: The unusual properties of the meteorite were eventually explained as the result of inorganic processes, the controversy over its discovery laid the groundwork for the development of astrobiology. An experiment on the two Viking Mars landers reported gas emissions from heated Martian soil samples that some scientists argue are consistent with the presence of living microorganisms. Lack of corroborating evidence from other experiments on
21000-616: The use of space technology, information, themes or resources in academic, cultural, industrial or other pursuits of broad benefit to humanity. In 2012, the Air Force Space Command awarded him the Air Force Space and Missile Pioneers Award. The award recognizes individuals for their significant role in the history of Air Force space and missile programs. Mark died from progressive dementia in Austin, Texas , on December 18, 2021, at
21150-439: Was able to later identify a cartoon which Jones found as the one involved in the conversation and thereby help to settle the time period as being the summer of 1950. The Fermi paradox is a conflict between the argument that scale and probability seem to favor intelligent life being common in the universe, and the total lack of evidence of intelligent life having ever arisen anywhere other than on Earth. The first aspect of
21300-624: Was also an Honorary Fellow of the American Institute of Aeronautics and Astronautics . He received the 1999 Joe J. King Engineering Achievement Award and the 1999 George E. Haddaway Medal for Achievement in Aviation. Dr. Mark was honored for his contributions to the U.S. military space program at the 2006 annual meeting of the American Astronautical Society. He received the 2006 Military Astronautics Award on November 14, 2006 at
21450-563: Was an expert and consultant in aerospace design and national defense policy. Mark retired from the Department of Aerospace Engineering and Engineering Mechanics at the University of Texas at Austin 's Cockrell School of Engineering in July 2014. Mark was born in the city of Mannheim , Baden , Germany. He lived in Vienna for a time before escaping the Nazi Anschluss via Switzerland. Before
21600-474: Was appointed Deputy Administrator of NASA by President Reagan , a position he served in from July 10, 1981 to September 1, 1984. He later returned to the Pentagon in 1999–2000 to serve as Director of Defense Research and Engineering (DDR&E). Upon leaving NASA in 1984, Mark served as Chancellor of the University of Texas system until 1992. He moved on to become a senior professor of aerospace engineering at
21750-417: Was habitable. It is unclear if life and intelligent life are ubiquitous in the cosmos or rare. The hypothesis of ubiquitous extraterrestrial life relies on three main ideas. The first one, the size of the universe allows for plenty of planets to have a similar habitability to Earth, and the age of the universe gives enough time for a long process analog to the history of Earth to happen there. The second
21900-479: Was not a paradox for others, who took this to imply the absence of extraterrestrial life. But it was one for him, since he believed in extraterrestrial life and the possibility of space travel. Therefore, he proposed what is now known as the zoo hypothesis and speculated that mankind is not yet ready for higher beings to contact us. In turn, Tsiolkovsky himself was not the first to discover the paradox, as shown by his reference to other people's reasons for not accepting
22050-410: Was not the temperature, but the scarcity of free heavy elements. Planetary systems emerged, and the first organic compounds may have formed in the protoplanetary disk of dust grains that would eventually create rocky planets like Earth. Although Earth was in a molten state after its birth and may have burned any organics that fell in it, it would have been more receptive once it cooled down. Once
22200-399: Was reported. Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites . Some claim to have identified evidence that microbial life has existed on Mars. In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001 , formed of rock ejected from Mars . Although all
22350-431: Was some speculation that a dimming of light from star KIC 8462852 , observed by the Kepler space telescope , could have been a result of Dyson sphere construction. However, in 2018, observations determined that the amount of dimming varied by the frequency of the light, pointing to dust, rather than an opaque object such as a Dyson sphere, as the cause of the dimming. Those who think that intelligent extraterrestrial life
22500-535: Was that led to the transition of (some) prokaryotic cells to eukaryotic cells , sexual reproduction and the Cambrian explosion . In his book Wonderful Life (1989), Stephen Jay Gould suggested that if the "tape of life" were rewound to the time of the Cambrian explosion, and one or two tweaks made, human beings most probably never would have evolved. Other thinkers such as Fontana, Buss, and Kauffman have written about
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