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Encyclopedia > Rare Earth hypothesis

The Rare Earth hypothesis is a hypothesis in planetary astronomy and astrobiology which argues that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances. Look up Hypothesis in Wiktionary, the free dictionary. ... Planetary science, also known as planetology or planetary astronomy, is the science of planets, or planetary systems, and the solar system. ... Pigments other than green might dominate plant life on exoplanets[1] The DNA structure might not be the only nucleic acid in the universe capable of supporting life[2] Astrobiology (from Greek: á¼€ÏƒÏ„ÏÎ¿, astro, constellation; Î²Î¯Î¿Ï‚, bios, life; and Î»ÏŒÎ³Î¿Ï‚, logos, knowledge) is the study of life in space, combining aspects of astronomy... This article does not adequately cite its references or sources. ... Multicellular organisms are those organisms consisting of more than one cell, and having differentiated cells that perform specialized functions. ... Phyla Radiata Cnidaria Ctenophora - Comb jellies Bilateria Protostomia Acoelomorpha Platyhelminthes - Flatworms Nemertina - Ribbon worms Gastrotricha Gnathostomulida - Jawed worms Micrognathozoa Rotifera - Rotifers Acanthocephala Priapulida Kinorhyncha Loricifera Entoprocta Nematoda - Roundworms Nematomorpha - Horsehair worms Cycliophora Mollusca - Mollusks Sipuncula - Peanut worms Annelida - Segmented worms Tardigrada - Water bears Onychophora - Velvet worms Arthropoda - Insects, etc. ... Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... Spiral Galaxy ESO 269-57 Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature, and chemical composition) of celestial objects such as stars, galaxies, and the interstellar medium, as well as their interactions. ... This article includes a list of works cited but its sources remain unclear because it lacks in-text citations. ...

The book Rare Earth: Why Complex Life Is Uncommon in the Universe, by Peter Ward, a geologist and paleontologist, and Donald Brownlee, an astronomer and astrobiologist, contains a good introduction to the hypothesis Peter D. Ward is a paleontologist and professor of Biology and of Earth and Space Sciences at the University of Washington. ... the are cool The Geologist by Carl Spitzweg A geologist is a contributor to the science of geology, studying the physical structure and processes of the Earth and planets of the solar system (see planetary geology). ... A paleontologist carefully chips rock from a column of dinosaur vertebrae. ... Pigments other than green might dominate plant life on exoplanets[1] The DNA structure might not be the only nucleic acid in the universe capable of supporting life[2] Astrobiology (from Greek: á¼€ÏƒÏ„ÏÎ¿, astro, constellation; Î²Î¯Î¿Ï‚, bios, life; and Î»ÏŒÎ³Î¿Ï‚, logos, knowledge) is the study of life in space, combining aspects of astronomy...

The rare earth hypothesis is the contrary of the principle of mediocrity (also called the Copernican principle), whose best known recent advocates include Carl Sagan and Frank Drake. The principle of mediocrity maintains that the universe is probably teeming with advanced life: the Earth is a typical rocky planet in a typical planetary system, located in an unexceptional region of a large but conventional barred-spiral galaxy. Ward and Brownlee argue to the contrary: planets, planetary systems, and galactic regions that are as friendly to complex life as are the Earth, the solar system, and our region of the Milky Way are probably extremely rare. If so, the Earth could be the only place in the Milky Way, and perhaps even in the entire universe, featuring complex life. The mediocrity principle is the notion in the philosophy of science that there is nothing special about Earth, and by implication the human race. ... The Copernican principle is the philosophical statement that no special observers should be proposed. ... Insert non-formatted text here Carl Edward Sagan (November 9, 1934 â€“ December 20, 1996) was an American astronomer and astrobiologist and a highly successful popularizer of astronomy, astrophysics, and other natural sciences. ... Professor Frank Drake Frank Drake (born May 28, 1930, Chicago, Illinois) is an American astronomer and astrophysicist. ... The eight planets and three dwarf planets of the Solar System. ... An artists concept of a protoplanetary disc. ... NGC 1300, viewed nearly face-on. ... An artists concept of a protoplanetary disc. ... It has been suggested that Andromeda-Milky Way collision be merged into this article or section. ...

If complex life can evolve only on an Earth-like planet, then the Rare Earth hypothesis solves the Fermi paradox (Webb 2002): "If extraterrestrial aliens exist, why aren't they obvious?" A graphical representation of the Arecibo message - Humanitys first attempt to use radio waves to communicate its existence to alien civilizations The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for or contact with...

## Complex life is probably very rare GA_googleFillSlot("encyclopedia_square");

The Rare Earth hypothesis argues that the emergence of complex life required a host of fortuitous circumstances. A number of such circumstances are set out below under the following headings: galactic habitable zone, a central star and planetary system having the requisite character, the circumstellar habitable zone, the size of the planet, the advantage of a large satellite, conditions needed to assure 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 still mysterious Cambrian explosion of animal phyla. The emergence of intelligent life may have required yet other rare events. In astronomy a habitable zone (HZ) is a region of space where conditions are favorable for life, as it can be found on earth. ... A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... The tectonic plates of the Lithosphere on Earth. ... View of Jupiters active atmosphere, including the Great Red Spot. ... Animated map exhibiting the worlds oceanic waters. ... A glaciation (a created composite term meaning Glacial Period, referring to the Period or Era of, as well as the process of High Glacial Activity), often called an ice age, is a geological phenomenon in which massive ice sheets form in the Arctic and Antarctic and advance toward the equator. ... The term bolide (from the Greek Î²Î¿Î»Î¹Ï‚, bolis, missile) can refer to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface. ... The Cambrian explosion is the geologically kukko sudden appearance in the fossil record of the ancestors of familiar animals, starting about 542 million years ago (Mya). ... For the linguistic term, see Phylum (linguistics). ...

In order for a small rocky planet to support complex life, the values of hundreds of variables must fall within narrow ranges as asserted by Hugh Ross. The universe is so vast that it could contain multiple 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. Hugh Ross Hugh N. Ross (born 1945) is a Canadian-born Old Earth Creationist. ... The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. ... A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in vacuum in one year. ... A graphical representation of the Arecibo message - Humanitys first attempt to use radio waves to communicate its existence to alien civilizations The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for or contact with...

### The galactic habitable zone

In an area lacking metals or an area near the center with high radiation, a planet would not be able to sustain life (galaxy shown is NGC 7331, often referred to as the "twin" of the Milky Way 1 )

Rare Earth suggests that much of the known universe, including large parts of our galaxy, cannot support complex life; Ward and Brownlee refer to such regions as "dead zones." Those parts of a galaxy where complex life is possible make up the galactic habitable zone. This zone is primarily a function of distance from the galactic center. As that distance increases: Image File history File links NASA/JPL-Caltech/M. Regan (STScI), and the SINGS Team File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links NASA/JPL-Caltech/M. Regan (STScI), and the SINGS Team File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Infrared photos of NGC 7331 as captured by NASAs Spitzer telescope NGC 7331 is a spiral galaxy that was discovered by William Herschel in 1784, and is sometimes referred to as the Milky Ways twin. ... It has been suggested that Andromeda-Milky Way collision be merged into this article or section. ... In astronomy a habitable zone (HZ) is a region of space where conditions are favorable for life, as it can be found on earth. ... The Galactic Center is the rotational center of the Milky Way galaxy. ...

1. The metal content of stars declines, and metals (which in astronomy means all elements other than hydrogen and helium) are necessary to the formation of terrestrial planets.
2. The X-ray and gamma ray radiation from the black hole at the galactic center, and from nearby neutron stars and quasars, becomes less intense. Radiation of this nature is considered dangerous to complex life. Hence the Rare Earth hypothesis deems unfit for life in the early universe, and regions where the stellar density is high and supernovae not rare.
3. 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, the less likely it is to be struck by a large bolide. A sufficiently large impact may extinguish all complex life on a planet.

While a planetary system may enjoy a location favorable to complex life, it must also maintain that location for a span of time sufficiently long for complex life to evolve. Hence a central star with a galactic orbit that steers clear of galactic regions where radiation levels are high, such as the galactic center and the spiral arms, would appear most favourable. If the central star's galactic orbit is eccentric (egg-shaped), it will pass through some spiral arms, but if the orbit is a near perfect circle and the orbital velocity equals the "rotational" velocity of the spiral arms, the star will drift into a spiral arm region only gradually, if at all. Therefore Rare Earth proponents conclude that a life-bearing star must have a galactic orbit that is nearly circular about the center of its galaxy. The required synchronization of the orbital velocity of a central star with the wave velocity of the spiral arms can occur only within a fairly narrow range of distances from the galactic center. This region is termed the "galactic habitable zone". Lineweaver et al (2004) calculate that the galactic habitable zone is an annular ring 7 to 9 kiloparsecs in diameter, that includes no more than 10% of the stars in the Milky Way. Based on conservative estimates of the total number of stars in the galaxy, this could represent something like 20 to 40 billion stars. Gonzalez (2001) would halve these numbers; he estimates that at most 5% of stars in the Milky Way fall in the galactic habitable zone. NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ... Two bodies with a slight difference in mass orbiting around a common barycenter. ... The Galactic Center is the rotational center of the Milky Way galaxy. ... A spiral galaxy presents a face-on view of its spiral arms. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ... It has been suggested that Andromeda-Milky Way collision be merged into this article or section. ...

The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with a period of 226 Ma, one closely matching the rotational period of the galaxy. However, Masters (2002) calculates that the orbit of the Sun takes it through a spiral arm approximately every 100 million years. In contrast, the Rare Earth hypothesis predicts that the Sun, since its formation, should have passed through no spiral arm at all. Annum is a Latin noun meaning year. ...

### A central star of the right character

The terrestrial example suggests complex life requires water in the liquid state and its planet must therefore be at an appropriate distance. This is the core of the notion of habitable zone. The habitable zone forms a ring around the central star. If a planet orbits its sun too closely or too far away, the surface temperature is incompatible with water being liquid (though sub-surface water, as suggested for Europa, may be possible at varying locations). Kasting et al (1993) estimate that the habitable zone for the Sun ranges from 0.95 to 1.15 astronomical units. In astronomy a habitable zone (HZ) is a region of space where conditions are favorable for life, as it can be found on earth. ... Apparent magnitude: 5. ...

A Habitable zone chart showing where Earth-like life could exist (however, microbial life could exist outside the habitable zone in the image)

The habitable zone varies with the type and age of the central star. The habitable zone for a main sequence star very gradually moves out over time until the star becomes a white dwarf, at which time the habitable zone vanishes. The habitable zone is closely connected to the greenhouse warming afforded by atmospheric carbon dioxide (CO2). Even though the Earth's atmosphere contains only 350 parts per million of CO2, that trace amount suffices to raise the average surface temperature of the Earth by about 40ºC from what it would otherwise be (Ward and Brownlee 2000: 18). Image File history File links Habitable_zone-en. ... Image File history File links Habitable_zone-en. ... The greenhouse effect first discovered by Jean Baptiste Joseph Fourier in 1824 is the process by which an atmosphere warms a planet. ... Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. ...

It is then presumed a star needs to have rocky planets within its habitable zone. While the habitable zone of hot stars, such as Sirius or Vega is wide, there are two problems: Sirius (Î± CMa / Î± Canis Majoris / Alpha Canis Majoris) (IPA: ) is the brightest star in the night-time sky with a visual apparent magnitude of âˆ’1. ... Vega (Î± Lyr / Î± Lyrae / Alpha Lyrae) is the brightest star in the constellation Lyra, and the fifth brightest star in the sky. ...

1. Given that rocky planets tend to form closer to their central stars, the planet likely forms too close to the star to lie within the habitable zone. This does not rule out life on a moon of a gas giant. Hot stars also emit much more ultraviolet radiation, which will ionize any planetary atmosphere.
2. Hot stars as mentioned above, have short lives, becoming red giants in as little as 1 Ga. This may not allow enough time for advanced life to evolve.

These considerations rule out the massive and powerful stars of type F6 to O (see stellar classification). A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ... Note: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. ... ... Atmosphere is the general name for a layer of gases that may surround a material body of sufficient mass. ... According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ... This article is about evolution in biology. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. ...

Small red dwarf stars, on the other hand, have habitable zones with a small radius. This proximity causes one face of the planet to constantly face the star, and the other to always remain dark, a situation known as tidal lock. Tidal lock rules out axial rotation; hence one side of a planet will be extremely hot, while the other will be extremely cold. Planets within a habitable zone with a small radius are also at increased risk of solar flares (see Aurelia), which would tend to ionize the atmosphere and are otherwise inimical to complex life. Rare Earth proponents argue that this rules out the possibility of life in such systems, though some exobiologists have suggested that habitability may exist under the right circumstances. This is a central point of contention for the theory, since these K and M category stars are estimated to make up 90% of all stars. An artists impression of a planet in orbit around a red dwarf According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. ... In astronomy a habitable zone (HZ) is a region of space where conditions are favorable for life, as it can be found on earth. ... Tidal locking makes one side of an astronomical body always face another, like the Moon facing the Earth. ... two Gulphogs take shelter during a solar flare Aurelia is a hypothetical Earth-sized extrasolar planet orbiting a red dwarf star in our local area of the galaxy. ... Understanding planetary habitability is partly an extrapolation of the Earths conditions, as it is the only planet currently known to support life. ...

Rare Earth proponents argue that the stellar type of central stars that are "just right" ranges from F7 to K1. Such stars are not common: G type stars such as the Sun (between the hotter F and cooler K) comprise only 5% of the stars in the Milky Way.

Globular clusters are unlikely to support life

Aged stars, such 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 gone through their red giant phase. The diameter of a red giant has substantially increased from its youth. If a planet was in the habitable zone during a star's youth and middle age, it will be fried when its parent star becomes a red giant (though theoretically planets at a much greater distance may become habitable). Download high resolution version (758x795, 157 KB)Image for M80 & Globular cluster Source: http://hubblesite. ... Download high resolution version (758x795, 157 KB)Image for M80 & Globular cluster Source: http://hubblesite. ... A globular cluster is a spherical bundle of stars (star cluster) that orbits a galaxy as a satellite. ... According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ... This article or section does not adequately cite its references or sources. ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ... An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties: The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies. ...

The energy output of a star over its lifespan should only change very gradually; variable stars such as Cepheid variables, for instance, are highly unlikely to support life. If the central star's energy output suddenly decreases, even for a relatively short while, the planet's water may freeze. Conversely, if the central star's energy output temporarily increases, the oceans may evaporate, resulting in a greenhouse effect; this may preclude the oceans from reforming. Cepheid in the Spiral Galaxy M100 A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. ... A schematic representation of the exchanges of energy between outer space, the Earths atmosphere, and the Earth surface. ...

There is no known way to achieve life without complex chemistry, and such chemistry requires metals, namely elements other than hydrogen, helium, and lithium. This suggests a condition for life is a solar system rich in metals. The only known mechanism for creating and dispersing metals is a supernova explosion. The presence of metals in stars is revealed by their absorption spectrum, and studies of stellar spectra reveal that many, perhaps most, stars are poor in metals. Low metallicity characterizes the early universe, globular clusters and other stars formed when the universe was young, stars in most galaxies other than large spirals, and stars in the outer regions of all galaxies. Thus metal-rich central stars capable of supporting complex life are believed most common in the quiet suburbs of the larger spiral galaxies, regions hospitable to complex life for another reason, namely the absence of high radiation. The globular cluster M80. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Standard atomic weight 4. ... General Name, Symbol, Number lithium, Li, 3 Chemical series alkali metals Group, Period, Block 1, 2, s Appearance silvery white/grey Standard atomic weight 6. ... The globular cluster M80. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... A materials absorption spectrum shows the fraction of incident electromagnetic radiation absorbed by the material over a range of frequencies. ... The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ... An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457) A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties: A considerable total angular momentum Composed of a central bulge surrounded by a...

If a star is poor in metals, any associated planetary system is likely poor in metals as well. In order to have rocky planets like the Earth, a central star must have condensed out of a nebula that was fairly metal-rich. Only gas giant planets will condense out of a metal-poor nebula; such a nebula simply lacks the material required to form terrestrial planets. Metal-poor is a term that is used to describe the chemical make up of an astronomical object. ... The Triangulum Emission Nebula NGC 604 The Pillars of Creation from the Eagle Nebula For other uses, see Nebula (disambiguation). ... In astronomy and cosmology, the term metal or metal-rich is used to refer to any element heavier than hydrogen and helium. ... This article or section does not cite any references or sources. ... The inner planets, Mercury, Venus, Earth, and Mars, their sizes to scale. ...

### Planetary system

A gas cloud capable of giving birth to a star can also give rise to gas giant (Jovian) planets like Jupiter and Saturn. But Jovian planets have no hard surface of the kind believed necessary for complex life (their satellites may have hard surfaces, though). Hence a planetary system capable of sustaining complex life must be structured more or less like the solar system, with small and rocky inner planets, and Jovian outer ones. This article or section does not cite any references or sources. ... From top: Neptune, Uranus, Saturn, and Jupiter. ...

A gas giant would help support life by keeping asteroids away from the life-bearing planet; however, "Hot Jupiters" (extrasolar gas giants very close to their host stars) would have a negative effect for life-bearing planets near the host star

Thanks to its gravitational force, a gas giant ejects the debris from planet formation into the equivalent of the Kuiper belt and Oort cloud. Hence a gas giant helps protect the inner rocky planets from asteroid bombardment. However, a gas giant must not be too close to a body upon which life is developing, unless that body is one of its moons. A gas giant must also not be too close to another gas giant.[citation needed] Either placement of the gas giant(s) could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone. Download high resolution version (840x840, 41 KB) Original Caption Released with Image: This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. ... Download high resolution version (840x840, 41 KB) Original Caption Released with Image: This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. ... Artists impression of roaster extrasolar planet HD 209458b (Osiris). ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... This image is an artists rendering of the Oort cloud and the Kuiper Belt. ... 253 Mathilde, a C-type asteroid. ...

Newtonian dynamics predict that all planetary orbits will tend to be chaotic.[citation needed] This tendency to chaos is much stronger when orbits are eccentric, especially the orbits of large planets. The need for stable orbits rules out planetary systems resembling those that have been discovered in recent years, namely systems with a large planet with a small orbit. Such planets are known as hot Jupiters. It is believed that hot Jupiters formed much further from their parent stars than they are now, and have gradually migrated inwards to their current orbits. In the process, they would have gravely disrupted the orbits of any inner planets in the habitable zone. To meet Wikipedias quality standards, this article may require cleanup. ... To meet Wikipedias quality standards, this article may require cleanup. ... Artists impression of roaster extrasolar planet HD 209458b (Osiris). ...

Planetary systems, especially their outer regions, are believed to be riddled with comets and asteroids which inevitably collide with planets.[citation needed] Such collisions, known as bolide impacts, can be highly disruptive for complex life. Hence bolide impacts must be rare (but nonexistent is not necessarily best either; see below) during the billions of years required for complex life to emerge. The frequency of bolide impacts on inner planets is reduced if there are lifeless planets at the right distance from the central star, and with sufficient gravity either to attract comets and asteroids to themselves or to eject them from the planetary system. Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... 253 Mathilde, a C-type asteroid. ... The term bolide (from the Greek Î²Î¿Î»Î¹Ï‚, bolis, missile) can refer to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface. ...

Hence a planetary system capable of supporting complex life must include at least one large outer planet. Jupiter's large mass has attracted many (nearly all?) of the bolides that would have otherwise hit Earth since the end of the late heavy bombardment about 3.8 Ga.[citation needed] But planetary systems with too many Jovian planets, or with a single one that is too large, are likely to be unstable, in which case the likely fate of a rocky inner planet able to support life is either to plunge into its central star or to be ejected into interstellar space. Adjectives: Jovian Atmosphere Surface pressure: 20â€“200 kPa[4] (cloud layer) Composition: ~86% Molecular hydrogen ~13% Helium 0. ... The term bolide (from the Greek Î²Î¿Î»Î¹Ï‚, bolis, missile) can refer to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface. ... The Late Heavy Bombardment (LHB) was a period approximately 3. ... From top: Neptune, Uranus, Saturn, and Jupiter. ...

### Size of planet

(Lissauer 1999, as summarized by Conway Morris 2003: 92; also see Comins 1993). A planet that is too small cannot hold much of an atmosphere. Hence the surface temperature becomes more variable and the average temperature drops. Water will either freeze, boil away, or decompose under the action of UV radiation;[citation needed] in any event, substantial and long-lasting oceans become 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 will either not last as long as they would on a larger planet or may not occur at all. View of Jupiters active atmosphere, including the Great Red Spot. ... Note: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. ... The tectonic plates of the world were mapped in the second half of the 20th century. ...

If a planet's size is such that its gravitational field substantially exceeds the Earth's, it will attract more bolides to itself. The stronger the gravitational field, the harder it is for mountains and continents to form.[citation needed] In the limit, such a planet would probably be covered with an ocean, in which case the lack of exposed rocks would rule out the feedback mechanism, described below, regulating atmospheric CO2. The gravitational field is a field (physics), generated by massive objects, that determines the magnitude and direction of gravitation experienced by other massive objects. ... The term bolide (from the Greek Î²Î¿Î»Î¹Ï‚, bolis, missile) can refer to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface. ... Lyskamm, 4 527 m, Pennine Alps A mountain is a landform that extends above the surrounding terrain in a limited area. ... Animated, colour-coded map showing the various continents. ... It has been suggested that this article or section be merged with Feedback loop. ...

### Large moon

The Moon is unusual because: For other moons in the solar system see natural satellite. ...

• The other rocky planets in the Solar System either have no satellites (Mercury and Venus), or have tiny satellites that are captured asteroids (Mars).
• As a fraction of its planet, it is much larger than any other satellite in the Solar System. The ex-dwarf planet Pluto's Charon would have been an exception, but it is no longer classified as a moon.

If the Earth had no Moon, the ocean tides resulting solely from the Sun's gravity would be very modest. A large satellite gives rise to substantial tides and the resulting tidal pools, which are likely to have been an important locus for the evolution of complex life.[citation needed] 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. It has been suggested that Theory of tides be merged into this article or section. ... Tide pools (also tidal pools or rock pools) are rocky pools by oceans that are filled with seawater. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ... The thickness of the Earths crust (km). ... Age of oceanic crust Oceanic crust is the part of Earths lithosphere which underlies the ocean basins. ... Mantle convection is the slow creeping motion of Earths rocky mantle in response to perpetual gravitationally unstable variations in its density. ...

If a giant impact is the only known way for a rocky inner planet to acquire a large satellite, any planet in the circumstellar habitable zone will need to form as a double planet in order that there be an impacting object sufficiently massive to give rise in due course to a large satellite. An impacting object of this nature is not necessarily improbable. Recent work by Edward Belbruno and J. Richard Gott of Princeton University suggests that a suitable impacting body could form in a planet's trojan points (L4 or L5).[1] In astronomy a habitable zone (HZ) is a region of space where conditions are favorable for life, as it can be found on earth. ... Pluto and Charon are sometimes informally considered to be a double (dwarf) planet. ... Edward Belbruno is a mathematician whose interests are in celestial mechanics, dynamical systems, dynamical astronomy, and aerospace engineering. ... J. Richard Gott is a professor of astrophysical sciences at Princeton University John Richard Gott III is especially well known for developing and advocating two cosmological theories with the flavour of science fiction: Time travel, and the Doomsday argument. ... Princeton University is a private coeducational research university located in Princeton, New Jersey, in the United States of America. ... The Trojan points are the L4 and L5 Lagrangian points. ... A contour plot of the effective potential (the Hills Surfaces) of a two-body system (the Sun and Earth here), showing the five Lagrange points. ... A contour plot of the effective potential (the Hills Surfaces) of a two-body system (the Sun and Earth here), showing the five Lagrange points. ...

### Magnetic field

A magnetosphere protects the biosphere from solar wind and cosmic rays, which are harmful to life. The magnetosphere results from a massive conductive planetary core made of molten iron, acting as a dynamo. The iron is molten because of heat given off by the decay of radioactive elements. If complex life can exist only on the surface of a planet surrounded by a magnetosphere, then complex life requires a planet whose interior contains radioactive elements. Moreover, these elements must have half lives long enough (e.g., uranium 238, thorium 232, and potassium 40) to sustain the magnetosphere for a time span long enough for complex life to evolve. Such elements are relatively rare in the universe.[citation needed] As the universe grows older, the frequency of the sort of supernovae that produces radioactive elements with long half lives is believed to decline.[citation needed] Hence these elements are fated to grow ever rarer as the universe grows older. Hence there is possibly an upper bound to the age of a universe capable of supporting complex life. A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ... A false-color composite of global oceanic and terrestrial photoautotroph abundance, from September 1997 to August 2000. ... The plasma in the solar wind meeting the heliopause For the British comic, see Solar Wind (comic). ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... The planetary core consists of the innermost layers of a planet. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ... Dynamo, or Dinamo, may refer to: Dynamo, an electrical generator Dynamo (sports society) of the Soviet Union Operation Dynamo, the 1940 mass evacuation at Dunkirk Dynamo, the rock band based in Belfast Dynamo theory, a theory relating to magnetic fields of celestial bodies Dynamo Open Air, annual heavy metal music... Radioactive decay is the set of various processes by which unstable atomic nuclei (nuclides) emit subatomic particles. ... The periodic table of the chemical elements A chemical element, or element for short, is a type of atom that is defined by its atomic number; that is, by the number of protons in its nucleus. ... The half-life of a quantity, subject to exponential decay, is the time required for the quantity to decay to half of its initial value. ... General Name, Symbol, Number uranium, U, 92 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery gray metallic; corrodes to a spalling black oxide coat in air Standard atomic weight 238. ... General Name, Symbol, Number thorium, Th, 90 Chemical series Actinides Group, Period, Block n/a, 7, f Appearance silvery white Atomic mass 232. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Standard atomic weight 39. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ...

The unusually massive iron core that generates the Earth's magnetosphere may have resulted from the merger of the proto-Earth's smaller core with that of an impacting body.[citation needed] This impacting body could have been the one that, under the giant impact theory (see above), gave rise to the Moon. The planetary core consists of the innermost layers of a planet. ... The Big Splash The giant impact theory (or Big Splash or Big Whack; cf. ...

### Plate tectonics

Plate tectonics and subduction, such as that on Earth (shown here), are considered essential in promoting biodiversity on a planet

This is the most original part of Ward and Brownlee's analysis (however this section owes much to Webb 2002: 180-84). They argue that in order for a rocky planet to support animal life, its crust must experience plate tectonics. That is, the lithosphere must consist of large crustal plates that, along certain margins, are continuously created from fluid matter carried from the deep interior in convection cells. Along other margins, called subduction zones, these crustal plates are reabsorbed into the planet's interior. Illustration by Jose F. Vigil. ... Illustration by Jose F. Vigil. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... The Juan de Fuca plate sinks below the North America plate at the Cascadia subduction zone. ... Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... Rainforests are among the most biodiverse ecosystems on earth Biodiversity is the variation of taxonomic life forms within a given ecosystem, biome or for the entire Earth. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... The tectonic plates of the Lithosphere on Earth. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... Earth cutaway from core to exosphere. ... A convection cell is a phenomenon of fluid dynamics which occurs in situations where there are temperature differences within a body of liquid or gas. ... Categories: Geology stubs | Plate tectonics ...

A planet will not experience plate tectonics unless its chemical composition allows it. The only known long lasting source of the required heat is radioactive decay occurring deep in the planet's interior. Continents must also be made up of less dense granitic rocks that "float" on underlying more dense basaltic rock. Taylor (1998) emphasizes that subduction zones (an essential part of plate tectonics) require the lubricating action of ample water; on Earth, such zones exist only at the bottom of oceans. The tectonic plates of the world were mapped in the second half of the 20th century. ... Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. ... Close-up of granite from Yosemite National Park, valley of the Merced River Quarrying granite for the Mormon Temple, Utah Territory. ... Basalt Basalt (IPA: ) is a common gray to black extrusive volcanic rock. ... The Juan de Fuca plate sinks below the North America plate at the Cascadia subduction zone. ...

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. At present, it is not known whether the organization of the large scale mantle convection needed to drive plate tectonics could develop in the absence of crustal inhomogeneity. The tectonic plates of the world were mapped in the second half of the 20th century. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ... The thickness of the Earths crust (km). ... Age of oceanic crust Oceanic crust is the part of Earths lithosphere which underlies the ocean basins. ... Earth cutaway from core to exosphere. ...

The reasons why convection-driven plate tectonics promotes the development of complex life include the following. Plate tectonics:

By drawing heat from the interior to the surface, convection driven plate tectonics assures that if a planet has a core of molten iron, that core keeps moving. That motion means that the core of the earth acts like a dynamo, generating a magnetic field. A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ... Magma is molten rock located beneath the surface of the Earth (or any other terrestrial planet), and which often collects in a magma chamber. ... Igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement of eruption. ... View of Jupiters active atmosphere, including the Great Red Spot. ... Dynamo, or Dinamo, may refer to: Dynamo, an electrical generator Dynamo (sports society) of the Soviet Union Operation Dynamo, the 1940 mass evacuation at Dunkirk Dynamo, the rock band based in Belfast Dynamo theory, a theory relating to magnetic fields of celestial bodies Dynamo Open Air, annual heavy metal music... A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ...

If the atmosphere contains too few greenhouse gases, the planet slides into a permanent ice age. Too much greenhouse gas, and the temperature becomes too high for complex life (many proteins denature at temperatures well short of the boiling point of water), and eventually the oceans turn to water vapor. The primary greenhouse gas in the Earth's atmosphere is carbon dioxide, CO2. It appears that plate tectonics play an important role in a complex feedback system (for details, see Ward and Brownlee) that stabilizes the Earth's temperature. Atmospheric CO2 combines with rainwater to form dilute carbonic acid. This acid interacts with surface rocks to form calcium carbonate, CaCO3, which is eventually deposited on the ocean bottom and carried into the Earth's interior at subduction zones. Thus CO2 is removed from the atmosphere. The high temperatures and pressures within the Earth's mantle transform CaCO3 into CO2 and CaO. This subterranean CO2 is eventually returned to the atmosphere via volcanism. Top: Increasing atmospheric CO2 levels as measured in the atmosphere and ice cores. ... Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. ... Carbonic acid (ancient name acid of air or aerial acid) has the formula H2CO3. ... Calcium carbonate is a chemical compound, with chemical formula CaCO3. ... Categories: Geology stubs | Plate tectonics ... This article is about volcanoes in geology. ...

Feedback occurs because a rise in atmospheric CO2 results in higher temperatures via the greenhouse effect, and more rainfall, and more acid rainwater. Hence the rate at which CO2 is removed from the atmosphere rises. When atmospheric CO2 falls, the rate at which it is removed from the atmosphere declines. Plate tectonics exposes and buries rocks in a way that automatically regulates the CO2 content of the atmosphere. The result has been an Earth with a more or less steady surface temperature, even though the sun's energy output is believed to be about 25% greater now than it was when the Earth was young. Absent this recycling of atmospheric carbon, the expected lifetime of the biosphere is not expected to exceed a few million years. Ice ages, by covering much of a planet's rocks and by reducing rainfall, interfere with this feedback process. A false-color composite of global oceanic and terrestrial photoautotroph abundance, from September 1997 to August 2000. ... Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years For the animated movie, see Ice Age (movie). ...

It is difficult to imagine how an aquatic species would smelt and shape metal ores or manipulate electricity (sea water is a fair electrical conductor thanks to its dissolved minerals). Hence it is likely that intelligent life with technology can only evolve on dry land; plate tectonics assures that a planet with ample water also has dry land. More generally, a planet with mountains, islands, and continents gives rise to more microclimates and evolutionary niches, which present evolution with more challenges. Hence plate tectonics promote biodiversity. Hot metal work from a blacksmith In chemistry, a metal (Greek: Metallon) is an element that readily loses electrons to form positive ions (cations) and has metallic bonds between metal atoms. ... Lightning strikes during a night-time thunderstorm. ... In science and engineering, conductors, such as a electrical connector, are materials that readily conduct electric current through electrical conduction. ... Minerals are natural compounds formed through geological processes. ... Tree ferns thrive in a protected dell at Heligan Gardens, in Cornwall, England, latitude 50Â° 15N A microclimate is a local atmospheric zone where the climate differs from the surrounding area. ... Two lichenes species on a rock, in two different ecological niches In ecology, a niche is a term describing the relational position of a species or population in an ecosystem. ... Rainforests are among the most biodiverse ecosystems on earth Biodiversity is the variation of taxonomic life forms within a given ecosystem, biome or for the entire Earth. ...

While plate tectonics appear to have helped complex life to evolve on Earth, how essential plate tectonics are for complex life in general, and the rarity of planets with plate tectonics, are both not well understood at present. The only object in the solar system other than the Earth believed to experience plate tectonics now is the Galilean moon Europa. Jupiters 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). ... Apparent magnitude: 5. ...

### Chemistry of the atmosphere

Carbon-based biochemistry clearly requires a large supply of atmospheric carbon dioxide and crustal carbon (in the form of carbonate compounds); however large amounts of carbon would give rise to a runaway greenhouse effect. Atmospheric oxygen is necessary to support the metabolism of Earthly animals and hence intelligent life. Hence something like photosynthesis had to evolve to shift the atmosphere from a reducing one to an oxidizing one. A schematic representation of the exchanges of energy between outer space, the Earths atmosphere, and the Earth surface. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... A few of the metabolic pathways in a cell. ... The leaf is the primary site of photosynthesis in plants. ...

Central stars invariably emit ultraviolet (UV) radiation. UV radiation whose wavelength falls in the range of 260-90 nm is efficiently absorbed by nucleic acids and proteins, and hence is lethal for all forms of terrestrial life. Fortunately, ozone efficiently absorbs UV radiation in the range 200-300 nm, and atmospheric oxygen is the building block for ozone. Hence a planet with complex life living on dry land must have an ozone layer in its upper atmosphere. Oxygen first appeared in the atmosphere when UV radiation in the range 100-200 nm broke water down into its atomic components. Once there was enough of an ozone layer to permit photosynthetic microbes to evolve on the planet's surface, the oxygen content of the atmosphere gradually rose through photosynthesis, and is believed to have reached its present (or even higher) level during the Cambrian era. Hence an atmosphere sufficiently rich in oxygen may have been a necessary condition for the Cambrian explosion. â€œUVâ€ redirects here. ... Look up nucleic acid in Wiktionary, the free dictionary. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... It has been suggested that Ozone generator be merged into this article or section. ... The ozone layer is the part of the Earths atmosphere which contains relatively high concentrations of ozone (O3). ... The leaf is the primary site of photosynthesis in plants. ... The Cambrian is a major division of the geologic timescale that begins about 542 mya (million years ago) at the end of the Proterozoic eon and ended about 488. ... The Cambrian explosion is the geologically kukko sudden appearance in the fossil record of the ancestors of familiar animals, starting about 542 million years ago (Mya). ...

Even if conditions on a planet's surface allow water in the liquid phase, we cannot conclude that there will in fact be any water present. The inner planets in our solar system were formed with little water. Much of the water in the oceans is believed to have been brought to Earth by the icy asteroid impacts during the early bombardment phase about 4.5 Ga. The oceans play a crucial role in moderating the seasonal swings in the Earth's temperature. The high specific heat of water enables oceans to warm slowly during the summer and then to give up their summer heat over the following winter. Too much water, on the other hand, leads to a planet with little or no land, and hence no weathering mechanism for regulating the carbon dioxide content of the atmosphere. A period early in the history of the solar system where the rubble and debris from the Suns formation had not yet cleared. ... The specific heat capacity (symbol c or s, also called specific heat) of a substance is defined as heat capacity per unit mass. ...

#### Global glaciation

1. Unicellular eukaryotes characterized by organelles, such as chromosomes, nuclei, and mitochondria;
2. Multicellular life with specialized biological tissues and organs, especially animals with calcified shells and skeletons, capable of leaving a clear fossil record.

The earliest unambiguous fossil evidence of multicellular life is the Ediacaran biota, about 580 Ma. Hence the better part of 2 billion years elapsed between the first and the second leaps. Meanwhile, only about 400 million years were required in order for the first multicellular animals (sponges and Ediacaran biota) to evolve into dinosaurs. Kingdoms Animalia - Animals Fungi Plantae - Plants Protista Alternative Phylogeny Unikonta    Opisthokonta    Amoebozoa Bikonta    Apusozoa    Cabozoa       Rhizaria       Excavata    Corticata       Archaeplastida       Chromalveolata Animals, plants, fungi, and protists are eukaryotes (IPA: ), organisms with a complex cell or cells, where the genetic material is organized into a membrane-bound nucleus or nuclei. ... Schematic of typical animal cell, showing subcellular components. ... Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division. ... HeLa cells stained for DNA with the Blue Hoechst dye. ... In cell biology, a mitochondrion is an organelle found in the cells of most eukaryotes. ... Biological tissue is a collection of interconnected cells that perform a similar function within an organism. ... In biology, an organ (Latin: organum, instrument, tool) is a group of tissues that perform a specific function or group of functions. ... Various seashells Danielle A shell is the hard, rigid outer covering, or integument, allanimals. ... This article or section does not cite any references or sources. ... FOSSIL is a standard for allowing serial communication for telecommunications programs under DOS. FOSSIL is an acronym for Fido Opus Seadog Standard Interface Layer. ... Dickinsonia costata, an Ediacaran organism of unknown affinity, with a quilted appearance. ... Annum is a Latin noun meaning year. ...

Curiously, both of these evolutionary transitions came hard on the heels of extended periods of glaciation so extensive that it is suspected that the earth was covered with ice, either entirely or over all but a narrow band about the Equator. This much ice cover would have raised the Earth's albedo to such an extent that the Earth's average temperature may have fallen to about -50°C. The thick ice covering almost all oceans ruled out any interactions between the oceans and the atmosphere. The continents were either covered with ice, or consisted of bare rock devoid of life. This scenario has been named Snowball Earth. World map showing the equator in red In tourist areas, the equator is often marked on the sides of roads The equator marked as it crosses IlhÃ©u das Rolas, in SÃ£o TomÃ© and PrÃ­ncipe. ... Albedo is the ratio of reflected to incident electromagnetic radiation. ... The Snowball Earth hypothesis is a controversial hypothesis (Sankaran, 2003) that attempts to explain a number of phenomena noted in the geological record by proposing that an ice age that took place in the Neoproterozoic was so severe that the Earths oceans froze over completely, with only heat from...

During such periods of catastrophic glaciation, life probably retreated to a narrow band near the equator, and to places warmed by tectonic activity, such as hydrothermal vents on the ocean floor, and volcanoes. Fortunately, glaciation interferes neither with plate tectonics nor with the resulting vulcanism. A hypothesized eventual rise in vulcanism increased atmospheric levels of greenhouse gases, which led to a dramatic increase in temperature and the end of the two apparent snowball earth episodes. Hydrothermal vents are fissures in a planets surface from which geothermally heated water issues. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... This article is about volcanoes in geology. ... Top: Increasing atmospheric CO2 levels as measured in the atmosphere and ice cores. ... The Snowball Earth hypothesis is a controversial hypothesis (Sankaran, 2003) that attempts to explain a number of phenomena noted in the geological record by proposing that an ice age that took place in the Neoproterozoic was so severe that the Earths oceans froze over completely, with only heat from...

The first Snowball Earth episode, the Huronian glaciation, began about 2.4 Ga, shortly after the appearance of the oldest known eukaryotic unicellular organisms. The second episode, the Cryogenian period, lasted from 850 Ma to 635 Ma, ending about 50 Ma before the emergence of the Ediacaran biota. It is an open question what role, if any, these ice ages played in triggering the emergence of complex life. In any event, when the glaciation ended, life eventually sprang back with renewed vigor and diversity. The Cambrian explosion began 542 Ma, in which representatives of all currently extant (and some now extinct) animal phyla suddenly appear in the fossil record. Just how or why the Cambrian explosion came about is still not understood, but it is likely to have resulted from one or more "evolutionary pumps." The Snowball Earth hypothesis is a controversial hypothesis (Sankaran, 2003) that attempts to explain a number of phenomena noted in the geological record by proposing that an ice age that took place in the Neoproterozoic was so severe that the Earths oceans froze over completely, with only heat from... The Huronian glaciation was from 2400 mya to 2100 mya, during the Siderian and Rhyacian periods of the Paleoproterozoic era. ... Kingdoms Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. ... The Cryogenian Period (from Greek cryos ice and genesis birth) is the second geologic period of the Neoproterozoic Era, followed by the Ediacaran Period. ... The Cambrian explosion is the geologically kukko sudden appearance in the fossil record of the ancestors of familiar animals, starting about 542 million years ago (Mya). ... For the linguistic term, see Phylum (linguistics). ...

More modest glaciations are also associated with rapid evolutionary change. The rapid evolution of hominids, which culminated in the appearance of homo sapiens about 200 ka, coincides with the oscillating Quarternary ice age that began about 1.5 Ma. Moreover, the agricultural revolution, when homo sapiens emerged as an aggressive discoverer of technology, began shortly after the last glacial retreat, around 12 ka. Homo sapiens (Latin: wise man) is the scientific name for the human species. ... The Quaternary Period is the geologic time period from the end of the Pliocene Epoch roughly 1. ...

#### Bolide impacts

The impact of a sufficiently massive asteroid or comet can act as an evolutionary pump. The evolution of complex life requires long periods of tranquility. Frequent impacts from large bolides, while not incompatible with the emergence and survival of microbes, make it unlikely that complex life will emerge and survive. Rare bolide impacts, however, while making many forms of complex life extinct, on balance appear to act as evolutionary pumps. A small number of mass-extinction events may be required to give evolution the chance to explore radical new approaches to the challenges of the environment, rather than remain trapped in a suboptimal local maximum. By "suboptimal" is meant "the likelihood that human-like intelligence will eventually emerge is not at a maximum." 253 Mathilde, a C-type asteroid. ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... For other uses, see Life (disambiguation). ... The Dodo, shown here in illustration, is an often-cited[1] example of modern extinction. ... An extinction event (also known as: mass extinction; extinction-level event, ELE) occurs when there is a sharp decrease in the number of species in a relatively short period of time. ... Global optimization is a branch of applied mathematics and numerical analysis that deals with the optimization of a function or a set of functions to some criteria. ...

A case in point is the asteroid impact that created the Chicxulub Crater, believed to have triggered the Cretaceous-Tertiary extinction event, when an estimated 70% of extant metazoans species, including all dinosaurs, became extinct. By removing dinosaurs from all niches they happened to fill, the K-T extinction opened the way for mammals to become large and take their place. Radar topography reveals the 180 kilometer (112 mile) wide ring of the crater (image courtesy NASA/JPL-Caltech) Chicxulub Crater (IPA: ) (cheek-shoo-LOOB) is an ancient impact crater buried underneath the YucatÃ¡n Peninsula, with its center located approximately underneath the town of Chicxulub, YucatÃ¡n, Mexico. ... Badlands near Drumheller, Alberta where erosion has exposed the KT boundary. ... Phyla Subkingdom Parazoa Porifera (sponges) Subkingdom Agnotozoa Placozoa Orthonectida Rhombozoa Subkingdom Metazoa Radiata Cnidaria Ctenophora - Comb jellies Bilateria Protostomia Acoelomorpha Platyhelminthes - Flatworms Nemertina - Ribbon worms Gastrotricha Gnathostomulida - Jawed worms Micrognathozoa Rotifera - Rotifers Acanthocephala Priapulida Kinorhyncha Loricifera Entoprocta Nematoda - Roundworms Nematomorpha - Horsehair worms Cycliophora Mollusca - Mollusks Sipuncula - Peanut worms Annelida - Segmented... Orders & Suborders Saurischia Sauropodomorpha Theropoda Ornithischia Thyreophora Ornithopoda Marginocephalia Dinosaurs were vertebrate animals that dominated the terrestrial ecosystem for over 160 million years, first appearing approximately 230 million years ago. ... Orders & Suborders Saurischia Sauropodomorpha Theropoda Ornithischia Thyreophora Ornithopoda Marginocephalia Dinosaurs were vertebrate animals that dominated the terrestrial ecosystem for over 160 million years, first appearing approximately 230 million years ago. ... The Cretaceous-Tertiary (KT) extinction event, also known as the KT boundary (from German: Kreide-Tertiär-Grenzschicht), was a period of massive extinction of species, about 65. ... Subclasses & Infraclasses Subclass â€ Allotheria* Subclass Prototheria Subclass Theria Infraclass â€ Trituberculata Infraclass Metatheria Infraclass Eutheria Mammals (class Mammalia) are warm-blooded, vertebrate animals characterized by the production of milk in female mammary glands and the presence of hair, three middle ear bones used in hearing, and a neocortex region in the...

#### Inertial interchange event

There is ample evidence that the rate of continental drift during the Cambrian explosion was unusually high. In fact, continents moved from Arctic to equatorial locations, and vice versa, in 15 million years or less. Kirschvink et al (1997)[citation needed] have proposed the following controversial explanation: a 90° change in the Earth's axis of rotation resulting from an imbalance in the distribution of continental masses relative to the axis. The result was huge changes in climates, ocean currents, and so on, occurring in a very short time and affecting the entire Earth. They named their explanation the "inertial interchange event." This scenario is not yet received science, but if such an event took place then it is a very unlikely occurrence, and if such an event was required for the evolution of animal life more complex than sponges and coral reefs, then we have yet another reason why complex life will be rare in the universe. Plates in the crust of the earth, according to the plate tectonics theory Continental drift refers to the movement of the Earths continents relative to each other. ... The axis of rotation of a rotating body is a line such that the distance between any point on the line and any point of the body remains constant under the rotation. ... Classes Calcarea Hexactinellida Demospongiae The sponges or poriferans (from Latin porus pore and ferre to bear) are animals of the phylum Porifera. ... Some of the biodiversity of a coral reef. ...

## Microbes may be common

Multicellular life does not include most microbes. The Rare Earth hypothesis permits microbial life to be far more common than complex life. This part of the hypothesis builds on the discovery, since 1980 or so, of extremophilic microorganisms that thrive in unusually hot, cold, dark, high pressure, salty, or acid conditions. Examples of such locations include rocks several kilometers under the surface of the Earth, hydrothermal vents on the ocean floor, and deep in Antarctic ice, as revealed by drilled ice cores. Some of these organisms are prokaryotes called Archaea that can gain energy from inorganic chemical reactions and do not require sunlight. Some of these extremophile Archaea also require an ambient temperature exceeding 80°C, and thrive in temperatures exceeding 100°C. Such conditions could well have been common deep in the oceans of the young Earth. A microorganism or microbe is an organism that is so small that it is microscopic (invisible to the naked eye). ... An extremophile is an organism, usually unicellular, which thrives in or requires extreme conditions that would exceed optimal conditions for growth and reproduction in the majority of mesophilic terrestrial organisms. ... A cluster of Escherichia coli bacteria magnified 10,000 times. ... A hydrothermal vent A hydrothermal vent is a fissure in a planets surface from which geothermally heated water issues. ... For other uses, see Antarctica (disambiguation). ... Prokaryotes (pro-KAR-ee-oht) (from Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, + suffix -otos, pl. ... Phyla Crenarchaeota Euryarchaeota Korarchaeota Nanoarchaeota Archaea are a major division of microorganisms. ... Inorganic chemistry is the branch of chemistry concerned with the properties and reactions of inorganic compounds. ... An extremophile is an organism, usually unicellular, which thrives in or requires extreme conditions that would exceed optimal conditions for growth and reproduction in the majority of mesophilic terrestrial organisms. ...

Evidence of single-celled microorganisms has been found in rocks dated about 3.5 Ga; hence these forms of life did not take very long to evolve, once the Earth's surface had cooled sufficiently. The domain Archaea suggests that microbial life can emerge fairly quickly in a much broader range of environments than those compatible with complex life. Hence the universe could well teem with simple microbes. Under the Rare Earth hypothesis, only eukaryotic, multicellular, animal, and intelligent life are rare, in that order. Giga-annum, usually abbreviated as Ga, is a unit of time equal to 10^9 years. ... Phyla Crenarchaeota Euryarchaeota Korarchaeota Nanoarchaeota Archaea are a major division of microorganisms. ... Kingdoms Animalia - Animals Fungi Plantae - Plants Protista Alternative Phylogeny Unikonta    Opisthokonta    Amoebozoa Bikonta    Apusozoa    Cabozoa       Rhizaria       Excavata    Corticata       Archaeplastida       Chromalveolata Animals, plants, fungi, and protists are eukaryotes (IPA: ), organisms with a complex cell or cells, where the genetic material is organized into a membrane-bound nucleus or nuclei. ... Intelligent Life is an upmarket magazine launched by the editors of The Economist in 2004. ...

## Rare Earth equation

Dr. Frank Drake, inventor of the Drake equation

The well-known Drake equation estimating the number of planets in the Milky Way harboring intelligent life, was set out some years before the scientific community became fully aware of many of the factors described above and now believed important. The more factors are included in a Drake-like equation, the greater the likelihood that any single factor is near zero. And if any factor is near zero, so is the product of all factors. Image File history File links Frankdrake. ... Image File history File links Frankdrake. ... Professor Frank Drake Frank Drake (born May 28, 1930, Chicago, Illinois) is an American astronomer and astrophysicist. ... The Drake equation (also known as the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of xenobiology, astrosociobiology and the search for extraterrestrial intelligence. ... The Drake equation (also known as the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of xenobiology, astrosociobiology and the search for extraterrestrial intelligence. ...

The following discussion is adapted from Cramer (2000). The Rare Earth equation is Ward and Brownlee's (W&B) riposte to the Drake equation. It calculates N, the number of Earth-like planets in the Milky Way having complex life forms, as: The Drake equation (also known as the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of xenobiology, astrosociobiology and the search for extraterrestrial intelligence. ...

$N = N^* cdot n_e cdot f_g cdot f_p cdot f_{pm} cdot f_i cdot f_c cdot f_l cdot f_m cdot f_j cdot f_{me}$.

Where:

• N* is the number of stars in the Milky Way. This number is not well-estimated, because the Milky Way's mass is not well estimated. Moreover, there is little information about the number of very small stars. N* is at least 100 billion, and may be as high as 500 billion, if there are many low visibility stars.
• ne is the average number of planets in a star's habitable zone. This zone is fairly narrow, because constrained by the requirement that the average planetary temperature be consistent with water remaining liquid throughout the time required for complex life to evolve. Thus ne = 1 is a likely upper bound.

We assume $N^* cdot n_e = 5cdot10^{11}$. The Rare Earth hypothesis can then be viewed as asserting that the product of the other nine Rare Earth equation factors listed below, which are all fractions, is no greater than 10-10 and could plausibly be as small as 10-12. In the latter case, N could be as small as 0 or 1. Ward and Brownlee do not actually calculate the value of N, because the numerical values of quite a few of the factors below can only be conjectured. They cannot be estimated simply because we have but one data point: the Earth, a rocky planet orbiting a G2 star in a quiet suburb of a large barred spiral galaxy, and the home of the only intelligent species we know, namely ourselves. It has been suggested that Andromeda-Milky Way collision be merged into this article or section. ... G2 or G-2 may be: The term G2 is used in many industries to refer to intelligence gathered from the field or customers about a competitors products or plans G2 (Generic Gamers), a Washington State based consortium of game and anime enthusiasts G2 (Guatemala), a government secret police... NGC 1300, viewed nearly face-on. ...

• fg is the fraction of stars in the galactic habitable zone. 0.1 at most.
• fp is the fraction of stars in the Milky Way with planets.
• fpm is the fraction of planets that are rocky ("metallic") rather than gaseous.
• fi is the fraction of habitable planets where microbial life arises. W&B believe this fraction is unlikely to be small.
• fc is the fraction of planets where complex life evolves. For 80% of the time since microbial life first appeared on the Earth, there was only bacterial life. Hence W&B argue that this fraction may be very small. Moreover, the Cambrian Explosion, when complex life really got off the ground, may have been triggered by extraordinary climatic and geological events.
• fl is the fraction of the total lifespan of a planet during which complex life is present. This fraction cannot be high because complex life takes so long to evolve. Complex life cannot endure indefinitely, because the energy put out by the sort of star that allows complex life to emerge gradually rises, and the central star eventually becomes a red giant, engulfing all planets in the planetary habitable zone. Also, given enough time, a catastrophic extinction of all complex life becomes ever more likely.
• fm is the fraction of habitable planets with a large moon. If the giant impact theory of the Moon's origin is correct, this fraction is small.
• fj is the fraction of planetary systems with large Jovian planets. This fraction could be large.
• fme is the fraction of planets with a sufficiently low number of extinction events. W&B argue that the low number of such events the Earth has experienced since the Cambrian explosion may be unusual, in which case this fraction would be small. Such a low number again requires a very stable planetary system, with outer planets having nearly circular orbits, no gravitational perturbations from passing stars, and no nearby supernovas, quasars, or gamma ray bursts.

The Rare Earth equation, unlike the Drake equation, does not factor the probability that complex life evolves into intelligent life that discovers technology. (Keep in mind that Ward and Brownlee are not evolutionary biologists.) Barrow and Tipler (1986: 3.2) review the consensus among such biologists that the evolutionary path from primitive Cambrian chordates, e.g. Pikaia, to homo sapiens was a highly improbable event. For example, the large brains of humans have marked adaptive disadvantages, requiring as they do an expensive metabolism, a long gestation period, and a childhood lasting more than 25% of the average total life span. Other improbable features of humans include: It has been suggested that Andromeda-Milky Way collision be merged into this article or section. ... The Cambrian explosion is the geologically kukko sudden appearance in the fossil record of the ancestors of familiar animals, starting about 542 million years ago (Mya). ... According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ... The Big Splash The giant impact theory (or Big Splash or Big Whack; cf. ... The Cambrian explosion is the geologically kukko sudden appearance in the fossil record of the ancestors of familiar animals, starting about 542 million years ago (Mya). ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. ... In astronomy, gamma-ray bursts (GRBs) are flashes of gamma rays that last from seconds to hours, the longer ones being followed by several days of X-ray afterglow. ... The Drake equation (also known as the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of xenobiology, astrosociobiology and the search for extraterrestrial intelligence. ... Intelligent Life is an upmarket magazine launched by the editors of The Economist in 2004. ... By the mid 20th century humans had achieved a mastery of technology sufficient to leave the surface of the Earth for the first time and explore space. ... This article or section does not cite any references or sources. ... Typical Classes See below Chordates (phylum Chordata) are a group of animals that includes the vertebrates, together with several closely related invertebrates. ... Pikaia is an extinct animal known from the Middle Cambrian fossil found near Mount Pika in the Burgess Shale of British Columbia. ... Homo sapiens (Latin: wise man) is the scientific name for the human species. ... Italic text // ahh addiing sum spiice iin hurr`` For other uses, see Brain (disambiguation). ... A few of the metabolic pathways in a cell. ... The Gestation period in a viviparous animal refers to the length of its pregnancy. ...

• Being the only extant bipedal land vertebrate. Combined with an unusual eye-hand coordination, this permits dextrous manipulations of the physical environment with the hands;
• A vocal apparatus far more expressive than that of any other mammal, enabling speech. Speech makes it possible for humans to interact cooperatively, to share knowledge, and to acquire a culture;
• The capability of formulating abstractions to a degree permitting the invention of mathematics, and the discovery of science and technology. Keep in mind how recently humans acquired anything like their current scientific and technological sophistication.

A biped is an animal that travels across surfaces supported by two legs. ... Classes and Clades See below Male and female Superb Fairy-wren Vertebrates are members of the subphylum Vertebrata (within the phylum Chordata), specifically, those chordates with backbones or spinal columns. ... abstraction in general. ... Euclid, Greek mathematician, 3rd century BC, as imagined by by Raphael in this detail from The School of Athens. ... Part of a scientific laboratory at the University of Cologne. ... By the mid 20th century humans had achieved a mastery of technology sufficient to leave the surface of the Earth for the first time and explore space. ...

Books that advocate the Rare Earth hypothesis, listed in order of increasing difficulty, include:

• Taylor (1998), a specialist on the solar system, firmly believes in the hypothesis, but its truth is not central to his purpose, which is to write a short introductory book on the solar system and its formation. Taylor concludes that the solar system is probably very unusual, because it resulted from so many chance factors and events.
• Webb (2002), a physicist, mainly presents and rejects candidate solutions for the Fermi paradox. The Rare Earth hypothesis emerges as one of the few solutions left standing by the end of the book.
• Ray Kurzweil, a computer pioneer and self-proclaimed Singularitarian, argues in The Singularity Is Near that the coming Singularity requires that Earth be the first planet on which sentient, technology-using life evolved. Although other Earth-like planets could exist, Earth must be the most evolutionarily advanced, because otherwise we would have seen evidence that another culture had experienced the Singularity and expanded to harness the full computational capacity of the physical universe.
• Simon Conway Morris (2003), a paleontologist, mainly argues that evolution is convergent. Morris devotes chapter 5 to the Rare Earth hypothesis, citing Ward and Brownlee (2000) with approval. Yet while Morris agrees that the Earth could well be the only planet in the Milky Way harboring complex life, he sees the evolution of complex life into intelligent life as fairly probable, contra Ernst Mayr's views as reported in section 3.2 of the following reference.
• John D. Barrow and Frank J. Tipler (1986. 3.2, 8.7, 9), cosmologists, vigorously defend the hypothesis that humans are likely to be the only intelligent life in the Milky Way, and perhaps the entire universe. But this hypothesis is not central to their book, a very thorough study of the anthropic principle, and of how the laws of physics are peculiarly suited to enable the emergence of complexity in nature. This book is beginning to date, because when it was written much of what is discussed in this entry was either unknown or insufficiently canonical.

## Criticism

The main objection to the Rare Earth hypothesis is that it is based on a single observation, namely the Earth and its lifeforms. Current scientific instrumentation and search techniques, such as Doppler shift, cannot detect most extrasolar planets of a size and mass similar to the Earth's. Extrasolar planets detected to date are predominantly of Jupiter-mass or larger, which thus renders detectable their effect on the motion of their parent star. Therefore, as of 2006, there is no data on how common Earth-like planets are in the Milky Way. However, on April 24, 2007 astronomers announced the possible discovery of a rocky planet lying in the habitable zone of a main sequence star, Gliese 581 c. Likewise, there is no evidence of extraterrestrial complex life. Although the hypothesis discusses a number of conditions appearing to favor the emergence and development of life under "Earth-like" conditions, the validity of the hypothesis remains open until other Earth-like worlds are found, or the scientific community formally concludes that they are indeed truly rare. The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... Adjectives: Jovian Atmosphere Surface pressure: 20â€“200 kPa[4] (cloud layer) Composition: ~86% Molecular hydrogen ~13% Helium 0. ... It has been suggested that Andromeda-Milky Way collision be merged into this article or section. ... In astronomy a habitable zone (HZ) is a region of space where conditions are favorable for life, as it can be found on earth. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... Gliese 581 c (IPA: ) is a super-earth extrasolar planet orbiting the red dwarf star Gliese 581. ...

According to Darling (2001), the Rare Earth hypothesis is neither hypothesis nor prediction, but merely a description of how life arose on Earth.[3] In his view Ward and Brownlee have done nothing more than select the factors that best suit their case. Look up Hypothesis in Wiktionary, the free dictionary. ... A prediction is a statement or claim that a particular event will occur in the future in more certain terms than a forecast. ...

"What matters is not whether there's anything unusual about the Earth; there's going to be something idiosyncratic about every planet in space. What matters is whether any of Earth's circumstances are not only unusual but also essential for complex life. So far we've seen nothing to suggest there is."

Other aspects of the Rare Earth hypothesis have come under attack. The hypothesis: This page is a candidate to be moved to Wiktionary. ...

• Sometimes relies on contested evidence. For example, while the giant impact theory of the Moon's origin has fair support, it is not universally accepted. Another example is the argument based on star metallicity. Tau Ceti has a metallicity estimated to lie between 22% and 70% of the Sun's, yet has recently been found to have more than ten times the cometary and asteroidal material that the Sun has, which suggests that it is very likely that Tau Ceti has terrestrial planets.
• Assumes the improbability of situations about which we have no hard evidence. Taking into account the size of the universe, the extremely long span of astronomical time, and the many possible ways life-friendly circumstances could arise, there may be more Earth-like planets than the Rare Earth hypothesis allows.
• Ignores the possibility that intelligent life could adapt to its environment. An intelligent space-faring species could, over a very long span of time, gradually transform and colonize many initially uninhabitable planets (in a process analogous to terraforming), or disseminate highly adaptable lifeforms into all environments capable of supporting them. Such a species would need, at the outset, a habitable planet on which to evolve, but could eventually transform many otherwise uninhabitable planets. Transformed planets would then become the predominant loci of intelligent life in the universe.

The Big Splash The giant impact theory (or Big Splash or Big Whack; cf. ... The globular cluster M80. ... Tau Ceti (Ï„ Cet / Ï„ Ceti) is a star commonly mentioned by science fiction authors since it is similar to the Sun in mass and spectral type in addition to being relatively close to us. ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... 253 Mathilde, a C-type asteroid. ... Artists conception of a terraformed Mars in four stages of development. ...

Alternative biochemistry is the biochemistry of alien life forms that differ radically from those on earth. ... In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe by way of coincidentally balanced features that are necessary and relevant to the existence of observers (usually assumed to be carbon-based life or even specifically human beings). ... Pigments other than green might dominate plant life on exoplanets[1] The DNA structure might not be the only nucleic acid in the universe capable of supporting life[2] Astrobiology (from Greek: á¼€ÏƒÏ„ÏÎ¿, astro, constellation; Î²Î¯Î¿Ï‚, bios, life; and Î»ÏŒÎ³Î¿Ï‚, logos, knowledge) is the study of life in space, combining aspects of astronomy... Astrochemistry is the study of the chemicals found in outer space, usually in molecular gas clouds, and their formation, interaction and destruction. ... Astrogeologist and NASA astronaut Harrison Jack Schmitt collecting lunar samples during the Apollo 17 mission Astrogeology is a planetary science discipline concerned with the geology of the celestial bodies such as the planets and their moons, asteroids, comets, and meteorites. ... The Drake equation (also known as the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of xenobiology, astrosociobiology and the search for extraterrestrial intelligence. ... Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... Evolving the Alien: The Science of Extraterrestrial Life (second edition publised as What Does a Martian Look Like? The Science of Extraterrestrial Life) is a book about xenobiology by biologist Jack Cohen and mathematician Ian Stewart. ... The deepest visible-light image of the cosmos. ... The planet Earth, photographed in the year 1972. ... The mediocrity principle is the notion in the philosophy of science that there is nothing special about humans or the Earth. ... Cosmic pluralism or the plurality of worlds describes the belief in numerous other worlds beyond the Earth which harbour extraterrestrial life. ... In classical philosophy, dialectic (Greek: Î´Î¹Î±Î»ÎµÎºÏ„Î¹ÎºÎ®) is an exchange of propositions (theses) and counter-propositions (antitheses) resulting in a synthesis of the opposing assertions, or at least a qualitative transformation in the direction of the dialogue. ... Metaphysical naturalism is any worldview in which nature is all there is and all things supernatural (which stipulatively includes as well as spirits and souls, non-natural values, and universals as they are commonly conceived) do not exist. ... This article does not adequately cite its references or sources. ... Panspermia is the hypothesis that the seeds of life are in the Universe, that they may have delivered life to Earth, and that they may deliver or have delivered life to other habitable bodies; also the process of such delivery. ... Understanding planetary habitability is partly an extrapolation of the Earths conditions, as it is the only planet currently known to support life. ... The Precambrian (Pre-Cambrian) is an informal name for the eons of the geologic timescale that came before the current Phanerozoic eon. ... The Snowball Earth hypothesis is a controversial hypothesis (Sankaran, 2003) that attempts to explain a number of phenomena noted in the geological record by proposing that an ice age that took place in the Neoproterozoic was so severe that the Earths oceans froze over completely, with only heat from... // This is a timeline of geological and relevant astronomical events on Earth before the Cambrian period started. ...

## References

1. ^ Belbruno, E.; J. Richard Gott III (2005). "Where Did The Moon Come From?". The Astronomical Journal 129 (3): 1724-1745. arXiv:astro-ph/0405372.
2. ^ For a detailed critique of the Rare Earth hypothesis along these lines, see Cohen and Ian Stewart (2002).
3. ^ Darling, David (2001). Life Everywhere: The Maverick Science of Astrobiology. Basic Books/Perseus.
• John D. Barrow and Frank J. Tipler, 1986. The Anthropic Cosmological Principle. Oxford Univ. Press.
• Cirkovic, Milan M., and Bradbury, Robert J., 2006, "Galactic Gradients, Postbiological Evolution, and the Apparent Failure of SETI," New Astronomy, vol. 11, pp. 628-639.
• Comins, Neil F., 1993. What if the moon didn't exist? Voyages to Earths that might have been. HarperCollins.
• Simon Conway Morris, 2003. Life's Solution. Cambridge Univ. Press. See chpt. 5; many references.
• Cohen, Jack, and Ian Stewart, 2004 (2002). What Does a Martian Look Like: The Science of Extraterrestrial Life. Ebury Press. ISBN 0-09-187927-2.
• Cramer, John G., 2000, "The 'Rare Earth' Hypothesis," Analog Science Fiction & Fact Magazine (September 2000).
• Guillermo Gonzalez, Brownlee, Donald, and Ward, Peter, 2001, "The Galactic Habitable Zone: Galactic Chemical Evolution," Icarus 152: 185-200.
• James Kasting, Whitmire, D. P., and Reynolds, R. T., 1993, "Habitable zones around main sequence stars," Icarus 101: 108-28.
• Kirschvink, Joseph L., Robert L. Ripperdan, and David A. Evans, 1997, "Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander," Science 277: 541-45.
• Knoll, Andrew H., 2003. Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Princeton Univ. Press.
• Lineweaver, Charles H., Fenner, Yeshe, and Gibson, Brad K., 2004, "The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way," Science 303: 59-62.
• Lissauer, 1999, "How common are habitable planets?" Nature 402: C11-14.
• Ross, Hugh, 1993, "Some of the parameters of the galaxy-sun-earth-moon system necessary for advanced life" in The Creator and the Cosmos, 2nd ed. Colorado Springs CO: NavPress, pp.113-114.
• Taylor, Stuart Ross, 1998. Destiny or Chance: Our Solar System and Its Place in the Cosmos. Cambridge Univ. Press.
• Frank J. Tipler, 2003, "Intelligent Life in Cosmology," International Journal of Astrobiology 2: 141-48.
• Ward, Peter D., and Brownlee, Donald, 2000. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books (Springer Verlag). ISBN 0-387-98701-0.
• Webb, Stephen, 2002. If the universe is teeming with aliens, where is everybody? Fifty solutions to the Fermi paradox and the problem of extraterrestrial life. Copernicus Books (Springer Verlag).

arXiv (pronounced archive, as if the X were the Greek letter Ï‡) is an archive for electronic preprints of scientific papers in the fields of physics, mathematics, computer science and quantitative biology which can be accessed via the Internet. ... Ian Stewart, FRS (b. ... John David Barrow FRS (born November 29, 1952, London) is an English cosmologist, theoretical physicist, and mathematician. ... Frank J. Tipler (born in 1947 in Andalusia, Alabama) is a professor of mathematical physics at Tulane University in New Orleans, Louisiana. ... In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe by way of coincidentally balanced features that are necessary and relevant to the existence of observers (usually assumed to be carbon-based life or even specifically human beings). ... Simon Conway Morris is a British paleontologist. ... Ian Stewart, FRS (b. ... Guillermo Memo Gonzalez (born January 4, 1986 in Paramount, California) is an American soccer player, who currently plays midfielder for the Los Angeles Galaxy in Major League Soccer. ... James F. Kasting (b. ... The Ancient and Medieval cosmos as depicted in Peter Apians Cosmographia (Antwerp, 1539). ... Frank J. Tipler (born in 1947 in Andalusia, Alabama) is a professor of mathematical physics at Tulane University in New Orleans, Louisiana. ...

Results from FactBites:

 Rare Earth hypothesis: Information from Answers.com (7611 words) Hence the Rare Earth hypothesis deems unfit for life the early universe, and regions where the stellar density is high and supernovae not rare. The Rare Earth equation is Ward and Brownlee's (WandB) riposte to the Drake equation. But this hypothesis is not central to their book, a very thorough study of the anthropic principle, and of how the laws of physics are peculiarly suited to enable the emergence of complexity in nature.
 Rare Earth hypothesis - Wikipedia, the free encyclopedia (7566 words) In planetary astronomy and astrobiology, the Rare Earth hypothesis asserts that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances. The Rare Earth hypothesis is the contrary of the principle of mediocrity (also called the Copernican principle), whose best known recent advocates include Carl Sagan and Frank Drake. The mediocrity principle and cosmic pluralism are the antithesis of the Rare Earth hypothesis.
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