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Extraterrestrial life

From Wikipedia, in a visual modern way
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)
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)
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)
Some major international efforts to search for extraterrestrial life. Clockwise from top left:

Extraterrestrial life, colloquially referred to as alien life, is life that may occur outside of Earth and which did not originate on Earth. No extraterrestrial life has yet been conclusively detected, although efforts are underway. Such life might range from simple forms like prokaryotes to intelligent beings, possibly bringing forth civilizations that might be far more advanced than humankind.[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" outside the planet Earth dates back to antiquity. Multiple 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" (originally expressed in his Letter to Herodotus) in The City of God.[4] In his first century poem De rerum natura (Book 2:1048–1076), the Epicurean philosopher Lucretius predicted that we would find innumerable exoplanets with life-forms similar to, and different from, the ones on Earth, and even other races of man.

Pre-modern writers typically assumed that extraterrestrial "worlds" would be inhabited by living beings. William Vorilong, in the 15th century, acknowledged the possibility that Christ could have visited extraterrestrial worlds to redeem their inhabitants.[5] Nicholas of Cusa wrote in 1440 that the Earth was "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 that all extraterrestrial bodies could be inhabited by men, plants, and animals, including the Sun.[6] Descartes wrote that there was no means to prove that the stars were not inhabited by "intelligent creatures," but their existence was a matter of speculation.[7] The writings of these thinkers show that interest in extraterrestrial life existed throughout history, but it is only recently that humans have had any means of investigating it.

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. Depending on the category of search, methods range from the analysis of telescope and specimen data[8] to radios used to detect and send communication signals.

The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, especially extraterrestrials in fiction. Over the years, science fiction has communicated scientific ideas, imagined a wide 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 wipe out less advanced societies—argue that it may be dangerous to actively call attention to Earth.[9][10]

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Earth

Earth

Earth is the third planet from the Sun and home to all known life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. Approximately 70.8% of Earth's surface is made up of the ocean, dwarfing Earth's polar ice, lakes, and rivers. The remaining 29.2% of Earth's surface is land, consisting of continents and islands. Earth's surface layer is formed of several slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes. Earth's liquid outer core generates the magnetic field that shapes the magnetosphere of Earth, deflecting destructive solar winds.

Extraterrestrial intelligence

Extraterrestrial intelligence

Extraterrestrial intelligence refers to hypothetical intelligent extraterrestrial life. The question of whether other inhabited worlds might exist has been debated since ancient times. The modern form of the concept emerged when the Copernican Revolution demonstrated that the Earth was a planet revolving around the Sun, and other planets were, conversely, other worlds. The question of whether other inhabited planets or moons exist was a natural consequence of this new understanding. It has become one of the most speculative questions in science and is a central theme of science fiction and popular culture.

Civilization

Civilization

A civilization is any society characterized by the existence of the state that has developed culture, language, a writing system, and currency.

Kardashev scale

Kardashev scale

The Kardashev scale is a method of measuring a civilization's level of technological advancement based on the amount of energy it is able to use. The measure was proposed by Soviet astronomer Nikolai Kardashev in 1964.

Drake equation

Drake equation

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy.

Astrobiology

Astrobiology

Astrobiology, and the related field of exobiology, is an interdisciplinary scientific field that studies the origins, early evolution, distribution, and future of life in the universe. Astrobiology is the multidisciplinary field that investigates the deterministic conditions and contingent events with which life arises, distributes, and evolves in the universe.

Democritus

Democritus

Democritus was an Ancient Greek pre-Socratic philosopher from Abdera, primarily remembered today for his formulation of an atomic theory of the universe. None of his work has survived.

Augustine of Hippo

Augustine of Hippo

Augustine of Hippo, also known as Saint Augustine, was a theologian and philosopher of Berber origin and the bishop of Hippo Regius in Numidia, Roman North Africa. His writings influenced the development of Western philosophy and Western Christianity, and he is viewed as one of the most important Church Fathers of the Latin Church in the Patristic Period. His many important works include The City of God, On Christian Doctrine, and Confessions.

Epicurus

Epicurus

Epicurus was an ancient Greek philosopher and sage who founded Epicureanism, a highly influential school of philosophy. He was born on the Greek island of Samos to Athenian parents. Influenced by Democritus, Aristippus, Pyrrho, and possibly the Cynics, he turned against the Platonism of his day and established his own school, known as "the Garden", in Athens. Epicurus and his followers were known for eating simple meals and discussing a wide range of philosophical subjects. He openly allowed women and slaves to join the school as a matter of policy. Of the over 300 works said to have been written by Epicurus about various subjects, the vast majority have been destroyed. Only three letters written by him—the letters to Menoeceus, Pythocles, and Herodotus—and two collections of quotes—the Principal Doctrines and the Vatican Sayings—have survived intact, along with a few fragments of his other writings. As a result of his work's destruction, most knowledge about his philosophy is due to later authors, particularly the biographer Diogenes Laërtius, the Epicurean Roman poet Lucretius and the Epicurean philosopher Philodemus, and with hostile but largely accurate accounts by the Pyrrhonist philosopher Sextus Empiricus, and the Academic Skeptic and statesman Cicero.

De rerum natura

De rerum natura

De rerum natura is a first-century BC didactic poem by the Roman poet and philosopher Lucretius with the goal of explaining Epicurean philosophy to a Roman audience. The poem, written in some 7,400 dactylic hexameters, is divided into six untitled books, and explores Epicurean physics through poetic language and metaphors. Namely, Lucretius explores the principles of atomism; the nature of the mind and soul; explanations of sensation and thought; the development of the world and its phenomena; and explains a variety of celestial and terrestrial phenomena. The universe described in the poem operates according to these physical principles, guided by fortuna ("chance"), and not the divine intervention of the traditional Roman deities.

Jesus

Jesus

Jesus, also referred to as Jesus Christ or Jesus of Nazareth, was a first-century Jewish preacher and religious leader; he is the central figure of Christianity, the world's largest religion. Most Christians believe he is the incarnation of God the Son and the awaited Messiah prophesied in the Hebrew Bible.

Extraterrestrials in fiction

Extraterrestrials in fiction

An extraterrestrial or alien is any extraterrestrial lifeform; a lifeform that did not originate on Earth. The word extraterrestrial means "outside Earth". The first published use of extraterrestrial as a noun occurred in 1956, during the Golden Age of Science Fiction.

Characteristics

Astronomers have discovered stars in the Milky Way galaxy that are almost 13.6 billion years old.[11]
Astronomers have discovered stars in the Milky Way galaxy that are almost 13.6 billion years old.[11]

Extraterrestrial life, such as microorganisms, has been hypothesized to exist in the Solar System and throughout the universe. This hypothesis relies on the vast size and consistent physical laws of the observable universe. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking[12] it would be improbable for life not to exist somewhere other than Earth.[13][14] 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.[15]

The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old.[16][17] Life may have emerged independently at many places throughout the universe, as it arose on Earth roughly 4.2 billion years ago through chemical processes. Alternatively, life may have formed less frequently, then spread—by meteoroids, for example—between habitable planets in a process called panspermia.[18][19] In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth.[20] According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars.[20]

Since the 1950s, astronomers have proposed that "habitable zones" around stars are the most likely places for life to exist. Numerous discoveries of such zones since 2007 have generated numerical estimates of many billions of planets with Earth-like compositions.[21] As of 2013, only a few planets had been discovered in these zones.[22] Nonetheless, on 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way,[23][24] 11 billion of which may be orbiting Sun-like stars.[25] The nearest such planet may be 12 light-years away, according to the scientists.[23][24]

Astrobiologists have also considered a "follow the energy" view of potential habitats.[26][27]

Life on Earth is quite ubiquitous and has adapted over time to almost all the available environments in it, even the most hostile ones. As a result, it is inferred that life in other celestial bodies may be equally adaptative. However, the origin of life is unrelated to its ease of adaptation, and may have stricter requirements. A planet or moon may not have any life on it, even if it was habitable.[28]

A study published in 2017 suggests that due to how complexity evolved in species on Earth, the level of predictability for alien evolution elsewhere would make them look similar to life on our planet. One of the study authors, Sam Levin, notes "Like humans, we predict that they are made-up of a hierarchy of entities, which all cooperate to produce an alien. At each level of the organism there will be mechanisms in place to eliminate conflict, maintain cooperation, and keep the organism functioning. We can even offer some examples of what these mechanisms will be."[29] 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.[30]

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Milky Way

Milky Way

The Milky Way is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλακτικός κύκλος, meaning "milky circle". From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

Microorganism

Microorganism

A omar abdelwahab, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells.

Observable universe

Observable universe

The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. There may be 2 trillion galaxies in the observable universe, although that number was reduced in 2021 to only several hundred billion based on data from New Horizons. Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe is a spherical region centered on the observer and is unique for every unique observational position.

Carl Sagan

Carl Sagan

Carl Edward Sagan was an American astronomer, planetary scientist, cosmologist, astrophysicist, astrobiologist, author, and science communicator. His best known scientific contribution is his research on the possibility of extraterrestrial life, including experimental demonstration of the production of amino acids from basic chemicals by radiation. Sagan assembled the first physical messages sent into space, the Pioneer plaque and the Voyager Golden Record, universal messages that could potentially be understood by any extraterrestrial intelligence that might find them. Sagan argued in favor of the hypothesis, accepted since, that the high surface temperatures of Venus are the result of the greenhouse effect.

Copernican principle

Copernican principle

In physical cosmology, the Copernican principle states that humans, on the Earth or in the Solar System, are not privileged observers of the universe, that observations from the Earth are representative of observations from the average position in the universe. Named for Copernican heliocentrism, it is a working assumption that arises from a modified cosmological extension of Copernicus' argument of a moving Earth.

Mediocrity principle

Mediocrity principle

The mediocrity principle is the philosophical notion that "if an item is drawn at random from one of several sets or categories, it's more likely to come from the most numerous category than from any one of the less numerous categories". The principle has been taken to suggest that there is nothing very unusual about the evolution of the Solar System, Earth's history, the evolution of biological complexity, human evolution, or any one nation. It is a heuristic in the vein of the Copernican principle, and is sometimes used as a philosophical statement about the place of humanity. The idea is to assume mediocrity, rather than starting with the assumption that a phenomenon is special, privileged, exceptional, or even superior.

Biochemistry

Biochemistry

Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and metabolism. Over the last decades of the 20th century, biochemistry has become successful at explaining living processes through these three disciplines. Almost all areas of the life sciences are being uncovered and developed through biochemical methodology and research. Biochemistry focuses on understanding the chemical basis which allows biological molecules to give rise to the processes that occur within living cells and between cells, in turn relating greatly to the understanding of tissues and organs, as well as organism structure and function. Biochemistry is closely related to molecular biology, which is the study of the molecular mechanisms of biological phenomena.

Big Bang

Big Bang

The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the earliest known periods through its subsequent large-scale form. These models offer a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure. The overall uniformity of the Universe, known as the flatness problem, is explained through cosmic inflation: a sudden and very rapid expansion of space during the earliest moments. However, physics currently lacks a widely accepted theory of quantum gravity that can successfully model the earliest conditions of the Big Bang.

Age of the universe

Age of the universe

In physical cosmology, the age of the universe is the time elapsed since the Big Bang. Astronomers have derived two different measurements of the age of the universe: a measurement based on direct observations of an early state of the universe, which indicate an age of 13.787±0.020 billion years as interpreted with the Lambda-CDM concordance model as of 2021; and a measurement based on the observations of the local, modern universe, which suggest a younger age. The uncertainty of the first kind of measurement has been narrowed down to 20 million years, based on a number of studies which all show similar figures for the age and which include studies of the microwave background radiation by the Planck spacecraft, the Wilkinson Microwave Anisotropy Probe and other space probes. Measurements of the cosmic background radiation give the cooling time of the universe since the Big Bang, and measurements of the expansion rate of the universe can be used to calculate its approximate age by extrapolating backwards in time. The range of the estimate is also within the range of the estimate for the oldest observed star in the universe.

Abiogenesis

Abiogenesis

In biology, abiogenesis or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but an evolutionary process that involved the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes. Many proposals have been made for different stages of the process.

Meteoroid

Meteoroid

A meteoroid is a small rocky or metallic body in outer space.

Panspermia

Panspermia

Panspermia is the hypothesis, first proposed in the 5th century BCE by the Greek philosopher Anaxagoras, that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecraft carrying unintended contamination by microorganisms. Panspermia is a fringe theory with little support amongst mainstream scientists. Critics argue that it does not answer the question of the origin of life but merely places it on another celestial body. It is also criticized because it cannot be tested experimentally.

Biochemical basis

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 volcanos, 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.[31] Magnetic fields and radioactivity have also been proposed as sources of energy, although they would be less efficient ones.[32]

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.[33] 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.[34][35] 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.[36]

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 potential replacement for carbon should 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's carbon and in the atmosphere, it's 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.[37]

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 on Earth started with a RNA world and later evolved to its current form, where some of the RNA tasks were transferred to the DNA and proteins. Extraterrestrial life may still be stuck on the RNA world, 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.[38] 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 organization 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.[39]

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. 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.[40]

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Hypothetical types of biochemistry

Hypothetical types of biochemistry

Hypothetical types of biochemistry are forms of biochemistry agreed to be scientifically viable but not proven to exist at this time. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons it may be chemically similar, though it is also possible that there are organisms with quite different chemistries – for instance, involving other classes of carbon compounds, compounds of another element, or another solvent in place of water.

Non-equilibrium thermodynamics

Non-equilibrium thermodynamics

Non-equilibrium thermodynamics is a branch of thermodynamics that deals with physical systems that are not in thermodynamic equilibrium but can be described in terms of macroscopic quantities that represent an extrapolation of the variables used to specify the system in thermodynamic equilibrium. Non-equilibrium thermodynamics is concerned with transport processes and with the rates of chemical reactions.

Plate tectonics

Plate tectonics

Plate tectonics is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large tectonic plates which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. Plate tectonics came to be generally accepted by geoscientists after seafloor spreading was validated in the mid to late 1960s.

Hydrothermal vent

Hydrothermal vent

A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspots. Hydrothermal deposits are rocks and mineral ore deposits formed by the action of hydrothermal vents.

Magnetic field

Magnetic field

A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field. A permanent magnet's magnetic field pulls on ferromagnetic materials such as iron, and attracts or repels other magnets. In addition, a nonuniform magnetic field exerts minuscule forces on "nonmagnetic" materials by three other magnetic effects: paramagnetism, diamagnetism, and antiferromagnetism, although these forces are usually so small they can only be detected by laboratory equipment. Magnetic fields surround magnetized materials, and are created by electric currents such as those used in electromagnets, and by electric fields varying in time. Since both strength and direction of a magnetic field may vary with location, it is described mathematically by a function assigning a vector to each point of space, called a vector field.

Abiogenesis

Abiogenesis

In biology, abiogenesis or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but an evolutionary process that involved the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes. Many proposals have been made for different stages of the process.

Ammonia

Ammonia

Ammonia is an inorganic compound of nitrogen and hydrogen with the formula NH3. A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous waste, particularly among aquatic organisms, and it contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to 45% of the world's food and fertilizers. Around 70% of ammonia is used to make fertilisers in various forms and composition, such as urea and Diammonium phosphate. Ammonia in pure form is also applied directly into the soil.

Hydrocarbon

Hydrocarbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or exemplified by the odors of gasoline and lighter fluid. They occur in a diverse range of molecular structures and phases: they can be gases, liquids, low melting solids or polymers.

Methane

Methane

Methane ( MEH-thayn, MEE-thayn) is a chemical compound with the chemical formula CH4 (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Earth makes it an economically attractive fuel, although capturing and storing it poses technical challenges due to its gaseous state under normal conditions for temperature and pressure.

Ethane

Ethane

Ethane is an organic chemical compound with chemical formula C2H6. At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petrochemical by-product of petroleum refining. Its chief use is as feedstock for ethylene production.

Chemical element

Chemical element

A chemical element is a chemical substance that cannot be broken down into other substances. The basic particle that constitutes a chemical element is the atom, and chemical elements are distinguished from each other by the number of protons in the nuclei of their atoms. This is in contrast to chemical compounds and mixtures.

Carbon

Carbon

Carbon is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up about 0.025 percent of Earth's crust. Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.

Planetary habitability in the Solar System

A series of artist's conceptions of past water coverage on Mars
A series of artist's conceptions of past water coverage on Mars

Some bodies in the Solar System have the potential for an environment in which extraterrestrial life can exist, particularly those with possible subsurface oceans.[41] Should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophile microorganisms. According to NASA's 2015 Astrobiology Strategy, "Life on other worlds is most likely to include microbes, and any complex living system elsewhere is likely to have arisen from and be founded upon microbial life. Important insights on the limits of microbial life can be gleaned from studies of microbes on modern Earth, as well as their ubiquity and ancestral characteristics."[42] Researchers found a stunning array of subterranean organisms, mostly microbial, deep underground and estimate that approximately 70 percent of the total number of Earth's bacteria and archaea organisms live within the Earth's crust.[43] Rick Colwell, a member of the Deep Carbon Observatory team from Oregon State University, told the BBC: "I think it’s probably reasonable to assume that the subsurface of other planets and their moons are habitable, especially since we’ve seen here on Earth that organisms can function far away from sunlight using the energy provided directly from the rocks deep underground".[44]

Mars may have niche subsurface environments where microbial life exists.[45][46][47] A subsurface marine environment on Jupiter's moon Europa might be the most likely habitat in the Solar System, outside Earth, for extremophile microorganisms.[48][49][50]

The panspermia hypothesis proposes that life elsewhere in the Solar System may have a common origin. If extraterrestrial life were found on another body in the Solar System, it could have originated from Earth just as life on Earth could have been seeded from elsewhere.[51] Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent from Earth to seed new stellar systems with life. The Nobel prize winner Francis Crick, along with Leslie Orgel, proposed that seeds of life may have been purposely spread by an advanced extraterrestrial civilization,[52] but considering an early "RNA world" Crick noted later that life may have originated on Earth.[53]

Mercury

The spacecraft MESSENGER found evidence of water ice on Mercury. There may be scientific support, based on studies reported in March 2020, for considering that parts of the planet Mercury may have been habitable, and perhaps that life forms, albeit likely primitive microorganisms, may have existed on the planet.[54][55]

Venus

In the early 20th century, Venus was considered to be similar to Earth for habitability, but observations since the beginning of the Space Age revealed that the Venusian surface temperature is around 467 °C (873 °F), making it inhospitable for Earth-like life.[56] Likewise, the atmosphere of Venus is almost completely carbon dioxide, which can be toxic to Earth-like life. Between the altitudes of 50 and 65 kilometers, the pressure and temperature are Earth-like, and it may accommodate thermoacidophilic extremophile microorganisms in the acidic upper layers of the Venusian atmosphere.[57][58][59][60] Furthermore, Venus likely had liquid water on its surface for at least a few million years after its formation.[61][62] The putative detection of an absorption line of phosphine in Venus's atmosphere, with no known pathway for abiotic production, led to speculation in September 2020 that there could be extant life currently present in the atmosphere.[63][64] Later research attributed the spectroscopic signal that was interpreted as phosphine to sulfur dioxide,[65] or found that in fact there was no absorption line.[66][67]

The Moon

3.5 to 4 billion years ago, the Moon could have had a magnetic field, an atmosphere, and liquid water sufficient to sustain life on its surface.[68][69] Warm and pressurized regions in the Moon's interior might still contain liquid water.[70]

As of 2021, no native lunar life has been found, including any signs of life in the samples of Moon rocks and soil.[71]

Mars

Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and now can occasionally be found as low-volume liquid brines in shallow Martian soil.[72] The origin of the potential biosignature of methane observed in the atmosphere of Mars is unexplained, although hypotheses not involving life have been proposed.[73]

There is evidence that Mars had a warmer and wetter past: Dried-up riverbeds, polar ice caps, volcanoes, and minerals that form in the presence of water have all been found. Evidence obtained by the Curiosity rover studying Aeolis Palus, Gale Crater in 2013 strongly suggests an ancient freshwater lake that could have been a hospitable environment for microbial life.[74][75] Furthermore, present conditions on the subsurface of Mars may support life.[76][77]

Current studies on Mars by the Curiosity and Perseverance rovers are searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable.[78][79][80][81] The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective.[78]

Ceres

Ceres, the only dwarf planet in the asteroid belt, has a thin water-vapor atmosphere.[82][83] The vapor could have been produced by ice volcanoes or by ice near the surface sublimating (transforming from solid to gas).[84] Nevertheless, the presence of water on Ceres had led to speculation that life may be possible there.[85][86][87] It is one of the few places in the Solar System where scientists would like to search for possible signs of life.[84] Although the dwarf planet might not have living things today, there could be signs it harbored life in the past.[84]

Jupiter system

Jupiter

Carl Sagan and others in the 1960s and 1970s computed conditions for hypothetical microorganisms living in the atmosphere of Jupiter.[88] The intense radiation and other conditions, however, do not appear to permit encapsulation and molecular biochemistry, so life there is thought unlikely.[89] In contrast, some of Jupiter's moons may have habitats capable of sustaining life. Scientists have indications that heated subsurface oceans of liquid water may exist deep under the crusts of the three outer Galilean moons—Europa,[48][49][90] Ganymede,[91][92][93][94] and Callisto.[95][96][97] The EJSM/Laplace mission was planned to determine the habitability of these environments; however, due to lack of funding, the program was not continued. Similar missions, like ESA's JUICE and NASA's Europa Clipper are currently in development and are slated for launch in 2023 and 2024, respectively.

Europa

Internal structure of Europa. The blue represents a subsurface ocean. Such subsurface oceans could possibly harbor life.[98]
Internal structure of Europa. The blue represents a subsurface ocean. Such subsurface oceans could possibly harbor life.[98]

Jupiter's moon Europa has been the subject of speculation about the existence of life, due to the strong possibility of a liquid water ocean beneath its ice surface.[48][50] Hydrothermal vents on the bottom of the ocean, if they exist, may warm the water and could be capable of supplying nutrients and energy to microorganisms.[99] It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice.[100]

The case for life on Europa was greatly enhanced in 2011 when it was discovered that vast lakes exist within Europa's thick, icy shell. Scientists found that ice shelves surrounding the lakes appear to be collapsing into them, thereby providing a mechanism through which life-forming chemicals created in sunlit areas on Europa's surface could be transferred to its interior.[101][102]

On 11 December 2013, NASA reported the detection of "clay-like minerals" (specifically, phyllosilicates), often associated with organic materials, on the icy crust of Europa.[103] The presence of the minerals may have been the result of a collision with an asteroid or comet, according to the scientists.[103] The Europa Clipper, which would assess the habitability of Europa, is planned for launch in 2024.[104][105] Europa's subsurface ocean is considered the best target for the discovery of life.[48][50]

Saturn system

Like Jupiter, Saturn is not likely to host life. However, its moons Titan and Enceladus have been speculated to have possible habitats supportive of life.[73][106][107][108]

Enceladus

Enceladus, a moon of Saturn, has some of the conditions for life, including geothermal activity and water vapor, as well as possible under-ice oceans heated by tidal effects.[109][110] The Cassini–Huygens probe detected carbon, hydrogen, nitrogen and oxygen—all key elements for supporting life—during its 2005 flyby through one of Enceladus's geysers spewing ice and gas. The temperature and density of the plumes indicate a warmer, watery source beneath the surface.[73] Of the bodies on which life is possible, living organisms could most easily enter the other bodies of the Solar System from Enceladus.[111]

Titan

Titan, the largest moon of Saturn, is the only known moon in the Solar System with a significant atmosphere. Data from the Cassini–Huygens mission refuted the hypothesis of a global hydrocarbon ocean, but later demonstrated the existence of liquid hydrocarbon lakes in the polar regions—the first stable bodies of surface liquid discovered outside Earth.[106][107][108] Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface that are consistent with—but do not prove—the hypothesis that organisms there, if present, could be consuming hydrogen, acetylene and ethane, and producing methane.[112][113][114] NASA's Dragonfly mission is slated to land on Titan in the mid-2030s with a VTOL-capable rotorcraft with a launch date set for 2027.

Other bodies

Models of heat retention and heating via radioactive decay in smaller icy Solar System bodies suggest that Rhea, Titania, Oberon, Triton, Pluto, Eris, Sedna, and Orcus may have oceans underneath solid icy crusts approximately 100 km thick.[115] Of particular interest in these cases is the fact that the models indicate that the liquid layers are in direct contact with the rocky core, which allows efficient mixing of minerals and salts into the water. This is in contrast with the oceans that may be inside larger icy satellites like Ganymede, Callisto, or Titan, where layers of high-pressure phases of ice are thought to underlie the liquid water layer.[115]

Hydrogen sulfide has been proposed as a hypothetical solvent for life and is quite plentiful on Jupiter's moon Io, and may be in liquid form a short distance below the surface.[116]

Discover more about Planetary habitability in the Solar System related topics

Planetary habitability

Planetary habitability

Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from another body, through a hypothetical process known as panspermia. Environments do not need to contain life to be considered habitable nor are accepted habitable zones (HZ) the only areas in which life might arise.

Habitability of natural satellites

Habitability of natural satellites

The habitability of natural satellites is a measure of their potential to sustain life in favorable circumstances. Habitable environments do not necessarily harbor life. Natural satellite habitability is a new area that is significant to astrobiology for various reasons, the most important of which being that natural satellites are expected to outnumber planets by a large margin, and it is projected that habitability parameters will be comparable to those of planets. There are, nevertheless, significant environmental variables that affect moons as prospective alien life locations. The strongest candidates for natural satellite habitability are currently icy satellites such as those of Jupiter and Saturn—Europa and Enceladus respectively, although if life exists in either place, it would probably be confined to subsurface habitats. Historically, life on Earth was thought to be strictly a surface phenomenon, but recent studies have shown that up to half of Earth's biomass could live below the surface. Europa and Enceladus exist outside the circumstellar habitable zone which has historically defined the limits of life within the Solar System as the zone in which water can exist as liquid at the surface. In the Solar System's habitable zone, there are only three natural satellites—the Moon, and Mars's moons Phobos and Deimos —none of which sustain an atmosphere or water in liquid form. Tidal forces are likely to play as significant a role providing heat as stellar radiation in the potential habitability of natural satellites.

Extremophile

Extremophile

An extremophile is an organism that is able to live in extreme environments, i.e. environments that make survival challenging such as due to extreme temperature, radiation, salinity, or pH level.

Microorganism

Microorganism

A omar abdelwahab, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells.

Jupiter

Jupiter

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, while being slightly less than one-thousandth the mass of the Sun. Jupiter is the third brightest natural object in the Earth's night sky after the Moon and Venus, and it has been observed since prehistoric times. It was named after Jupiter, the chief deity of ancient Roman religion.

Europa (moon)

Europa (moon)

Europa, or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 91 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus.

Panspermia

Panspermia

Panspermia is the hypothesis, first proposed in the 5th century BCE by the Greek philosopher Anaxagoras, that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecraft carrying unintended contamination by microorganisms. Panspermia is a fringe theory with little support amongst mainstream scientists. Critics argue that it does not answer the question of the origin of life but merely places it on another celestial body. It is also criticized because it cannot be tested experimentally.

Directed panspermia

Directed panspermia

Directed panspermia is the deliberate transport of microorganisms into space to be used as introduced species on lifeless but habitable astronomical objects.

Francis Crick

Francis Crick

Francis Harry Compton Crick was an English molecular biologist, biophysicist, and neuroscientist. He, James Watson, Rosalind Franklin, and Maurice Wilkins played crucial roles in deciphering the helical structure of the DNA molecule. Crick and Watson's paper in Nature in 1953 laid the groundwork for understanding DNA structure and functions. Together with Maurice Wilkins, they were jointly awarded the 1962 Nobel Prize in Physiology or Medicine "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material".

Leslie Orgel

Leslie Orgel

Leslie Eleazer Orgel FRS was a British chemist. He is known for his theories on the origin of life.

RNA world

RNA world

The RNA world is a hypothetical stage in the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins. The term also refers to the hypothesis that posits the existence of this stage.

MESSENGER

MESSENGER

MESSENGER was a NASA robotic space probe that orbited the planet Mercury between 2011 and 2015, studying Mercury's chemical composition, geology, and magnetic field. The name is a backronym for "Mercury Surface, Space Environment, Geochemistry, and Ranging", and a reference to the messenger god Mercury from Roman mythology.

Scientific search

The scientific search for extraterrestrial life is being carried out both directly and indirectly. As of September 2017, 3,667 exoplanets in 2,747 systems have been identified, and other planets and moons in the Solar System hold the potential for hosting primitive life such as microorganisms. As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms on Venus (via phosphine) and Mars (via methane) was reported.[117]

Direct search

Lifeforms produce a variety of biosignatures that may be detectable by telescopes.[118][119]
Lifeforms produce a variety of biosignatures that may be detectable by telescopes.[118][119]

Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites.[16][17] Some claim to have identified evidence that microbial life has existed on Mars.[120][121][122][123] 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.[124] Lack of corroborating evidence from other experiments on the same samples suggests that a non-biological reaction is a more likely hypothesis.[124][125][126][127] In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001, formed of rock ejected from Mars.[120][121]

Electron micrograph of Martian meteorite ALH84001 showing structures that some scientists think could be fossilized bacteria-like life forms
Electron micrograph of Martian meteorite ALH84001 showing structures that some scientists think could be fossilized bacteria-like life forms

In February 2005 NASA scientists reported they may have found some evidence of extraterrestrial life on Mars.[128] 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.[129] Though such methane findings are still debated, support among some scientists for the existence of life on Mars exists.[130]

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.[131][132]

The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium, which is relatively easy to determine from a distance by spectroscopy. However, significant advances in the ability to find and resolve light from smaller rocky worlds near their stars are necessary before such spectroscopic methods can be used to analyze extrasolar planets. To that effect, the Carl Sagan Institute was founded in 2014 and is dedicated to the atmospheric characterization of exoplanets in circumstellar habitable zones.[133][134] Planetary spectroscopic data will be obtained from telescopes like WFIRST and ELT.[135]

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

In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggest DNA and RNA components (adenine, guanine and related organic molecules), building blocks for life as we know it, may be formed extraterrestrially in outer space.[136][137][138] In October 2011, scientists reported that cosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.[139][140][141] One of the scientists suggested that these compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."[139]

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.[142][143] 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.[144]

Indirect search

Projects such as SETI are monitoring the galaxy for electromagnetic interstellar communications from civilizations on other worlds.[145][146] If there is an advanced extraterrestrial civilization, there is no guarantee that it is transmitting radio communications in the direction of Earth or that this information could be interpreted as such by humans. The length of time required for a signal to travel across the vastness of space means that any signal detected would come from the distant past.[147]

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.[148]

Extrasolar planets

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

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.[149][150] Since 1992, over four thousand exoplanets have been discovered (5,297 planets in 3,904 planetary systems including 850 multiple planetary systems as of 1 January 2023).[151] 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.[151] The number of observed exoplanets is expected to increase greatly in the coming years.[152]

The Kepler space telescope has also detected a few thousand[153][154] candidate planets,[155][156] of which about 11% may be false positives.[157]

There is at least one planet on average per star.[158] 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.[159][160] 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.[25] The rogue planets in the Milky Way possibly number in the trillions.[161]

The nearest known exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 pc) from Earth in the southern constellation of Centaurus.[162]

As of March 2014, 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,[163][164] 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.[8]

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 analyze 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.[165]

Terrestrial analysis

The science of astrobiology considers life on Earth as well, and in the broader astronomical context. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia, when the young Earth was about 400 million years old.[166][167] According to one of the researchers, "If life arose relatively quickly on Earth, then it could be common in the universe."[166]

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Astrobiology

Astrobiology

Astrobiology, and the related field of exobiology, is an interdisciplinary scientific field that studies the origins, early evolution, distribution, and future of life in the universe. Astrobiology is the multidisciplinary field that investigates the deterministic conditions and contingent events with which life arises, distributes, and evolves in the universe.

Exoplanet

Exoplanet

An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, initially detected in 1988, was confirmed in 2003. As of 1 January 2023, there are 5,297 confirmed exoplanets in 3,904 planetary systems, with 850 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and also much more about exoplanets, including composition, environmental conditions and potential for life.

Planetary system

Planetary system

A planetary system is a set of gravitationally bound non-stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems may also consist of bodies such as dwarf planets, asteroids, natural satellites, meteoroids, comets, planetesimals and circumstellar disks. The Sun together with the planetary system revolving around it, including Earth, forms the Solar System. The term exoplanetary system is sometimes used in reference to other planetary systems.

Microorganism

Microorganism

A omar abdelwahab, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells.

Phosphine

Phosphine

Phosphine is a colorless, flammable, highly toxic compound with the chemical formula PH3, classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting fish, due to the presence of substituted phosphine and diphosphane. With traces of P2H4 present, PH3 is spontaneously flammable in air (pyrophoric), burning with a luminous flame. Phosphine is a highly toxic respiratory poison, and is immediately dangerous to life or health at 50 ppm. Phosphine has a trigonal pyramidal structure.

Methane

Methane

Methane ( MEH-thayn, MEE-thayn) is a chemical compound with the chemical formula CH4 (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Earth makes it an economically attractive fuel, although capturing and storing it poses technical challenges due to its gaseous state under normal conditions for temperature and pressure.

Biosignature

Biosignature

A biosignature is any substance – such as an element, isotope, or molecule – or phenomenon that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical or chemical structures and its use of free energy and the production of biomass and wastes. A biosignature can provide evidence for living organisms outside the Earth and can be directly or indirectly detected by searching for their unique byproducts.

Meteoroid

Meteoroid

A meteoroid is a small rocky or metallic body in outer space.

Nanobacterium

Nanobacterium

Nanobacterium is the unit or member name of a former proposed class of living organisms, specifically cell-walled microorganisms, now discredited, with a size much smaller than the generally accepted lower limit for life. Originally based on observed nano-scale structures in geological formations, the status of nanobacteria was controversial, with some researchers suggesting they are a new class of living organism capable of incorporating radiolabeled uridine, and others attributing to them a simpler, abiotic nature. One skeptic dubbed them "the cold fusion of microbiology", in reference to a notorious episode of supposed erroneous science. The term "calcifying nanoparticles" (CNPs) has also been used as a conservative name regarding their possible status as a life form.

Martian meteorite

Martian meteorite

A Martian meteorite is a rock that formed on Mars, was ejected from the planet by an impact event, and traversed interplanetary space before landing on Earth as a meteorite. As of September 2020, 277 meteorites had been classified as Martian, less than half a percent of the 72,000 meteorites that have been classified. The largest complete, uncut Martian meteorite, Taoudenni 002, was recovered in Mali in early 2021. It weighs 14.5 kilograms and is on display at the Maine Mineral & Gem Museum.

Ames Research Center

Ames Research Center

The Ames Research Center (ARC), also known as NASA Ames, is a major NASA research center at Moffett Federal Airfield in California's Silicon Valley. It was founded in 1939 as the second National Advisory Committee for Aeronautics (NACA) laboratory. That agency was dissolved and its assets and personnel transferred to the newly created National Aeronautics and Space Administration (NASA) on October 1, 1958. NASA Ames is named in honor of Joseph Sweetman Ames, a physicist and one of the founding members of NACA. At last estimate NASA Ames had over US$3 billion in capital equipment, 2,300 research personnel and a US$860 million annual budget.

Mars Science Laboratory

Mars Science Laboratory

Mars Science Laboratory (MSL) is a robotic space probe mission to Mars launched by NASA on November 26, 2011, which successfully landed Curiosity, a Mars rover, in Gale Crater on August 6, 2012. The overall objectives include investigating Mars' habitability, studying its climate and geology, and collecting data for a human mission to Mars. The rover carries a variety of scientific instruments designed by an international team.

Drake equation

In 1961, University of California, Santa Cruz, 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).[168] The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. The equation is best understood not as an equation in the strictly mathematical sense, but to summarize all the various concepts which scientists must contemplate when considering the question of life elsewhere.[169] The Drake equation is:

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 (civilizations)
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:

[170]

The Drake equation has proved controversial since several of its factors are uncertain and based on conjecture, not allowing conclusions to be made.[171] This has led critics to label the equation a guesstimate, or even meaningless.

Based on observations from the Hubble Space Telescope, there are between 125 and 250 billion galaxies in the observable universe.[172] It is estimated that at least ten percent of all Sun-like stars have a system of planets,[173] 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.[174][175] Also based on Kepler data, scientists estimate that at least one in six stars has an Earth-sized planet.[176]

The apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for such civilizations is known as the Fermi paradox.[177]

Discover more about Drake equation related topics

Drake equation

Drake equation

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy.

Extraterrestrial intelligence

Extraterrestrial intelligence

Extraterrestrial intelligence refers to hypothetical intelligent extraterrestrial life. The question of whether other inhabited worlds might exist has been debated since ancient times. The modern form of the concept emerged when the Copernican Revolution demonstrated that the Earth was a planet revolving around the Sun, and other planets were, conversely, other worlds. The question of whether other inhabited planets or moons exist was a natural consequence of this new understanding. It has become one of the most speculative questions in science and is a central theme of science fiction and popular culture.

Astronomer

Astronomer

An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either observational or theoretical astronomy. Examples of topics or fields astronomers study include planetary science, solar astronomy, the origin or evolution of stars, or the formation of galaxies. A related but distinct subject is physical cosmology, which studies the Universe as a whole.

Frank Drake

Frank Drake

Frank Donald Drake was an American astrophysicist and astrobiologist.

Probability theory

Probability theory

Probability theory is the branch of mathematics concerned with probability. Although there are several different probability interpretations, probability theory treats the concept in a rigorous mathematical manner by expressing it through a set of axioms. Typically these axioms formalise probability in terms of a probability space, which assigns a measure taking values between 0 and 1, termed the probability measure, to a set of outcomes called the sample space. Any specified subset of the sample space is called an event. Central subjects in probability theory include discrete and continuous random variables, probability distributions, and stochastic processes . Although it is not possible to perfectly predict random events, much can be said about their behavior. Two major results in probability theory describing such behaviour are the law of large numbers and the central limit theorem.

Milky Way

Milky Way

The Milky Way is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλακτικός κύκλος, meaning "milky circle". From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

Galaxy

Galaxy

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.

Civilization

Civilization

A civilization is any society characterized by the existence of the state that has developed culture, language, a writing system, and currency.

Planet

Planet

A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion. The Solar System has at least eight planets: the terrestrial planets Mercury, Venus, Earth and Mars, and the giant planets Jupiter, Saturn, Uranus and Neptune. These planets each rotate around an axis tilted with respect to its orbital pole. All of them possess an atmosphere, although that of Mercury is tenuous, and some share such features as ice caps, seasons, volcanism, hurricanes, tectonics, and even hydrology. Apart from Venus and Mars, the Solar System planets generate magnetic fields, and all except Venus and Mercury have natural satellites. The giant planets bear planetary rings, the most prominent being those of Saturn.

Intelligence

Intelligence

Intelligence has been defined in many ways: the capacity for abstraction, logic, understanding, self-awareness, learning, emotional knowledge, reasoning, planning, creativity, critical thinking, and problem-solving. More generally, it can be described as the ability to perceive or infer information, and to retain it as knowledge to be applied towards adaptive behaviors within an environment or context.

Guesstimate

Guesstimate

Guesstimate is an informal English portmanteau of guess and estimate, first used by American statisticians in 1934 or 1935. It is defined as an estimate made without using adequate or complete information, or, more strongly, as an estimate arrived at by guesswork or conjecture. Like the words estimate and guess, guesstimate may be used as a verb or a noun. A guesstimate may be a first rough approximation pending a more accurate estimate, or it may be an educated guess at something for which no better information will become available.

Hubble Space Telescope

Hubble Space Telescope

The Hubble Space Telescope is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versatile, renowned both as a vital research tool and as a public relations boon for astronomy. The Hubble telescope is named after astronomer Edwin Hubble and is one of NASA's Great Observatories. The Space Telescope Science Institute (STScI) selects Hubble's targets and processes the resulting data, while the Goddard Space Flight Center (GSFC) controls the spacecraft.

History and cultural impact

Cosmic pluralism

The statue of Simandhara, an enlightened man in Jain mythology who is believed to be residing on another planet
The statue of Simandhara, an enlightened man in Jain mythology who is believed to be residing on another planet

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. Before the development of the heliocentric theory and a recognition that the Sun is just one of many stars,[178] the notion of pluralism was largely mythological and philosophical. 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 Bharat Kshetra, Mahavideh Kshetra, Airavat Kshetra, Hari kshetra, etc.[179][180][181][182] Medieval Muslim writers like Fakhr al-Din al-Razi and Muhammad al-Baqir supported cosmic pluralism on the basis of the Qur'an.[183]

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.[184]

With the scientific and Copernican revolutions, and later, during the Enlightenment, cosmic pluralism became a mainstream notion, supported by the likes of Bernard le Bovier de Fontenelle in his 1686 work Entretiens sur la pluralité des mondes.[185] Pluralism was also championed by philosophers such as John Locke and astronomers such as William Herschel. The astronomer Camille Flammarion promoted the notion of cosmic pluralism in his 1862 book La pluralité des mondes habités.[186] None of these notions of pluralism were based on any specific observation or scientific information.

Early modern period

There was a dramatic shift in thinking initiated by the invention of the telescope and the Copernican assault on geocentric cosmology. Once it became clear that Earth was merely one planet amongst countless bodies in the universe, the theory of extraterrestrial life started to become a topic in the scientific community. The best known early-modern proponent of such ideas 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".[187] Bruno's belief in the plurality of worlds was one of the charges leveled against him by the Venetian Holy Inquisition, which trialed and executed him.[188]

In the early 17th century, the Czech astronomer Anton Maria Schyrleus of Rheita mused that "if Jupiter has (...) inhabitants (...) they must be larger and more beautiful than the inhabitants of Earth, in proportion to the [characteristics] of the two spheres".[189]

In Baroque literature such as The Other World: The Societies and Governments of the Moon by Cyrano de Bergerac, extraterrestrial societies are presented as humoristic or ironic parodies of earthly society. The didactic poet Henry More took up the classical theme of the Greek Democritus in "Democritus Platonissans, or an Essay Upon the Infinity of Worlds" (1647). In "The Creation: a Philosophical Poem in Seven Books" (1712), Sir Richard Blackmore observed: "We may pronounce each orb sustains a race / Of living things adapted to the place". With the new relative viewpoint that the Copernican revolution had wrought, he suggested "our world's sunne / Becomes a starre elsewhere". Fontanelle's "Conversations on the Plurality of Worlds" (translated into English in 1686) offered similar excursions on the possibility of extraterrestrial life, expanding, rather than denying, the creative sphere of a Maker.

The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. William Herschel, the discoverer of Uranus, was one of many 18th–19th-century astronomers 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.

19th century

Artificial Martian channels, depicted by Percival Lowell
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.[190] 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 civilization.[191] The idea of life on Mars led British writer H. G. Wells to write the novel The War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet's desiccation.

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.[192] By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis.

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 civilizations 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.[193] The end of the spontaneous generation belief forced to investigate 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.[184] Some of those authors are Jöns Jacob Berzelius (1834),[194] Kelvin (1871),[195] Hermann von Helmholtz (1879)[196] and, somewhat later, by Svante Arrhenius (1903).[197]

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.[188] A famous case is the Cydonia region of Mars, first imagined 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.[198]

Recent history

The Arecibo message is a digital message sent to Messier 13, and is a well-known symbol of human attempts to contact extraterrestrials.
The Arecibo message is a digital message sent to Messier 13, and is a well-known symbol of human attempts to contact extraterrestrials.

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.[199] Astrobiology, however, remains constrained by the current lack of extraterrestrial lifeforms 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 analyze.[200]

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.[201] Most UFOs or UFO sightings[202] can be readily explained as sightings of Earth-based aircraft (including top-secret aircraft), known astronomical objects or weather phenomenons, or as hoaxes.[203]

The possibility of extraterrestrial life on the Moon was ruled out in the 1960s, and during the 1970s it became clear that most of the other bodies of the Solar System do not harbor highly developed life, although the question of primitive life on bodies in the Solar System remains open.

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".[204] 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.

The Wow! signal represented as "6EQUJ5". The original printout with Ehman's handwritten exclamation is preserved by Ohio History Connection. It was pointed towards the Proxima Centauri system. The signal was used to support the search for extraterrestrial intelligence.[205]
The Wow! signal represented as "6EQUJ5". The original printout with Ehman's handwritten exclamation is preserved by Ohio History Connection. It was pointed towards the Proxima Centauri system. The signal was used to support the search for extraterrestrial intelligence.[205]

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".[206] 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.[207] 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.[208] Jared Diamond had earlier expressed similar concerns.[209] 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.[210] 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;[211][212] one result was a statement, signed by many, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent".[213]

Discover more about History and cultural impact related topics

Extraterrestrials in fiction

Extraterrestrials in fiction

An extraterrestrial or alien is any extraterrestrial lifeform; a lifeform that did not originate on Earth. The word extraterrestrial means "outside Earth". The first published use of extraterrestrial as a noun occurred in 1956, during the Golden Age of Science Fiction.

Potential cultural impact of extraterrestrial contact

Potential cultural impact of extraterrestrial contact

The cultural impact of extraterrestrial contact is the corpus of changes to terrestrial science, technology, religion, politics, and ecosystems resulting from contact with an extraterrestrial civilization. This concept is closely related to the search for extraterrestrial intelligence (SETI), which attempts to locate intelligent life as opposed to analyzing the implications of contact with that life.

Cosmic pluralism

Cosmic pluralism

Cosmic pluralism, the plurality of worlds, or simply pluralism, describes the belief in numerous "worlds" in addition to Earth, which may harbour extraterrestrial life.

Jainism

Jainism

Jainism, also known as Jain Dharma, is an Indian religion. Jainism traces its spiritual ideas and history through the succession of twenty-four tirthankaras, with the first in the current time cycle being Rishabhadeva, whom the tradition holds to have lived millions of years ago, the twenty-third tirthankara Parshvanatha, whom historians date to the 9th century BCE, and the twenty-fourth tirthankara Mahavira, around 600 BCE. Jainism is considered to be an eternal dharma with the tirthankaras guiding every time cycle of the cosmology. The three main pillars of Jainism are ahiṃsā (non-violence), anekāntavāda (non-absolutism), and aparigraha (asceticism).

Fakhr al-Din al-Razi

Fakhr al-Din al-Razi

Fakhr al-Dīn al-Rāzī or Fakhruddin Razi, often known by the sobriquet Sultan of the Theologians, was an influential Muslim polymath and one of the pioneers of inductive logic. He wrote various works in the fields of medicine, chemistry, physics, astronomy, cosmology, literature, theology, ontology, philosophy, history and jurisprudence. He was one of the earliest proponents and skeptics that came up with the concept of multiverse, and compared it with the astronomical teachings of Quran. A rejector of the geocentric model and the Aristotelian notions of a single universe revolving around a single world, al-Razi argued about the existence of the outer space beyond the known world.

Muhammad al-Baqir

Muhammad al-Baqir

Muḥammad al-Bāqir, with the full name Muḥammad ibn ʿAlī ibn al-Ḥusayn ibn ʿAlī ibn Abī Ṭālib, also known as Abū Jaʿfar or simply al-Bāqir was the fifth Imam in Shia Islam, succeeding his father, Zayn al-Abidin, and succeeded by his son, Ja'far al-Sadiq. His mother, Fatima Umm Abd Allah, was the daughter of Hasan, making al-Baqir the first Imam who descended from both grandsons of Muhammad, namely, Hasan and Husayn.

Ancient Greece

Ancient Greece

Ancient Greece was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity, that comprised a loose collection of culturally and linguistically related city-states and other territories. Most of these regions were officially unified only once, for 13 years, under Alexander the Great's empire from 336 to 323 BC. In Western history, the era of classical antiquity was immediately followed by the Early Middle Ages and the Byzantine period.

Anaxagoras

Anaxagoras

Anaxagoras was a Pre-Socratic Greek philosopher. Born in Clazomenae at a time when Asia Minor was under the control of the Persian Empire, Anaxagoras came to Athens. According to Diogenes Laërtius and Plutarch, in later life he was charged with impiety and went into exile in Lampsacus; the charges may have been political, owing to his association with Pericles, if they were not fabricated by later ancient biographers.

Age of Enlightenment

Age of Enlightenment

The Age of Enlightenment or the Enlightenment was an intellectual and philosophical movement that dominated Europe in the 17th and 18th centuries with global influences and effects. The Enlightenment included a range of ideas centered on the value of human happiness, the pursuit of knowledge obtained by means of reason and the evidence of the senses, and ideals such as natural law, liberty, progress, toleration, fraternity, constitutional government, and separation of church and state.

John Locke

John Locke

John Locke was an English philosopher and physician, widely regarded as one of the most influential of Enlightenment thinkers and commonly known as the "father of liberalism". Considered one of the first of the British empiricists, following the tradition of Francis Bacon, Locke is equally important to social contract theory. His work greatly affected the development of epistemology and political philosophy. His writings influenced Voltaire and Jean-Jacques Rousseau, and many Scottish Enlightenment thinkers, as well as the American Revolutionaries. His contributions to classical republicanism and liberal theory are reflected in the United States Declaration of Independence. Internationally, Locke’s political-legal principles continue to have a profound influence on the theory and practice of limited representative government and the protection of basic rights and freedoms under the rule of law.

William Herschel

William Herschel

Frederick William Herschel was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel (1750–1848). Born in the Electorate of Hanover, William Herschel followed his father into the military band of Hanover, before emigrating to Great Britain in 1757 at the age of nineteen.

Government responses

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.[214] 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 doesn't have response mechanisms for the case of an extraterrestrial contact.[215]

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."[216]

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.[217]

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.[218] It is also one of the objectives of the Chinese Five-hundred-meter Aperture Spherical Telescope (FAST) program.[219]

The French space agency has an office for the study of "non-identified aero spatial phenomena".[220][221] 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.[220]

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 civilization and some of Earth's governments.[222]

Discover more about Government responses related topics

Planetary protection

Planetary protection

Planetary protection is a guiding principle in the design of an interplanetary mission, aiming to prevent biological contamination of both the target celestial body and the Earth in the case of sample-return missions. Planetary protection reflects both the unknown nature of the space environment and the desire of the scientific community to preserve the pristine nature of celestial bodies until they can be studied in detail.

Outer Space Treaty

Outer Space Treaty

The Outer Space Treaty, formally the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, is a multilateral treaty that forms the basis of international space law. Negotiated and drafted under the auspices of the United Nations, it was opened for signature in the United States, the United Kingdom, and the Soviet Union on 27 January 1967, entering into force on 10 October 1967. As of February 2022, 112 countries are parties to the treaty—including all major spacefaring nations—and another 23 are signatories.

United Nations Office for Outer Space Affairs

United Nations Office for Outer Space Affairs

The United Nations Office for Outer Space Affairs (UNOOSA) is an office of the U.N. Secretariat that promotes and facilitates peaceful international cooperation in outer space. It works to establish or strengthen the legal and regulatory frameworks for space activities, and assists developing countries in using space science and technology for sustainable socioeconomic development. Another purpose of UNOOSA is to keep affairs proper should the world encounter an alien race in outer space, hence the name.

Dmitry Rogozin

Dmitry Rogozin

Dmitry Olegovich Rogozin is a Russian politician who served as director general of Roscosmos from 2018 to July 2022. He previously served as deputy prime minister in charge of the defense industry from 2011 to 2018, and as Russia's ambassador to NATO from 2008 to 2011.

Five-hundred-meter Aperture Spherical Telescope

Five-hundred-meter Aperture Spherical Telescope

The Five-hundred-meter Aperture Spherical radio Telescope, nicknamed Tianyan, is a radio telescope located in the Dawodang depression (大窝凼洼地), a natural basin in Pingtang County, Guizhou, southwest China. FAST has a 500 m (1,600 ft) diameter dish constructed in a natural depression in the landscape. It is the world's largest filled-aperture radio telescope and the second-largest single-dish aperture, after the sparsely-filled RATAN-600 in Russia.

CNES

CNES

The National Centre for Space Studies is the French government space agency. Its headquarters are located in central Paris and it is under the supervision of the French Ministries of Defence and Research.

Israel Space Agency

Israel Space Agency

The Israel Space Agency is a governmental body, a part of Israel's Ministry of Science and Technology, that coordinates all Israeli space research programs with scientific and commercial goals.

Isaac Ben-Israel

Isaac Ben-Israel

Isaac Ben-Israel is an Israeli military scientist, general, politician and state official. He currently serves as the chairman of the Israeli Space Agency and the National Council for Research and Development, under the auspices of the Ministry of Science, Technology and Space of Israel. He finished his service in the IDF ranked General, serving as head of the military Administration for the Development of Weapons and the Technological Industry. Between 2010 and 2012 he served as chief Cybernetics adviser to PM Netanyahu, during which period he founded the National Cyber Bureau in the PM office and launched the National Cyber Initiative. Ben-Israel is now head of the Security Studies program in Tel Aviv University, where he also heads the annual international Cyber Security conference. Between 2007 and 2009 he served as a member of the Knesset for Kadima. Ben-Israel is one of Israel's top experts on Space, Cyber and technological related security. He holds a PhD in Philosophy and a BSc in Physics and Mathematics from Tel Aviv University. In 2020, he declared that Covid-19 would play itself out within seventy days, regardless of intervention levels.

Haim Eshed

Haim Eshed

Haim Eshed alternatively romanized as Chaim Eshed is a visiting professor of aeronautics and astronautics at various space technology research institutions. A retired brigadier general in Israeli Military Intelligence, Eshed was director of space programs for Israel Ministry of Defense for nearly 30 years, is former chair of the Space Committee of the National Council for Research and Development for the Ministry of Science, Technology and Space and a member of the steering committee of Israel Space Agency. Eshed is responsible for the launch of 20 Israeli made satellites, and he is widely cited as the father of Israel's space program.

Source: "Extraterrestrial life", Wikipedia, Wikimedia Foundation, (2023, January 25th), https://en.wikipedia.org/wiki/Extraterrestrial_life.

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See also
Notes
  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.
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