The Rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone , a central star and planetary system having the requisite character , the circumstellar habitable zone , a right-sized terrestrial planet , the advantage of a gas giant guardian like Jupiter and  a large natural satellite , conditions needed to ensure the planet has a magnetosphere and plate tectonics , the chemistry of the lithosphere, atmosphere, and oceans , the role of "evolutionary pumps" such as massive glaciation and rare Bolide impacts, and whatever led to the appearance of the eukaryote cell , sexual reproduction  and the Cambrian explosion of animal, plant, and fungi phyla. The evolution of human  intelligence may have required yet further events, which are extremely unlikely to have happened were it not for the Cretaceous–Paleogene extinction event 66 million years ago removing dinosaurs as the dominant terrestrial vertebrates. In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The universe is so vast that it could contain many Earth-like planets . But if such planets exist, they are likely to be separated from each other by many thousands of light years . Such distances may preclude communication among any intelligent species evolving on such planets, which would solve the Fermi paradox: "If extraterrestrial aliens are common, why aren't they obvious?" The right location in the right kind of galaxy Rare Earth suggests that much of the known universe, including large parts of our galaxy, are "dead zones" unable to support complex life. Those parts of a galaxy where complex life is possible make up the galactic habitable zone,  primarily characterized by distance from the Galactic Center. As that distance increases  star metallicity declines. Metals (which in astronomy means all elements other than hydrogen and helium) are necessary to the formation of terrestrial planets. The X-ray and gamma ray radiation from the black hole at the galactic center, and from nearby neutron stars, becomes less intense  . Thus the early universe, and present-day galactic regions where stellar density is high and supernovae are common, will be dead zones. Gravitational perturbation of planets and planetesimals by nearby stars becomes less likely as the density of stars decreases. Hence the further a planet lies from the Galactic Center or a spiral arm, the less likely it is to be struck by a large bolide which could extinguish all complex life on a planet. Item #1 rules out the outer reaches of a galaxy; #2 and #3 rule out galactic inner regions. Hence a galaxy's habitable zone may be a ring sandwiched between its uninhabitable center and outer reaches. Also, a habitable planetary system must maintain its favorable location long enough for complex life to evolve. A star with an eccentric ( elliptic or hyperbolic) galactic orbit will pass through some spiral arms, unfavorable regions of high star density; thus a life-bearing star must have a galactic orbit that is nearly circular, with a close synchronization between the orbital velocity of the star and of the spiral arms. This further restricts the galactic habitable zone within a fairly narrow range of distances from the Galactic Center. Lineweaver et al. calculate this zone to be a ring 7 to 9 kiloparsecs in radius, including no more than 10\% of the stars in the Milky Way, about 20 to 40 billion stars. Gonzalez , et al. would halve these numbers; they estimate that at most 5\% of stars in the Milky Way fall in the galactic habitable zone. The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with a period of 226 Ma (million years), closely matching the rotational period of the galaxy. However, the majority of stars in barred spiral galaxies populate the spiral arms rather than the halo and tend to move in gravitationally aligned orbits, so there is little that is unusual about the Sun's orbit. While the Rare Earth hypothesis predicts that the Sun should rarely, if ever, have passed through a spiral arm since its formation, astronomer Karen Masters has calculated that the orbit of the Sun takes it through a major spiral arm approximately every 100 million years. Some researchers have suggested that several mass extinctions do  correspond with previous crossings of the spiral arms. Orbiting at the right distance from the right type of star According to the hypothesis, Earth has an improbable orbit in the very narrow habitable zone (dark green) around the Sun. The terrestrial example suggests that complex life requires liquid water, requiring an orbital distance neither too close nor too far from the central star, another scale of habitable zone or Goldilocks Principle : The habitable zone varies with the star's type and age. For advanced life, the star must also be highly stable, which is typical of middle star life, about 4.6 billion years old. Proper metallicity and size are also important to stability. The Sun has a low  0.1\%  luminosity variation. To date  no solar twin star, with an exact match of the sun's luminosity variation, has been found, though some come close. The star must  have no stellar companions, as in binary systems, which would disrupt the orbits of  planets. Estimates suggest 50\% or more of all star systems are binary.University of Nebraska-Lincoln astronomy education group, Stellar Luminosity CalculatorNational Center for Atmospheric Research, The Effects of Solar Variability on Earth's Climate, 2012 ReportMost of Earth’s twins aren’t identical, or even close!, by Ethan on 5 June 2013 The habitable zone for a main sequence star very gradually moves out over its lifespan until it becomes a white dwarf and the habitable zone vanishes. The liquid water and other gases available in the habitable zone bring the benefit of greenhouse warming. Even though the Earth's atmosphere contains a water vapor concentration from 0\% (in arid regions) to 4\% (in rain forest and ocean regions) and – as of February 2018 – only 408.05 parts per million of , these small amounts suffice to raise the average surface temperature by about 40 °C, with the dominant contribution being due to water vapor. Rocky planets must orbit within the habitable zone for life to form. Although the habitable zone of such hot stars as Sirius or Vega is wide, hot stars also emit much more ultraviolet radiation that ionizes any planetary atmosphere. They may become red giants before advanced life evolves on their planets. These considerations rule out the massive and powerful stars of type F6 to O (see stellar classification) as homes to evolved metazoan life. Small red dwarf stars conversely have small habitable zones wherein planets are in tidal lock, with one very hot side always facing the star and another very cold side ; and they are also at increased risk of solar flares (see Aurelia). Life probably cannot arise in such systems. Rare Earth proponents claim that only stars from F7 to K1 types are hospitable. Such stars are rare: G type stars such as the Sun (between the hotter F and cooler K) comprise only 9\% The One Hundred Nearest Star Systems, Research Consortium on Nearby Stars. of the hydrogen-burning stars in the Milky Way. Such aged stars as red giants and white dwarfs are also unlikely to support life. Red giants are common in globular clusters and elliptical galaxies. White dwarfs are mostly dying stars that have already completed their red giant phase. Stars that become red giants expand into or overheat the habitable zones of their youth and middle age (though theoretically planets at a much greater distance may become habitable). The right arrangement of planets Depiction of the Sun and planets of the Solar System and the sequence of planets. Rare Earth argues that without such an arrangement, in particular the presence of the massive gas giant Jupiter (  fifth planet from the Sun and the largest), complex life on Earth would not have arisen. Rare Earth proponents argue that a planetary system capable of sustaining complex life must be structured more or less like the Solar System, with small, rocky inner planets and massive outer gas giants. Without the protection of ' celestial vacuum cleaner ' planets with strong gravitational pull, a planet would be subject to more  catastrophic asteroid collisions. Observations of exo-planets have shown that arrangements of planets similar to the Solar System are rare. Most planetary systems have super Earths , several times larger than Earth, close to their star, whereas the Solar System's inner region has only a few small rocky planets and none inside Mercury's orbit. Only 10\% of stars have giant planets similar to Jupiter and Saturn, and those few rarely have stable  nearly circular orbits distant from their star. Konstantin Batygin and colleagues argue that these features can be explained if, early in the history of the Solar System, Jupiter and Saturn drifted towards the Sun, sending showers of planetesimals towards the super-Earths which sent them spiralling into the Sun, and ferrying icy building blocks into the terrestrial region of the Solar System which provided the building blocks for the rocky planets. The two giant planets then drifted out again to their present position. However, in the view of Batygin and his colleagues: "The concatenation of chance events required for this delicate choreography suggest that small, Earth-like rocky planets – and perhaps life itself – could be rare throughout the cosmos."Batygin et al., pp. 23-24 A continuously stable orbit Rare Earth argues that a gas giant  must not be too close to a body where life is developing. Close placement of gas giant(s) could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone. The need for stable orbits rules out stars with systems of planets that contain large planets with orbits close to the host star (called "hot Jupiters"). It is believed that hot Jupiters have migrated inwards to their current orbits. In the process, they would have catastrophically disrupted the orbits of any planets in the habitable zone."Once you realize that most of the known extrasolar planets have highly eccentric orbits (like the planets in Upsilon Andromedae), you begin to wonder if there might be something special about our solar system" (UCBerkeleyNews quoting Extra solar planetary researcher Eric Ford.) To exacerbate matters, hot Jupiters are much more common orbiting F and G class stars.Sol Company, Stars and Habitable Planets, 2012 A terrestrial planet of the right size upright=1.4|Planets of the Solar System  to scale. Rare Earth argues that complex life cannot exist on large gaseous planets like Jupiter and Saturn (top row) or Uranus and Neptune (top middle) or smaller planets such as Mars and Mercury It is argued that life requires terrestrial planets like Earth and as gas giants lack such a surface, that complex life cannot arise there. A planet that is too small cannot hold much atmosphere, making surface temperature low and variable and oceans impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics may be brief or entirely absent. A planet that is too large will retain too dense an atmosphere  like Venus. Although Venus is similar in size and mass to Earth, its surface atmospheric pressure is 92 times that of Earth, and surface temperature of 735 K (462 °C; 863 °F). Earth had a similar early atmosphereto Venus , but may have lost it in the giant impact event which formed the Moon., as summarized by ; also see With plate tectonics The Great American Interchange on Earth, around ~ 3.5 to 3 Ma, an example of species competition, resulting from continental plate interaction An artist's rendering of the structure of Earth's magnetic field-magnetosphere that protects Earth's life from solar radiation. 1) Bow shock. 2) Magnetosheath. 3) Magnetopause. 4) Magnetosphere. 5) Northern tail lobe. 6) Southern tail lobe. 7) Plasmasphere. Rare Earth proponents argue that plate tectonics and a strong magnetic field are essential for biodiversity, global temperature regulation, and the carbon cycle. The lack of mountain chains elsewhere in the Solar System is direct evidence that Earth is the only body with plate tectonics, and thus the only nearby body capable of supporting life. Plate tectonics depend on the right chemical composition and a long-lasting source of heat from radioactive decay. Continents must be made of less dense felsic rocks that "float" on underlying denser mafic rock. Taylor emphasizes that tectonic subduction zones require the lubrication of oceans of water. Plate tectonics also provides a means of biochemical cycling.Plate Tectonics Could Be Essential for Alien Life, By Lee Pullen, Astrobiology Magazine, February 19, 2009, SPACE.com Plate tectonics and as a result continental drift and the creation of separate land masses would create diversified ecosystems and biodiversity, one of the strongest defences against extinction. 2000. Plate tectonics essential for complex evolution - Rare Earth - Copernicus Books An example of species diversification and later competition on Earth's continents is the Great American Interchange. North and Middle America drifted into South America at around 3.5 to 3 Ma. The fauna of South America  evolved separately for about 30 million years, since Antarctica separated . Many species were subsequently wiped outin mainly South America by competing Northern American animals. A large moon Tide pools resulting from tidal interaction of the Moon are said to have promoted the evolution of complex life. The Moon is unusual because the other rocky planets in the Solar System either have no satellites (Mercury and Venus), or only  tiny satellites which are probably captured asteroids (Mars). The Giant-impact theory hypothesizes that the Moon resulted from the impact of a Mars-sized body, dubbed Theia, with the young Earth. This giant impact also gave the Earth its axial tilt (inclination) and velocity of rotation. Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable. The Rare Earth hypothesis further argues that the axial tilt cannot be too large or too small (relative to the orbital plane). A planet with a large tilt will experience extreme seasonal variations in climate. A planet with little or no tilt will lack the stimulus to evolution that climate variation provides. In this view, the Earth's tilt is "just right". The gravity of a large satellite also stabilizes the planet's tilt; without this effect the variation in tilt would be chaotic, probably making complex life forms on land impossible. A large satellite also increases the likelihood of plate tectonics through the effect of tidal forces on the planet's crust. The impact that formed the Moon may also have initiated plate tectonics, without which the continental crust would cover the entire planet, leaving no room for oceanic crust. It is possible that the large scale mantle convection needed to drive plate tectonics could not have emerged in the absence of crustal inhomogeneity. A further theory indicates that such a large moon may also contribute to maintaining a planet's magnetic shield by continually acting upon a metallic planetary core as dynamo, thus protecting the surface of the planet from charged particles and cosmic rays, and helping to ensure the atmosphere is not stripped over time by solar winds.  Earth's atmosphere %DIFDELCMD < %DIFDELCMD < %%% Atmosphere A terrestrial planet of the right size is needed to retain an atmosphere , like Earth and Venus  . On Earth, once the giant impact of Theia thinned Earth's atmosphere, other events were needed to make the atmosphere capable of sustaining life. The Late Heavy Bombardment reseeded Earth with water lost after the impact of Theia. The development of an ozone layer formed protection from ultraviolet (UV) sunlight.NASA, Ozone and the Atmosphere, Goddard Earth Sciences (GES) Data and Information Services Center Nitrogen and carbon dioxide are needed in a correct ratio for life to form.Emsley, p. 360 Lightning is needed for nitrogen fixation. The carbon dioxide gas needed for life comes from sources such as volcanoes and geysers. Carbon dioxide is only needed at low levels (currently at 400 ppm); at high levels it is poisonous.The International Volcanic Health Hazard Network, Carbon Dioxide (CO2) Precipitation is needed to have a stable water cycle. A proper atmosphere must reduce diurnal temperature variation. One or more evolutionary triggers for complex life This diagram illustrates the twofold cost of sex. If each individual were to contribute to the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size each generation  Regardless of whether planets with similar physical attributes to the Earth are rare or not, some argue that life usually remains simple bacteria . Biochemist Nick Lane argues that simple cells (prokaryotes) emerged soon after Earth's formation, but since almost half the planet's life had passed before they evolved into complex ones (eukaryotes)  all of whom share a common ancestor, this event can only have happened once. In some views, prokaryotes lack the cellular architecture to evolve into eukaryotes because a bacterium expanded up to eukaryotic proportions would have tens of thousands of times less energy available ; two billion years ago, one simple cell incorporated itself into another, multiplied, and evolved into mitochondria that supplied the vast increase in available energy that enabled the evolution of complex  life. If this incorporation occurred only once in four billion years or is otherwise unlikely, then life on most planets remains simple.Lane, 2012 An alternative view is that mitochondria evolution was environmentally triggered, and that mitochondria-containing organisms appeared soon after the first traces of atmospheric oxygen.Martin, W. & Mentel, M. (2010)The Origin of Mitochondria. Nature Education 3(9):58 The right time in evolution Timeline of evolution; human writing exists for only 0.000218\% of Earth's history. While life on Earth is regarded to have spawned relatively early in the planet's history, the evolution from multicellular to intelligent organisms took around 800 million years. 800 million years for complex organ evolution - Heidelberg University Civilizations on Earth have existed for about 12,000 years  and radio communication reaching space has existed for less than 100 years. Relative to the age of the Solar System (~4.57 Ga) this is a short time, in which extreme climatic variations, super volcanoes, and large meteorite impacts were absent. These events would severely harm intelligent life, as well as life in general. For example, the Permian-Triassic mass extinction, caused by widespread and continuous volcanic eruptions in an area the size of Western Europe, led to the extinction of 95\% of known species around 251.2 Ma ago. About 65 million years ago, the Chicxulub impact at the Cretaceous–Paleogene boundary (~65.5 Ma) on the Yucatán peninsula in Mexico led to a mass extinction of the most advanced species at that time.
The Rare Earth hypothesis argues that the evolution of biological complexity anywhere in the universe requires the coincidence of a large number of fortuitous circumstances, such as a galactic habitable zone ; a central star and planetary system having the requisite character (i.e. a circumstellar habitable zone ); a terrestrial planet of the right mass; the advantage of one or more gas giant guardians like Jupiter and possibly a large natural satellite to shield the planet from frequent impact events; conditions needed to ensure the planet has a magnetosphere and plate tectonics ; the apparently unique chemistry of the lithosphere, atmosphere, and oceans ; the influence of "evolutionary pumps" such as massive glaciations and occasional bolide impacts; and whatever factors may have led to the emergence of eukaryotic cells , sexual reproduction , and the Cambrian explosion of animal, plant, and fungi phyla. The evolution of human beings and of human intelligence may have required yet further specific events and circumstances, all of which are extremely unlikely to have happened were it not for the Cretaceous–Paleogene extinction event 66 million years ago removing dinosaurs as the dominant terrestrial vertebrates. In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The universe is so vast that it might still contain many Earth-like planets , but if such planets exist, they are likely to be separated from each other by many thousands of light-years . Such distances may preclude communication among any intelligent species that may evolve on such planets, which would solve the Fermi paradox: "If extraterrestrial aliens are common, why aren't they obvious?" The right location in the right kind of galaxy Rare Earth suggests that much of the known universe, including large parts of our galaxy, are "dead zones" unable to support complex life. Those parts of a galaxy where complex life is possible make up the galactic habitable zone, which is primarily characterized by distance from the Galactic Center. As that distance increases , star metallicity declines. Metals (which in astronomy refers to all elements other than hydrogen and helium) are necessary for the formation of terrestrial planets. The X-ray and gamma ray radiation from the black hole at the galactic center, and from nearby neutron stars, becomes less intense as distance increases . Thus the early universe, and present-day galactic regions where stellar density is high and supernovae are common, will be dead zones. Gravitational perturbation of planets and planetesimals by nearby stars becomes less likely as the density of stars decreases. Hence the further a planet lies from the Galactic Center or a spiral arm, the less likely it is to be struck by a large bolide which could extinguish all complex life on a planet. Item #1 rules out the outermost reaches of a galaxy; #2 and #3 rule out galactic inner regions. Hence a galaxy's habitable zone may be a relatively narrow ring of adequate conditions sandwiched between its uninhabitable center and outer reaches. Also, a habitable planetary system must maintain its favorable location long enough for complex life to evolve. A star with an eccentric ( elliptical or hyperbolic) galactic orbit will pass through some spiral arms, unfavorable regions of high star density; thus a life-bearing star must have a galactic orbit that is nearly circular, with a close synchronization between the orbital velocity of the star and of the spiral arms. This further restricts the galactic habitable zone within a fairly narrow range of distances from the Galactic Center. Lineweaver et al. calculate this zone to be a ring 7 to 9 kiloparsecs in radius, including no more than 10\% of the stars in the Milky Way, about 20 to 40 billion stars. Gonzalez et al. would halve these numbers; they estimate that at most 5\% of stars in the Milky Way fall within the galactic habitable zone. The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with a period of 226 Ma (million years), closely matching the rotational period of the galaxy. However, the majority of stars in barred spiral galaxies populate the spiral arms rather than the halo and tend to move in gravitationally aligned orbits, so there is little that is unusual about the Sun's orbit. While the Rare Earth hypothesis predicts that the Sun should rarely, if ever, have passed through a spiral arm since its formation, astronomer Karen Masters has calculated that the orbit of the Sun takes it through a major spiral arm approximately every 100 million years. Some researchers have suggested that several mass extinctions do indeed correspond with previous crossings of the spiral arms. Orbiting at the right distance from the right type of star According to the hypothesis, Earth has an improbable orbit in the very narrow habitable zone (dark green) around the Sun. The terrestrial example suggests that complex life requires liquid water, the maintenance of which requires an orbital distance neither too close nor too far from the central star, another scale of habitable zone or Goldilocks Principle . The habitable zone varies with the star's type and age. For advanced life, the star must also be highly stable, which is typical of middle star life, about 4.6 billion years old. Proper metallicity and size are also important to stability. The Sun has a low ( 0.1\% ) luminosity variation. To date , no solar twin star, with an exact match of the sun's luminosity variation, has been found, though some come close. The star must also have no stellar companions, as in binary systems, which would disrupt the orbits of any planets. Estimates suggest 50\% or more of all star systems are binary.University of Nebraska-Lincoln astronomy education group, Stellar Luminosity CalculatorNational Center for Atmospheric Research, The Effects of Solar Variability on Earth's Climate, 2012 ReportMost of Earth’s twins aren’t identical, or even close!, by Ethan on 5 June 2013 The habitable zone for a main sequence star very gradually moves out over its lifespan until the star becomes a white dwarf and the habitable zone vanishes. The liquid water and other gases available in the habitable zone bring the benefit of greenhouse warming. Even though the Earth's atmosphere contains a water vapor concentration from 0\% (in arid regions) to 4\% (in rainforest and ocean regions) and – as of February 2018 – only 408.05 parts per million of , these small amounts suffice to raise the average surface temperature by about 40 °C, with the dominant contribution being due to water vapor. Rocky planets must orbit within the habitable zone for life to form. Although the habitable zone of such hot stars as Sirius or Vega is wide, hot stars also emit much more ultraviolet radiation that ionizes any planetary atmosphere. Such stars may also become red giants before advanced life evolves on their planets. These considerations rule out the massive and powerful stars of type F6 to O (see stellar classification) as homes to evolved metazoan life. Conversely, small red dwarf stars have small habitable zones wherein planets are in tidal lock, with one very hot side always facing the star and another very cold side always facing away, and they are also at increased risk of solar flares (see Aurelia). Life probably cannot arise in such systems. Rare Earth proponents claim that only stars from F7 to K1 types are hospitable. Such stars are rare: G type stars such as the Sun (between the hotter F and cooler K) comprise only 9\% The One Hundred Nearest Star Systems, Research Consortium on Nearby Stars. of the hydrogen-burning stars in the Milky Way. Such aged stars as red giants and white dwarfs are also unlikely to support life. Red giants are common in globular clusters and elliptical galaxies. White dwarfs are mostly dying stars that have already completed their red giant phase. Stars that become red giants expand into or overheat the habitable zones of their youth and middle age (though theoretically planets at much greater distances may then become habitable). The right arrangement of planets Depiction of the Sun and planets of the Solar System and the sequence of planets. Rare Earth argues that without such an arrangement, in particular the presence of the massive gas giant Jupiter ( the fifth planet from the Sun and the largest), complex life on Earth would not have arisen. Rare Earth proponents argue that a planetary system capable of sustaining complex life must be structured more or less like the Solar System, with small, rocky inner planets and massive outer gas giants. Without the protection of such " celestial vacuum cleaner " planets with strong gravitational pulls, other planets would be subject to more frequent catastrophic asteroid collisions. Observations of exoplanets have shown that arrangements of planets similar to the Solar System are rare. Most planetary systems have super-Earths , several times larger than Earth, close to their star, whereas the Solar System's inner region has only a few small rocky planets and none inside Mercury's orbit. Only 10\% of stars have giant planets similar to Jupiter and Saturn, and those few rarely have stable , nearly circular orbits distant from their star. Konstantin Batygin and colleagues argue that these features can be explained if, early in the history of the Solar System, Jupiter and Saturn drifted towards the Sun, sending showers of planetesimals towards the super-Earths which sent them spiralling into the Sun, and ferrying icy building blocks into the terrestrial region of the Solar System which provided the building blocks for the rocky planets. The two giant planets then drifted out again to their present positions. In the view of Batygin and his colleagues: "The concatenation of chance events required for this delicate choreography suggest that small, Earth-like rocky planets – and perhaps life itself – could be rare throughout the cosmos."Batygin et al., pp. 23–24 A continuously stable orbit Rare Earth argues that a gas giant also must not be too close to a body where life is developing. Close placement of one or more gas giants could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone. The need for stable orbits rules out stars with planetary systems that contain large planets with orbits close to the host star (called "hot Jupiters"). It is believed that hot Jupiters have migrated inwards to their current orbits. In the process, they would have catastrophically disrupted the orbits of any planets in the habitable zone."Once you realize that most of the known extrasolar planets have highly eccentric orbits (like the planets in Upsilon Andromedae), you begin to wonder if there might be something special about our solar system" (UCBerkeleyNews quoting extrasolar planetary researcher Eric Ford.) To exacerbate matters, hot Jupiters are much more common orbiting F and G class stars.Sol Company, Stars and Habitable Planets, 2012 A terrestrial planet of the right size upright=1.4|Planets of the Solar System , shown to scale. Rare Earth argues that complex life cannot exist on large gaseous planets like Jupiter and Saturn (top row) or Uranus and Neptune (top middle) or smaller planets such as Mars and Mercury . The Rare Earth hypothesis argues that life requires terrestrial planets like Earth , and since gas giants lack such a surface, that complex life cannot arise there. A planet that is too small cannot maintain much atmosphere, rendering its surface temperature low and variable and oceans impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics may be brief or entirely absent. A planet that is too large will retain too dense an atmosphere , like Venus. Although Venus is similar in size and mass to Earth, its surface atmospheric pressure is 92 times that of Earth, and its surface temperature is 735 K (462 °C; 863 °F). The early Earth once had a similar atmosphere , but may have lost it in the giant impact event which formed the Moon., as summarized by ; also see With plate tectonics The Great American Interchange on Earth, approximately 3.5 to 3 Ma, an example of species competition, resulting from continental plate interaction An artist's rendering of the structure of Earth's magnetic field-magnetosphere that protects Earth's life from solar radiation. 1) Bow shock. 2) Magnetosheath. 3) Magnetopause. 4) Magnetosphere. 5) Northern tail lobe. 6) Southern tail lobe. 7) Plasmasphere. Rare Earth proponents argue that plate tectonics and a strong magnetic field are essential for biodiversity, global temperature regulation, and the carbon cycle. The lack of mountain chains elsewhere in the Solar System is direct evidence that Earth is the only body with plate tectonics, and thus the only nearby body capable of supporting life. Plate tectonics depend on the right chemical composition and a long-lasting source of heat from radioactive decay. Continents must be made of less dense felsic rocks that "float" on underlying denser mafic rock. Taylor emphasizes that tectonic subduction zones require the lubrication of oceans of water. Plate tectonics also provide a means of biochemical cycling.Plate Tectonics Could Be Essential for Alien Life, By Lee Pullen, Astrobiology Magazine, February 19, 2009, SPACE.com Plate tectonics and as a result continental drift and the creation of separate landmasses would create diversified ecosystems and biodiversity, one of the strongest defences against extinction. 2000. Plate tectonics essential for complex evolution - Rare Earth - Copernicus Books An example of species diversification and later competition on Earth's continents is the Great American Interchange. North and Middle America drifted into South America at around 3.5 to 3 Ma. The fauna of South America had already evolved separately for about 30 million years, since Antarctica separated , but after the merger many species were wiped out, mainly in South America, by competing North American animals. A large moon Tide pools resulting from the tidal interactions of the Moon are said to have promoted the evolution of complex life. The Moon is unusual because the other rocky planets in the Solar System either have no satellites (Mercury and Venus), or only relatively tiny satellites which are probably captured asteroids (Mars). The giant-impact theory hypothesizes that the Moon resulted from the impact of a Mars-sized body, dubbed Theia, with the young Earth. This giant impact also gave the Earth its axial tilt (inclination) and velocity of rotation. Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable. The Rare Earth hypothesis further argues that the axial tilt cannot be too large or too small (relative to the orbital plane). A planet with a large tilt will experience extreme seasonal variations in climate. A planet with little or no tilt will lack the stimulus to evolution that climate variation provides. In this view, the Earth's tilt is "just right". The gravity of a large satellite also stabilizes the planet's tilt; without this effect the variation in tilt would be chaotic, probably making complex life forms on land impossible. A large satellite also increases the likelihood of plate tectonics through the effect of tidal forces on the planet's crust. The impact that formed the Moon may also have initiated plate tectonics, without which the continental crust would cover the entire planet, leaving no room for oceanic crust. It is possible that the large-scale mantle convection needed to drive plate tectonics could not have emerged in the absence of crustal inhomogeneity. A further theory indicates that such a large moon may also contribute to maintaining a planet's magnetic shield by continually acting upon a metallic planetary core as dynamo, thus protecting the surface of the planet from charged particles and cosmic rays, and helping to ensure the atmosphere is not stripped over time by solar winds. Atmosphere Earth's atmosphere %DIFDELCMD < %DIFDELCMD < %%% A terrestrial planet must be the right size , like Earth and Venus , in order to retain an atmosphere . On Earth, once the giant impact of Theia thinned Earth's atmosphere, other events were needed to make the atmosphere capable of sustaining life. The Late Heavy Bombardment reseeded Earth with water lost after the impact of Theia. The development of an ozone layer generated a protective shield against ultraviolet (UV) sunlight.NASA, Ozone and the Atmosphere, Goddard Earth Sciences (GES) Data and Information Services Center Nitrogen and carbon dioxide are needed in a correct ratio for life to form.Emsley, p. 360 Lightning is needed for nitrogen fixation. The carbon dioxide gas needed for life comes from sources such as volcanoes and geysers. Carbon dioxide is only needed at low levels (currently at 400 ppm); at high levels it is poisonous.The International Volcanic Health Hazard Network, Carbon Dioxide (CO2) Precipitation is needed to have a stable water cycle. A proper atmosphere must reduce diurnal temperature variation. One or more evolutionary triggers for complex life This diagram illustrates the twofold cost of sex. If each individual were to contribute to the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size each generation . Regardless of whether planets with similar physical attributes to the Earth are rare or not, some argue that life tends not to evolve into anything more complex than simple bacteria without being provoked by rare and specific circumstances . Biochemist Nick Lane argues that simple cells (prokaryotes) emerged soon after Earth's formation, but since almost half the planet's life had passed before they evolved into complex ones (eukaryotes) , all of whom share a common ancestor, this event can only have happened once. According to some views, prokaryotes lack the cellular architecture to evolve into eukaryotes because a bacterium expanded up to eukaryotic proportions would have tens of thousands of times less energy available to power its metabolism. Two billion years ago, one simple cell incorporated itself into another, multiplied, and evolved into mitochondria that supplied the vast increase in available energy that enabled the evolution of complex eukaryotic life. If this incorporation occurred only once in four billion years or is otherwise unlikely, then life on most planets remains simple.Lane, 2012 An alternative view is that the evolution of mitochondria was environmentally triggered, and that mitochondria-containing organisms appeared soon after the first traces of atmospheric oxygen.Martin, W. & Mentel, M. (2010)The Origin of Mitochondria. Nature Education 3(9):58 The right time in evolution Timeline of evolution; human writing exists for only 0.000218\% of Earth's history. While life on Earth is regarded to have spawned relatively early in the planet's history, the evolution from multicellular to intelligent organisms took around 800 million years. 800 million years for complex organ evolution - Heidelberg University Civilizations on Earth have existed for about 12,000 years , and radio communication reaching space has existed for less than 100 years. Relative to the age of the Solar System (~4.57 Ga) this is a short time, in which extreme climatic variations, super volcanoes, and large meteorite impacts were absent. These events would severely harm intelligent life, as well as life in general. For example, the Permian-Triassic mass extinction, caused by widespread and continuous volcanic eruptions in an area the size of Western Europe, led to the extinction of 95\% of known species around 251.2 Ma ago. About 65 million years ago, the Chicxulub impact at the Cretaceous–Paleogene boundary (~65.5 Ma) on the Yucatán peninsula in Mexico led to a mass extinction of the most advanced species at that time.
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(i.e. a
R
, a right-sized terrestrial planet , the advantage of a gas giant guardian
); a terrestrial planet of the right mass; the advantage of one or more gas giant guardians
A
possibly
R
,
to shield the planet from frequent impact events;
R
, the
; the apparently unique
R
, the role
; the influence
R
glaciation and rare Bolide impacts, and whatever
glaciations and occasional bolide impacts; and whatever factors may have
R
appearance of the eukaryote cell
emergence of eukaryotic cells
A
,
A
beings and of human
R
events,
specific events and circumstances, all of
R
could
might still
R
. But
, but
R
light years
light-years
R
evolving
that may evolve
A
which is
A
,
R
means
refers to
R
to
for
A
as distance increases
R
outer
outermost
R
ring
relatively narrow ring of adequate conditions
R
elliptic
elliptical
D
,
R
in
within
A
indeed
R
requiring
the maintenance of which requires
R
:
.
A
(
A
)
A
,
A
also
A
any
R
it
the star
R
rain forest
rainforest
R
They may
Such stars may also
R
Small
Conversely, small
D
conversely
R
;
always facing away,
R
a much greater distance may
much greater distances may then
A
the
R
'
such "
R
'
"
R
pull, a planet
pulls, other planets
A
frequent
R
exo-planets
exoplanets
R
super Earths
super-Earths
A
,
R
position. However, in
positions. In
R
23-24
23–24
A
also
R
gas giant(s)
one or more gas giants
R
systems of planets
planetary systems
R
Extra solar
extrasolar
A
, shown
R
It is argued
. The Rare Earth hypothesis argues
R
and as
, and since
R
hold
maintain
R
making
rendering its
A
,
R
surface temperature of
its surface temperature is
R
Earth
The early Earth once
R
early atmosphereto Venus
atmosphere
R
around ~
approximately
R
provides
provide
R
land masses
landmasses
A
had already
R
. Many species were subsequently wiped outin mainly South America by competing Northern
, but after the merger many species were wiped out, mainly in South America, by competing North
R
tidal interaction
the tidal interactions
A
relatively
R
Giant-impact
giant-impact
R
large scale
large-scale
A
Atmosphere
D
Atmosphere
R
of
must be
D
is needed to retain an atmosphere
A
, in order to retain an atmosphere
R
formed protection from
generated a protective shield against
A
.
R
usually remains simple bacteria
tends not to evolve into anything more complex than simple bacteria without being provoked by rare and specific circumstances
A
,
R
In
According to
R
; two
to power its metabolism. Two
A
eukaryotic
R
mitochondria evolution
the evolution of mitochondria
A
,