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152-443: A planet , in astronomy, is one of a class of celestial bodies that orbit stars. (A dwarf planet is a similar, but officially mutually exclusive, class of body.) Planet or Planets may also refer to: Planet A planet is a large, rounded astronomical body that is generally required to be in orbit around a star , stellar remnant , or brown dwarf , and is not one itself. The Solar System has eight planets by

304-449: A Sun-like star . With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet. Gliese 667 Cc , discovered in 2011 but announced in 2012, is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C . It is one of the most Earth-like planets known. Gliese 163 c , discovered in September 2012 in orbit around the red dwarf Gliese 163

456-469: A geodynamo that generates a magnetic field . Similar differentiation processes are believed to have occurred on some of the large moons and dwarf planets, though the process may not always have been completed: Ceres, Callisto, and Titan appear to be incompletely differentiated. The asteroid Vesta, though not a dwarf planet because it was battered by impacts out of roundness, has a differentiated interior similar to that of Venus, Earth, and Mars. All of

608-406: A rogue planet , is believed to be orbited by a tiny protoplanetary disc , and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses. The idea of planets has evolved over the history of astronomy, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in

760-516: A triaxial ellipsoid . The exoplanet Tau Boötis b and its parent star Tau Boötis appear to be mutually tidally locked. The defining dynamic characteristic of a planet, according to the IAU definition, is that it has cleared its neighborhood . A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with

912-494: A boundary, even though deuterium burning does not last very long and most brown dwarfs have long since finished burning their deuterium. This is not universally agreed upon: the exoplanets Encyclopaedia includes objects up to 60 M J , and the Exoplanet Data Explorer up to 24 M J . The smallest known exoplanet with an accurately known mass is PSR B1257+12A , one of the first exoplanets discovered, which

1064-485: A circumstellar habitable zone centered at 1.34 AU from the Sun, a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at 0.25 {\displaystyle {\sqrt {0.25}}} , or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating factors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation of

1216-441: A dehydrated state temperature between 0.150 K (−273 °C) and 424 K (151 °C). Life on a planetary object orbiting outside HZ might hibernate on the cold side as the planet approaches the apastron where the planet is coolest and become active on approach to the periastron when the planet is sufficiently warm. A 2015 review concluded that the exoplanets Kepler-62f , Kepler-186f and Kepler-442b were likely

1368-679: A disk remnant left over from the supernova that produced the pulsar. The first confirmed discovery of an exoplanet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of 51 Pegasi b , an exoplanet around 51 Pegasi . From then until the Kepler space telescope mission, most of the known exoplanets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets

1520-615: A given period. Like the general circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative and extended region. In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes and huge starspots which can cover 20% of the star's surface area, have the potential to strip an otherwise habitable planet of its atmosphere and water. As with more massive stars, though, stellar evolution changes their nature and energy flux, so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for

1672-443: A habitable zone in the outer parts of stellar systems may exist during the pre-main-sequence phase of stellar evolution, especially around M-dwarfs, potentially lasting for billion-year timescales. Circumstellar habitable zones change over time with stellar evolution. For example, hot O-type stars, which may remain on the main sequence for fewer than 10 million years, would have rapidly changing habitable zones not conducive to

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1824-515: A helium-burning star, important life processes like photosynthesis could only happen around planets where the atmosphere has carbon dioxide, as by the time a solar-mass star becomes a red giant, planetary-mass bodies would have already absorbed much of their free carbon dioxide. Moreover, as Ramirez and Kaltenegger (2016) showed, intense stellar winds would completely remove the atmospheres of such smaller planetary bodies, rendering them uninhabitable anyway. Thus, Titan would not be habitable even after

1976-467: A high component of greenhouse gas and terrestrial planets much more massive than Earth ( super-Earth class planets), that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study; not enough is known about the nature of atmospheres of these kinds of extrasolar objects, and their position in the habitable zone cannot determine

2128-503: A larger "extended habitable zone" in which a planet like Venus, with stronger greenhouse effects , can have the right temperature for liquid water to exist at the surface. Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units , though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above

2280-486: A liquid. Although traditional definitions of the habitable zone assume that carbon dioxide and water vapor are the most important greenhouse gases (as they are on the Earth), a study led by Ramses Ramirez and co-author Lisa Kaltenegger has shown that the size of the habitable zone is greatly increased if prodigious volcanic outgassing of hydrogen is also included along with the carbon dioxide and water vapor. The outer edge in

2432-442: A list of omens and their relationships with various celestial phenomena including the motions of the planets. The inferior planets Venus and Mercury and the superior planets Mars , Jupiter , and Saturn were all identified by Babylonian astronomers . These would remain the only known planets until the invention of the telescope in early modern times. The ancient Greeks initially did not attach as much significance to

2584-411: A mass 5.5–10.4 times the mass of Earth, attracted attention upon its discovery for potentially being in the habitable zone, though later studies concluded that it is actually too close to its star to be habitable. Planets more massive than Jupiter are also known, extending seamlessly into the realm of brown dwarfs. Exoplanets have been found that are much closer to their parent star than any planet in

2736-479: A metallic or rocky core today, or a reaccumulation of the resulting debris. Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle that either is or was a fluid . The terrestrial planets' mantles are sealed within hard crusts , but in

2888-517: A multitude of similar-sized objects. As described above, this characteristic was mandated as part of the IAU 's official definition of a planet in August 2006. Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs . Gravity causes planets to be pulled into

3040-402: A negligible axial tilt as a result of their proximity to their stars. Similarly, the axial tilts of the planetary-mass moons are near zero, with Earth's Moon at 6.687° as the biggest exception; additionally, Callisto's axial tilt varies between 0 and about 2 degrees on timescales of thousands of years. The planets rotate around invisible axes through their centres. A planet's rotation period

3192-413: A planet reaches a mass somewhat larger than Mars's mass, it begins to accumulate an extended atmosphere , greatly increasing the capture rate of the planetesimals by means of atmospheric drag . Depending on the accretion history of solids and gas, a giant planet , an ice giant , or a terrestrial planet may result. It is thought that the regular satellites of Jupiter, Saturn, and Uranus formed in

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3344-478: A planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away. It has been noted that the current term of 'circumstellar habitable zone' poses confusion as the name suggests that planets within this region will possess a habitable environment. However, surface conditions are dependent on a host of different individual properties of that planet. This misunderstanding

3496-426: A plausible base for future human exploration . Titan has the only nitrogen -rich planetary atmosphere in the Solar System other than Earth's. Just as Earth's conditions are close to the triple point of water, allowing it to exist in all three states on the planet's surface, so Titan's are to the triple point of methane . Planetary atmospheres are affected by the varying insolation or internal energy, leading to

3648-495: A radius estimated at 1.1 Earth, Kepler-186f , discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method though its mass remains unknown and its parent star is not a Solar analog. Kapteyn b , discovered in June 2014 is a possible rocky world of about 4.8 Earth masses and about 1.5 Earth radii were found orbiting the habitable zone of

3800-457: A roughly spherical shape, so a planet's size can be expressed roughly by an average radius (for example, Earth radius or Jupiter radius ). However, planets are not perfectly spherical; for example, the Earth's rotation causes it to be slightly flattened at the poles with a bulge around the equator . Therefore, a better approximation of Earth's shape is an oblate spheroid , whose equatorial diameter

3952-454: A significant impact on mythology , religious cosmology , and ancient astronomy . In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the " fixed stars ", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες ( planētes asteres ) ' wandering stars ' or simply πλανῆται ( planētai ) ' wanderers ' from which today's word "planet"

4104-403: A significantly lower mass than the gas giants (only 14 and 17 Earth masses). Dwarf planets are gravitationally rounded, but have not cleared their orbits of other bodies . In increasing order of average distance from the Sun, the ones generally agreed among astronomers are Ceres , Orcus , Pluto , Haumea , Quaoar , Makemake , Gonggong , Eris , and Sedna . Ceres is the largest object in

4256-407: A similar way; however, Triton was likely captured by Neptune, and Earth's Moon and Pluto's Charon might have formed in collisions. When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation , the solar wind , Poynting–Robertson drag and other effects. Thereafter there still may be many protoplanets orbiting

4408-568: A star with high orbital eccentricity may spend only some of its year in the HZ and experience a large variation in temperature and atmospheric pressure. This would result in dramatic seasonal phase shifts where liquid water may exist only intermittently. It is possible that subsurface habitats could be insulated from such changes and that extremophiles on or near the surface might survive through adaptions such as hibernation ( cryptobiosis ) and/or hyperthermostability . Tardigrades , for example, can survive in

4560-467: A terrestrial planet could sustain liquid water on its surface, given enough atmospheric pressure. One in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone, which suggests that the nearest would be expected to be within 12  light-years distance from Earth. The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation , which estimates

4712-438: Is 43 kilometers (27 mi) larger than the pole -to-pole diameter. Generally, a planet's shape may be described by giving polar and equatorial radii of a spheroid or specifying a reference ellipsoid . From such a specification, the planet's flattening, surface area, and volume can be calculated; its normal gravity can be computed knowing its size, shape, rotation rate, and mass. A planet's defining physical characteristic

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4864-409: Is a star that resembles the Sun. No solar twin with an exact match as that of the Sun has been found. However, some stars are nearly identical to the Sun and are considered solar twins. An exact solar twin would be a G2V star with a 5,778 K temperature, be 4.6 billion years old, with the correct metallicity and a 0.1% solar luminosity variation. Stars with an age of 4.6 billion years are at

5016-403: Is coincidental with the red-dwarf habitable zone, it has been suggested that the volcanism caused by tidal heating could cause a "tidal Venus" planet with high temperatures and no hospitable environment for life. Others maintain that circumstellar habitable zones are more common and that it is indeed possible for water to exist on planets orbiting cooler stars. Climate modeling from 2013 supports

5168-543: Is delineated by a set of elements: Planets have varying degrees of axial tilt; they spin at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the Northern Hemisphere points away from its star, the Southern Hemisphere points towards it, and vice versa. Each planet therefore has seasons , resulting in changes to

5320-458: Is known as a stellar day . Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's north pole . The exceptions are Venus and Uranus, which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise. Regardless of which convention

5472-471: Is located 49 light years from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percent more stellar radiation than Earth, leading to surface temperatures of about 60° C . HD 40307 g , a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307 . In December 2012, Tau Ceti e and Tau Ceti f were found in

5624-419: Is reflected in excited reports of 'habitable planets'. Since it is completely unknown whether conditions on these distant HZ worlds could host life, different terminology is needed. Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit), the mass of the body itself, and the radiative flux of

5776-470: Is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium . This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until

5928-437: Is the largest known detached object , a population that never comes close enough to the Sun to interact with any of the classical planets; the origins of their orbits are still being debated. All nine are similar to terrestrial planets in having a solid surface, but they are made of ice and rock rather than rock and metal. Moreover, all of them are smaller than Mercury, with Pluto being the largest known dwarf planet and Eris being

6080-500: Is the smallest object generally agreed to be a geophysical planet , at about six millionths of Earth's mass, though there are many larger bodies that may not be geophysical planets (e.g. Salacia ). An exoplanet is a planet outside the Solar System. As of 24 July 2024, there are 7,026 confirmed exoplanets in 4,949 planetary systems , with 1007 systems having more than one planet . Known exoplanets range in size from gas giants about twice as large as Jupiter down to just over

6232-421: Is the smallest, at 0.055 Earth masses. The planets of the Solar System can be divided into categories based on their composition. Terrestrials are similar to Earth, with bodies largely composed of rock and metal: Mercury, Venus, Earth, and Mars. Earth is the largest terrestrial planet. Giant planets are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune. They differ from

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6384-528: Is unclear whether such satellites could form in the first place. HD 69830 d , a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellar habitable zone of HD 69830 , 41 light years away from Earth. The following year, 55 Cancri f was discovered within the HZ of its host star 55 Cancri A . Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces. Though, in theory, such giant planets could possess moons,

6536-457: Is used, Uranus has a retrograde rotation relative to its orbit. The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets contributes to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter

6688-602: The Hill radius of the planet so that they are not pulled out of the orbit of their host planet. Red dwarfs that have masses less than 20% of that of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to the star that it would be stripped from its host planet. In such a system, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as to eliminate any prospects of habitability. A planetary object that orbits

6840-645: The Kuiper belt . The discovery of other large objects in the Kuiper belt, particularly Eris , spurred debate about how exactly to define a planet. In 2006, the International Astronomical Union (IAU) adopted a definition of a planet in the Solar System, placing the four terrestrial planets and the four giant planets in the planet category; Ceres, Pluto, and Eris are in the category of dwarf planet . Many planetary scientists have nonetheless continued to apply

6992-458: The Moon , Mars , and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods. At Hellas Basin , for example, atmospheric pressures can reach 1,115 Pa and temperatures above zero Celsius (about

7144-423: The asteroid belt , located between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the Kuiper belt , which is a second belt of small Solar System bodies beyond the orbit of Neptune. Gonggong and Eris orbit in the scattered disc , which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna

7296-454: The climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice . Each planet has two in the course of its orbit; when one hemisphere has its summer solstice with its day being the longest, the other has its winter solstice when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of

7448-576: The lithosphere , and photolysis . For an extrasolar system, an icy body from beyond the frost line could migrate into the habitable zone of its star, creating an ocean planet with seas hundreds of kilometers deep such as GJ 1214 b or Kepler-22b may be. Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorised to outgassing, impact degassing and ingassing. Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and

7600-514: The orbital period , causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was thought to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable; however, three-dimensional climate models in 2013 showed that the side of a red dwarf planet facing the host star could have extensive cloud cover, increasing its bond albedo and reducing significantly temperature differences between

7752-409: The solar wind make it impossible for these bodies to sustain liquid water on their surface. Despite this, studies are strongly suggestive of past liquid water on the surface of Venus, Mars, Vesta and Ceres, suggesting a more common phenomenon than previously thought. Since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from

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7904-400: The 16th and 17th centuries. With the development of the telescope , the meaning of planet broadened to include objects only visible with assistance: the moons of the planets beyond Earth; the ice giants Uranus and Neptune; Ceres and other bodies later recognized to be part of the asteroid belt ; and Pluto , later found to be the largest member of the collection of icy bodies known as

8056-408: The HZ concept is more complex. Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain spectral types . Binary systems, for example, have circumstellar habitable zones that differ from those of single-star planetary systems, in addition to the orbital stability concerns inherent with a three-body configuration. If

8208-470: The HZ might outnumber planets. In subsequent decades, the HZ concept began to be challenged as a primary criterion for life, so the concept is still evolving. Since the discovery of evidence for extraterrestrial liquid water , substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres , like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given

8360-409: The HZ, such an orbit would causes extreme seasonal effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round. Gliese 876 b , discovered in 1998, and Gliese 876 c , discovered in 2001, are both gas giants discovered in the habitable zone around Gliese 876 that may also have large moons. Another gas giant, Upsilon Andromedae d

8512-455: The Milky Way. However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered. Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way. NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised

8664-610: The Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. After the fall of the Western Roman Empire , astronomy developed further in India and the medieval Islamic world. In 499 CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of

8816-588: The Moon. The smallest object in the Solar System generally agreed to be a geophysical planet is Saturn's moon Mimas, with a radius about 3.1% of Earth's and a mass about 0.00063% of Earth's. Saturn's smaller moon Phoebe , currently an irregular body of 1.7% Earth's radius and 0.00014% Earth's mass, is thought to have attained hydrostatic equilibrium and differentiation early in its history before being battered out of shape by impacts. Some asteroids may be fragments of protoplanets that began to accrete and differentiate, but suffered catastrophic collisions, leaving only

8968-452: The Solar System is to the Sun. Mercury, the closest planet to the Sun at 0.4  AU , takes 88 days for an orbit, but ultra-short period planets can orbit in less than a day. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. There are hot Jupiters , such as 51 Pegasi b, that orbit very close to their star and may evaporate to become chthonian planets , which are

9120-404: The Solar System planets except Mercury have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. Saturn's largest moon Titan also has a substantial atmosphere thicker than that of Earth; Neptune's largest moon Triton and the dwarf planet Pluto have more tenuous atmospheres. The larger giant planets are massive enough to keep large amounts of

9272-410: The Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU. With regard to spectral types, Zoltán Balog proposes that O-type stars cannot form planets due to the photoevaporation caused by their strong ultraviolet emissions. Studying ultraviolet emissions, Andrea Buccino found that only 40% of stars studied (including

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9424-473: The Solar System would extend out as far as 2.4 AU in that case. Similar increases in the size of the habitable zone were computed for other stellar systems. An earlier study by Ray Pierrehumbert and Eric Gaidos had eliminated the CO 2 -H 2 O concept entirely, arguing that young planets could accrete many tens to hundreds of bars of hydrogen from the protoplanetary disc, providing enough of a greenhouse effect to extend

9576-491: The Solar System, but in multitudes of other extrasolar systems. The consensus as to what counts as a planet, as opposed to other objects, has changed several times. It previously encompassed asteroids , moons , and dwarf planets like Pluto , and there continues to be some disagreement today. The five classical planets of the Solar System , being visible to the naked eye, have been known since ancient times and have had

9728-534: The Solar System, whereas others are commonly observed in exoplanets. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates : counter-clockwise as seen from above the Sun's north pole. At least one exoplanet, WASP-17b , has been found to orbit in the opposite direction to its star's rotation. The period of one revolution of a planet's orbit is known as its sidereal period or year . A planet's year depends on its distance from its star;

9880-454: The Sun becomes a red giant. Nevertheless, life need not originate during this stage of stellar evolution for it to be detected. Once the star becomes a red giant, and the habitable zone extends outward, the icy surface would melt, forming a temporary atmosphere that can be searched for signs of life that may have been thriving before the start of the red giant stage. A planet's atmospheric conditions influence its ability to retain heat so that

10032-469: The Sun) had overlapping liquid water and ultraviolet habitable zones. Stars smaller than the Sun, on the other hand, have distinct impediments to habitability. For example, Michael Hart proposed that only main-sequence stars of spectral class K0 or brighter could offer habitable zones, an idea which has evolved in modern times into the concept of a tidal locking radius for red dwarfs . Within this radius, which

10184-480: The atmospheric dynamics that affect the day-night temperature difference are complex. One important characteristic of the planets is their intrinsic magnetic moments , which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change

10336-408: The background of the stars—namely, Mercury, Venus, Mars, Jupiter, and Saturn. Planets have historically had religious associations: multiple cultures identified celestial bodies with gods, and these connections with mythology and folklore persist in the schemes for naming newly discovered Solar System bodies. Earth itself was recognized as a planet when heliocentrism supplanted geocentrism during

10488-404: The best candidates for being potentially habitable. These are at a distance of 990, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is closest in size to Earth with 1.2 times Earth's radius, and it is located towards the outer edge of the habitable zone around its red dwarf star. Among nearest terrestrial exoplanet candidates , Tau Ceti e is 11.9 light-years away. It is in

10640-459: The case of the Sun would range from 7 to 22 AU. At such stage, Saturn's moon Titan would likely be habitable in Earth's temperature sense. Given that this new equilibrium lasts for about 1 Gyr , and because life on Earth emerged by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants. However, around such

10792-791: The children's fairy tale of " Goldilocks and the Three Bears ", in which a little girl chooses from sets of three items, rejecting the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right". Since the concept was first presented in 1953, many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets. Most such planets, being either super-Earths or gas giants , are more massive than Earth, because massive planets are easier to detect . On November 4, 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sized planets orbiting in

10944-581: The circumstellar habitable zone of Tau Ceti , a Sun-like star 12 light years away. Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone; however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study. It is now considered as uninhabitable. Recent discoveries have uncovered planets that are thought to be similar in size or mass to Earth. "Earth-sized" ranges are typically defined by mass. The lower range used in many definitions of

11096-559: The collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk . Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets . After

11248-406: The concept be extended to other solvents, including dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, among others, which would support hypothetical life forms that use an alternative biochemistry . In 2013, further developments in habitable zone concepts were made with the proposal of a circum planetary habitable zone, also known as the "habitable edge", to encompass the region around

11400-406: The definition, regarding where exactly to draw the line between a planet and a star. Multiple exoplanets have been found to orbit in the habitable zones of their stars (where liquid water can potentially exist on a planetary surface ), but Earth remains the only planet known to support life . It is not known with certainty how planets are formed. The prevailing theory is that they coalesce during

11552-423: The development of life. Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size. For example, the Sun is expected to engulf the previously habitable Earth as a red giant. However, once a red giant star reaches the horizontal branch , it achieves a new equilibrium and can sustain a new circumstellar habitable zone, which in

11704-464: The development of life. Red dwarf stars, on the other hand, which can live for hundreds of billions of years on the main sequence, would have planets with ample time for life to develop and evolve. Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther out; our Sun, for example, was 75% as bright in the Archaean as it is now, and in

11856-434: The effect that a repositioned orbit would have on the habitability of Earth or Venus as their surface gravity allows sufficient atmosphere to be retained for several billion years. According to the extended habitable zone concept, planetary-mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain

12008-473: The eight planets in the Solar System, only Venus and Mars lack such a magnetic field. Of the magnetized planets, the magnetic field of Mercury is the weakest and is barely able to deflect the solar wind . Jupiter's moon Ganymede has a magnetic field several times stronger, and Jupiter's is the strongest in the Solar System (so intense in fact that it poses a serious health risk to future crewed missions to all its moons inward of Callisto ). The magnetic fields of

12160-418: The farther a planet is from its star, the longer the distance it must travel and the slower its speed, since it is less affected by its star's gravity . No planet's orbit is perfectly circular, and hence the distance of each from the host star varies over the course of its year. The closest approach to its star is called its periastron , or perihelion in the Solar System, whereas its farthest separation from

12312-522: The formation of dynamic weather systems such as hurricanes (on Earth), planet-wide dust storms (on Mars), a greater-than-Earth-sized anticyclone on Jupiter (called the Great Red Spot ), and holes in the atmosphere (on Neptune). Weather patterns detected on exoplanets include a hot region on HD 189733 b twice the size of the Great Red Spot, as well as clouds on the hot Jupiter Kepler-7b ,

12464-401: The freezing point of water. However, their atmospheric conditions vary substantially. The aphelion of Venus, for example, touches the inner edge of the zone in most estimates and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to 462 °C (864 °F) at which water can only exist as vapor. The entire orbits of

12616-403: The future, continued increases in energy output will put Earth outside the Sun's habitable zone, even before it reaches the red giant phase. In order to deal with this increase in luminosity, the concept of a continuously habitable zone has been introduced. As the name suggests, the continuously habitable zone is a region around a star in which planetary-mass bodies can sustain liquid water for

12768-626: The giant planets have numerous moons in complex planetary-type systems. Except for Ceres and Sedna, all the consensus dwarf planets are known to have at least one moon as well. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa and Enceladus). The four giant planets are orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny ' moonlets ' whose gravity shapes and maintains their structure. Although

12920-467: The giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel and mantles of silicates . Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen . Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water, ammonia , methane , and other ices . The fluid action within these planets' cores creates

13072-448: The gradual accumulation of material driven by gravity , a process called accretion . The word planet comes from the Greek πλανήται ([[[wikt:πλανήτης|planḗtai]]] Error: {{Transliteration}}: transliteration text not Latin script (pos 8) ( help ) ) ' wanderers ' . In antiquity , this word referred to the Sun , Moon , and five points of light visible to the naked eye that moved across

13224-415: The gravitational instabilities of those systems. The concept of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man , in which he discussed the concept of the circumstellar habitable zone as well as various other determinants of planetary habitability, eventually estimating the number of habitable planets in the Milky Way to be about 600 million. At

13376-417: The grounds that the internal physics of objects does not change between approximately one Saturn mass (beginning of significant self-compression) and the onset of hydrogen burning and becoming a red dwarf star. Beyond roughly 13 M J (at least for objects with solar-type isotopic abundance ), an object achieves conditions suitable for nuclear fusion of deuterium : this has sometimes been advocated as

13528-530: The habitable zones of Sun-like stars and red dwarfs in the Milky Way . About 11 billion of these may be orbiting Sun-like stars. Proxima Centauri b , located about 4.2 light-years (1.3 parsecs ) from Earth in the constellation of Centaurus , is the nearest known exoplanet, and is orbiting in the habitable zone of its star. The HZ is also of particular interest to the emerging field of habitability of natural satellites , because planetary-mass moons in

13680-411: The host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets which can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth can remain habitable, complemented by

13832-404: The idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces in spite of tidal locking. Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration. At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs . Also,

13984-488: The inner edge of its planetary system's habitable zone, giving it an estimated average surface temperature of 68 °C (154 °F). Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zone tend to reflect the availability of scientific data. A 2013 study by Ravi Kumar Kopparapu put η e , the fraction of stars with planets in the HZ, at 0.48, meaning that there may be roughly 95–180 billion habitable planets in

14136-416: The interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet. Of

14288-728: The large amount of liquid water known to exist in lithospheres and asthenospheres of the Solar System. Sustained by other energy sources, such as tidal heating or radioactive decay or pressurized by non-atmospheric means, liquid water may be found even on rogue planets , or their moons. Liquid water can also exist at a wider range of temperatures and pressures as a solution , for example with sodium chlorides in seawater on Earth, chlorides and sulphates on equatorial Mars , or ammoniates, due to its different colligative properties . In addition, other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at

14440-520: The leftover cores. There are also exoplanets that are much farther from their star. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are thousands of AU from their star and take more than a million years to orbit (e.g. COCONUTS-2b ). Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in

14592-447: The light gases hydrogen and helium, whereas the smaller planets lose these gases into space . Analysis of exoplanets suggests that the threshold for being able to hold on to these light gases occurs at about 2.0 +0.7 −0.6 M E , so that Earth and Venus are near the maximum size for rocky planets. The composition of Earth's atmosphere is different from the other planets because the various life processes that have transpired on

14744-501: The likelihood that a star will have planets. Hence, a metal-rich population I star is more likely to have a substantial planetary system than a metal-poor, population II star . According to the IAU definition , there are eight planets in the Solar System, which are (in increasing distance from the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, at 318 Earth masses , whereas Mercury

14896-426: The location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect , meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so

15048-480: The magnetosphere of an orbiting hot Jupiter. Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies. This is common in satellite systems (e.g. the resonance between Io, Europa , and Ganymede around Jupiter, or between Enceladus and Dione around Saturn). All except Mercury and Venus have natural satellites , often called "moons". Earth has one, Mars has two, and

15200-411: The mitigation of atmospheric escape . In a 2013 study led by Italian astronomer Giovanni Vladilo , it was shown that the size of the circumstellar habitable zone increased with greater atmospheric pressure. Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained because even a small shift in pressure or temperature could render water unable to form as

15352-418: The most conservative estimates, only Earth lies within the zone; in the most permissive estimates, even Saturn at perihelion, or Mercury at aphelion, might be included. Astronomers use stellar flux and the inverse-square law to extrapolate circumstellar habitable zone models created for the Solar System to other stars. For example, according to Kopparapu's habitable zone estimate, although the Solar System has

15504-582: The most massive. There are at least nineteen planetary-mass moons or satellite planets—moons large enough to take on ellipsoidal shapes: The Moon, Io, and Europa have compositions similar to the terrestrial planets; the others are made of ice and rock like the dwarf planets, with Tethys being made of almost pure ice. Europa is often considered an icy planet, though, because its surface ice layer makes it difficult to study its interior. Ganymede and Titan are larger than Mercury by radius, and Callisto almost equals it, but all three are much less massive. Mimas

15656-421: The most restrictive definition of the term: the terrestrial planets Mercury , Venus , Earth , and Mars , and the giant planets Jupiter , Saturn , Uranus , and Neptune . The best available theory of planet formation is the nebular hypothesis , which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk . Planets grow in this disk by

15808-537: The most stable state. Proper metallicity and size are also critical to low luminosity variation. Using data collected by NASA's Kepler space telescope and the W. M. Keck Observatory , scientists have estimated that 22% of solar-type stars in the Milky Way galaxy have Earth-sized planets in their habitable zone. On 7 January 2013, astronomers from the Kepler team announced the discovery of Kepler-69c (formerly KOI-172.02 ), an Earth-size exoplanet candidate (1.7 times

15960-435: The net temperature effect of such atmospheres including induced albedo , anti-greenhouse or other possible heat sources. For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone is: Mercury, 0.39 AU; Venus, 0.72 AU; Earth, 1.00 AU; Mars, 1.52 AU; Vesta, 2.36 AU; Ceres and Pallas, 2.77 AU; Jupiter, 5.20 AU; Saturn, 9.58 AU. In

16112-485: The number of intelligent, communicating civilizations that exist in the Milky Way. There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes , which have masses between that of Earth and Neptune. Objects less than about twice the mass of Earth are expected to be rocky like Earth; beyond that, they become a mixture of volatiles and gas like Neptune. The planet Gliese 581c , with

16264-415: The number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class F , G , and K are expected to have planets in their HZs. The first discoveries of extrasolar planets in the HZ occurred just a few years after the first extrasolar planets were discovered. However, these early detections were all gas giant-sized, and many were in eccentric orbits. Despite this, studies indicate

16416-414: The object collapses into a sphere. Mass is the prime attribute by which planets are distinguished from stars. No objects between the masses of the Sun and Jupiter exist in the Solar System, but there are exoplanets of this size. The lower stellar mass limit is estimated to be around 75 to 80 times that of Jupiter ( M J ). Some authors advocate that this be used as the upper limit for planethood, on

16568-401: The origins of planetary rings are not precisely known, they are believed to be the result of natural satellites that fell below their parent planets' Roche limits and were torn apart by tidal forces . The dwarf planets Haumea and Quaoar also have rings. No secondary characteristics have been observed around exoplanets. The sub-brown dwarf Cha 110913−773444 , which has been described as

16720-486: The other giant planets, measured at their surfaces, are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative to the planets' rotational axes and displaced from the planets' centres. In 2003, a team of astronomers in Hawaii observing the star HD 179949 detected a bright spot on its surface, apparently created by

16872-715: The other in perpetual night. Mercury and Venus, the closest planets to the Sun, similarly exhibit very slow rotation: Mercury is tidally locked into a 3:2 spin–orbit resonance (rotating three times for every two revolutions around the Sun), and Venus's rotation may be in equilibrium between tidal forces slowing it down and atmospheric tides created by solar heating speeding it up. All the large moons are tidally locked to their parent planets; Pluto and Charon are tidally locked to each other, as are Eris and Dysnomia, and probably Orcus and its moon Vanth . The other dwarf planets with known rotation periods rotate faster than Earth; Haumea rotates so fast that it has been distorted into

17024-422: The outer edge of desert-planet habitable zones is further out. A planet cannot have a hydrosphere —a key ingredient for the formation of carbon-based life—unless there is a source for water within its stellar system. The origin of water on Earth is still not completely understood; possible sources include the result of impacts with icy bodies, outgassing , mineralization , leakage from hydrous minerals from

17176-420: The planet have introduced free molecular oxygen . The atmospheres of Mars and Venus are both dominated by carbon dioxide , but differ drastically in density: the average surface pressure of Mars's atmosphere is less than 1% that of Earth's (too low to allow liquid water to exist), while the average surface pressure of Venus's atmosphere is about 92 times that of Earth's. It is likely that Venus's atmosphere

17328-597: The planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either continually in sunlight or continually in darkness around the time of its solstices . In the Solar System, Mercury, Venus, Ceres, and Jupiter have very small tilts; Pallas, Uranus, and Pluto have extreme ones; and Earth, Mars, Vesta, Saturn, and Neptune have moderate ones. Among exoplanets, axial tilts are not known for certain, though most hot Jupiters are believed to have

17480-572: The planets as the Babylonians. In the 6th and 5th centuries BC, the Pythagoreans appear to have developed their own independent planetary theory , which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides is said to have been the first to identify the evening star ( Hesperos ) and morning star ( Phosphoros ) as one and

17632-475: The positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century CE. So complete

17784-442: The possibility of large, Earth-like moons around these planets supporting liquid water. One of the first discoveries was 70 Virginis b , a gas giant initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold". Later study revealed temperatures analogous to Venus, ruling out any potential for liquid water. 16 Cygni Bb , also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in

17936-404: The radius of Earth) orbiting Kepler-69 , a star similar to the Sun, in the HZ and expected to offer habitable conditions. The discovery of two planets orbiting in the habitable zone of Kepler-62 , by the Kepler team was announced on April 19, 2013. The planets, named Kepler-62e and Kepler-62f , are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively. With

18088-503: The red subdwarf Kapteyn's Star , 12.8 light-years away. On 6 January 2015, NASA announced the 1000th confirmed exoplanet discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related stars : two of the three, Kepler-438b and Kepler-442b , are near-Earth-size and likely rocky ; the third, Kepler-440b , is a super-Earth . However, Kepler-438b

18240-432: The region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a galactic center that stars there are enriched with heavier elements , but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers. Subsequently, some astrobiologists propose that

18392-736: The same ( Aphrodite , Greek corresponding to Latin Venus ), though this had long been known in Mesopotamia. In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the Scientific Revolution . By the 1st century BC, during the Hellenistic period , the Greeks had begun to develop their own mathematical schemes for predicting

18544-432: The same system and thought to be a better candidate for habitability, was also announced in 2007. Its existence was later disconfirmed in 2014, but only for a short time. As of 2015, the planet has no newer disconfirmations. Gliese 581 g , yet another planet thought to have been discovered in the circumstellar habitable zone of the system, was considered to be more habitable than both Gliese 581 c and d. However, its existence

18696-416: The same time, science-fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to the general public through his various explorations of space colonization . The term " Goldilocks zone " emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase. In 1993, astronomer James Kasting introduced

18848-463: The second millennium BC. The MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year. Late Babylonian astronomy is the origin of Western astronomy and indeed all Western efforts in the exact sciences . The Enuma anu enlil , written during the Neo-Assyrian period in the 7th century BC, comprises

19000-423: The series of planets, in the rank which was necessary, and at exactly the right distance from the sun, to accomplish its secondary object of vegetation; and from this we might infer that the other planets are either too near or too remote from the sun, to vegetate." The concept of a circumstellar habitable zone was first introduced in 1913, by Edward Maunder in his book "Are The Planets Inhabited?". The concept

19152-421: The size of Neptune and smaller, down to smaller than Mercury. In 2011, the Kepler space telescope team reported the discovery of the first Earth-sized exoplanets orbiting a Sun-like star , Kepler-20e and Kepler-20f . Since that time, more than 100 planets have been identified that are approximately the same size as Earth , 20 of which orbit in the habitable zone of their star—the range of orbits where

19304-450: The size of the Moon . Analysis of gravitational microlensing data suggests a minimum average of 1.6 bound planets for every star in the Milky Way . In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12 . This discovery was confirmed and is generally considered to be the first definitive detection of exoplanets. Researchers suspect they formed from

19456-413: The smaller planetesimals (as well as radioactive decay ) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by density, with higher density materials sinking toward the core . Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from

19608-451: The solar system outer edge to 10 AU. In this case, though, the hydrogen is not continuously replenished by volcanism and is lost within millions to tens of millions of years. In the case of planets orbiting in the HZs of red dwarf stars, the extremely close distances to the stars cause tidal locking , an important factor in habitability. For a tidally locked planet, the sidereal day is as long as

19760-470: The spin axis of the planet. There is great variation in the length of day between the planets, with Venus taking 243  days to rotate, and the giant planets only a few hours. The rotational periods of exoplanets are not known, but for hot Jupiters , their proximity to their stars means that they are tidally locked (that is, their orbits are in sync with their rotations). This means, they always show one face to their stars, with one side in perpetual day,

19912-400: The star is called its apastron ( aphelion ). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy , just as a falling object on Earth accelerates as it falls. As the planet nears apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory . Each planet's orbit

20064-523: The star or each other, but over time many will collide, either to form a larger, combined protoplanet or release material for other protoplanets to absorb. Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies . The energetic impacts of

20216-524: The stars. He also theorized that the orbits of planets were elliptical . Aryabhata's followers were particularly strong in South India , where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them. Circumstellar habitable zone The habitable zone is also called the Goldilocks zone , a metaphor , allusion and antonomasia of

20368-460: The subsequent impact of comets (smaller planets will lose any atmosphere they gain through various escape mechanisms ). With the discovery and observation of planetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity —an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 ( helium )—appears to determine

20520-412: The super-Earth Gliese 1214 b , and others. Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. These planets may have vast differences in temperature between their day and night sides that produce supersonic winds, although multiple factors are involved and the details of

20672-603: The super-Earth class is 1.9 Earth masses; likewise, sub-Earths range up to the size of Venus (~0.815 Earth masses). An upper limit of 1.5 Earth radii is also considered, given that above 1.5  R 🜨 the average planet density rapidly decreases with increasing radius, indicating these planets have a significant fraction of volatiles by volume overlying a rocky core. A genuinely Earth-like planet – an Earth analog or "Earth twin" – would need to meet many conditions beyond size and mass; such properties are not observable using current technology. A solar analog (or "solar twin")

20824-498: The surface, have been proposed. An estimate of the range of distances from the Sun allowing the existence of liquid water appears in Newton's Principia (Book III, Section 1, corol. 4). The philosopher Louis Claude de Saint-Martin speculated in his 1802 work Man: His True Nature and Ministry , "... we may presume, that, being susceptible of vegetation, it [the Earth] has been placed, in

20976-505: The technology did not exist to detect moons around them, and no extrasolar moons had been discovered. Planets within the zone with the potential for solid surfaces were therefore of much higher interest. The 2007 discovery of Gliese 581c , the first super-Earth in the circumstellar habitable zone, created significant interest in the system by the scientific community, although the planet was later found to have extreme surface conditions that may resemble Venus. Gliese 581 d, another planet in

21128-550: The term planet more broadly, including dwarf planets as well as rounded satellites like the Moon. Further advances in astronomy led to the discovery of over five thousand planets outside the Solar System, termed exoplanets . These often show unusual features that the Solar System planets do not show, such as hot Jupiters —giant planets that orbit close to their parent stars, like 51 Pegasi b —and extremely eccentric orbits , such as HD 20782 b . The discovery of brown dwarfs and planets larger than Jupiter also spurred debate on

21280-470: The term "circumstellar habitable zone" to refer more precisely to the region then (and still) known as the habitable zone. Kasting was the first to present a detailed model for the habitable zone for exoplanets. An update to habitable zone concept came in 2000 when astronomers Peter Ward and Donald Brownlee introduced the idea of the " galactic habitable zone ", which they later developed with Guillermo Gonzalez . The galactic habitable zone, defined as

21432-439: The term "habitable zone" in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life. A major early contributor to the habitable zone concept, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems , given

21584-433: The terrestrial planets in composition. The gas giants , Jupiter and Saturn, are primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Saturn is one third as massive as Jupiter, at 95 Earth masses. The ice giants , Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane , and ammonia , with thick atmospheres of hydrogen and helium. They have

21736-474: The triple point for water) for 70 days in the Martian year. Despite indirect evidence in the form of seasonal flows on warm Martian slopes , no confirmation has been made of the presence of liquid water there. While other objects orbit partly within this zone, including comets, Ceres is the only one of planetary mass. A combination of low mass and an inability to mitigate evaporation and atmosphere loss against

21888-433: The two sides. Planetary mass natural satellites have the potential to be habitable as well. However, these bodies need to fulfill additional parameters, in particular being located within the circumplanetary habitable zones of their host planets. More specifically, moons need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds like Io , but must remain within

22040-489: Was also disconfirmed in 2014, and astronomers are divided about its existence. Discovered in August 2011, HD 85512 b was initially speculated to be habitable, but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone. Kepler-22 b , discovered in December 2011 by the Kepler space probe, is the first transiting exoplanet discovered around

22192-413: Was derived. In ancient Greece , China , Babylon , and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the apparently common-sense perceptions that Earth was solid and stable and that it

22344-416: Was discovered in 1999 orbiting Upsilon Andromidae's habitable zone. Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellar habitable zone and has a low orbital eccentricity, comparable to that of Mars in the Solar System. Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years, though it

22496-416: Was found in 1992 in orbit around a pulsar . Its mass is roughly half that of the planet Mercury. Even smaller is WD 1145+017 b , orbiting a white dwarf; its mass is roughly that of the dwarf planet Haumea, and it is typically termed a minor planet. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b , with a mass (and radius) that is probably slightly higher than that of

22648-606: Was later discussed in 1953 by Hubertus Strughold , who in his treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars , coined the term "ecosphere" and referred to various "zones" in which life could emerge. In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same concept in further scientific detail. Both works stressed the importance of liquid water to life. Su-Shu Huang , an American astrophysicist, first introduced

22800-569: Was not moving but at rest. The first civilization known to have a functional theory of the planets were the Babylonians , who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa , a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as

22952-508: Was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries. To the Greeks and Romans, there were seven known planets, each presumed to be circling Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names):

23104-458: Was the result of a runaway greenhouse effect in its history, which today makes it the hottest planet by surface temperature, hotter even than Mercury. Despite hostile surface conditions, temperature, and pressure at about 50–55 km altitude in Venus's atmosphere are close to Earthlike conditions (the only place in the Solar System beyond Earth where this is so), and this region has been suggested as

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