Extraterrestrial life

Life that does not originate on Earth

This article is about any kind of extraterrestrial life. For aliens with human-like intelligence, see Extraterrestrial intelligence.

Unsolved problem in astronomy

Could life have arisen elsewhere?
What are the requirements for life?
Are there exoplanets like Earth?
How likely is the evolution of intelligent life?

More unsolved problems in astronomy

Extraterrestrial life, or alien life (colloquially, aliens), 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, or far less, advanced than humans.[1][2][3] The Drake equation speculates about 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 the boundless immensity of space" in The City of God.[4]

Pre-modern writers typically assumed extraterrestrial "worlds" were inhabited by living beings. William Vorilong, in the 15th century, acknowledged the possibility Jesus could have visited extraterrestrial worlds to redeem their inhabitants.[4]: 26 Nicholas of Cusa wrote in 1440 that Earth is "a brilliant star" like other celestial objects visible in space; which would appear similar to the Sun, from an exterior perspective, due to a layer of "fiery brightness" in the outer layer of the atmosphere. He theorized all extraterrestrial bodies could be inhabited by men, plants, and animals, including the Sun.[5] Descartes wrote that there were no means to prove the stars were not inhabited by "intelligent creatures", but their existence was a matter of speculation.[4]: 67

In comparison to the life-abundant Earth, the vast majority of intrasolar and extrasolar planets and moons have harsh surface conditions and disparate atmospheric chemistry, or lack an atmosphere.[6] However, there are many extreme and chemically harsh ecosystems on Earth that do support forms of life and are often hypothesized to be the origin of life on Earth. Examples include life surrounding hydrothermal vents,[7] acidic hot springs,[8] and volcanic lakes,[9] as well as halophiles[10] and the deep biosphere.[11]

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 a narrower search for extraterrestrial intelligent life. Solar system exploration has investigated conditions for life, especially on Venus, Mars, Europa, and Titan. Exoplanets were first detected in 1992. As of 7 August 2025, there are 5,972 confirmed exoplanets in 4,460 planetary systems, with 1,000 systems having more than one planet.[12] Depending on the category of search, methods range from analysis of telescope and specimen data[13] to radios used to detect and transmit interstellar communication.[14] Interstellar travel remains largely hypothetical, with only the Voyager 1 and Voyager 2 probes confirmed to have entered the interstellar medium.

The concept of extraterrestrial life, particularly extraterrestrial intelligence, has had a major cultural impact, especially extraterrestrials in fiction. Science fiction has communicated scientific ideas, imagined 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.[15][16]

## Context

[edit]

This article is one of a series on:

Life in the universe
Outline
Planetary habitability in the Solar System

* Venus
* Earth
* Mars
* Europa
* Enceladus
* Titan

Life outside the Solar System

* Habitable zone
* Potentially habitable exoplanets
* Galactic habitable zone

Habitability of...

* Binary star systems
* Natural satellites
* Neutron star systems
* Red dwarf systems
* K-type main-sequence star systems
* Yellow dwarf systems
* F-type main-sequence star systems

* v
* t
* e

Initially, after the Big Bang, the universe was too hot to allow life. It is estimated that the temperature of the universe was around 10 billion K at the one second mark.[17] 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 was not the temperature, but the scarcity of free heavy elements.[18] 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.[19] Once 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.[20][21]

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 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 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.[22]

Earth is a planet in the Solar System, 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.[23] The immense distances between celestial objects 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.[24] No probe has ever reached extrasolar planetary systems. The Voyager 2 has left 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.[24] 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 the stellar evolution of stars and planets, it is usually not taken into account by astrobiology.[25]

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 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.[26] The actual distances for 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 star's stellar evolution.[27]

The Big Bang took place 13.8 billion years ago, the Solar System was formed 4.6 billion years ago, and the first hominids appeared 6 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 a scale.[28]

During a period of about 7 million years, from about 10 to 17 million after the Big Bang, the background temperature was between 373 and 273 K (100 and 0 °C; 212 and 32 °F), allowing the possibility of liquid water if any planets existed. Avi Loeb (2014) speculated that primitive life might in principle have appeared during this window, which he called "the Habitable Epoch of the Early Universe".[29][30]

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 were habitable.[31]

## Likelihood of existence

[edit]

Main articles: Drake equation and Extraterrestrial intelligence

It is unclear if life, and more importantly, intelligent life in the cosmos is ubiquitous 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 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.[32] 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.[33][34] This argument 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.[35]

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 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 a reasonable scientific explanation for any gathered data.[36][37]

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).[38][better source needed] The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. The Drake equation is:

N = R ∗ ⋅ f p ⋅ n e ⋅ f ℓ ⋅ f i ⋅ f c ⋅ L {\displaystyle N=R_{\ast }\cdot f_{p}\cdot n_{e}\cdot f_{\ell }\cdot f_{i}\cdot f_{c}\cdot L}

where:

N = the number of Milky Way galaxy civilizations already capable of communicating across interplanetary space

and

R* = the average rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life
fl = the fraction of planets that actually support life
fi = the fraction of planets with life that evolves to become intelligent life (civilisations)
fc = the fraction of civilizations that develop a technology to broadcast detectable signs of their existence into space
L = the length of time over which such civilizations broadcast detectable signals into space

Drake's proposed estimates are as follows, but numbers on 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} [39][better source needed]

The Drake equation has proved controversial since, although it is written as 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.[40] This does not allow one to make noteworthy conclusions from the equation.[41]

Based on observations from the Hubble Space Telescope, there are nearly 2 trillion galaxies in the observable universe.[42] It is estimated that at least ten percent of all Sun-like stars have a system of planets,[43] i.e. there are 6.25×1018 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.[44][45] The Nebular hypothesis that explains the formation of the Solar System and other planetary systems would suggest that those can have several configurations, and not all of them may have rocky planets within the habitable zone.[46]

The apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilisations and the lack of evidence for such civilisations is known as the Fermi paradox.[47] 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 a potential explanation to the Fermi paradox.[48]

## Biochemical basis

[edit]

Main article: Hypothetical types of biochemistry

See also: Water § Effects on life

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 the hierarchy of lifeforms from Earth for simplicity, as it is the only one known to exist.[49]

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 the ocean that do not receive sunlight, and take energy from black smokers instead.[50] Magnetic fields and radioactivity have also been proposed as sources of energy, although they would be less efficient ones.[51]

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: 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.[52] 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.[53][54] Life based on ammonia rather than water has been suggested as an alternative, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.[55]

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 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 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 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.[56] 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 the problems of survival of life on other planets.[57] 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 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.[58] Life on Earth may have 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 is unclear if our biochemistry is the most efficient one that could be generated, or which elements would follow a similar pattern.[59] 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 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.[60]

The evolution from simple cells to eukaryotes, and from them to multicellular lifeforms, 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.[61] Scientists from 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.[62] 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 a star's light and heat, and so its lifeforms would not grow beyond a certain complexity.[61] 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.[63]

### Harsh environmental conditions on Earth harboring life

[edit]

It 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.[64] The environmental conditions on these planets can have intense UV radiation paired with extreme temperatures, lack of water,[65] and much more that can lead to conditions that don't seem to favor 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[66] 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 the first places that life could have originated on Earth.[67] 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 the area surrounding these hydrothermal vents[68] 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, the hydrothermal vents found on the ocean floor are known to support many chemosynthetic processes[7] which allow organisms to utilize energy through reduced chemical compounds that fix carbon.[68] 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[69] and therefore, these carbon fixing compounds were necessary for 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,[70] 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 the origin of extraterrestrial life.

## Planetary habitability in the Solar System

[edit]

Main article: Planetary habitability in the Solar System

Besides Earth, Mars, Europa and Enceladus are the most likely places in the Solar System to find 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.[71] Astrobiologist Mary Voytek points out that it would be unlikely to find large ecosystems, as they would have already been detected by now.[26]

The inner Solar System 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.[72] Comparing both helps to understand 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.[26]

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 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.[26]

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.[26]

Although 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 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 the probe to study ocean water. Still, Cassini detected complex organic molecules, salts, evidence of hydrothermal activity, hydrogen, and methane.[26]

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 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.[26]

## Scientific search

[edit]

Main article: Astrobiology

The science that searches and studies life in the universe, both on Earth and elsewhere, is called astrobiology. With 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.[73]

The scientific search for extraterrestrial life is being carried out both directly and indirectly. As of September 2017[update], 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) was reported.[74]

### Search for basic life

[edit]

Lifeforms produce a variety of biosignatures that may be detectable by telescopes.[75][76]

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.[77][78][79][80] In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001, formed of rock ejected from Mars.[77][78] Although all 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.[77]

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.[81] Lack of corroborating evidence from other experiments on the same samples suggests that a non-biological reaction is a more likely hypothesis.[81][82][83][84]

In February 2005 NASA scientists reported they may have found some evidence of extraterrestrial life on Mars.[85] 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.[86]

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.[87][88]

A group of scientists at Cornell University started 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.[89]

In August 2011, NASA studied meteorites found on Antarctica, finding adenine, guanine, hypoxanthine and xanthine. Adenine and guanine are components of DNA, and 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.[90][91] In October 2011, scientists reported that cosmic dust contains complex organic compounds ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.[92][93][94] 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."[92]

In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth.[95] 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.[96]

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[97] as we know it, as well as other organic molecules, some of which are yet to be better identified and understood. According to the researchers, "these [newly discovered] compounds could potentially support extant microbial communities or drive complex organic synthesis leading to the origin of life."[98][99]

### Search for extraterrestrial intelligences

[edit]

Main article: Search for extraterrestrial intelligence

The Green Bank Telescope is one of the radio telescopes used by the Breakthrough Listen project to search for alien communications.

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 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.[100]

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 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.[100] Besides, even if there is an advanced extraterrestrial civilization, there 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.[101]

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 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 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.[100]

The Kardashev scale proposes that a civilization may eventually start consuming energy directly from its local star. This would require giant structures built next to it, called Dyson spheres. Those speculative structures would cause an excess infrared radiation, that telescopes may notice. The infrared radiation is typical of young stars, surrounded by dusty protoplanetary disks that will eventually form planets. An older star such as the Sun would have no natural reason to have excess infrared radiation.[100] The presence of heavy elements in a star's light-spectrum is another potential biosignature; such elements would (in theory) be found if the star were being used as an incinerator/repository for nuclear waste products.[102]

### Extrasolar planets

[edit]

Main article: Exoplanet

See also: List of potentially habitable exoplanets

Artist's impression of Gliese 581 c, the first terrestrial extrasolar planet discovered within its star's habitable zone

Some astronomers search for extrasolar planets that may be conducive to life, narrowing the search to terrestrial planets within the habitable zones of their stars.[103][104] Since 1992, over four thousand exoplanets have been discovered (6,032 planets in 4,530 planetary systems including 989 multiple planetary systems as of 29 July 2025).[105]

The extrasolar planets so far discovered range in size from that of terrestrial planets similar to Earth's size to that of gas giants larger than Jupiter.[105] The number of observed exoplanets is expected to increase greatly in the coming years.[106][better source needed] The Kepler space telescope has also detected a few thousand[107][108] candidate planets,[109][110] of which about 11% may be false positives.[111]

There is at least one planet on average per star.[112] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone,[c] with the nearest expected to be within 12 light-years distance from Earth.[113][114] Assuming 200 billion stars in the Milky Way,[d] that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs are included.[115] The rogue planets in the Milky Way possibly number in the trillions.[116]

The nearest known exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 pc) from Earth in the southern constellation of Centaurus.[117]

As of March 2014[update], the least massive exoplanet known is PSR B1257+12 A, which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is DENIS-P J082303.1−491201 b,[118][119] about 29 times the mass of Jupiter, although according to most definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. Almost all of the planets detected so far are within the Milky Way, but there have also been a few possible detections of extragalactic planets. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[13]

One sign that a planet probably already contains life is the presence of an atmosphere with significant amounts of oxygen, since that gas is highly reactive and generally would not last long without constant replenishment. This replenishment occurs on Earth through photosynthetic organisms. One way to analyse the atmosphere of an exoplanet is through spectrography when it transits its star, though this might only be feasible with dim stars like white dwarfs.[120]

## History and cultural impact

[edit]

Main article: History of the extraterrestrial life debate

See also: History of astronomy and Potential cultural impact of extraterrestrial contact

### Cosmic pluralism

[edit]

Main article: Cosmic pluralism

The Greek Epicurus proposed that other worlds may have their own animals and plants.

The modern concept of extraterrestrial life is based on assumptions that were not commonplace during the early days of astronomy. The first explanations for the celestial objects seen in the night sky were based on mythology. Scholars from Ancient Greece were the first to consider that the universe is inherently understandable and rejected explanations based on supernatural incomprehensible forces, such as the myth of the Sun being pulled across the sky in the chariot of Apollo. They had not developed the scientific method yet and based their ideas on pure thought and speculation, but they developed precursor ideas to it, such as that explanations had to be discarded if they contradict observable facts. The discussions of those Greek scholars established many of the pillars that would eventually lead to the idea of extraterrestrial life, such as Earth being round and not flat. The cosmos was first structured in a geocentric model that considered that the sun and all other celestial bodies revolve around Earth. However, they did not consider them as worlds. In Greek understanding, the world was composed by both Earth and the celestial objects with noticeable movements. Anaximander thought that the cosmos was made from apeiron, a substance that created the world, and that the world would eventually return to the cosmos.

Eventually two groups emerged, the atomists that thought that matter at both Earth and the cosmos was equally made of small atoms of the classical elements (earth, water, fire and air), and the Aristotelians who thought that those elements were exclusive of Earth and that the cosmos was made of a fifth one, the aether. Atomist Epicurus thought that the processes that created the world, its animals and plants should have created other worlds elsewhere, along with their own animals and plants. Aristotle thought instead that all the earth element naturally fell towards the center of the universe, and that would make it impossible for other planets to exist elsewhere. Under that reasoning, Earth was not only in the center, it was also the only planet in the universe.[121]

Cosmic pluralism, the plurality of worlds, or simply pluralism, describes the philosophical belief in numerous "worlds" in addition to Earth, which might harbor extraterrestrial life. The earliest recorded assertion of extraterrestrial human life is found in ancient scriptures of Jainism. There are multiple "worlds" mentioned in Jain scriptures that support human life. These include, among others, Bharat Kshetra, Mahavideh Kshetra, Airavat Kshetra, and Hari kshetra.[122][123][124] Medieval Muslim writers like Fakhr al-Din al-Razi and Muhammad al-Baqir supported cosmic pluralism on the basis of the Qur'an.[125] Chaucer's poem The House of Fame engaged in medieval thought experiments that postulated the plurality of worlds.[126] However, those ideas about other worlds were different from the current knowledge about the structure of the universe, and did not postulate the existence of planetary systems other than the Solar System. When those authors talk about other worlds, they talk about places located at the center of their own systems, and with their own stellar vaults and cosmos surrounding them.[127]

The Greek ideas and the disputes between atomists and Aristotelians outlived the fall of the Greek empire. The Great Library of Alexandria compiled information about it, part of which was translated by Islamic scholars and thus survived the end of the Library. Baghdad combined the knowledge of the Greeks, the Indians, the Chinese and its own scholars, and the knowledge expanded through the Byzantine Empire. From there it eventually returned to Europe by the time of the Middle Ages. However, as the Greek atomist doctrine held that the world was created by random movements of atoms, with no need for a creator deity, it became associated with atheism, and the dispute intertwined with religious ones.[128] Still, the Church did not react to those topics in a homogeneous way, and there were stricter and more permissive views within the church itself.[129]

The first known mention of the term 'panspermia' was in the writings of the 5th-century BC Greek philosopher Anaxagoras. He proposed the idea that life exists everywhere.[130]

### Early modern period

[edit]

Galileo before the Holy Office, a 19th-century painting by Joseph-Nicolas Robert-Fleury

By the time of the late Middle Ages there were many known inaccuracies in the geocentric model, but it was kept in use because naked eye observations provided limited data. Nicolaus Copernicus started the Copernican Revolution by proposing that the planets revolve around the sun rather than Earth. His proposal had little acceptance at first because, as he kept the assumption that orbits were perfect circles, his model led to as many inaccuracies as the geocentric one. Tycho Brahe improved the available data with naked-eye observatories, which worked with highly complex sextants and quadrants. Tycho could not make sense of his observations, but Johannes Kepler did: orbits were not perfect circles, but ellipses. This knowledge benefited the Copernican model, which worked now almost perfectly. The invention of the telescope a short time later, perfected by Galileo Galilei, clarified the final doubts, and the paradigm shift was completed.[131] Under this new understanding, the notion of extraterrestrial life became feasible: if Earth is but just a planet orbiting around a star, there may be planets similar to Earth elsewhere. The astronomical study of distant bodies also proved that physical laws are the same elsewhere in the universe as on Earth, with nothing making the planet truly special.[132]

The new ideas were met with resistance from the Catholic church. Galileo was tried for the heliocentric model, which was considered heretical, and forced to recant it.[133] The best-known early-modern proponent of ideas of extraterrestrial life was the Italian philosopher Giordano Bruno, who argued in the 16th century for an infinite universe in which every star is surrounded by its own planetary system. Bruno wrote that other worlds "have no less virtue nor a nature different to that of our earth" and, like Earth, "contain animals and inhabitants".[134] Bruno's belief in the plurality of worlds was one of the charges leveled against him by the Venetian Holy Inquisition, which tried and executed him.[135]

The heliocentric model was further strengthened by the postulation of the theory of gravity by Sir Isaac Newton. This theory provided the mathematics that explains the motions of all things in the universe, including planetary orbits. By this point, the geocentric model was definitely discarded. By this time, the use of the scientific method had become a standard, and new discoveries were expected to provide evidence and rigorous mathematical explanations. Science also took a deeper interest in the mechanics of natural phenomena, trying to explain not just the way nature works but also the reasons for working that way.[136]

There was very little actual discussion about extraterrestrial life before this point, as the Aristotelian ideas remained influential while geocentrism was still accepted. When it was finally proved wrong, it not only meant that Earth was not the center of the universe, but also that the lights seen in the sky were not just lights, but physical objects. The notion that life may exist in them as well soon became an ongoing topic of discussion, although one with no practical ways to investigate.[137]

The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. William Herschel, the discoverer of Uranus, was one of many 18th–19th-century astronomers who believed that the Solar System is populated by alien life. Other scholars of the period who championed "cosmic pluralism" included Immanuel Kant and Benjamin Franklin. At the height of the Enlightenment, even the Sun and Moon were considered candidates for extraterrestrial inhabitants.[138][139]

### 19th century

[edit]

Artificial Martian channels, depicted by Percival Lowell

Speculation about life on Mars increased in the late 19th century, following telescopic observation of apparent Martian canals – which soon, however, turned out to be optical illusions.[140] Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilisation.[141]

Spectroscopic analysis of Mars's atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen was present in the Martian atmosphere.[142] By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis.[143]

As a consequence of the belief in the spontaneous generation there was little thought about the conditions of each celestial body: it was simply assumed that life would thrive anywhere. This theory was disproved by Louis Pasteur in the 19th century. Popular belief in thriving alien civilisations elsewhere in the solar system still remained strong until Mariner 4 and Mariner 9 provided close images of Mars, which debunked forever the idea of the existence of Martians and decreased the previous expectations of finding alien life in general.[144] The end of the spontaneous generation belief forced investigation into the origin of life. Although abiogenesis is the more accepted theory, a number of authors reclaimed the term "panspermia" and proposed that life was brought to Earth from elsewhere.[130] Some of those authors are Jöns Jacob Berzelius (1834),[145] Kelvin (1871),[146] Hermann von Helmholtz (1879)[147] and, somewhat later, by Svante Arrhenius (1903).[148]

The science fiction genre, although not so named during the time, developed during the late 19th century. The expansion of the genre of extraterrestrials in fiction influenced the popular perception over the real-life topic, making people eager to jump to conclusions about the discovery of aliens. Science marched at a slower pace, some discoveries fueled expectations and others dashed excessive hopes. For example, with the advent of telescopes, most structures seen on the Moon or Mars were immediately attributed to Selenites or Martians, and later ones (such as more powerful telescopes) revealed that all such discoveries were natural features.[135] A famous case is the Cydonia region of Mars, first imaged by the Viking 1 orbiter. The low-resolution photos showed a rock formation that resembled a human face, but later spacecraft took photos in higher detail that showed that there was nothing special about the site.[149]

### Recent history

[edit]

See also: Space exploration

Some major international efforts to search for extraterrestrial life, clockwise from top left:

* The search for extrasolar planets (image: Kepler telescope)
* Listening for extraterrestrial signals indicating intelligence (image: Allen array)
* Robotic exploration of the Solar System (image: Curiosity rover on Mars)

The search and study of extraterrestrial life became a science of its own, astrobiology. Also known as exobiology, this discipline is studied by the NASA, the ESA, the INAF, and others. Astrobiology studies life from Earth as well, but with a cosmic perspective. For example, abiogenesis is of interest to astrobiology, not because of the origin of life on Earth, but for the chances of a similar process taking place in other celestial bodies. Many aspects of life, from its definition to its chemistry, are analyzed as either likely to be similar in all forms of life across the cosmos or only native to Earth.[150] Astrobiology, however, remains constrained by the current lack of extraterrestrial life-forms to study, as all life on Earth comes from the same ancestor, and it is hard to infer general characteristics from a group with a single example to analyse.[151]

The 20th century came with great technological advances, speculations about future hypothetical technologies, and an increased basic knowledge of science by the general population thanks to science divulgation through the mass media. The public interest in extraterrestrial life and the lack of discoveries by mainstream science led to the emergence of pseudosciences that provided affirmative, if questionable, answers to the existence of aliens. Ufology claims that many unidentified flying objects (UFOs) would be spaceships from alien species, and ancient astronauts hypothesis claim that aliens would have visited Earth in antiquity and prehistoric times but people would have failed to understand it by then.[152] Most UFOs or UFO sightings[153] can be readily explained as sightings of Earth-based aircraft (including top-secret aircraft), known astronomical objects or weather phenomenons, or as hoaxes.[154]

Looking beyond the pseudosciences, Lewis White Beck strove to elevate the level of public discourse on the topic of extraterrestrial life by tracing the evolution of philosophical thought over the centuries from ancient times into the modern era. His review of the contributions made by Lucretius, Plutarch, Aristotle, Copernicus, Immanuel Kant, John Wilkins, Charles Darwin and Karl Marx demonstrated that even in modern times, humanity could be profoundly influenced in its search for extraterrestrial life by subtle and comforting archetypal ideas which are largely derived from firmly held religious, philosophical and existential belief systems. On a positive note, however, Beck further argued that even if the search for extraterrestrial life proves to be unsuccessful, the endeavor itself could have beneficial consequences by assisting humanity in its attempt to actualize superior ways of living here on Earth.[155]

By the 21st century, it was accepted that multicellular life in the Solar System can only exist on Earth, but the interest in extraterrestrial life increased regardless. This is a result of the advances in several sciences. The knowledge of planetary habitability allows to consider on scientific terms the likelihood of finding life at each specific celestial body, as it is known which features are beneficial and harmful for life. Astronomy and telescopes also improved to the point exoplanets can be confirmed and even studied, increasing the number of search places. Life may still exist elsewhere in the Solar System in unicellular form, but the advances in spacecraft allow to send robots to study samples in situ, with tools of growing complexity and reliability. Although no extraterrestrial life has been found and life may still be just a rarity from Earth, there are scientific reasons to suspect that it can exist elsewhere, and technological advances that may detect it if it does.[156]

Many scientists are optimistic about the chances of finding alien life. In the words of SETI's Frank Drake, "All we know for sure is that the sky is not littered with powerful microwave transmitters".[157] Drake noted that it is entirely possible that advanced technology results in communication being carried out in some way other than conventional radio transmission. At the same time, the data returned by space probes, and giant strides in detection methods, have allowed science to begin delineating habitability criteria on other worlds, and to confirm that at least other planets are plentiful, though aliens remain a question mark. The Wow! signal, detected in 1977 by a SETI project, remains a subject of speculative debate.[158]

On the other hand, other scientists are pessimistic. Jacques Monod wrote that "Man knows at last that he is alone in the indifferent immensity of the universe, whence which he has emerged by chance".[159] In 2000, geologist and paleontologist Peter Ward and astrobiologist Donald Brownlee published a book entitled Rare Earth: Why Complex Life is Uncommon in the Universe.[160][better source needed] In it, they discussed the Rare Earth hypothesis, in which they claim that Earth-like life is rare in the universe, whereas microbial life is common. Ward and Brownlee are open to the idea of evolution on other planets that is not based on essential Earth-like characteristics such as DNA and carbon.

As for the possible risks, theoretical physicist Stephen Hawking warned in 2010 that humans should not try to contact alien life forms. He warned that aliens might pillage Earth for resources. "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans", he said.[161] Jared Diamond had earlier expressed similar concerns.[162] On 20 July 2015, Hawking and Russian billionaire Yuri Milner, along with the SETI Institute, announced a well-funded effort, called the Breakthrough Initiatives, to expand efforts to search for extraterrestrial life. The group contracted the services of the 100-meter Robert C. Byrd Green Bank Telescope in West Virginia in the United States and the 64-meter Parkes Telescope in New South Wales, Australia.[163] On 13 February 2015, scientists (including Geoffrey Marcy, Seth Shostak, Frank Drake and David Brin) at a convention of the American Association for the Advancement of Science, discussed Active SETI and whether transmitting a message to possible intelligent extraterrestrials in the Cosmos was a good idea;[164][165] one result was a statement, signed by many, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent".[166]

## Government responses

[edit]

See also: Planetary protection

The 1967 Outer Space Treaty and the 1979 Moon Agreement define rules of planetary protection against potentially hazardous extraterrestrial life. COSPAR also provides guidelines for planetary protection.[167] A committee of the United Nations Office for Outer Space Affairs had in 1977 discussed for a year strategies for interacting with extraterrestrial life or intelligence. The discussion ended without any conclusions. As of 2010, the UN lacks response mechanisms for the case of an extraterrestrial contact.[168]

One of the NASA divisions is the Office of Safety and Mission Assurance (OSMA), also known as the Planetary Protection Office. A part of its mission is to "rigorously preclude backward contamination of Earth by extraterrestrial life."[169]

In 2016, the Chinese Government released a white paper detailing its space program. According to the document, one of the research objectives of the program is the search for extraterrestrial life.[170] It is also one of the objectives of the Chinese Five-hundred-meter Aperture Spherical Telescope (FAST) program.[171]

In 2020, Dmitry Rogozin, the head of the Russian space agency, said the search for extraterrestrial life is one of the main goals of deep space research. He also acknowledged the possibility of existence of primitive life on other planets of the Solar System.[172]

The French space agency has an office for the study of "non-identified aero spatial phenomena".[173][174] The agency is maintaining a publicly accessible database of such phenomena, with over 1600 detailed entries. According to the head of the office, the vast majority of entries have a mundane explanation; but for 25% of entries, their extraterrestrial origin can neither be confirmed nor denied.[173]

In 2020, chairman of the Israel Space Agency Isaac Ben-Israel stated that the probability of detecting life in outer space is "quite large". But he disagrees with his former colleague Haim Eshed who stated that there are contacts between an advanced alien civilisation and some of Earth's governments.[175]

## In fiction

[edit]

Main article: Extraterrestrials in fiction

Grey aliens are a common way to depict extraterrestrials in fiction.

Although the idea of extraterrestrial peoples became feasible once astronomy developed enough to understand the nature of planets, they were not thought of as being any different from humans. Having no scientific explanation for the origin of mankind and its relation to other species, there was no reason to expect them to be any other way. This was changed by the 1859 book On the Origin of Species by Charles Darwin, which proposed the theory of evolution. Now with the notion that evolution on other planets may take other directions, science fiction authors created bizarre aliens, clearly distinct from humans. A usual way to do that was to add body features from other animals, such as insects or octopuses. Costuming and special effects feasibility alongside budget considerations forced films and TV series to tone down the fantasy, but these limitations lessened since the 1990s with the advent of computer-generated imagery (CGI), and later on as CGI became more effective and less expensive.[176]

Real-life events sometimes captivate people's imagination and this influences the works of fiction. For example, during the Barney and Betty Hill incident, the first recorded claim of an alien abduction, the couple reported that they were abducted and experimented on by aliens with oversized heads, big eyes, pale grey skin, and small noses, a description that eventually became the grey alien archetype once used in works of fiction.[176]

## See also

[edit]

* Europa Clipper
* Europa (moon)
* Assembly theory
* Carbon chauvinism
* First contact (anthropology)
* Hemolithin
* Hypothetical types of biochemistry
* Life origination beyond planets
* Outline of extraterrestrial life
* Quiet and loud aliens
* Sentiocentrism
* Speciesism
* Uncontacted peoples

## Notes

[edit]

1. ^ For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars wasn't available so this statistic is an extrapolation from data about K-type stars
2. ^ For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
3. ^ For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).
4. ^ About 1/4 of stars are GK Sun-like stars. The number of stars in the galaxy is not accurately known, but assuming 200 billion stars in total, the Milky Way would have about 50 billion Sun-like (GK) stars, of which about 1 in 5 (22%) or 11 billion would be Earth-sized in the habitable zone. Including red dwarfs would increase this to 40 billion.

## References

[edit]

1. ^ Frank, Adam (31 December 2020). "A new frontier is opening in the search for extraterrestrial life – The reason we haven't found life elsewhere in the universe is simple: We haven't really looked until now". The Washington Post. Retrieved 1 January 2021.
2. ^ Davies, Paul (18 November 2013). "Are We Alone in the Universe?". The New York Times. Archived from the original on 1 January 2022. Retrieved 20 November 2013.
3. ^ Pickrell, John (4 September 2006). "Top 10: Controversial pieces of evidence for extraterrestrial life". New Scientist. Retrieved 18 February 2011.
4. ^ a b c Crowe, Michael J. (2008). The extraterrestrial life debate, antiquity to 1915: a sourcebook. Notre Dame, Ind: University of Notre Dame. ISBN 978-0-268-02368-3.
5. ^ Nicholas of Cusa. (1954). Of Learned Ignorance. Translated by Germain Heron. Routledge. pp. 111–118.
6. ^ Catling, D.C. (2015), "Planetary Atmospheres", Treatise on Geophysics, Elsevier, pp. 429–472, Bibcode:2015trge.book..429C, doi:10.1016/b978-0-444-53802-4.00185-8, ISBN 978-0-444-53803-1, retrieved 17 April 2024
7. ^ a b Shibuya, Takazo; Takai, Ken (16 November 2022). "Liquid and supercritical CO2 as an organic solvent in Hadean seafloor hydrothermal systems: implications for prebiotic chemical evolution". Progress in Earth and Planetary Science. 9 (1). doi:10.1186/s40645-022-00510-6. ISSN 2197-4284.
8. ^ Damer, Bruce; Deamer, David (1 April 2020). "The Hot Spring Hypothesis for an Origin of Life". Astrobiology. 20 (4): 429–452\. Bibcode:2020AsBio..20..429D. doi:10.1089/ast.2019.2045. ISSN 1531-1074. PMC 7133448. PMID 31841362.
9. ^ Mapelli, Francesca; Marasco, Ramona; Rolli, Eleonora; Daffonchio, Daniele; Donachie, Stuart; Borin, Sara (2015), Rouwet, Dmitri; Christenson, Bruce; Tassi, Franco; Vandemeulebrouck, Jean (eds.), "Microbial Life in Volcanic Lakes", Volcanic Lakes, Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 507–522, doi:10.1007/978-3-642-36833-2_23, hdl:2434/266460, ISBN 978-3-642-36832-5, retrieved 17 April 2024
10. ^ Marion, Giles M.; Fritsen, Christian H.; Eicken, Hajo; Payne, Meredith C. (1 December 2003). "The search for life on Europa: Limiting environmental factors, potential habitats, and Earth analogues". Astrobiology. 3 (4): 785–811\. Bibcode:2003AsBio...3..785M. doi:10.1089/153110703322736105. ISSN 1531-1074. PMID 14987483.
11. ^ Heuer, Verena B.; Inagaki, Fumio; Morono, Yuki; Kubo, Yusuke; Spivack, Arthur J.; Viehweger, Bernhard; Treude, Tina; Beulig, Felix; Schubotz, Florence; Tonai, Satoshi; Bowden, Stephen A. (4 December 2020). "Temperature limits to deep subseafloor life in the Nankai Trough subduction zone". Science. 370 (6521): 1230–1234\. Bibcode:2020Sci...370.1230H. doi:10.1126/science.abd7934. hdl:2164/15700. ISSN 0036-8075. PMID 33273103. S2CID 227257205.
12. ^ "Planetary Systems Composite Data". NASA Exoplanet Archive. Retrieved 7 August 2025.
13. ^ a b Overbye, Dennis (6 January 2015). "So Many Earth-Like Planets, So Few Telescopes". The New York Times. Archived from the original on 1 January 2022. Retrieved 6 January 2015.
14. ^ Mann, Adam (1 December 2020). "Want to Talk to Aliens? Try Changing the Technological Channel beyond Radio". Scientific American. Retrieved 10 May 2024.
15. ^ Ghosh, Pallab (12 February 2015). "Scientists in US are urged to seek contact with aliens". BBC News.
16. ^ Baum, Seth; Haqq-Misra, Jacob; Domagal-Goldman, Shawn (June 2011). "Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis". Acta Astronautica. 68 (11): 2114–2129\. arXiv:1104.4462. Bibcode:2011AcAau..68.2114B. doi:10.1016/j.actaastro.2010.10.012. ISSN 0094-5765. S2CID 16889489.
17. ^ "Overview - NASA Science". 1 September 2020. Retrieved 12 February 2025.
18. ^ Avi Loeb (4 April 2021). "When Did Life First Emerge in the Universe?". Scientific American. Retrieved 17 April 2023.
19. ^ Moskowitz, Clara (29 March 2012). "Life's Building Blocks May Have Formed in Dust Around Young Sun". Space.com. Retrieved 30 March 2012.
20. ^ Rampelotto, P. H. (April 2010). Panspermia: A Promising Field of Research (PDF). Astrobiology Science Conference 2010: Evolution and Life: Surviving Catastrophes and Extremes on Earth and Beyond. 20–26 April 2010. League City, Texas. Bibcode:2010LPICo1538.5224R.
21. ^ Gonzalez, Guillermo; Richards, Jay Wesley (2004). The privileged planet: how our place in the cosmos is designed for discovery. Regnery Publishing. pp. 343–345\. ISBN 978-0-89526-065-9.
22. ^ Bennet, pp. 60-63
23. ^ Bennett, p. 53
24. ^ a b Bennet, p. 55
25. ^ Bennet, pp. 57-58
26. ^ a b c d e f g Pat Brennan (10 November 2020). "Life in Our Solar System? Meet the Neighbors". NASA. Retrieved 30 March 2023.
27. ^ Vicky Stein (16 February 2023). "Goldilocks zone: Everything you need to know about the habitable sweet spot". Space.com. Retrieved 22 April 2023.
28. ^ Bennet, p. 65
29. ^ Loeb, Abraham (October 2014). "The Habitable Epoch of the Early Universe" (PDF). International Journal of Astrobiology. 13 (4): 337–339\. arXiv:1312.0613. Bibcode:2014IJAsB..13..337L. doi:10.1017/S1473550414000196. S2CID 2777386. Retrieved 15 December 2014.
30. ^ Dreifus, Claudia (2 December 2014). "Much-Discussed Views That Go Way Back – Avi Loeb Ponders the Early Universe, Nature and Life". The New York Times. Retrieved 3 December 2014.
31. ^ Aguilera Mochon, pp. 9–10
32. ^ Bennet, p. 51
33. ^ Steiger, Brad; White, John, eds. (1986). Other Worlds, Other Universes. Health Research Books. p. 3\. ISBN 978-0-7873-1291-6.
34. ^ Filkin, David; Hawking, Stephen W. (1998). Stephen Hawking's universe: the cosmos explained. Art of Mentoring Series. Basic Books. p. 194. ISBN 978-0-465-08198-1.
35. ^ Rauchfuss, Horst (2008). Chemical Evolution and the Origin of Life. trans. Terence N. Mitchell. Springer. ISBN 978-3-540-78822-5.
36. ^ Aguilera Mochón, p. 66
37. ^ Morgan Kelly (26 April 2012). "Expectation of extraterrestrial life built more on optimism than evidence, study finds". Princeton University. Retrieved 22 April 2023.
38. ^ "Chapter 3 – Philosophy: "Solving the Drake Equation". SETI League. December 2002. Retrieved 24 July 2015.
39. ^ Aguirre, L. (1 July 2008). "The Drake Equation". Nova ScienceNow. PBS. Retrieved 7 March 2010.
40. ^ Burchell, M. J. (2006). "W(h)ither the Drake equation?". International Journal of Astrobiology. 5 (3): 243–250\. Bibcode:2006IJAsB...5..243B. doi:10.1017/S1473550406003107. S2CID 121060763.
41. ^ Cohen, Jack; Stewart, Ian (2002). "Chapter 6: What does a Martian look like?". Evolving the Alien: The Science of Extraterrestrial Life. Hoboken, NJ: John Wiley and Sons. ISBN 978-0-09-187927-3.
42. ^ Macrobert, Alan (13 October 2016). "About those 2 trillion new galaxies..." Sky & Telescope. Retrieved 24 May 2023.
43. ^ Marcy, G.; Butler, R.; Fischer, D.; et al. (2005). "Observed Properties of Exoplanets: Masses, Orbits and Metallicities". Progress of Theoretical Physics Supplement. 158: 24–42\. arXiv:astro-ph/0505003. Bibcode:2005PThPS.158...24M. doi:10.1143/PTPS.158.24. S2CID 16349463. Archived from the original on 2 October 2008.
44. ^ Swift, Jonathan J.; Johnson, John Asher; Morton, Timothy D.; Crepp, Justin R.; Montet, Benjamin T.; et al. (January 2013). "Characterizing the Cool KOIs. IV. Kepler-32 as a Prototype for the Formation of Compact Planetary Systems throughout the Galaxy". The Astrophysical Journal. 764 (1). 105. arXiv:1301.0023. Bibcode:2013ApJ...764..105S. doi:10.1088/0004-637X/764/1/105. S2CID 43750666.
45. ^ "100 Billion Alien Planets Fill Our Milky Way Galaxy: Study". Space.com. 2 January 2013. Archived from the original on 3 January 2013. Retrieved 10 March 2016.
46. ^ Bennet, p. 98
47. ^ Overbye, Dennis (3 August 2015). "The Flip Side of Optimism About Life on Other Planets". The New York Times. Archived from the original on 1 January 2022. Retrieved 29 October 2015.
48. ^ Wang, Zhi-Wei; Braunstein, Samuel L. (2023). "Sciama's argument on life in a random universe and distinguishing apples from oranges". Nature Astronomy. 7 (2023): 755–756\. arXiv:2109.10241. Bibcode:2023NatAs...7..755W. doi:10.1038/s41550-023-02014-9.
49. ^ Bennett, p. 3
50. ^ Aguilera Mochón, p. 42
51. ^ Aguilera Mochón, p. 58
52. ^ Aguilera Mochón, p. 51
53. ^ Bond, Jade C.; O'Brien, David P.; Lauretta, Dante S. (June 2010). "The Compositional Diversity of Extrasolar Terrestrial Planets. I. In Situ Simulations". The Astrophysical Journal. 715 (2): 1050–1070\. arXiv:1004.0971. Bibcode:2010ApJ...715.1050B. doi:10.1088/0004-637X/715/2/1050. S2CID 118481496.
54. ^ Pace, Norman R. (20 January 2001). "The universal nature of biochemistry". Proceedings of the National Academy of Sciences of the United States of America. 98 (3): 805–808\. Bibcode:2001PNAS...98..805P. doi:10.1073/pnas.98.3.805. PMC 33372. PMID 11158550.
55. ^ National Research Council (2007). "6.2.2: Nonpolar Solvents". The Limits of Organic Life in Planetary Systems. The National Academies Press. p. 74\. doi:10.17226/11919. ISBN 978-0-309-10484-5.
56. ^ Aguilera Mochón, pp. 43–49
57. ^ Horowitz, N.H. (1986). Utopia and Back and the search for life in the solar system. New York: W.H. Freeman and Company. ISBN 0-7167-1766-2
58. ^ Bernstein, Harris; Byerly, Henry C.; Hopf, Frederick A.; et al. (June 1983). "The Darwinian Dynamic". The Quarterly Review of Biology. 58 (2): 185–207. doi:10.1086/413216. JSTOR 2828805\. S2CID 83956410
59. ^ Aguilera Mochón, pp. 58–59
60. ^ Aguilera Mochón, pp. 42–43
61. ^ a b Aguilera Mochón, pp. 61–66
62. ^ "Aliens may be more like us than we think". University of Oxford. 31 October 2017.
63. ^ Stevenson, David S.; Large, Sean (25 October 2017). "Evolutionary exobiology: Towards the qualitative assessment of biological potential on exoplanets". International Journal of Astrobiology. 18 (3): 204–208\. doi:10.1017/S1473550417000349. S2CID 125275411.
64. ^ "Atmosphere - Planets, Composition, Pressure | Britannica". www.britannica.com. Retrieved 17 April 2024.
65. ^ Amils, Ricardo; González-Toril, Elena; Fernández-Remolar, David; Gómez, Felipe; Aguilera, Ángeles; Rodríguez, Nuria; Malki, Mustafá; García-Moyano, Antonio; Fairén, Alberto G.; de la Fuente, Vicenta; Luis Sanz, José (February 2007). "Extreme environments as Mars terrestrial analogs: The Rio Tinto case". Planetary and Space Science. 55 (3): 370–381\. Bibcode:2007P&SS...55..370A. doi:10.1016/j.pss.2006.02.006.
66. ^ Daniel, Isabelle; Oger, Philippe; Winter, Roland (2006). "Origins of life and biochemistry under high-pressure conditions". Chemical Society Reviews. 35 (10): 858–875\. doi:10.1039/b517766a. ISSN 0306-0012. PMID 17003893.
67. ^ Dong, Hailiang; Yu, Bingsong (1 September 2007). "Geomicrobiological processes in extreme environments: A review". Episodes. 30 (3): 202–216\. doi:10.18814/epiiugs/2007/v30i3/003. ISSN 0705-3797.
68. ^ a b Georgieva, Magdalena N.; Little, Crispin T.S.; Maslennikov, Valeriy V.; Glover, Adrian G.; Ayupova, Nuriya R.; Herrington, Richard J. (June 2021). "The history of life at hydrothermal vents". Earth-Science Reviews. 217: 103602. Bibcode:2021ESRv..21703602G. doi:10.1016/j.earscirev.2021.103602.
69. ^ Zahnle, Kevin J.; Lupu, Roxana; Catling, David C.; Wogan, Nick (1 June 2020). "Creation and Evolution of Impact-generated Reduced Atmospheres of Early Earth". The Planetary Science Journal. 1 (1): 11. arXiv:2001.00095. Bibcode:2020PSJ.....1...11Z. doi:10.3847/PSJ/ab7e2c. ISSN 2632-3338.
70. ^ Atreya, S.K; Mahaffy, P.R; Niemann, H.B; Wong, M.H; Owen, T.C (February 2003). "Composition and origin of the atmosphere of Jupiter—an update, and implications for the extrasolar giant planets". Planetary and Space Science. 51 (2): 105–112\. Bibcode:2003P&SS...51..105A. doi:10.1016/S0032-0633(02)00144-7.
71. ^ Bennett, pp. 3-4
72. ^ Marcq, Emmanuel; Mills, Franklin P.; Parkinson, Christopher D.; Vandaele, Ann Carine (30 November 2017). "Composition and Chemistry of the Neutral Atmosphere of Venus" (PDF). Space Science Reviews. 214 (1): 10. doi:10.1007/s11214-017-0438-5. ISSN 1572-9672. S2CID 255067610.
73. ^ "What Is Astrobiology?". University of Washington. Retrieved 28 April 2023.
74. ^ Chang, Kenneth; Stirone, Shannon (8 February 2021). "Life on Venus? The Picture Gets Cloudier – Despite doubts from many scientists, a team of researchers who said they had detected an unusual gas in the planet's atmosphere were still confident of their findings". The New York Times. Retrieved 8 February 2021.
75. ^ Cofield, Calla; Chou, Felicia (25 June 2018). "NASA Asks: Will We Know Life When We See It?". NASA. Retrieved 26 June 2018.
76. ^ Nightingale, Sarah (25 June 2018). "UCR Team Among Scientists Developing Guidebook for Finding Life Beyond Earth". UCR Today. University of California, Riverside. Retrieved 26 June 2018.
77. ^ a b c Crenson, Matt (6 August 2006). "Experts: Little Evidence of Life on Mars". Associated Press. Archived from the original on 16 April 2011. Retrieved 8 March 2011.
78. ^ a b McKay, David S.; Gibson, Everett K. Jr.; Thomas-Keprta, Kathie L.; Vali, Hojatollah; Romanek, Christopher S.; et al. (August 1996). "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001". Science. 273 (5277): 924–930\. Bibcode:1996Sci...273..924M. doi:10.1126/science.273.5277.924. PMID 8688069. S2CID 40690489.
79. ^ Webster, Guy (27 February 2014). "NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars". NASA. Retrieved 27 February 2014.
80. ^ Gannon, Megan (28 February 2014). "Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life". Space.com. Retrieved 28 February 2014.
81. ^ a b Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 978-0-7137-2747-0.
82. ^ Klein, Harold P.; Levin, Gilbert V.; Levin, Gilbert V.; Oyama, Vance I.; Lederberg, Joshua; Rich, Alexander; Hubbard, Jerry S.; Hobby, George L.; Straat, Patricia A.; Berdahl, Bonnie J.; Carle, Glenn C.; Brown, Frederick S.; Johnson, Richard D. (1 October 1976). "The Viking Biological Investigation: Preliminary Results". Science. 194 (4260): 99–105\. Bibcode:1976Sci...194...99K. doi:10.1126/science.194.4260.99. PMID 17793090. S2CID 24957458.
83. ^ Beegle, Luther W.; Wilson, Michael G.; Abilleira, Fernando; Jordan, James F.; Wilson, Gregory R. (August 2007). "A Concept for NASA's Mars 2016 Astrobiology Field Laboratory". Astrobiology. 7 (4): 545–577\. Bibcode:2007AsBio...7..545B. doi:10.1089/ast.2007.0153. PMID 17723090.
84. ^ "ExoMars rover". ESA. Archived from the original on 19 October 2012. Retrieved 14 April 2014.
85. ^ Berger, Brian (16 February 2005). "Exclusive: NASA Researchers Claim Evidence of Present Life on Mars". Space.com.
86. ^ "NASA denies Mars life reports". spacetoday.net. 19 February 2005.
87. ^ Chow, Dennis (22 July 2011). "NASA's Next Mars Rover to Land at Huge Gale Crater". Space.com. Retrieved 22 July 2011.
88. ^ Amos, Jonathan (22 July 2011). "Mars rover aims for deep crater". BBC News. Retrieved 22 July 2011.
89. ^ Cofield, Calla (30 March 2015). "Catalog of Earth Microbes Could Help Find Alien Life". Space.com. Retrieved 11 May 2015.
90. ^ Callahan, M.P.; Smith, K.E.; Cleaves, H.J.; Ruzica, J.; Stern, J.C.; Glavin, D.P.; House, C.H.; Dworkin, J.P. (11 August 2011). "Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases". Proceedings of the National Academy of Sciences. 108 (34): 13995–13998\. Bibcode:2011PNAS..10813995C. doi:10.1073/pnas.1106493108. PMC 3161613. PMID 21836052.
91. ^ Steigerwald, John (8 August 2011). "NASA Researchers: DNA Building Blocks Can Be Made in Space". NASA. Archived from the original on 11 May 2020. Retrieved 10 August 2011.
92. ^ a b Chow, Denise (26 October 2011). "Discovery: Cosmic Dust Contains Organic Matter from Stars". Space.com. Retrieved 26 October 2011.
93. ^ "Astronomers Discover Complex Organic Matter Exists Throughout the Universe". ScienceDaily. 26 October 2011. Retrieved 27 October 2011.
94. ^ Kwok, Sun; Zhang, Yong (26 October 2011). "Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features". Nature. 479 (7371): 80–3\. Bibcode:2011Natur.479...80K. doi:10.1038/nature10542. PMID 22031328. S2CID 4419859.
95. ^ Ker Than (30 August 2012). "Sugar Found In Space: A Sign of Life?". National Geographic. Retrieved 4 July 2023.
96. ^ Jørgensen, Jes K.; Favre, Cécile; Bisschop, Suzanne E.; Bourke, Tyler L.; van Dishoeck, Ewine F.; Schmalzl, Markus (September 2012). "Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA" (PDF). The Astrophysical Journal Letters. 757 (1). L4. arXiv:1208.5498. Bibcode:2012ApJ...757L...4J. doi:10.1088/2041-8205/757/1/L4. S2CID 14205612.
97. ^ Green, Jaime (5 December 2023). "What Is Life? - The answer matters in space exploration. But we still don't really know". The Atlantic. Archived from the original on 5 December 2023. Retrieved 15 December 2023.
98. ^ Chang, Kenneth (14 December 2023). "Poison Gas Hints at Potential for Life on an Ocean Moon of Saturn - A researcher who has studied the icy world said "the prospects for the development of life are getting better and better on Enceladus."". The New York Times. Archived from the original on 14 December 2023. Retrieved 15 December 2023.
99. ^ Peter, Jonah S.; et al. (14 December 2023). "Detection of HCN and diverse redox chemistry in the plume of Enceladus". Nature Astronomy. 8 (2): 164–173\. arXiv:2301.05259. Bibcode:2024NatAs...8..164P. doi:10.1038/s41550-023-02160-0. S2CID 255825649. Archived from the original on 15 December 2023. Retrieved 15 December 2023.
100. ^ a b c d Pat Brennan. "Searching for Signs of Intelligent Life: Technosignatures". NASA. Retrieved 4 July 2023.
101. ^ "The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum". The Columbus Optical SETI Observatory.
102. ^ Whitmire, Daniel P.; Wright, David P. (April 1980). "Nuclear waste spectrum as evidence of technological extraterrestrial civilizations". Icarus. 42 (1): 149–156\. Bibcode:1980Icar...42..149W. doi:10.1016/0019-1035(80)90253-5.
103. ^ "Discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy exoplanet". IAP.fr. 25 January 2006.
104. ^ Than, Ker (24 April 2007). "Major Discovery: New Planet Could Harbor Water and Life". Space.com.
105. ^ a b Schneider, Jean (10 September 2011). "Interactive Extra-solar Planets Catalog". Extrasolar Planets Encyclopaedia. Retrieved 30 January 2012.
106. ^ Wall, Mike (4 April 2012). "NASA Extends Planet-Hunting Kepler Mission Through 2016". Space.com.
107. ^ "NASA – Kepler". Archived from the original on 5 November 2013. Retrieved 4 November 2013.
108. ^ Harrington, J. D.; Johnson, M. (4 November 2013). "NASA Kepler Results Usher in a New Era of Astronomy".
109. ^ Tenenbaum, P.; Jenkins, J. M.; Seader, S.; Burke, C. J.; Christiansen, J. L.; Rowe, J. F.; Caldwell, D. A.; Clarke, B. D.; Li, J.; Quintana, E. V.; Smith, J. C.; Thompson, S. E.; Twicken, J. D.; Borucki, W. J.; Batalha, N. M.; Cote, M. T.; Haas, M. R.; Hunter, R. C.; Sanderfer, D. T.; Girouard, F. R.; Hall, J. R.; Ibrahim, K.; Klaus, T. C.; McCauliff, S. D.; Middour, C. K.; Sabale, A.; Uddin, A. K.; Wohler, B.; Barclay, T.; Still, M. (2013). "Detection of Potential Transit Signals in the First 12 Quarters of Kepler Mission Data". The Astrophysical Journal Supplement Series. 206 (1): 5. arXiv:1212.2915. Bibcode:2013ApJS..206....5T. doi:10.1088/0067-0049/206/1/5. S2CID 250885680.
110. ^ "My God, it's full of planets! They should have sent a poet" (Press release). Planetary Habitability Laboratory, University of Puerto Rico at Arecibo. 3 January 2012. Archived from the original on 25 July 2015. Retrieved 25 July 2015.
111. ^ Santerne, A.; Díaz, R. F.; Almenara, J.-M.; Lethuillier, A.; Deleuil, M.; Moutou, C. (2013). "Astrophysical false positives in exoplanet transit surveys: Why do we need bright stars?". Sf2A-2013: Proceedings of the Annual Meeting of the French Society of Astronomy and Astrophysics: 555. arXiv:1310.2133. Bibcode:2013sf2a.conf..555S.
112. ^ Cassan, A.; et al. (11 January 2012). "One or more bound planets per Milky Way star from microlensing observations". Nature. 481 (7380): 167–169\. arXiv:1202.0903. Bibcode:2012Natur.481..167C. doi:10.1038/nature10684. PMID 22237108. S2CID 2614136.
113. ^ Sanders, R. (4 November 2013). "Astronomers answer key question: How common are habitable planets?". newscenter.berkeley.edu.
114. ^ Petigura, E. A.; Howard, A. W.; Marcy, G. W. (2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences. 110 (48): 19273–19278\. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC 3845182. PMID 24191033.
115. ^ Khan, Amina (4 November 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved 5 November 2013.
116. ^ Strigari, L. E.; Barnabè, M.; Marshall, P. J.; Blandford, R. D. (2012). "Nomads of the Galaxy". Monthly Notices of the Royal Astronomical Society. 423 (2): 1856–1865\. arXiv:1201.2687. Bibcode:2012MNRAS.423.1856S. doi:10.1111/j.1365-2966.2012.21009.x. S2CID 119185094. estimates 700 objects >10−6 solar masses (roughly the mass of Mars) per main-sequence star between 0.08 and 1 Solar mass, of which there are billions in the Milky Way.
117. ^ Chang, Kenneth (24 August 2016). "One Star Over, a Planet That Might Be Another Earth". The New York Times. Archived from the original on 1 January 2022. Retrieved 4 September 2016.
118. ^ "DENIS-P J082303.1-491201 b". Caltech. Retrieved 8 March 2014.
119. ^ Sahlmann, J.; Lazorenko, P. F.; Ségransan, D.; Martín, Eduardo L.; Queloz, D.; Mayor, M.; Udry, S. (August 2013). "Astrometric orbit of a low-mass companion to an ultracool dwarf". Astronomy & Astrophysics. 556: 133. arXiv:1306.3225. Bibcode:2013A&A...556A.133S. doi:10.1051/0004-6361/201321871. S2CID 119193690.
120. ^ Aguilar, David A.; Pulliam, Christine (25 February 2013). "Future Evidence for Extraterrestrial Life Might Come from Dying Stars". Harvard-Smithsonian Center for Astrophysics. Release 2013-06. Retrieved 9 June 2017.
121. ^ Bennett, pp. 16-23
122. ^ Crowe, Michael J. (1999). The Extraterrestrial Life Debate, 1750–1900. Courier Dover Publications. ISBN 978-0-486-40675-6.
123. ^ Wiker, Benjamin D. (4 November 2002). "Alien Ideas: Christianity and the Search for Extraterrestrial Life". Crisis Magazine. Archived from the original on 10 February 2003.
124. ^ Irwin, Robert (2003). The Arabian Nights: A Companion. Tauris Parke Paperbacks. p. 204 & 209\. ISBN 978-1-86064-983-7.
125. ^ David A. Weintraub (2014). "Islam," Religions and Extraterrestrial Life (pp 161–168). Springer International Publishing.
126. ^ Gabrovsky, A.N. (2016). Chaucer the Alchemist: Physics, Mutability, and the Medieval Imagination. The New Middle Ages. Palgrave Macmillan US. p. 83\. ISBN 978-1-137-52391-4. Retrieved 14 May 2023.
127. ^ Crowe, p. 4
128. ^ Bennett, p. 24
129. ^ Bennett, p. 31
130. ^ a b J. William Schopf (2002). Life's Origin: The Beginnings of Biological Evolution. University of California Press. ISBN 9780520233911. Retrieved 6 August 2022.
131. ^ Bennet, pp. 24-27
132. ^ Bennet, p. 5
133. ^ Bennett, p. 29
134. ^ "Giordano Bruno: On the Infinite Universe and Worlds (De l'Infinito Universo et Mondi) Introductory Epistle: Argument of the Third Dialogue". Archived from the original on 13 October 2014. Retrieved 4 October 2014.
135. ^ a b Aguilera Mochon, p. 8
136. ^ Bennet, p. 30
137. ^ Bennet, pp. 30-32
138. ^ "Peoples & Creatures of the Moon | Life on Other Worlds | Articles and Essays | Finding Our Place in the Cosmos: From Galileo to Sagan and Beyond | Digital Collections | Library of Congress". Library of Congress, Washington, D.C. 20540 USA. Retrieved 10 May 2024.
139. ^ Parkyn, Joel L. (April 2019). "The Devine Pedagogy: Theological Explorations of Intelligent Extraterrestrial Life" (PDF). ore.exeter.ac.uk. Retrieved 10 May 2024.
140. ^ Evans, J. E.; Maunder, E. W. (June 1903). "Experiments as to the actuality of the "Canals" observed on Mars". Monthly Notices of the Royal Astronomical Society. 63 (8): 488–499\. Bibcode:1903MNRAS..63..488E. doi:10.1093/mnras/63.8.488.
141. ^ Wallace, Alfred Russel (1907). Is Mars Habitable? A Critical Examination of Professor Lowell's Book "Mars and Its Canals," With an Alternative Explanation. London: Macmillan. OCLC 8257449.
142. ^ Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 978-0-7137-2747-0.
143. ^ "Seeing and Interpreting Martian Oceans and Canals | Life on Other Worlds | Articles and Essays | Finding Our Place in the Cosmos: From Galileo to Sagan and Beyond | Digital Collections | Library of Congress". Library of Congress, Washington, D.C. 20540 USA. Retrieved 10 May 2024.
144. ^ Aguilera Mochon, pp. 8–9
145. ^ Berzelius, Jöns Jacob (1834). "Analysis of the Alais meteorite and implications about life in other worlds". Annalen der Chemie und Pharmacie. 10: 134–135.
146. ^ Thomson, William (August 1871). "The British Association Meeting at Edinburgh". Nature. 4 (92): 261–278\. Bibcode:1871Natur...4..261.. doi:10.1038/004261a0. PMC 2070380. "We must regard it as probably to the highest degree that there are countless seed-bearing meteoritic stones moving through space."
147. ^ Demets, René (October 2012). "Darwin's Contribution to the Development of the Panspermia Theory". Astrobiology. 12 (10): 946–950\. Bibcode:2012AsBio..12..946D. doi:10.1089/ast.2011.0790. PMID 23078643.
148. ^ Arrhenius, Svante (March 1908). Worlds in the Making: The Evolution of the Universe. trans. H. Borns. Harper & Brothers. OCLC 1935295.
149. ^ Nola Taylor Tillman (20 August 2012). "The Face on Mars: Fact & Fiction". Space.com. Retrieved 18 September 2022.
150. ^ Aguilera Mochon, pp. 10–11
151. ^ "Life's Working Definition: Does It Work?". NASA. 2002. Archived from the original on 26 May 2018. Retrieved 17 January 2022.
152. ^ Aguilera Mochon, p. 10
153. ^ Cross, Anne (2004). "The Flexibility of Scientific Rhetoric: A Case Study of UFO Researchers". Qualitative Sociology. 27 (1): 3–34\. doi:10.1023/B:QUAS.0000015542.28438.41. S2CID 144197172.
154. ^ Ailleris, Philippe (January–February 2011). "The lure of local SETI: Fifty years of field experiments". Acta Astronautica. 68 (1–2): 2–15\. Bibcode:2011AcAau..68....2A. doi:10.1016/j.actaastro.2009.12.011.1–2&rft.pages=2-15&rft.date=2011-01/2011-02&rft_id=info:doi/10.1016/j.actaastro.2009.12.011&rft_id=info:bibcode/2011AcAau..68....2A&rft.aulast=Ailleris&rft.aufirst=Philippe&rfr_id=info:sid/en.wikipedia.org:Extraterrestrial+life" class="Z3988">
155. ^ Beck, Lewis White (1971). "Extraterrestrial Intelligent Life". Proceedings and Addresses of the American Philosophical Association. 45: 5–21\. doi:10.2307/3129745. JSTOR 3129745.
156. ^ Bennett, p. 4
157. ^ "LECTURE 4: MODERN THOUGHTS ON EXTRATERRESTRIAL LIFE". The University of Antarctica. Retrieved 25 July 2015.
158. ^ "Did the Wow! signal come from this star? | Space | EarthSky". earthsky.org. 2 December 2020. Retrieved 10 May 2024.
159. ^ Paul Davies (1 September 2016). "The Cosmos Might Be Mostly Devoid of Life". Scientific American. Retrieved 8 July 2022.
160. ^ Ward, Peter; Brownlee, Donald (2000). Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus. Bibcode:2000rewc.book.....W. ISBN 978-0-387-98701-9.
161. ^ "Hawking warns over alien beings". BBC News. 25 April 2010. Retrieved 2 May 2010.
162. ^ Diamond, Jared M. (2006). "Chapter 12". The Third Chimpanzee: The Evolution and Future of the Human Animal. Harper Perennial. ISBN 978-0-06-084550-6.
163. ^ Katz, Gregory (20 July 2015). "Searching for ET: Hawking to look for extraterrestrial life". Excite!. Associated Press. Retrieved 20 July 2015.
164. ^ Borenstein, Seth (13 February 2015). "Should We Call the Cosmos Seeking ET? Or Is That Risky?". The New York Times. Associated Press. Archived from the original on 14 February 2015.
165. ^ Ghosh, Pallab (12 February 2015). "Scientist: 'Try to contact aliens'". BBC News. Retrieved 12 February 2015.
166. ^ "Regarding Messaging To Extraterrestrial Intelligence (METI) / Active Searches For Extraterrestrial Intelligence (Active SETI)". University of California, Berkeley. 13 February 2015. Retrieved 14 February 2015.
167. ^ Matignon, Louis (29 May 2019). "The French anti-UFO Municipal Law of 1954". Space Legal Issues. Archived from the original on 27 April 2021. Retrieved 26 March 2021.
168. ^ "Press Conference by Director of Office for Outer Space Affairs". UN Press. 14 October 2010.
169. ^ Kluger, Jeffrey (2 March 2020). "Coronavirus Could Preview What Will Happen When Alien Life Reaches Earth". Time.
170. ^ Wheeler, Michelle (14 July 2017). "Is China The Next Space Superpower?". Particle.
171. ^ "China Focus: Earth's largest radio telescope to search for "new worlds" outside solar system". Archived from the original on 11 July 2019.
172. ^ "Рогозин допустил существование жизни на Марсе и других планетах Солнечной системы". ТАСС.
173. ^ a b "France opens up its UFO files". New Scientist. 22 March 2007.
174. ^ Bockman, Chris (4 November 2014). "Why the French state has a team of UFO hunters". BBC News.
175. ^ Jeffay, Nathan (10 December 2020). "Israeli space chief says aliens may well exist, but they haven't met humans". The Times of Israel.
176. ^ a b Zaria Gorvett (22 October 2023). "The weird aliens of early science fiction". BBC. Retrieved 25 January 2024.

## Further reading

[edit]

* Aguilera Mochón, Juan Antonio (2016). La vida no terrestre [The non-terrestrial life] (in Spanish). RBA. ISBN 978-84-473-8665-9.
* Baird, John C. (1987). The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings. Hanover: University Press of New England. ISBN 978-0-87451-406-3.
* Bennett, Jeffrey (2017). Life in the universe. United States: Pearson. pp. 3–4\. ISBN 978-0-13-408908-9.
* Cohen, Jack; Stewart, Ian (2002). Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. ISBN 978-0-09-187927-3.
* Crowe, Michael J. (1986). The Extraterrestrial Life Debate, 1750–1900. Cambridge. ISBN 978-0-521-26305-4.
* Dick, Steven J. (1984). Plurality of Worlds: The Extraterrestrial Life Debate from Democratus to Kant. Cambridge.
* Dick, Steven J. (1996). The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science. Cambridge. ISBN 978-0-521-34326-8.
* Dick, Steven J. (2001). Life on Other Worlds: The 20th Century Extraterrestrial Life Debate. Cambridge. ISBN 978-0-521-79912-6.
* Dick, Steven J.; Strick, James E. (2004). The Living Universe: NASA And the Development of Astrobiology. Rutgers. ISBN 978-0-8135-3447-3.
* Fasan, Ernst (1970). Relations with alien intelligences – the scientific basis of metalaw. Berlin: Berlin Verlag.
* Goldsmith, Donald (1997). The Hunt for Life on Mars. New York: A Dutton Book. ISBN 978-0-525-94336-5.
* Gribbin, John, "Alone in the Milky Way: Why we are probably the only intelligent life in the galaxy", Scientific American, vol. 319, no. 3 (September 2018), pp. 94–99.
* Grinspoon, David (2003). Lonely Planets: The Natural Philosophy of Alien Life. HarperCollins. ISBN 978-0-06-018540-4.
* Lemnick, Michael T. (1998). Other Worlds: The Search for Life in the Universe. New York: A Touchstone Book. Bibcode:1998owsl.book.....L.
* Michaud, Michael (2006). Contact with Alien Civilizations – Our Hopes and Fears about Encountering Extraterrestrials. Berlin: Springer. ISBN 978-0-387-28598-6.
* Pickover, Cliff (2003). The Science of Aliens. New York: Basic Books. ISBN 978-0-465-07315-3.
* Roth, Christopher F. (2005). "Ufology as Anthropology: Race, Extraterrestrials, and the Occult". In Debbora Battaglia (ed.). E.T. Culture: Anthropology in Outerspaces. Durham, NC: Duke University Press.
* Sagan, Carl; Shklovskii, I. S. (1966). Intelligent Life in the Universe. Random House.
* Sagan, Carl (1973). Communication with Extraterrestrial Intelligence. MIT Press. ISBN 978-0-262-19106-7.
* Ward, Peter D. (2005). Life as we do not know it-the NASA search for (and synthesis of) alien life. New York: Viking. ISBN 978-0-670-03458-1.
* Tumminia, Diana G. (2007). Alien Worlds – Social and Religious Dimensions of Extraterrestrial Contact. Syracuse: Syracuse University Press. ISBN 978-0-8156-0858-5.

## External links

[edit]

Wikimedia Commons has media related to Extraterrestrial life.

Wikiquote has quotations related to Extraterrestrial life.

Wikisource has original works on the topic: Extraterrestrial life

* Astrobiology at NASA
* European Astrobiology Institute

* v
* t
* e

Extraterrestrial life

Events and objects|

* Shergotty meteorite (1865)
* Nakhla meteorite (1911)
* Murchison meteorite (1969)
* Viking lander biological experiments (1976)
* Allan Hills 77005 (1977)
* Allan Hills 84001 (1984)
* Yamato 000593 (2000)
* CI1 fossils (2011)

Signals of interest|

| Misidentified|

* CP 1919 (misidentified pulsar)
* CTA-102 (misidentified quasar)

|

Stars|

* KIC 8462852 (unusual light fluctuations)
* EPIC 204278916 (unusual light fluctuations)
* VVV-WIT-07 (unusual light fluctuations)
* HD 164595 signal (unknown origin)

Other|

* SHGb02+14a (radio source)
* Wow! signal (inconclusive)
* Fast radio burst (unknown origin)
* BLC1 (radio signal)

Life in the Universe|

* Earliest known life forms
* Planetary habitability in the Solar System
* Life on Venus
* Life on Mars
* Life on Titan

Planetary
habitability|

* Catalog of Nearby Habitable Systems
* Circumstellar habitable zone
* Earth analog
* Extraterrestrial liquid water
* Galactic habitable zone
* Habitability of binary star systems
* Habitability of natural satellites
* Habitability of neutron star systems
* Habitability of red dwarf systems
* Habitability of K-type main-sequence star systems
* Habitability of yellow dwarf systems
* Habitability of F-type main-sequence star systems
* List of potentially habitable exoplanets
* Planetary habitability
* Superhabitable planet
* Tholin

Space missions|

* Beagle 2
* Biological Oxidant and Life Detection
* BioSentinel
* Curiosity rover
* Darwin
* Dragonfly
* Enceladus Explorer
* Enceladus Life Finder
* Europa Clipper
* ExoMars
* Rosalind Franklin rover
* ExoLance
* EXPOSE
* Foton-M3
* Icebreaker Life
* Journey to Enceladus and Titan
* Laplace-P
* Life Investigation For Enceladus
* Living Interplanetary Flight Experiment
* Mars Geyser Hopper
* Mars sample-return mission
* Mars 2020
* Northern Light
* Opportunity rover
* Perseverance rover
* SpaceX Red Dragon
* Spirit rover
* Tanpopo
* Titan Mare Explorer
* Venus In Situ Explorer
* Viking 1
* Viking 2

Interstellar
communication|

* Active SETI
* Allen Telescope Array
* Arecibo message
* Arecibo Observatory
* Berkeley SETI Research Center
* Bracewell probe
* Breakthrough Initiatives
* Breakthrough Listen
* Breakthrough Message
* Communication with extraterrestrial intelligence
* Gauss's Pythagorean right triangle proposal
* Astrolinguistics
* Lincos language
* NIROSETI
* Pioneer plaque
* Project Cyclops
* Project Ozma
* Project Phoenix
* SERENDIP
* Search for extraterrestrial intelligence
* SETI@home
* setiQuest
* Voyager Golden Record
* Water hole
* Xenolinguistics

Types of alleged
extraterrestrial beings|

* Energy beings
* Grey aliens
* Insectoids
* Little green men
* Nordic aliens
* Reptilian humanoids

Hypotheses|

* Cosmic pluralism
* Directed panspermia
* Drake equation
* Extraterrestrial hypothesis
* Fermi paradox
* Hypothetical types of biochemistry
* Interplanetary contamination
* Kardashev scale
* Mediocrity principle
* Non-planetary abiogenesis
* Panspermia

Fermi paradox solutions|

* Aestivation hypothesis
* Berserker hypothesis
* Dark forest hypothesis
* Firstborn hypothesis
* Great Filter
* Hart–Tipler conjecture
* Neocatastrophism
* Planetarium hypothesis
* Rare Earth hypothesis
* Zoo hypothesis

Related topics|

* Ancient astronauts
* Astrobiology
* Astroecology
* Biosignature
* Brookings Report
* Exotheology
* Extraterrestrials in fiction
* Extremophile
* Hemolithin
* History of the extraterrestrial life debate
* Nexus for Exoplanet System Science
* Noogenesis
* Planetary protection
* Potential cultural impact of extraterrestrial contact
* Post-detection policy
* San Marino Scale
* Technosignature
* UFO religion
* Xenoarchaeology

* v
* t
* e

Astrobiology

Disciplines|

* Astrochemistry
* Astrophysics
* Atmospheric sciences
* Biochemistry
* Evolutionary biology
* Exoplanetology
* Geomicrobiology
* Microbiology
* Paleontology
* Planetary oceanography
* Planetary science

Main topics|

* Abiogenesis
* Allan Hills 84001
* Biomolecule
* Biosignature
* Drake equation
* Earliest known life forms
* Earth analog
* Extraterrestrial life
* Extraterrestrial sample curation
* Extremophiles
* Hypothetical types of biochemistry
* List of microorganisms tested in outer space
* Ocean planet
* Panspermia
* Planetary protection
* Search for extraterrestrial intelligence (SETI)
* Yamato meteorite

Planetary
habitability|

* Circumstellar habitable zone
* Earth analog
* Extraterrestrial liquid water
* Galactic habitable zone
* Habitability of binary star systems
* Habitability of natural satellites
* Habitability of neutron star systems
* Habitability of red dwarf systems
* Habitability of K-type main-sequence star systems
* Habitability of yellow dwarf systems
* Habitability of F-type main-sequence star systems
* Habitable zone for complex life
* List of potentially habitable exoplanets
* Tholin
* Superhabitable planet

Space
missions|

| Earth orbit|

* BIO
* BIOCORE
* Biolab
* Bion
* BIOPAN
* Biosatellite program
* E-MIST
* ERA
* Eu:CROPIS
* EXOSTACK
* EXPOSE
* Lunar Micro Ecosystem
* O/OREOS
* OREOcube
* Tanpopo
* VEGGIE

|

Mars|

* Beagle 2
* Fobos-Grunt
* Mars Science Laboratory
* Curiosity rover
* Mars 2020
* Perseverance rover
* Phoenix
* Tianwen-1
* Zhurong rover
* Trace Gas Orbiter
* Viking

Comets and
asteroids|

* Hayabusa2
* OSIRIS-REx
* Rosetta

Heliocentric|

* BioSentinel

Planned|

* Dragonfly
* Europa Clipper
* ExoMars
* Rosalind Franklin rover

Proposed|

* Breakthrough Enceladus
* BRUIE
* CAESAR
* Enceladus Explorer
* Enceladus Life Finder‎
* Enceladus Life Signatures and Habitability
* Enceladus Orbilander
* Europa Lander
* ExoLance
* Explorer of Enceladus and Titan
* Icebreaker Life
* Journey to Enceladus and Titan
* Laplace-P
* Life Investigation For Enceladus
* Mars sample return mission
* Oceanus
* THEO
* Trident

Cancelled and
undeveloped|

* Astrobiology Field Laboratory
* Beagle 3
* Biological Oxidant and Life Detection
* Kazachok
* Living Interplanetary Flight Experiment
* Mars Astrobiology Explorer-Cacher
* MELOS
* Northern Light
* Red Dragon
* Terrestrial Planet Finder

Institutions
and programs|

* Astrobiology Society of Britain
* Astrobiology Science and Technology for Exploring Planets
* Breakthrough Initiatives
* Breakthrough Listen
* Breakthrough Message
* Breakthrough Starshot
* Carl Sagan Institute
* Center for Life Detection Science
* European Astrobiology Network Association
* NASA Astrobiology Institute
* Nexus for Exoplanet System Science
* Ocean Worlds Exploration Program
* Spanish Astrobiology Center‎

* Category
* Commons

* v
* t
* e

Molecules detected in outer space

Molecules|

| Diatomic|

* Aluminium monochloride
* Aluminium monofluoride
* Aluminium(II) oxide
* Argonium
* Carbon cation
* Carbon monophosphide
* Carbon monosulfide
* Carbon monoxide
* Cyano radical
* Diatomic carbon
* Fluoromethylidynium
* Helium hydride ion
* Hydrogen chloride
* Hydrogen fluoride
* Hydrogen (molecular)
* Hydroxyl radical
* Imidogen
* Iron(II) oxide
* Magnesium monohydride
* Methylidyne radical
* Nitric oxide
* Nitrogen (molecular)
* Oxygen (molecular)
* Phosphorus monoxide
* Phosphorus mononitride
* Potassium chloride
* Silicon carbide
* Silicon monoxide
* Silicon monosulfide
* Sodium chloride
* Sodium iodide
* Sulfanyl
* Sulfur mononitride
* Sulfur monoxide
* Titanium(II) oxide

|
|

Triatomic|

* Aluminium(I) hydroxide
* Aluminium isocyanide
* Amino radical
* Carbon dioxide
* Carbonyl sulfide
* CCP radical
* Chloronium
* Diazenylium
* Dicarbon monoxide
* Disilicon carbide
* Ethynyl radical
* Formyl radical
* Hydrogen cyanide (HCN)
* Hydrogen isocyanide (HNC)
* Hydrogen sulfide
* Hydroperoxyl
* Iron cyanide
* Isoformyl
* Magnesium cyanide
* Magnesium isocyanide
* Methylene
* Methylidynephosphane
* N2H+
* Nitrous oxide
* Nitroxyl
* Ozone
* Potassium cyanide
* Sodium cyanide
* Sodium hydroxide
* Silicon carbonitride
* c-Silicon dicarbide
* SiNC
* Sulfur dioxide
* Thioformyl
* Thioxoethenylidene
* Titanium dioxide
* Tricarbon
* Trihydrogen cation
* Water

Four
atoms|

* Acetylene
* Ammonia
* Cyanoethynyl
* Formaldehyde
* Fulminic acid
* HCCN
* Hydrogen peroxide
* Hydromagnesium isocyanide
* Isocyanic acid
* Isothiocyanic acid
* Ketenyl
* Methyl cation
* Methyl radical
* Methylene amidogen
* Propynylidyne
* Protonated carbon dioxide
* Protonated hydrogen cyanide
* Silicon tricarbide
* Thiocyanic acid
* Thioformaldehyde
* Tricarbon monosulfide
* Tricarbon monoxide

Five
atoms|

* Ammonium ion
* Butadiynyl
* Carbodiimide
* Cyanamide
* Cyanoacetylene
* Cyanoformaldehyde
* Cyanomethyl
* Cyclopropenylidene
* Formic acid
* Isocyanoacetylene
* Ketene
* Methane
* Methoxy radical
* Methylenimine
* Propadienylidene
* Protonated formaldehyde
* Silane
* Silicon-carbide cluster

Six
atoms|

* Acetonitrile
* Cyanobutadiynyl radical
* Cyclopropenone
* Diacetylene
* E-Cyanomethanimine
* Ethylene
* Formamide
* HC4N
* Ketenimine
* Methanethiol
* Methanol
* Methyl isocyanide
* Pentynylidyne
* Propynal
* Protonated cyanoacetylene

Seven
atoms|

* Acetaldehyde
* Acrylonitrile
* Vinyl cyanide
* Cyanodiacetylene
* Ethylene oxide
* Glycolonitrile
* Hexatriynyl radical
* Methyl isocyanate
* Methylamine
* Propyne
* Vinyl alcohol

Eight
atoms|

* Acetic acid
* Acrolein
* Aminoacetonitrile
* Cyanoallene
* Ethanimine
* Glycolaldehyde
* Hexapentaenylidene
* Methyl formate
* Methylcyanoacetylene

Nine
atoms|

* Acetamide
* Cyanohexatriyne
* Dimethyl ether
* Ethanethiol
* Ethanol
* Methyldiacetylene
* N-Methylformamide
* Octatetraynyl radical
* Propene
* Propionitrile

Ten
atoms
or more|

* Acetone
* Benzene
* Benzonitrile
* Buckminsterfullerene (C60, C60+, fullerene, buckyball)
* Butyronitrile
* C70 fullerene
* Cyanodecapentayne
* Ethyl formate
* Ethylene glycol
* Heptatrienyl radical
* Methyl acetate
* Methyl-cyano-diacetylene
* Methyltriacetylene
* Propionaldehyde
* Pyrimidine

Deuterated
molecules|

* Ammonia
* Ammonium ion
* Formaldehyde
* Formyl radical
* Heavy water
* Hydrogen cyanide
* Hydrogen deuteride
* Hydrogen isocyanide
* N2D+
* Propyne
* Trihydrogen cation

Unconfirmed|

* Anthracene
* Dihydroxyacetone
* Glycine
* Graphene
* H2NCO+
* Hemolithin
* Linear C5
* Methoxyethane
* Naphthalene cation
* Phosphine
* Pyrene
* Silylidyne

Related|

* Abiogenesis
* Astrobiology
* Astrochemistry
* Atomic and molecular astrophysics
* Chemical formula
* Circumstellar dust
* Circumstellar envelope
* Cosmic dust
* Cosmic ray
* Cosmochemistry
* Diffuse interstellar band
* Earliest known life forms
* Extraterrestrial life
* Extraterrestrial liquid water
* Forbidden mechanism
* Homochirality
* Intergalactic dust
* Interplanetary medium
* Interstellar medium
* Iron–sulfur world theory
* Kerogen
* Molecules in stars
* Nexus for Exoplanet System Science
* Organic compound
* Outer space
* PAH world hypothesis
* Photodissociation region
* Polycyclic aromatic hydrocarbon (PAH)
* Pseudo-panspermia
* RNA world hypothesis
* Spectroscopy
* Tholin

* Category:Astrochemistry
* Outer space portal
* Astronomy portal
* Chemistry portal

* v
* t
* e

Interstellar communications

Programs|

* Active SETI
* Breakthrough Initiatives
* Breakthrough Listen
* Breakthrough Message
* Communication with extraterrestrial intelligence
* Search for extraterrestrial intelligence

Messages|

* List of interstellar radio messages
* A Message from Earth
* Across the Universe
* Arecibo message
* Bracewell probe
* Cosmic Call
* CosmicOS
* Hello from Earth
* Pioneer plaque
* Teen Age Message
* Voyager Golden Record

People|

* Frank Drake
* Ann Druyan
* Hans Freudenthal
* Sebastian von Hoerner
* Nikolai Kardashev
* Jon Lomberg
* Carl Sagan
* Linda Salzman Sagan
* Iosif Shklovsky
* Aleksandr Zaitsev

Other|

* Alien language
* Archaeology, Anthropology, and Interstellar Communication
* Astrobiology
* Astrolinguistics
* Drake equation
* Extraterrestrial life
* Lincos
* Fermi paradox
* Prix Guzman
* San Marino Scale

* Category
* Space Portal

Portals:

* Biology
* Astronomy
* Stars
* Spaceflight

Authority control databases

International|

* FAST

National|

* Germany
* United States
* France
* BnF data
* Czech Republic
* Spain
* Israel

Other|

* IdRef
* NARA
* Yale LUX

*[v]: View this template
*[t]: Discuss this template
*[e]: Edit this template