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175-618: Mercury is the first planet from the Sun and the smallest in the Solar System . In English, it is named after the ancient Roman god Mercurius ( Mercury ), god of commerce and communication, and the messenger of the gods. Mercury is classified as a terrestrial planet , with roughly the same surface gravity as Mars . The surface of Mercury is heavily cratered , as a result of countless impact events that have accumulated over billions of years. Its largest crater, Caloris Planitia , has

350-469: 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

525-406: 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

700-519: 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

875-406: A 3:2 ratio. This relationship is called spin–orbit resonance , and sidereal here means "relative to the stars". Consequently, one solar day (sunrise to sunrise) on Mercury lasts for around 176 Earth days: twice the planet's sidereal year. This means that one side of Mercury will remain in sunlight for one Mercurian year of 88 Earth days; while during the next orbit, that side will be in darkness all

1050-504: 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

1225-495: A circular orbit having the same semi-major axis . Mercury's higher velocity when it is near perihelion is clear from the greater distance it covers in each 5-day interval. In the diagram, the varying distance of Mercury to the Sun is represented by the size of the planet, which is inversely proportional to Mercury's distance from the Sun. This varying distance to the Sun leads to Mercury's surface being flexed by tidal bulges raised by

1400-420: A conduit. Scientists could not quantify the age of the volcanic complex system but reported that it could be on the order of a billion years. The surface temperature of Mercury ranges from 100 to 700 K (−173 to 427 °C; −280 to 800 °F). It never rises above 180 K at the poles, due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. At perihelion ,

1575-451: A diameter of 1,550 km (960 mi), which is about one-third the diameter of the planet (4,880 km or 3,030 mi). Similarly to the Earth 's Moon , Mercury's surface displays an expansive rupes system generated from thrust faults and bright ray systems formed by impact event remnants . Mercury's sidereal year (88.0 Earth days) and sidereal day (58.65 Earth days) are in

1750-689: 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

1925-429: A full excess turn. Similar, but much smaller, effects exist for other Solar System bodies: 8.6247 arcseconds per century for Venus, 3.8387 for Earth, 1.351 for Mars, and 10.05 for 1566 Icarus . 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

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2100-472: A general paucity of smaller craters below about 30 km (19 mi) in diameter. Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to lunar maria. Unlike lunar maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localization and rounded, lobate shape of these plains strongly support volcanic origins. All

2275-418: A high density, its core must be large and rich in iron. The radius of Mercury's core is estimated to be 2,020 ± 30 km (1,255 ± 19 mi), based on interior models constrained to be consistent with a moment of inertia factor of 0.346 ± 0.014 . Hence, Mercury's core occupies about 57% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has

2450-561: A layer of regolith that inhibits sublimation . By comparison, the Antarctic ice sheet on Earth has a mass of about 4 × 10 kg, and Mars's south polar cap contains about 10 kg of water. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet's interior and deposition by impacts of comets. Mercury is too small and hot for its gravity to retain any significant atmosphere over long periods of time; it does have

2625-528: A layered, chemically heterogeneous crust with large-scale variations in chemical composition observed on the surface. The crust is low in iron but high in sulfur, resulting from the stronger early chemically reducing conditions than is found on other terrestrial planets. The surface is dominated by iron-poor pyroxene and olivine , as represented by enstatite and forsterite , respectively, along with sodium-rich plagioclase and minerals of mixed magnesium, calcium, and iron-sulfide. The less reflective regions of

2800-442: 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

2975-411: 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

3150-479: 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

3325-409: A model of the planet's structure. Where there have been landers or multiple orbiting spacecraft, these models are constrained by seismological data and also moment of inertia data derived from the spacecraft's orbits. Where such data is not available, uncertainties are inevitably higher. The uncompressed densities of the rounded terrestrial bodies directly orbiting the Sun trend towards lower values as

3500-457: A molten core. The mantle-crust layer is in total 420 km (260 mi) thick. Projections differ as to the size of the crust specifically; data from the Mariner 10 and MESSENGER probes suggests a thickness of 35 km (22 mi), whereas an Airy isostacy model suggests a thickness of 26 ± 11 km (16.2 ± 6.8 mi). One distinctive feature of Mercury's surface

3675-537: A much smaller iron core. Another Jovian moon Europa has a similar density but has a significant ice layer on the surface: for this reason, it is sometimes considered an icy planet instead. Terrestrial planets can have surface structures such as canyons , craters , mountains , volcanoes , and others, depending on the presence at any time of an erosive liquid or tectonic activity or both. Terrestrial planets have secondary atmospheres , generated by volcanic out-gassing or from comet impact debris. This contrasts with

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3850-585: 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

4025-402: 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

4200-426: 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

4375-484: A possibly separate subsequent episode called the Late Heavy Bombardment that ended 3.8 billion years ago. Mercury received impacts over its entire surface during this period of intense crater formation, facilitated by the lack of any atmosphere to slow impactors down. During this time Mercury was volcanically active; basins were filled by magma , producing smooth plains similar to the maria found on

4550-492: A process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies . The energetic impacts of 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

4725-549: A prolonged interval. A "rimless depression" inside the southwest rim of the Caloris Basin consists of at least nine overlapping volcanic vents, each individually up to 8 km (5.0 mi) in diameter. It is thus a " compound volcano ". The vent floors are at least 1 km (0.62 mi) below their brinks and they bear a closer resemblance to volcanic craters sculpted by explosive eruptions or modified by collapse into void spaces created by magma withdrawal back down into

4900-410: 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

5075-428: A series of smaller "corpuscules") might exist in an orbit even closer to the Sun than that of Mercury, to account for this perturbation. Other explanations considered included a slight oblateness of the Sun. The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place faith in this possible explanation, and the hypothetical planet was named Vulcan , but no such planet

5250-454: 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"

5425-403: 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

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5600-607: A solid surface, but are composed of ice and rock rather than of rock and metal. These include the dwarf planets, such as Ceres , Pluto and Eris , which are found today only in the regions beyond the formation snow line where water ice was stable under direct sunlight in the early Solar System. It also includes the other round moons, which are ice-rock (e.g. Ganymede , Callisto , Titan , and Triton ) or even almost pure (at least 99%) ice ( Tethys and Iapetus ). Some of these bodies are known to have subsurface hydrospheres (Ganymede, Callisto, Enceladus , and Titan), like Europa, and it

5775-483: A solid, metallic outer core layer, a deeper liquid core layer, and a solid inner core. The composition of the iron-rich core remains uncertain, but it likely contains nickel, silicon and perhaps sulfur and carbon, plus trace amounts of other elements. The planet's density is the second highest in the Solar System at 5.427 g/cm, only slightly less than Earth's density of 5.515 g/cm. If the effect of gravitational compression were to be factored out from both planets,

5950-405: A tenuous surface-bounded exosphere at a surface pressure of less than approximately 0.5 nPa (0.005 picobars). It includes hydrogen , helium , oxygen , sodium , calcium , potassium , magnesium , silicon , and hydroxide , among others. This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen atoms and helium atoms probably come from

6125-471: 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

6300-497: A terrestrial planet is the average density its materials would have at zero pressure . A greater uncompressed density indicates a greater metal content. Uncompressed density differs from the true average density (also often called "bulk" density) because compression within planet cores increases their density; the average density depends on planet size, temperature distribution, and material stiffness as well as composition. Calculations to estimate uncompressed density inherently require

6475-480: A third of the radius of the planet. These twisted magnetic flux tubes, technically known as flux transfer events , form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface via magnetic reconnection . This also occurs in Earth's magnetic field. The MESSENGER observations showed the reconnection rate was ten times higher at Mercury, but its proximity to

6650-438: 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

6825-422: Is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometres (1,516.0 mi). Mercury is also smaller —albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan . Mercury consists of approximately 70% metallic and 30% silicate material. Mercury appears to have a solid silicate crust and mantle overlying

7000-585: Is also possible for some others (e.g. Ceres, Mimas , Dione , Miranda , Ariel , Triton, and Pluto). Titan even has surface bodies of liquid, albeit liquid methane rather than water. Jupiter's Ganymede, though icy, does have a metallic core like the Moon, Io, Europa, and the terrestrial planets. The name Terran world has been suggested to define all solid worlds (bodies assuming a rounded shape), without regard to their composition. It would thus include both terrestrial and icy planets. The uncompressed density of

7175-561: 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

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7350-509: Is highly homogeneous, which suggests that Mercury had a magma ocean early in its history, like the Moon. According to current models , Mercury may have a solid silicate crust and mantle overlying a solid outer core, a deeper liquid core layer, and a solid inner core. There are many competing hypotheses about Mercury's origins and development, some of which incorporate collision with planetesimals and rock vaporization. Historically, humans knew Mercury by different names depending on whether it

7525-468: Is inclined by 7 degrees to the plane of Earth's orbit (the ecliptic ), the largest of all eight known solar planets. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between Earth and the Sun, which is in May or November. This occurs about every seven years on average. Mercury's axial tilt is almost zero, with

7700-458: 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

7875-422: Is slowly declining: The next approach to within 82,100,000 km (51 million mi) is in 2679, and to within 82,000,000 km (51 million mi) in 4487, but it will not be closer to Earth than 80,000,000 km (50 million mi) until 28,622. Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of an inferior conjunction. This large range arises from

8050-435: Is stabilized by the variance of the tidal force along Mercury's eccentric orbit, acting on a permanent dipole component of Mercury's mass distribution. In a circular orbit there is no such variance, so the only resonance stabilized in such an orbit is at 1:1 (e.g., Earth–Moon), when the tidal force, stretching a body along the "center-body" line, exerts a torque that aligns the body's axis of least inertia (the "longest" axis, and

8225-425: Is that Mercury originally had a metal–silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass. Early in the Solar System's history, Mercury may have been struck by a planetesimal of approximately 1 ⁄ 6 Mercury's mass and several thousand kilometers across. The impact would have stripped away much of

8400-470: 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

8575-437: 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

8750-595: Is the largest terrestrial planet. Giant planets are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune. They differ from 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

8925-416: Is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified. Mercury's core has a higher iron content than that of any other planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory

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9100-500: 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

9275-733: Is unknown whether extrasolar terrestrial planets in general will follow such a trend. The data in the tables below are mostly taken from a list of gravitationally rounded objects of the Solar System and planetary-mass moon . All distances from the Sun are averages. Most of the planets discovered outside the Solar System are giant planets, because they are more easily detectable. But since 2005, hundreds of potentially terrestrial extrasolar planets have also been found, with several being confirmed as terrestrial. Most of these are super-Earths , i.e. planets with masses between Earth's and Neptune's; super-Earths may be gas planets or terrestrial, depending on their mass and other parameters. During

9450-457: 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

9625-464: The Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable. It is likely that this magnetic field is generated by a dynamo effect, in a manner similar to the magnetic field of Earth. This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal heating effects caused by the planet's high orbital eccentricity would serve to keep part of

9800-680: The Milky Way galaxy . The following exoplanets have a density of at least 5 g/cm and a mass below Neptune's and are thus very likely terrestrial: Kepler-10b , Kepler-20b , Kepler-36b , Kepler-48d , Kepler 68c , Kepler-78b , Kepler-89b , Kepler-93b , Kepler-97b , Kepler-99b , Kepler-100b , Kepler-101c , Kepler-102b , Kepler-102d , Kepler-113b , Kepler-131b , Kepler-131c , Kepler-138c , Kepler-406b , Kepler-406c , Kepler-409b . In 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth- and super-Earth-sized planets orbiting in

9975-520: The Solar System have the same basic structure, such as a central metallic core (mostly iron ) with a surrounding silicate mantle . The large rocky asteroid 4 Vesta has a similar structure; possibly so does the smaller one 21 Lutetia . Another rocky asteroid 2 Pallas is about the same size as Vesta, but is significantly less dense; it appears to have never differentiated a core and a mantle. The Earth's Moon and Jupiter's moon Io have similar structures to terrestrial planets, but Earth's Moon has

10150-451: The Sun that are about 17 times stronger than the Moon's on Earth. Combined with a 3:2 spin–orbit resonance of the planet's rotation around its axis, it also results in complex variations of the surface temperature. The resonance makes a single solar day (the length between two meridian transits of the Sun) on Mercury last exactly two Mercury years, or about 176 Earth days. Mercury's orbit

10325-480: The VLA in the early 1990s revealed that there are patches of high radar reflection near the poles. Although ice was not the only possible cause of these reflective regions, astronomers thought it to be the most likely explanation. The presence of water ice was confirmed using MESSENGER images of craters at the north pole. The icy crater regions are estimated to contain about 10–10 kg of ice, and may be covered by

10500-458: The antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around Mercury, converging at the basin's antipode (180 degrees away). The resulting high stresses fractured the surface. Alternatively, it has been suggested that this terrain formed as a result of

10675-528: 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

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10850-497: The asteroid belt ; and Pluto , later found to be the largest member of the collection of icy bodies known as 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

11025-454: 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

11200-607: The habitable zones of Sun-like stars and red dwarfs within the Milky Way . Eleven billion of these estimated planets may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. However, this does not give estimates for the number of extrasolar terrestrial planets, because there are planets as small as Earth that have been shown to be gas planets (see Kepler-138d ). Estimates show that about 80% of potentially habitable worlds are covered by land, and about 20% are ocean planets. Planets with rations more like those of Earth, which

11375-484: 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 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

11550-484: The outer , giant planets , whose atmospheres are primary; primary atmospheres were captured directly from the original solar nebula . The Solar System has four terrestrial planets under the dynamical definition: Mercury , Venus , Earth and Mars . The Earth's Moon as well as Jupiter's moons Io and Europa would also count geophysically, as well as perhaps the large protoplanet-asteroids Pallas and Vesta (though those are borderline cases). Among these bodies, only

11725-486: The solar wind . A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting , which meant that lighter particles were lost from the accreting material and not gathered by Mercury. Each hypothesis predicts a different surface composition, and two space missions have been tasked with making observations of this composition. The first MESSENGER , which ended in 2015, found higher-than-expected potassium and sulfur levels on

11900-545: The transit method. In the same year, the Kepler space telescope mission team released a list of 1235 extrasolar planet candidates , including six that are "Earth-size" or "super-Earth-size" (i.e. they have a radius less than twice that of the Earth) and in the habitable zone of their star. Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range (see image). In 2016, statistical modeling of

12075-400: The 1980s–1990s, and are thought to result primarily from the vaporization of surface rock struck by micrometeorite impacts including presently from Comet Encke . In 2008, magnesium was discovered by MESSENGER . Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere and

12250-427: The Caloris Basin was so powerful that it caused lava eruptions and left a concentric mountainous ring ~2 km (1.2 mi) tall surrounding the impact crater . The floor of the Caloris Basin is filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they were volcanic lava flows induced by the impact or a large sheet of impact melt. At

12425-403: The Earth has an active surface hydrosphere . Europa is believed to have an active hydrosphere under its ice layer. During the formation of the Solar System, there were many terrestrial planetesimals and proto-planets , but most merged with or were ejected by the four terrestrial planets, leaving only Pallas and Vesta to survive more or less intact. These two were likely both dwarf planets in

12600-517: The Greek Hermes, because it moves across the sky faster than any other planet, though some associated the planet with Apollo instead, as detailed by Pliny the Elder . The astronomical symbol for Mercury is a stylized version of Hermes' caduceus ; a Christian cross was added in the 16th century: [REDACTED] . Mercury is one of four terrestrial planets in the Solar System , which means it

12775-580: 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

12950-438: The Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including solar wind and micrometeorite impacts. There are two geologically distinct plains regions on Mercury. Gently rolling, hilly plains in the regions between craters are Mercury's oldest visible surfaces, predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show

13125-588: 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

13300-521: The Moon. One of the most unusual craters is Apollodorus , or "the Spider", which hosts a series of radiating troughs extending outwards from its impact site. Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that

13475-452: 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

13650-404: 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

13825-408: 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 the definition, regarding where exactly to draw the line between a planet and a star. Multiple exoplanets have been found to orbit in

14000-437: 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

14175-489: 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;

14350-487: The Sun is at an angle of about 25 degrees past noon due to diurnal temperature lag , at 0.4 Mercury days and 0.8 Mercury years past sunrise. Conversely, there are two other points on the equator, 90 degrees of longitude apart from the first ones, where the Sun passes overhead only when the planet is at aphelion in alternate years, when the apparent motion of the Sun in Mercury's sky is relatively rapid. These points, which are

14525-452: The Sun only accounts for about a third of the reconnection rate observed by MESSENGER . Mercury has the most eccentric orbit of all the planets in the Solar System; its eccentricity is 0.21 with its distance from the Sun ranging from 46,000,000 to 70,000,000 km (29,000,000 to 43,000,000 mi). It takes 87.969 Earth days to complete an orbit. The diagram illustrates the effects of the eccentricity, showing Mercury's orbit overlaid with

14700-410: The Sun peek up a little more than two-thirds of the way over the horizon, then reverse and set before rising again, all within the same Mercurian day . This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity equals its angular rotational velocity so that the Sun's apparent motion ceases; closer to perihelion, Mercury's angular orbital velocity then exceeds

14875-463: The Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, at 318 Earth masses , whereas Mercury 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

15050-549: The Sun, collide with Venus, be ejected from the Solar System, or even disrupt the rest of the inner Solar System. In 1859, the French mathematician and astronomer Urbain Le Verrier reported that the slow precession of Mercury's orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. He suggested, among possible explanations, that another planet (or perhaps instead

15225-412: The abundance of chemical elements with an atomic number greater than 2 ( helium )—appears to determine 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

15400-408: The angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes. A similar effect would have occurred if Mercury had been in synchronous rotation: the alternating gain and loss of rotation over a revolution would have caused a libration of 23.65° in longitude. For

15575-460: The area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity. According to International Astronomical Union rules, each new crater must be named after an artist who was famous for more than fifty years, and dead for more than three years, before the date the crater is named. The largest known crater is Caloris Planitia , or Caloris Basin, with a diameter of 1,550 km (960 mi). The impact that created

15750-480: 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

15925-409: The axis of the aforementioned dipole) to always point at the center. However, with noticeable eccentricity, like that of Mercury's orbit, the tidal force has a maximum at perihelion and therefore stabilizes resonances, like 3:2, ensuring that the planet points its axis of least inertia roughly at the Sun when passing through perihelion. The original reason astronomers thought it was synchronously locked

16100-501: The best measured value as low as 0.027 degrees. This is significantly smaller than that of Jupiter , which has the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury's poles, the center of the Sun never rises more than 2.1 arcminutes above the horizon. By comparison, the angular size of the Sun as seen from Mercury ranges from 1 + 1 ⁄ 4 to 2 degrees across. At certain points on Mercury's surface, an observer would be able to see

16275-433: 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 a similar way; however, Triton was likely captured by Neptune, and Earth's Moon and Pluto's Charon might have formed in collisions. When

16450-502: The catalog of known exoplanets has increased significantly, and there have been several published refinements of the mass-radius model. As of 2024, the expected transition point between rocky and intermediate-mass planets sits at roughly 4.4 earth masses, and roughly 1.6 earth radii. In September 2020, astronomers using microlensing techniques reported the detection , for the first time, of an Earth-mass rogue planet (named OGLE-2016-BLG-1928 ) unbounded by any star, and free-floating in

16625-430: The convergence of ejecta at this basin's antipode. Overall, 46 impact basins have been identified. A notable basin is the 400 km (250 mi)-wide, multi-ring Tolstoj Basin that has an ejecta blanket extending up to 500 km (310 mi) from its rim and a floor that has been filled by smooth plains materials. Beethoven Basin has a similar-sized ejecta blanket and a 625 km (388 mi)-diameter rim. Like

16800-445: The core in the liquid state necessary for this dynamo effect. Mercury's magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within Earth, is strong enough to trap solar wind plasma . This contributes to the space weathering of the planet's surface. Observations taken by the Mariner 10 spacecraft detected this low energy plasma in

16975-438: The craters. Above the planet's surface is an extremely tenuous exosphere and a faint magnetic field that is strong enough to deflect solar winds . Mercury has no natural satellite . As of the early 2020s, many broad details of Mercury's geological history are still under investigation or pending data from space probes. Like other planets in the Solar System, Mercury was formed approximately 4.5 billion years ago. Its mantle

17150-453: The crust are high in carbon, most likely in the form of graphite. Names for features on Mercury come from a variety of sources and are set according to the IAU planetary nomenclature system. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to

17325-471: The data is needed. Mercury's surface is similar in appearance to that of the Moon, showing extensive mare -like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. It is more heterogeneous than the surface of Mars or the Moon, both of which contain significant stretches of similar geology, such as maria and plateaus. Albedo features are areas of markedly different reflectivity, which include impact craters,

17500-475: 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 a planet reaches a mass somewhat larger than Mars's mass, it begins to accumulate an extended atmosphere , greatly increasing

17675-475: The distance from the Sun increases, consistent with the temperature gradient that would have existed within the primordial solar nebula. The Galilean satellites show a similar trend going outwards from Jupiter; however, no such trend is observable for the icy satellites of Saturn or Uranus. The icy worlds typically have densities less than 2 g·cm . Eris is significantly denser ( 2.43 ± 0.05 g·cm ), and may be mostly rocky with some surface ice, like Europa. It

17850-453: The early 1990s, the first extrasolar planets were discovered orbiting the pulsar PSR B1257+12 , with masses of 0.02, 4.3, and 3.9 times that of Earth, by pulsar timing . When 51 Pegasi b , the first planet found around a star still undergoing fusion , was discovered, many astronomers assumed it to be a gigantic terrestrial, because it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did. It

18025-438: The early 20th century, Albert Einstein 's general theory of relativity provided the explanation for the observed precession, by formalizing gravitation as being mediated by the curvature of spacetime. The effect is small: just 42.980 ± 0.001 arcseconds per century (or 0.43 arcsecond per year, or 0.1035 arcsecond per orbital period) for Mercury; it therefore requires a little over 12.5 million orbits, or 3 million years, for

18200-473: 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

18375-422: The equatorial subsolar point is located at latitude 0°W or 180°W, and it climbs to a temperature of about 700 K . During aphelion , this occurs at 90° or 270°W and reaches only 550 K . On the dark side of the planet, temperatures average 110 K . The intensity of sunlight on Mercury's surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m). Although daylight temperatures at

18550-418: 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

18725-418: The features has suggested a total shrinkage of Mercury's radius in the range of ~1–7 km (0.62–4.35 mi). Most activity along the major thrust systems probably ended about 3.6–3.7 billion years ago. Small-scale thrust fault scarps have been found, tens of meters in height and with lengths in the range of a few kilometers, that appear to be less than 50 million years old, indicating that compression of

18900-536: 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 ,

19075-451: 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 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

19250-626: 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

19425-467: 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

19600-609: The gradual accumulation of material driven by gravity , a process called accretion . The word planet comes from the Greek πλανήται ( planḗtai ) ' wanderers ' . In antiquity , this word referred to the Sun , Moon , and five points of light visible to the naked eye that moved across 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

19775-417: 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

19950-475: The habitable zone of the star, so they could potentially be habitable, with Earth-like temperatures. Another possibly terrestrial planet, HD 85512 b , was discovered in 2011; it has at least 3.6 times the mass of Earth. The radius and composition of all these planets are unknown. The first confirmed terrestrial exoplanet , Kepler-10b , was found in 2011 by the Kepler space telescope , specifically designed to discover Earth-size planets around other stars using

20125-416: 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

20300-422: The interior and consequent surface geological activity continue to the present. There is evidence for pyroclastic flows on Mercury from low-profile shield volcanoes . Fifty-one pyroclastic deposits have been identified, where 90% of them are found within impact craters. A study of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over

20475-520: 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

20650-447: 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

20825-424: The lost gases can be replaced by outgassing from the mantle and from 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

21000-480: 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

21175-479: The magnetosphere of the planet's nightside. Bursts of energetic particles in the planet's magnetotail indicate a dynamic quality to the planet's magnetosphere. During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury's magnetic field can be extremely "leaky". The spacecraft encountered magnetic "tornadoes"—twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space—that were up to 800 km wide or

21350-496: The mass of Earth and orbits a star about 21,000 light-years away in the constellation Scorpius. From 2007 to 2010, three (possibly four) potential terrestrial planets were found orbiting within the Gliese 581 planetary system . The smallest, Gliese 581e , is only about 1.9 Earth masses, but orbits very close to the star. Two others, Gliese 581c and the disputed Gliese 581d , are more-massive super-Earths orbiting in or close to

21525-432: The materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm versus Earth's 4.4 g/cm. Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression, particularly at the core , Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such

21700-582: 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

21875-421: 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

22050-548: 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

22225-414: 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

22400-530: The ones on the equator where the apparent retrograde motion of the Sun happens when it is crossing the horizon as described in the preceding paragraph, receive much less solar heat than the first ones described above. Mercury attains an inferior conjunction (nearest approach to Earth) every 116 Earth days on average, but this interval can range from 105 days to 129 days due to the planet's eccentric orbit. Mercury can come as near as 82,200,000 km (0.549 astronomical units; 51.1 million miles) to Earth, and that

22575-452: The orbital eccentricity of Mercury varies chaotically from nearly zero (circular) to more than 0.45 over millions of years due to perturbations from the other planets. This was thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more usual 1:1), because this state is more likely to arise during a period of high eccentricity. However, accurate modeling based on a realistic model of tidal response has demonstrated that Mercury

22750-675: The original crust and mantle, leaving the core behind as a relatively major component. A similar process, known as the giant impact hypothesis , has been proposed to explain the formation of Earth's Moon. Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. It would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K. Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by

22925-401: 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

23100-490: 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

23275-715: 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

23450-464: The past, but have been battered out of equilibrium shapes by impacts. Some other protoplanets began to accrete and differentiate but suffered catastrophic collisions that left only a metallic or rocky core, like 16 Psyche or 8 Flora respectively. Many S-type and M-type asteroids may be such fragments. The other round bodies from the asteroid belt outward are geophysically icy planets . They are similar to terrestrial planets in that they have

23625-400: The permanently shadowed polar craters. The detection of high amounts of water-related ions like O, OH, and H 3 O was a surprise. Because of the quantities of these ions that were detected in Mercury's space environment, scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind. Sodium, potassium, and calcium were discovered in the atmosphere during

23800-420: 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

23975-434: The planet's high orbital eccentricity. Essentially, because Mercury is closest to the Sun, when taking an average over time, Mercury is most often the closest planet to the Earth, and—in that measure—it is the closest planet to each of the other planets in the Solar System. The longitude convention for Mercury puts the zero of longitude at one of the two hottest points on the surface, as described above. However, when this area

24150-440: The planet's surface. According to NASA, Mercury is not a suitable planet for Earth-like life. It has a surface boundary exosphere instead of a layered atmosphere, extreme temperatures, and high solar radiation. It is unlikely that any living beings can withstand those conditions. Some parts of the subsurface of Mercury may have been habitable , and perhaps life forms , albeit likely primitive microorganisms , may have existed on

24325-475: The planet. Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field . According to measurements taken by Mariner 10 , it is about 1.1% the strength of Earth's . The magnetic-field strength at Mercury's equator is about 300 nT . Like that of Earth, Mercury's magnetic field is dipolar and nearly aligned with the planet's spin axis (10° dipolar tilt, compared to 11° for Earth). Measurements from both

24500-597: 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

24675-589: 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

24850-480: 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

25025-632: 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 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

25200-450: The relationship between a planet's mass and radius using a broken power law appeared to suggest that the transition point between rocky, terrestrial worlds and mini-Neptunes without a defined surface was in fact very close to Earth and Venus's, suggesting that rocky worlds much larger than our own are in fact quite rare. This resulted in some advocating for the retirement of the term "super-earth" as being scientifically misleading. Since 2016

25375-447: The resulting ejecta, and ray systems . Larger albedo features correspond to higher reflectivity plains. Mercury has " wrinkle-ridges " (dorsa), Moon-like highlands , mountains (montes), plains (planitiae), escarpments (rupes), and valleys ( valles ). The planet's mantle is chemically heterogeneous, suggesting the planet went through a magma ocean phase early in its history. Crystallization of minerals and convective overturn resulted in

25550-765: 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

25725-444: The same face directed towards the Sun, in the same way that the same side of the Moon always faces Earth. Radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun. The eccentricity of Mercury's orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly stationary in Mercury's sky. The 3:2 resonant tidal locking

25900-435: The same reason, there are two points on Mercury's equator, 180 degrees apart in longitude , at either of which, around perihelion in alternate Mercurian years (once a Mercurian day), the Sun passes overhead, then reverses its apparent motion and passes overhead again, then reverses a second time and passes overhead a third time, taking a total of about 16 Earth-days for this entire process. In the other alternate Mercurian years,

26075-424: The same thing happens at the other of these two points. The amplitude of the retrograde motion is small, so the overall effect is that, for two or three weeks, the Sun is almost stationary overhead, and is at its most brilliant because Mercury is at perihelion, its closest to the Sun. This prolonged exposure to the Sun at its brightest makes these two points the hottest places on Mercury. Maximum temperature occurs when

26250-442: The schemes for naming newly discovered Solar System bodies. Earth itself was recognized as a planet when heliocentrism supplanted geocentrism during 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

26425-469: 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

26600-421: 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

26775-450: 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

26950-752: The smooth plains of Mercury formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket. An unusual feature of Mercury's surface is the numerous compression folds, or rupes , that crisscross the plains. These exist on the Moon, but are much more prominent on Mercury. As Mercury's interior cooled, it contracted and its surface began to deform, creating wrinkle ridges and lobate scarps associated with thrust faults . The scarps can reach lengths of 1,000 km (620 mi) and heights of 3 km (1.9 mi). These compressional features can be seen on top of other features, such as craters and smooth plains, indicating they are more recent. Mapping of

27125-452: The solar wind, diffusing into Mercury's magnetosphere before later escaping back into space. The radioactive decay of elements within Mercury's crust is another source of helium, as well as sodium and potassium. Water vapor is present, released by a combination of processes such as comets striking its surface, sputtering creating water out of hydrogen from the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in

27300-470: 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,

27475-400: 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

27650-637: 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. Terrestrial planet Terrestrial planets have a solid planetary surface , making them substantially different from larger gaseous planets , which are composed mostly of some combination of hydrogen , helium , and water existing in various physical states . All terrestrial planets in

27825-513: The study of Mercury. Depressions or fossae are named for works of architecture. Montes are named for the word "hot" in a variety of languages. Plains or planitiae are named for Mercury in various languages. Escarpments or rupēs are named for ships of scientific expeditions. Valleys or valles are named for abandoned cities, towns, or settlements of antiquity. Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during

28000-412: 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

28175-444: The surface of Mercury are generally extremely high, observations strongly suggest that ice (frozen water) exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K, far lower than the global average. This creates a cold trap where ice can accumulate. Water ice strongly reflects radar , and observations by the 70-meter Goldstone Solar System Radar and

28350-404: The surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because said potassium and sulfur would have been driven off by the extreme heat of these events. BepiColombo , which will arrive at Mercury in 2025, will make observations to test these hypotheses. The findings so far would seem to favor the third hypothesis; however, further analysis of

28525-456: The time until the next sunrise after another 88 Earth days. Combined with its high orbital eccentricity , the planet's surface has widely varying sunlight intensity and temperature, with the equatorial regions ranging from −170 °C (−270 °F) at night to 420 °C (790 °F) during sunlight. Due to the very small axial tilt , the planet's poles are permanently shadowed . This strongly suggests that water ice could be present in

28700-478: The westerly direction on Mercury. The two hottest places on the equator are therefore at longitudes 0° W and 180° W, and the coolest points on the equator are at longitudes 90° W and 270° W. However, the MESSENGER project uses an east-positive convention. For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and always keeping

28875-484: Was an evening star or a morning star. By about 350 BC, the ancient Greeks had realized the two stars were one. They knew the planet as Στίλβων Stilbōn , meaning "twinkling", and Ἑρμής Hermēs , for its fleeting motion, a name that is retained in modern Greek ( Ερμής Ermis ). The Romans named the planet after the swift-footed Roman messenger god, Mercury (Latin Mercurius ), whom they equated with

29050-463: Was captured into the 3:2 spin-orbit state at a very early stage of its history, within 20 (more likely, 10) million years after its formation. Numerical simulations show that a future secular orbital resonant interaction with the perihelion of Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the orbit will be destabilized in the next five billion years. If this happens, Mercury may fall into

29225-413: 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

29400-517: Was ever found. The observed perihelion precession of Mercury is 5,600 arcseconds (1.5556°) per century relative to Earth, or 574.10 ± 0.65 arcseconds per century relative to the inertial ICRF . Newtonian mechanics, taking into account all the effects from the other planets and including 0.0254 arcseconds per century due to the oblateness of the Sun, predicts a precession of 5,557 arcseconds (1.5436°) per century relative to Earth, or 531.63 ± 0.69 arcseconds per century relative to ICRF. In

29575-470: Was first visited, by Mariner 10 , this zero meridian was in darkness, so it was impossible to select a feature on the surface to define the exact position of the meridian. Therefore, a small crater further west was chosen, called Hun Kal , which provides the exact reference point for measuring longitude. The center of Hun Kal defines the 20° west meridian. A 1970 International Astronomical Union resolution suggests that longitudes be measured positively in

29750-416: 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

29925-426: Was later found to be a gas giant. In 2005, the first planets orbiting a main-sequence star and which showed signs of being terrestrial planets were found: Gliese 876 d and OGLE-2005-BLG-390Lb . Gliese 876 d orbits the red dwarf Gliese 876 , 15 light years from Earth, and has a mass seven to nine times that of Earth and an orbital period of just two Earth days. OGLE-2005-BLG-390Lb has about 5.5 times

30100-589: 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

30275-458: Was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury's 3:2 spin-orbit resonance, a solar day lasts about 176 Earth days. A sidereal day (the period of rotation) lasts about 58.7 Earth days. Simulations indicate that

30450-518: 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):

30625-462: 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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