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109-453: The Tenzing Montes / ˈ t ɛ n z ɪ ŋ ˈ m ɒ n t iː z / (formerly Norgay Montes ) are a range of icy mountains on Pluto , bordering the southwest region of Sputnik Planitia and the nearby Hillary Montes and Wright Mons. With peaks reaching 6.2 km (3.9 mi; 20,000 ft) in height, they are the highest mountain range on Pluto, and also the steepest, with a mean slope of 19.2 degrees. The mountains, first viewed by

218-474: A Tisserand parameter (relative to Neptune) greater than 3 and have a time-averaged eccentricity greater than 0.2. An alternative classification, introduced by B. J. Gladman , B. G. Marsden and C. Van Laerhoven in 2007, uses 10-million-year orbit integration instead of the Tisserand parameter. An object qualifies as an SDO if its orbit is not resonant, has a semi-major axis no greater than 2000 AU, and, during

327-573: A blink comparator were used in astronomy to detect objects in the Solar System, because these objects would move between two exposures—this involved time-consuming steps like exposing and developing photographic plates or films , and people then using a blink comparator to manually detect prospective objects. During the 1980s, the use of CCD -based cameras in telescopes made it possible to directly produce electronic images that could then be readily digitized and transferred to digital images . Because

436-521: A 'dwarf' planet" by the IAU." Some members of the public have also rejected the change, citing the disagreement within the scientific community on the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision. In 2006, in its 17th annual words-of-the-year vote, the American Dialect Society voted plutoed as

545-445: A Plutonian diameter greater than 2,360 km, with a "best guess" of 2,368 km. On July 13, 2015, images from NASA's New Horizons mission Long Range Reconnaissance Imager (LORRI), along with data from the other instruments, determined Pluto's diameter to be 2,370 km (1,473 mi), which was later revised to be 2,372 km (1,474 mi) on July 24, and later to 2374 ± 8 km . Using radio occultation data from

654-472: A clear distinction between the Kuiper belt and the scattered disc, separating those objects in stable orbits (the Kuiper belt) from those in scattered orbits (the scattered disc and the centaurs). However, the difference between the Kuiper belt and the scattered disc is not clear-cut, and many astronomers see the scattered disc not as a separate population but as an outward region of the Kuiper belt. Another term used

763-493: A collision could happen. However, Pluto is also protected by its 2:3 orbital resonance with Neptune : for every two orbits that Pluto makes around the Sun, Neptune makes three, in a frame of reference that rotates at the rate that Pluto's perihelion precesses (about 0.97 × 10 degrees per year ). Each cycle lasts about 495 years. (There are many other objects in this same resonance, called plutinos .) At present, in each 495-year cycle,

872-410: A member of the scattered disc, because, with a perihelion distance of 76 AU, it is too remote to be affected by the gravitational attraction of the outer planets. Under this definition, an object with a perihelion greater than 40 AU could be classified as outside the scattered disc. Sedna is not the only such object: (148209) 2000 CR 105 (discovered before Sedna) and 474640 Alicanto have

981-492: A passing star. A scheme introduced by a 2005 report from the Deep Ecliptic Survey by J. L. Elliott et al. distinguishes between two categories: scattered-near (i.e. typical SDOs) and scattered-extended (i.e. detached objects). Scattered-near objects are those whose orbits are non-resonant, non-planetary-orbit-crossing and have a Tisserand parameter (relative to Neptune) less than 3. Scattered-extended objects have

1090-484: A perihelion too far away from Neptune to be influenced by it. This led to a discussion among astronomers about a new minor planet set, called the extended scattered disc ( E-SDO ). 2000 CR 105 may also be an inner Oort-cloud object or (more likely) a transitional object between the scattered disc and the inner Oort cloud. More recently, these objects have been referred to as "detached" , or distant detached objects ( DDO ). There are no clear boundaries between

1199-607: A period equal to the entire age of the Solar System; a second posits that the scattering took place relatively quickly, during Neptune's early migration epoch. Models for a continuous formation throughout the age of the Solar System illustrate that at weak resonances within the Kuiper belt (such as 5:7 or 8:1), or at the boundaries of stronger resonances, objects can develop weak orbital instabilities over millions of years. The 4:7 resonance in particular has large instability. KBOs can also be shifted into unstable orbits by close passage of massive objects, or through collisions. Over time,

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1308-427: A planet. Entitled "The Great Planet Debate", the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of planet. In June 2008, the IAU had announced in a press release that the term " plutoid " would henceforth be used to refer to Pluto and other planetary-mass objects that have an orbital semi-major axis greater than that of Neptune, though

1417-538: A possible ninth planet, which he termed " Planet X ". By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search, using mathematical calculations made by Elizabeth Williams , until his death in 1916, but to no avail. Unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7, 1915, but they were not recognized for what they were. There are fourteen other known precovery observations, with

1526-514: A resolution in honor of Clyde Tombaugh, the discoverer of Pluto and a longtime resident of that state, that declared that Pluto will always be considered a planet while in New Mexican skies and that March 13, 2007, was Pluto Planet Day. The Illinois Senate passed a similar resolution in 2009 on the basis that Tombaugh was born in Illinois. The resolution asserted that Pluto was "unfairly downgraded to

1635-460: A role in the creation of the superresonance. The 2nd-largest known plutino , Orcus , has a diameter around 900 km and is in a very similar orbit to that of Pluto. However, the orbits of Pluto and Orcus are out of phase, so that the two never approach each other. It has been termed the "anti-Pluto", and is named for the Etruscan counterpart to the god Pluto . Pluto's rotation period , its day,

1744-447: A sample of his astronomical drawings. Tombaugh's task was to systematically image the night sky in pairs of photographs, then examine each pair and determine whether any objects had shifted position. Using a blink comparator , he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly

1853-590: A small increase in Pluto's surface temperature can lead to exponential increases in Pluto's atmospheric density; from 18 hPa to as much as 280 hPa (three times that of Mars to a quarter that of the Earth). At such densities, nitrogen could flow across the surface as liquid. Just like sweat cools the body as it evaporates from the skin, the sublimation of Pluto's atmosphere cools its surface. Pluto has no or almost no troposphere ; observations by New Horizons suggest only

1962-532: A small region of Pluto's orbit lies closer to the Sun than Neptune's. The Pluto–Charon barycenter came to perihelion on September 5, 1989, and was last closer to the Sun than Neptune between February 7, 1979, and February 11, 1999. Although the 3:2 resonance with Neptune (see below) is maintained, Pluto's inclination and eccentricity behave in a chaotic manner. Computer simulations can be used to predict its position for several million years (both forward and backward in time), but after intervals much longer than

2071-543: A team of researchers proposed that the mountains Wright Mons and Piccard Mons are actually a merger of many smaller cryovolcanic domes, suggesting a source of heat on the body at levels previously thought not possible. Pluto's diameter is 2 376 .6 ± 3.2 km and its mass is (1.303 ± 0.003) × 10  kg , 17.7% that of the Moon (0.22% that of Earth). Its surface area is 1.774 443 × 10  km , or just slightly bigger than Russia or Antarctica (particularly including

2180-501: A thin tropospheric boundary layer . Its thickness in the place of measurement was 4 km, and the temperature was 37±3 K. The layer is not continuous. In July 2019, an occultation by Pluto showed that its atmospheric pressure, against expectations, had fallen by 20% since 2016. In 2021, astronomers at the Southwest Research Institute confirmed the result using data from an occultation in 2018, which showed that light

2289-424: A white or greyish appearance. One explanation is the exposure of whiter subsurface layers by impacts; another is that the scattered objects' greater distance from the Sun creates a composition gradient, analogous to the composition gradient of the terrestrial and gas giant planets. Michael E. Brown, discoverer of the scattered object Eris, suggests that its paler colour could be because, at its current distance from

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2398-581: A year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and 29. A lesser-quality photograph taken on January 21 helped confirm the movement. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. One Plutonian year corresponds to 247.94 Earth years; thus, in 2178, Pluto will complete its first orbit since its discovery. The name Pluto came from

2507-405: Is "scattered Kuiper-belt object" (or SKBO) for bodies of the scattered disc. Morbidelli and Brown propose that the difference between objects in the Kuiper belt and scattered-disc objects is that the latter bodies "are transported in semi-major axis by close and distant encounters with Neptune," but the former experienced no such close encounters. This delineation is inadequate (as they note) over

2616-531: Is a dwarf planet in the Kuiper belt , a ring of bodies beyond the orbit of Neptune . It is the ninth-largest and tenth-most- massive known object to directly orbit the Sun . It is the largest known trans-Neptunian object by volume, by a small margin, but is less massive than Eris . Like other Kuiper belt objects, Pluto is made primarily of ice and rock and is much smaller than the inner planets . Pluto has roughly one-sixth

2725-429: Is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of water ice. The pre– New Horizons estimate for the diameter of the core is 1700 km , 70% of Pluto's diameter. It is possible that such heating continues, creating a subsurface ocean of liquid water 100 to 180 km thick at the core–mantle boundary. In September 2016, scientists at Brown University simulated

2834-405: Is divided into roughly 20 regularly spaced haze layers up to 150 km high, thought to be the result of pressure waves created by airflow across Pluto's mountains. Pluto has five known natural satellites . The largest and closest to Pluto is Charon . First identified in 1978 by astronomer James Christy , Charon is the only moon of Pluto that may be in hydrostatic equilibrium . Charon's mass

2943-495: Is equal to 6.387 Earth days. Like Uranus and 2 Pallas , Pluto rotates on its "side" in its orbital plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at its solstices , one-fourth of its surface is in continuous daylight, whereas another fourth is in continuous darkness. The reason for this unusual orientation has been debated. Research from the University of Arizona has suggested that it may be due to

3052-414: Is highly compact: the five known satellites orbit within the inner 3% of the region where prograde orbits would be stable. The orbital periods of all Pluto's moons are linked in a system of orbital resonances and near-resonances . When precession is accounted for, the orbital periods of Styx, Nix, and Hydra are in an exact 18:22:33 ratio. There is a sequence of approximate ratios, 3:4:5:6, between

3161-426: Is less massive than the dwarf planet Eris , a trans-Neptunian object discovered in 2005, though Pluto has a larger diameter of 2,376.6 km compared to Eris's approximate diameter of 2,326 km. With less than 0.2 lunar masses, Pluto is much less massive than the terrestrial planets , and also less massive than seven moons : Ganymede , Titan , Callisto , Io , the Moon , Europa , and Triton . The mass

3270-527: Is less than 10 million years old. Latest studies have shown that the surface has an age of 180 000 +90 000 −40 000 years. The New Horizons science team summarized initial findings as "Pluto displays a surprisingly wide variety of geological landforms, including those resulting from glaciological and surface–atmosphere interactions as well as impact, tectonic , possible cryovolcanic , and mass-wasting processes." In Western parts of Sputnik Planitia there are fields of transverse dunes formed by

3379-410: Is more similar to that of Io with slightly more orange and significantly less red than Mars . Notable geographical features include Tombaugh Regio, or the "Heart" (a large bright area on the side opposite Charon), Belton Regio , or the "Whale" (a large dark area on the trailing hemisphere), and the " Brass Knuckles " (a series of equatorial dark areas on the leading hemisphere). Sputnik Planitia,

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3488-735: Is much less than thought before Charon was discovered. The discovery of Pluto's satellite Charon in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton's formulation of Kepler's third law . Observations of Pluto in occultation with Charon allowed scientists to establish Pluto's diameter more accurately, whereas the invention of adaptive optics allowed them to determine its shape more accurately. Determinations of Pluto's size have been complicated by its atmosphere and hydrocarbon haze. In March 2014, Lellouch, de Bergh et al. published findings regarding methane mixing ratios in Pluto's atmosphere consistent with

3597-443: Is one of the defining characteristics of scattered objects, as it allows Neptune to exert its gravitational influence. The classical objects ( cubewanos ) are very different from the scattered objects: more than 30% of all cubewanos are on low-inclination, near-circular orbits whose eccentricities peak at 0.25. Classical objects possess eccentricities ranging from 0.2 to 0.8. Though the inclinations of scattered objects are similar to

3706-478: Is still poorly understood: no model of the formation of the Kuiper belt and the scattered disc has yet been proposed that explains all their observed properties. According to contemporary models, the scattered disc formed when Kuiper belt objects (KBOs) were "scattered" into eccentric and inclined orbits by gravitational interaction with Neptune and the other outer planets . The amount of time for this process to occur remains uncertain. One hypothesis estimates

3815-552: Is sufficient to cause the barycenter of the Pluto–Charon system to be outside Pluto. Beyond Charon there are four much smaller circumbinary moons. In order of distance from Pluto they are Styx, Nix, Kerberos, and Hydra. Nix and Hydra were both discovered in 2005, Kerberos was discovered in 2011, and Styx was discovered in 2012. The satellites' orbits are circular (eccentricity < 0.006) and coplanar with Pluto's equator (inclination < 1°), and therefore tilted approximately 120° relative to Pluto's orbit. The Plutonian system

3924-538: Is sufficiently delayed that at the second perihelion of each cycle it will not be far ahead of Neptune coming behind it, and Neptune will start to decrease Pluto's period again. The whole cycle takes about 20,000 years to complete. Numerical studies have shown that over millions of years, the general nature of the alignment between the orbits of Pluto and Neptune does not change. There are several other resonances and interactions that enhance Pluto's stability. These arise principally from two additional mechanisms (besides

4033-475: Is therefore moving faster, so during the first of two orbits in the 495-year cycle, it is approaching Neptune from behind. At present it remains between 50° and 65° behind Neptune for 100 years (e.g. 1937–2036). The gravitational pull between the two causes angular momentum to be transferred to Pluto. This situation moves Pluto into a slightly larger orbit, where it has a slightly longer period, according to Kepler's third law . After several such repetitions, Pluto

4142-518: The Antarctic sea ice during winter). Its surface gravity is 0.063 g (compared to 1 g for Earth and 0.17 g for the Moon). This gives Pluto an escape velocity of 4,363.2 km per hour / 2,711.167 miles per hour (as compared to Earth's 40,270 km per hour / 25,020 miles per hour). Pluto is more than twice the diameter and a dozen times the mass of Ceres , the largest object in the asteroid belt . It

4251-620: The Hayden Planetarium in New York City displayed a Solar System model of only eight planets, which made headlines almost a year later. Ceres , Pallas , Juno and Vesta lost their planet status among most astronomers after the discovery of many other asteroids in the 1840s. On the other hand, planetary geologists often regarded Ceres, and less often Pallas and Vesta, as being different from smaller asteroids because they were large enough to have undergone geological evolution. Although

4360-466: The Kozai mechanism , which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this case, Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to the orbit of Neptune is always greater than 52° (90°–38°) . The closest such angular separation occurs every 10,000 years. Second, the longitudes of ascending nodes of

4469-633: The Lyapunov time of 10–20 million years, calculations become unreliable: Pluto is sensitive to immeasurably small details of the Solar System, hard-to-predict factors that will gradually change Pluto's position in its orbit. The semi-major axis of Pluto's orbit varies between about 39.3 and 39.6  AU with a period of about 19,951 years, corresponding to an orbital period varying between 246 and 249 years. The semi-major axis and period are presently getting longer. Despite Pluto's orbit appearing to cross that of Neptune when viewed from north or south of

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4578-557: The New Horizons Radio Science Experiment (REX), the diameter was found to be 2 376 .6 ± 3.2 km . Pluto has a tenuous atmosphere consisting of nitrogen (N 2 ), methane (CH 4 ), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto's surface. According to the measurements by New Horizons , the surface pressure is about 1  Pa (10  μbar ), roughly one million to 100,000 times less than Earth's atmospheric pressure. It

4687-546: The New Horizons spacecraft on 14 July 2015, and announced by NASA on 15 July 2015, are named after the Nepalese mountaineer Tenzing Norgay , who, along with Sir Edmund Hillary , made the first successful ascent of the highest peak on Earth , Mount Everest (29 May 1953). The mountains were informally called Norgay Montes by the New Horizons team, but that name was later changed from Norgay to Tenzing . On 7 September 2017,

4796-474: The University of Hawaiʻi calculated Pluto's albedo for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of Earth. (Pluto's albedo is 1.4–1.9 times that of Earth. ) In 1978, the discovery of Pluto's moon Charon allowed the measurement of Pluto's mass for the first time: roughly 0.2% that of Earth, and far too small to account for

4905-526: The centaurs , a class of icy planetoids that orbit between Jupiter and Neptune, may simply be SDOs thrown into the inner reaches of the Solar System by Neptune, making them "cis-Neptunian" rather than trans-Neptunian scattered objects. Some objects, like (29981) 1999 TD 10 , blur the distinction and the Minor Planet Center (MPC), which officially catalogues all trans-Neptunian objects , now lists centaurs and SDOs together. The MPC, however, makes

5014-581: The centaurs , a population of icy bodies between Jupiter and Neptune, being the intermediate stage in an object's migration from the disc to the inner Solar System. Eventually, perturbations from the giant planets send such objects towards the Sun, transforming them into periodic comets. Many objects of the proposed Oort cloud are also thought to have originated in the scattered disc. Detached objects are not sharply distinct from scattered disc objects, and some such as Sedna have sometimes been considered to be included in this group. Traditionally, devices like

5123-402: The resonant Kuiper-belt objects , those which Neptune has locked into a precise orbital ratio such as 2:3 (the object goes around twice for every three Neptune orbits) and 1:2 (the object goes around once for every two Neptune orbits). These ratios, called orbital resonances , allow KBOs to persist in regions which Neptune's gravitational influence would otherwise have cleared out over the age of

5232-609: The 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analyzing perturbations in the orbit of Uranus . Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune. In 1906, Percival Lowell —a wealthy Bostonian who had founded Lowell Observatory in Flagstaff, Arizona , in 1894—started an extensive project in search of

5341-402: The 2:3 mean-motion resonance). First, Pluto's argument of perihelion , the angle between the point where it crosses the ecliptic (or the invariant plane ) and the point where it is closest to the Sun, librates around 90°. This means that when Pluto is closest to the Sun, it is at its farthest north of the plane of the Solar System, preventing encounters with Neptune. This is a consequence of

5450-605: The CCD captured more light than film (about 90% versus 10% of incoming light) and the blinking could now be done at an adjustable computer screen, the surveys allowed for higher throughput. A flood of new discoveries was the result: over a thousand trans-Neptunian objects were detected between 1992 and 2006. The first scattered-disc object (SDO) to be recognised as such was 1996 TL 66 , originally identified in 1996 by astronomers based at Mauna Kea in Hawaii. Three more were identified by

5559-684: The God of Death in Hinduism. Polynesian languages also tend to use the indigenous god of the underworld, as in Māori Whiro . Vietnamese might be expected to follow Chinese, but does not because the Sino-Vietnamese word 冥 minh "dark" is homophonous with 明 minh "bright". Vietnamese instead uses Yama, which is also a Buddhist deity, in the form of Sao Diêm Vương 星閻王 "Yama's Star", derived from Chinese 閻王 Yán Wáng / Yìhm Wòhng "King Yama". Once Pluto

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5668-410: The IAU included Pluto, and Eris and its moon Dysnomia , in their Minor Planet Catalogue , giving them the official minor-planet designations "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia". Had Pluto been included upon its discovery in 1930, it would have likely been designated 1164, following 1163 Saga , which was discovered a month earlier. There has been some resistance within

5777-613: The Oort cloud but to have been drawn into the inner Solar System by the gravity of the giant planets, whereas the JFCs are thought to have originated in the scattered disc. The centaurs are thought to be a dynamically intermediate stage between the scattered disc and the Jupiter family. There are many differences between SDOs and JFCs, even though many of the Jupiter-family comets may have originated in

5886-459: The Oort cloud have been made. Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with " detached objects ". The Kuiper belt is a relatively thick torus (or "doughnut") of space, extending from about 30 to 50 AU comprising two main populations of Kuiper belt objects (KBOs): the classical Kuiper-belt objects (or "cubewanos"), which lie in orbits untouched by Neptune, and

5995-545: The Roman god of the underworld ; and it is also an epithet for Hades (the Greek equivalent of Pluto). Upon the announcement of the discovery, Lowell Observatory received over a thousand suggestions for names. Three names topped the list: Minerva , Pluto and Cronus . 'Minerva' was the Lowell staff's first choice but was rejected because it had already been used for an asteroid ; Cronus

6104-552: The Solar System, since the objects are never close enough to Neptune to be scattered by its gravity. Those in 2:3 resonances are known as " plutinos ", because Pluto is the largest member of their group, whereas those in 1:2 resonances are known as " twotinos ". In contrast to the Kuiper belt, the scattered-disc population can be disturbed by Neptune. Scattered-disc objects come within gravitational range of Neptune at their closest approaches (~30 AU) but their farthest distances reach many times that. Ongoing research suggests that

6213-482: The Solar System, the two objects' orbits do not intersect. When Pluto is closest to the Sun, and close to Neptune's orbit as viewed from such a position, it is also the farthest north of Neptune's path. Pluto's orbit passes about 8 AU north of that of Neptune, preventing a collision. This alone is not enough to protect Pluto; perturbations from the planets (especially Neptune) could alter Pluto's orbit (such as its orbital precession ) over millions of years so that

6322-409: The Sun than Neptune , but a stable orbital resonance prevents them from colliding. Pluto has five known moons : Charon , the largest, whose diameter is just over half that of Pluto; Styx ; Nix ; Kerberos ; and Hydra . Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body, and they are tidally locked . New Horizons

6431-429: The Sun, its atmosphere of methane is frozen over its entire surface, creating an inches-thick layer of bright white ice. Pluto, conversely, being closer to the Sun, would be warm enough that methane would freeze only onto cooler, high- albedo regions, leaving low-albedo tholin -covered regions bare of ice. The Kuiper belt was initially thought to be the source of the Solar System's ecliptic comets . However, studies of

6540-487: The age of the Solar System, since bodies "trapped in resonances" could "pass from a scattering phase to a non-scattering phase (and vice versa) numerous times." That is, trans-Neptunian objects could travel back and forth between the Kuiper belt and the scattered disc over time. Therefore, they chose instead to define the regions, rather than the objects, defining the scattered disc as "the region of orbital space that can be visited by bodies that have encountered Neptune" within

6649-497: The anti-Charon face of Pluto (around 180° longitude, where Tombaugh Regio 's western lobe, Sputnik Planitia , is located), whereas methane is most abundant near 300° east. The mountains are made of water ice. Pluto's surface is quite varied, with large differences in both brightness and color. Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn 's moon Iapetus . The color varies from charcoal black, to dark orange and white. Pluto's color

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6758-495: The astronomical community toward the reclassification, and in particular planetary scientists often continue to reject it, considering Pluto, Charon, and Eris to be planets for the same reason they do so for Ceres. In effect, this amounts to accepting only the second clause of the IAU definition. Alan Stern , principal investigator with NASA 's New Horizons mission to Pluto, derided the IAU resolution. He also stated that because less than five percent of astronomers voted for it,

6867-596: The body to reorient itself, leading to its unusual axial tilt of 120°. The buildup of nitrogen is due to Pluto's vast distance from the Sun. At the equator, temperatures can drop to −240 °C (−400.0 °F; 33.1 K), causing nitrogen to freeze as water would freeze on Earth. The same polar wandering effect seen on Pluto would be observed on Earth were the Antarctic ice sheet several times larger. The plains on Pluto's surface are composed of more than 98 percent nitrogen ice , with traces of methane and carbon monoxide . Nitrogen and carbon monoxide are most abundant on

6976-598: The decision was not representative of the entire astronomical community. Marc W. Buie , then at the Lowell Observatory, petitioned against the definition. Others have supported the IAU, for example Mike Brown , the astronomer who discovered Eris. Public reception to the IAU decision was mixed. A resolution introduced in the California State Assembly facetiously called the IAU decision a "scientific heresy". The New Mexico House of Representatives passed

7085-447: The disc extends from the orbit of Neptune to 2000 AU, the region referred to as the inner Oort cloud. The scattered disc is a very dynamic environment. Because they are still capable of being perturbed by Neptune, SDOs' orbits are always in danger of disruption; either of being sent outward to the Oort cloud or inward into the centaur population and ultimately the Jupiter family of comets. For this reason Gladman et al. prefer to refer to

7194-411: The discrepancies in the orbit of Uranus. Subsequent searches for an alternative Planet X, notably by Robert Sutton Harrington , failed. In 1992, Myles Standish used data from Voyager 2 ' s flyby of Neptune in 1989, which had revised the estimates of Neptune's mass downward by 0.5%—an amount comparable to the mass of Mars—to recalculate its gravitational effect on Uranus. With the new figures added in,

7303-461: The discrepancies, and with them the need for a Planet X, vanished. As of 2000 the majority of scientists agree that Planet X, as Lowell defined it, does not exist. Lowell had made a prediction of Planet X's orbit and position in 1915 that was fairly close to Pluto's actual orbit and its position at that time; Ernest W. Brown concluded soon after Pluto's discovery that this was a coincidence. From 1992 onward, many bodies were discovered orbiting in

7412-555: The earliest made by the Yerkes Observatory on August 20, 1909. Percival's widow, Constance Lowell, entered into a ten-year legal battle with the Lowell Observatory over her husband's legacy, and the search for Planet X did not resume until 1929. Vesto Melvin Slipher , the observatory director, gave the job of locating Planet X to 23-year-old Clyde Tombaugh , who had just arrived at the observatory after Slipher had been impressed by

7521-523: The early evolution of the Solar System, perhaps through exchanges of angular momentum with scattered objects. Once the orbits of Jupiter and Saturn shifted to a 2:1 resonance (two Jupiter orbits for each orbit of Saturn), their combined gravitational pull disrupted the orbits of Uranus and Neptune, sending Neptune into the temporary "chaos" of the proto-Kuiper belt. As Neptune traveled outward, it scattered many trans-Neptunian objects into higher and more eccentric orbits. This model states that 90% or more of

7630-470: The entire trans-Neptunian population would show a similar red surface colour, as they were thought to have originated in the same region and subjected to the same physical processes. Specifically, SDOs were expected to have large amounts of surface methane, chemically altered into tholins by sunlight from the Sun. This would absorb blue light, creating a reddish hue. Most classical objects display this colour, but scattered objects do not; instead, they present

7739-422: The first Kuiper belt objects discovered were quite small, objects increasingly closer in size to Pluto were soon discovered, some large enough (like Pluto itself) to satisfy geological but not dynamical ideas of planethood. On July 29, 2005, the debate became unavoidable when astronomers at Caltech announced the discovery of a new trans-Neptunian object , Eris , which was substantially more massive than Pluto and

7848-444: The first time Pluto is at perihelion (such as in 1989), Neptune is 57° ahead of Pluto. By Pluto's second passage through perihelion, Neptune will have completed a further one and a half of its own orbits, and will be 123° behind Pluto. Pluto and Neptune's minimum separation is over 17 AU, which is greater than Pluto's minimum separation from Uranus (11 AU). The minimum separation between Pluto and Neptune actually occurs near

7957-412: The god and the planet inhabited "gloomy" regions, and the god was able to make himself invisible, as the planet had been for so long. The choice was further helped by the fact that the first two letters of Pluto were the initials of Percival Lowell; indeed, 'Percival' had been one of the more popular suggestions for a name for the new planet. Pluto's planetary symbol ⟨ [REDACTED] ⟩

8066-406: The impact thought to have formed Sputnik Planitia , and showed that it might have been the result of liquid water upweling from below after the collision, implying the existence of a subsurface ocean at least 100 km deep. In June 2020, astronomers reported evidence that Pluto may have had a subsurface ocean , and consequently may have been habitable , when it was first formed. In March 2022,

8175-400: The integration, its semi-major axis shows an excursion of 1.5 AU or more. Gladman et al. suggest the term scattering disk object to emphasize this present mobility. If the object is not an SDO as per the above definition, but the eccentricity of its orbit is greater than 0.240, it is classified as a detached TNO . (Objects with smaller eccentricity are considered classical.) In this scheme,

8284-442: The mass of the Moon , and one-third its volume. Pluto has a moderately eccentric and inclined orbit, ranging from 30 to 49 astronomical units (4.5 to 7.3  billion kilometres ; 2.8 to 4.6 billion miles ) from the Sun. Light from the Sun takes 5.5 hours to reach Pluto at its orbital distance of 39.5 AU (5.91 billion km; 3.67 billion mi). Pluto's eccentric orbit periodically brings it closer to

8393-463: The more extreme KBOs, very few scattered objects have orbits as close to the ecliptic as much of the KBO population. Although motions in the scattered disc are random, they do tend to follow similar directions, which means that SDOs can become trapped in temporary resonances with Neptune. Examples of possible resonant orbits within the scattered disc include 1:3, 2:7, 3:11, 5:22 and 4:79. The scattered disc

8502-521: The most massive object discovered in the Solar System since Triton in 1846. Its discoverers and the press initially called it the tenth planet , although there was no official consensus at the time on whether to call it a planet. Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet. The debate came to a head in August 2006, with an IAU resolution that created an official definition for

8611-778: The name Tenzing Montes was officially approved together with the names of Tombaugh Regio and twelve other nearby surface features. Several massifs within Tenzing Montes reach elevations of more than 4 km above the surrounding terrain. The table below is based on Table 3 in. The Tenzing Montes rise up to 6.2 km (3.9 mi; 20,000 ft) high, about twice as high as the Hillary Montes . In comparison, Mount Everest rises 4.6 km (2.9 mi; 15,000 ft) base-to-peak (though to an elevation of 8.8 km (5.5 mi; 29,000 ft) above sea level). Pluto Pluto ( minor-planet designation : 134340 Pluto )

8720-500: The numbers of objects in the Kuiper belt and the scattered disc are hypothesized to be roughly equal, observational bias due to their greater distance means that far fewer SDOs have been observed to date. Known trans-Neptunian objects are often divided into two subpopulations: the Kuiper belt and the scattered disc. A third reservoir of trans-Neptunian objects, the Oort cloud , has been hypothesized, although no confirmed direct observations of

8829-565: The objects in the scattered disc may have been "promoted into these eccentric orbits by Neptune's resonances during the migration epoch...[therefore] the scattered disc might not be so scattered." Scattered objects, like other trans-Neptunian objects, have low densities and are composed largely of frozen volatiles such as water and methane . Spectral analysis of selected Kuiper belt and scattered objects has revealed signatures of similar compounds. Both Pluto and Eris, for instance, show signatures for methane. Astronomers originally supposed that

8938-423: The periods of Styx, Nix, Kerberos, and Hydra with that of Charon; the ratios become closer to being exact the further out the moons are. Scattered disk Although the closest scattered-disc objects approach the Sun at about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered disc objects among the coldest and most distant objects in the Solar System. The innermost portion of

9047-443: The radius of a Hill sphere , and the Kuiper belt as its "complement ... in the a > 30 AU region"; the region of the Solar System populated by objects with semi-major axes greater than 30 AU. The Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered-disc object. Its discoverer Michael E. Brown has suggested instead that it should be considered an inner Oort-cloud object rather than

9156-417: The region as the scattering disc, rather than scattered. Unlike Kuiper-belt objects (KBOs), the orbits of scattered-disc objects can be inclined as much as 40° from the ecliptic . SDOs are typically characterized by orbits with medium and high eccentricities with a semi-major axis greater than 50 AU, but their perihelia bring them within influence of Neptune. Having a perihelion of roughly 30 AU

9265-555: The region since 1992 have shown that the orbits within the Kuiper belt are relatively stable, and that ecliptic comets originate from the scattered disc, where orbits are generally less stable. Comets can loosely be divided into two categories: short-period and long-period—the latter being thought to originate in the Oort cloud. The two major categories of short-period comets are Jupiter-family comets (JFCs) and Halley-type comets . Halley-type comets, which are named after their prototype, Halley's Comet , are thought to have originated in

9374-547: The same survey in 1999: 1999 CV 118 , 1999 CY 118 , and 1999 CF 119 . The first object presently classified as an SDO to be discovered was 1995 TL 8 , found in 1995 by Spacewatch . As of 2011, over 200 SDOs have been identified, including Gǃkúnǁʼhòmdímà (discovered by Schwamb, Brown, and Rabinowitz), Gonggong (Schwamb, Brown, and Rabinowitz) 2002 TC 302 ( NEAT ), Eris (Brown, Trujillo, and Rabinowitz), Sedna (Brown, Trujillo, and Rabinowitz), and 474640 Alicanto ( Deep Ecliptic Survey ). Although

9483-480: The same volume as Pluto, showing that Pluto is part of a population of objects called the Kuiper belt . This made its official status as a planet controversial, with many questioning whether Pluto should be considered together with or separately from its surrounding population. Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the Solar System . In February 2000

9592-505: The scattered and detached regions. Gomes et al. define SDOs as having "highly eccentric orbits, perihelia beyond Neptune, and semi-major axes beyond the 1:2 resonance." By this definition, all distant detached objects are SDOs. Since detached objects' orbits cannot be produced by Neptune scattering, alternative scattering mechanisms have been put forward, including a passing star or a distant, planet-sized object . Alternatively, it has been suggested that these objects have been captured from

9701-405: The scattered disc overlaps with a torus -shaped region of orbiting objects traditionally called the Kuiper belt , but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the Kuiper belt proper. Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets in the Solar System, with

9810-467: The scattered disc would gradually form from these isolated events. Computer simulations have also suggested a more rapid and earlier formation for the scattered disc. Modern theories indicate that neither Uranus nor Neptune could have formed in situ beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets, and Saturn, may have formed closer to Jupiter, but were flung outwards during

9919-428: The suggestion to astronomy professor Herbert Hall Turner , who cabled it to colleagues at Lowell on March 16, three days after the announcement. The name 'Pluto' was mythologically appropriate: the god Pluto was one of six surviving children of Saturn , and the others had already all been chosen as names of major or minor planets (his brothers Jupiter and Neptune , and his sisters Ceres , Juno and Vesta ). Both

10028-550: The term "planet". According to this resolution, there are three conditions for an object in the Solar System to be considered a planet: Pluto fails to meet the third condition. Its mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times, in contrast to Earth, which is 1.7 million times the remaining mass in its orbit (excluding the moon). The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2, but do not meet criterion 3 would be called dwarf planets . In September 2006,

10137-430: The term has not seen significant use. Pluto's orbital period is about 248 years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference plane called the ecliptic . In contrast, Pluto's orbit is moderately inclined relative to the ecliptic (over 17°) and moderately eccentric (elliptical). This eccentricity means

10246-510: The time of Pluto's aphelion. The 2:3 resonance between the two bodies is highly stable and has been preserved over millions of years. This prevents their orbits from changing relative to one another, so the two bodies can never pass near each other. Even if Pluto's orbit were not inclined, the two bodies could never collide. When Pluto's period is slightly different from 3/2 of Neptune's, the pattern of its distance from Neptune will drift. Near perihelion Pluto moves interior to Neptune's orbit and

10355-561: The two bodies—the points where they cross the invariant plane —are in near-resonance with the above libration. When the two longitudes are the same—that is, when one could draw a straight line through both nodes and the Sun—Pluto's perihelion lies exactly at 90°, and hence it comes closest to the Sun when it is furthest north of Neptune's orbit. This is known as the 1:1 superresonance . All the Jovian planets (Jupiter, Saturn, Uranus, and Neptune) play

10464-471: The way that a body's spin will always adjust to minimize energy. This could mean a body reorienting itself to put extraneous mass near the equator and regions lacking mass tend towards the poles. This is called polar wander . According to a paper released from the University of Arizona, this could be caused by masses of frozen nitrogen building up in shadowed areas of the dwarf planet. These masses would cause

10573-434: The western lobe of the "Heart", is a 1,000 km-wide basin of frozen nitrogen and carbon monoxide ices, divided into polygonal cells, which are interpreted as convection cells that carry floating blocks of water ice crust and sublimation pits towards their margins; there are obvious signs of glacial flows both into and out of the basin. It has no craters that were visible to New Horizons , indicating that its surface

10682-433: The winds blowing from the center of Sputnik Planitia in the direction of surrounding mountains. The dune wavelengths are in the range of 0.4–1 km and likely consist of methane particles 200–300 μm in size. Pluto's density is 1.853 ± 0.004 g/cm . Because the decay of radioactive elements would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto's internal structure

10791-503: The word of the year. To "pluto" is to "demote or devalue someone or something". In April 2024, Arizona (where Pluto was first discovered in 1930) passed a law naming Pluto as the official state planet. Researchers on both sides of the debate gathered in August 2008, at the Johns Hopkins University Applied Physics Laboratory for a conference that included back-to-back talks on the IAU definition of

10900-464: Was appearing less gradually from behind Pluto's disc, indicating a thinning atmosphere. The presence of methane, a powerful greenhouse gas , in Pluto's atmosphere creates a temperature inversion , with the average temperature of its atmosphere tens of degrees warmer than its surface, though observations by New Horizons have revealed Pluto's upper atmosphere to be far colder than expected (70 K, as opposed to about 100 K). Pluto's atmosphere

11009-569: Was disfavored because it was promoted by an unpopular and egocentric astronomer, Thomas Jefferson Jackson See . A vote was then taken and 'Pluto' was the unanimous choice. To make sure the name stuck, and that the planet would not suffer changes in its name as Uranus had, Lowell Observatory proposed the name to the American Astronomical Society and the Royal Astronomical Society ; both approved it unanimously. The name

11118-543: Was found to be much smaller than expected. These doubts increased following the discovery of additional objects in the Kuiper belt starting in the 1990s, and particularly the more massive scattered disk object Eris in 2005. In 2006, the International Astronomical Union (IAU) formally redefined the term planet to exclude dwarf planets such as Pluto. Many planetary astronomers, however, continue to consider Pluto and other dwarf planets to be planets. In

11227-497: Was found, its faintness and lack of a viewable disc cast doubt on the idea that it was Lowell's Planet X . Estimates of Pluto's mass were revised downward throughout the 20th century. Astronomers initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1931, Pluto was calculated to be roughly the mass of Earth , with further calculations in 1948 bringing the mass down to roughly that of Mars . In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of

11336-437: Was initially thought that, as Pluto moves away from the Sun, its atmosphere should gradually freeze onto the surface; studies of New Horizons data and ground-based occultations show that Pluto's atmospheric density increases, and that it likely remains gaseous throughout Pluto's orbit. New Horizons observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected. Alan Stern has contended that even

11445-507: Was named after Uranus, and neptunium , which was named after Neptune. Most languages use the name "Pluto" in various transliterations. In Japanese, Houei Nojiri suggested the calque Meiōsei ( 冥王星 , "Star of the King (God) of the Underworld") , and this was borrowed into Chinese and Korean. Some languages of India use the name Pluto, but others, such as Hindi , use the name of Yama ,

11554-477: Was published on May 1, 1930. The name Pluto had received some 150 nominations among the letters and telegrams sent to Lowell. The first had been from Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford , England, who was interested in classical mythology . She had suggested it to her grandfather Falconer Madan when he read the news of Pluto's discovery to his family over breakfast; Madan passed

11663-442: Was soon embraced by wider culture. In 1930, Walt Disney was apparently inspired by it when he introduced for Mickey Mouse a canine companion named Pluto , although Disney animator Ben Sharpsteen could not confirm why the name was given. In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following uranium , which

11772-408: Was the first spacecraft to visit Pluto and its moons, making a flyby on July 14, 2015, and taking detailed measurements and observations. Pluto was discovered in 1930 by Clyde W. Tombaugh , making it by far the first known object in the Kuiper belt. It was immediately hailed as the ninth planet , but it never fit well with the other eight, and its planetary status was questioned when it

11881-412: Was then created as a monogram of the letters "PL". This symbol is rarely used in astronomy anymore, though it is still common in astrology. However, the most common astrological symbol for Pluto, occasionally used in astronomy as well, is an orb (possibly representing Pluto's invisibility cap) over Pluto's bident ⟨ [REDACTED] ⟩ , which dates to the early 1930s. The name 'Pluto'

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