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102-414: Sputnik Planitia / ˈ s p ʌ t n ɪ k p l ə ˈ n ɪ ʃ i ə , ˈ s p ʊ t -/ (formerly Sputnik Planum ) is a large, partially glaciated basin on Pluto . About 1,400 by 1,200 km (870 by 750 mi) in size, Sputnik Planitia is partially submerged in large, bright glaciers of nitrogen ice . Named after Earth's first artificial satellite, Sputnik 1 , it constitutes the western lobe of

204-472: 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 was found to be much smaller than expected. These doubts increased following

306-521: 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

408-445: 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

510-493: 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,

612-408: 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 the Sun than Neptune , but

714-427: 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

816-499: A positive gravity anomaly , but by itself would not have been sufficient to overcome the topographic depression associated with the basin. However, other effects of an impact event (see below) could have also contributed to such an anomaly. A positive gravity anomaly could have caused polar wander , reorienting the spin axis of Pluto to put the planitia near the Pluto-Charon tidal axis (the minimum-energy configuration). Sputnik Planitia

918-453: A possible cryovolcano . The informal name Sputnik Planum was first announced by the New Horizons team on a press conference on 24 July 2015. A planum is a flat region of higher elevation (a plateau). When topographical data was analyzed in early 2016,it became clear that Sputnik is actually a basin , and the informal name was changed to Sputnik Planitia later that year. The name

1020-538: 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

1122-514: 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

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1224-460: 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,

1326-443: A runaway process due to the positive feedback association of increased albedo, cooling and further ice deposition (similar to the ice segregation that occurred on Iapetus ). Simulations suggest that over a period of about a million years, the runaway process would collect much of the ice into a single cap even in the absence of a preexisting basin. The accumulation of dense nitrogen ice would have contributed to making Sputnik Planitia

1428-447: 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

1530-590: 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

1632-532: 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

1734-445: 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 was the first spacecraft to visit Pluto and its moons, making

1836-423: A surface age of 180 000 +90 000 −40 000 years. Other scientists have proposed that sublimation at the surface of the planitia is responsible for its convection by cooling the surface through latent heat consumption, instead of the driving source from the core originally proposed. Other obvious indications of ice flow visible in images of the planitia include examples of valley-type glaciers flowing down into

1938-543: 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

2040-501: 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

2142-581: 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

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2244-409: Is a sign of convection of the nitrogen/carbon monoxide ice, with ice warmed by heat from the interior welling up in the center of cells, spreading and then sinking at the ridged margins. Convection cells have about 100 m of vertical relief, with the highest points being at their centers. Modeling of nitrogen ice convection cells suggests a depth of about one tenth their width, or 3–4 km for most of

2346-443: Is about 5–10 times less than that of water ice (it also has a viscosity roughly an order of magnitude greater than that of water ice). The additional insulation would help to maintain the layer of water below it in a liquid state, as well as keep the ice crust above it cool. A similar mechanism may contribute to the formation of subsurface oceans on other outer Solar System satellites and trans-Neptunian objects. The polygonal structure

2448-429: 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

2550-405: 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

2652-495: 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

2754-414: 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

2856-426: 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

2958-527: 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

3060-410: 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,

3162-735: 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

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3264-433: Is presently close to the anti-Charon point on Pluto, a result that has less than a 5% probability of arising by chance. If Sputnik Planitia was created by an impact, then explaining the positive gravity anomaly requires the presence of a subsurface liquid water ocean below Pluto's water ice crust; isostatic uplift of the thinned crust and consequent intrusion of denser liquid water below the basin would account for most of

3366-552: 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

3468-538: 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

3570-424: Is the most volatile. The nitrogen ice of the basin rests on Pluto's crust mostly composed of much more rigid water ice. Sputnik Planitia likely originated as an impact basin that subsequently collected volatile ices . The size of the hypothetical impactor has been estimated as 150–300 km. Alternatively, it has been suggested that the accumulation of ices in this location depressed the surface there, leading to

3672-452: 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 the mass of the Moon , and one-third its volume. Pluto has

3774-475: 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

3876-476: Is where different parts of the Earth's crust move in different directions because of circulation in the mantle . The effect should further not be confused with the effect known as geomagnetic reversal that describes the repeated proven reversal of the magnetic field of the Earth . Paleomagnetism is used to create tectonic plate reconstructions by finding the paleolatitude of a particular site. This paleolatitude

3978-519: Is where the axis of rotation moves, in other words the North Pole points toward a different star. There are also smaller and faster variations in the axis of rotation going under the term nutation . Precession is caused by the gravitational attraction of the Moon and Sun , and occurs all the time and at a much faster rate than polar wander. It does not result in changes of latitude. True polar wander has to be distinguished from continental drift , which

4080-518: 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

4182-620: 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

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4284-466: 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

4386-688: 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

4488-557: 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

4590-474: 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

4692-402: 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

4794-421: The Earth by an amount that depends on how far they are from the axis near the equator that does not move. Cases of true polar wander have occurred several times in the course of the Earth's history. It has been suggested that east Asia moved south due to true polar wander by 25° between about 174 and 157 million years ago. Mars , Europa , and Enceladus are also believed to have undergone true pole wander, in

4896-684: 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

4998-410: 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

5100-545: 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

5202-482: 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

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5304-671: The al-Idrisi Montes, which may have formed via the collapse of adjacent water ice highlands onto the planitia. On its southwest, the planitia is bordered by the Hillary Montes , rising 1.6 km (0.99 mi; 5,200 ft) above the surface, and, further south, the Tenzing Montes , rising 3.4 km (2.1 mi; 11,000 ft) above the surface. These mountains also have a chaotic, blocky character. The mountains were named after Sir Edmund Hillary , New Zealand mountaineer , and Nepalese Sherpa mountaineer Tenzing Norgay , who were

5406-452: The anomaly. Gradual freezing of such an ocean, in combination with polar wander and the loading of Sputnik Planitia with ice, would also explain the extensional tectonic features seen across Pluto. Alternatively, if the accumulation of ice in a single cap (without an impact) created a positive gravity anomaly that reoriented Pluto prior to formation of a basin, the tidal bulge raised by Charon might then have maintained Pluto's orientation even if

5508-497: 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

5610-517: The article Polar motion . The mass distribution of the Earth is not spherically symmetric, and the Earth has three different moments of inertia . The axis around which the moment of inertia is greatest is closely aligned with the rotation axis (the axis going through the geographic North and South Poles). The other two axes are near the equator . That is similar to a brick rotating around an axis going through its shortest dimension (a vertical axis when

5712-495: 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,

5814-434: The basin from adjacent eastern highlands (the right lobe of Tombaugh Regio), presumably in response to deposition of nitrogen ice there, as well as ice from the planitia flowing into and filling adjacent depressions. The planitia has numerous blocky hills (one to several km across) that form aggregations at cell margins up to 20 km across; these may represent floating chunks of detached water ice crust that were carried onto

5916-596: 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

6018-416: The brick is lying flat). However, if the moment of inertia around one of the two axes close to the equator becomes nearly equal to that around the polar axis, the constraint on the orientation of the object (the Earth) is relaxed. This situation is like a rugby football or an American football spinning around an axis running through its "equator". (Note that the "equator" of the ball does not correspond to

6120-424: The case of Europa by 80°. Uranus ' extreme inclination with respect to the ecliptic is not an instance of true polar wander (a shift of the body relative to its rotational axis), but instead a large shift of the rotational axis itself. This axis shift is believed to be the result of a catastrophic series of impacts that occurred billions of years ago. Polar wander should not be confused with precession , which

6222-598: 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

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6324-588: 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 the 1840s, Urbain Le Verrier used Newtonian mechanics to predict

6426-411: 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,

6528-461: 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

6630-555: 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

6732-421: The equator of the Earth.) Small perturbations can move the football, which then spins around another axis through the same "equator". In the same way, conditions can make the Earth (both the crust and the mantle) slowly reorient until a new geographic point moves to the North Pole, with the axis of low moment of inertia being kept very near the equator. Such a reorientation changes the latitudes of most points on

6834-422: 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

6936-484: The first climbers to reach the summit of the highest peak on Earth , Mount Everest , on 29 May 1953. Some groups of hills in the basin are named after spacecraft; for example, “ Coleta de Dados ”, in honor of the first Brazilian satellite launched into space . Immediately to the southwest of the Tenzing Montes ( context ) is a large, circular mountain with a central depression, Wright Mons . It has been identified as

7038-444: 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

7140-792: The focusing there of seismic energy from the formative impact. While this suggestion is tentative in view of the poor resolution of the imaging of the antipodal region, the concept is similar to what has been proposed for areas antipodal to the Caloris basin on Mercury and Mare Orientale on the Moon. A high seasonal thermal inertia of Pluto's surface is an important driver of deposition of nitrogen ice at low latitudes. These latitudes receive less annual insolation than Pluto's polar regions due to its high obliquity (122.5°). The coldest regions on Pluto, on average, are at 30° N. and S. latitude; early in Pluto's history, ice would tend to accumulate at these latitudes in

7242-452: The formation of a basin via a positive feedback process without an impact. The accumulation of several kilometers of nitrogen ice in the basin was in part a consequence of its higher surface pressure, which leads to a higher N 2 condensation temperature. The positive temperature gradient of Pluto's atmosphere contributes to making a topographic depression a cold trap. The terrain on Pluto antipodal to Sputnik Planitia may have been altered by

7344-412: 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] ⟩

7446-452: The heart-shaped Tombaugh Regio . Sputnik Planitia lies mostly in the northern hemisphere, but extends across the equator. Much of it has a surface of irregular polygons separated by troughs, interpreted as convection cells in the relatively soft nitrogen ice. The polygons average about 33 km (21 mi) across. In some cases troughs are populated by blocky mountains or hills, or contain darker material. There appear to be windstreaks on

7548-406: 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,

7650-429: The margins of convection cells. Often the bottoms of the pits are dark, which may represent an accumulation of tholins left behind by the subliming ice, or a dark substrate below the planitia, if the pits penetrate all the way through the ice. In regions of the planitia where convection cells are not apparent, the pits are more numerous. On its northwest, Sputnik Planitia is bordered by a chaotic set of blocky mountains,

7752-521: 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

7854-643: The northwest margin is a field of transverse dunes (perpendicular to the windstreaks), spaced about 0.4 to 1 km apart, that are thought to be composed of 200-300 μm diameter particles of methane ice derived from the nearby Al-Idrisi Montes . The ice composing the basin is thought to consist primarily of nitrogen ice , with smaller fractions of carbon monoxide and methane ice, although relative proportions are uncertain. At Pluto's ambient temperature of 38 K (−235.2 °C; −391.3 °F), nitrogen and carbon monoxide ices are denser and much less rigid than water ice, making glacial-like flows possible; nitrogen ice

7956-420: 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. True polar wander If the body is near the steady state but with the angular momentum not exactly lined up with the largest moment of inertia axis, the pole position will oscillate. Weather and water movements can also induce small changes. These subjects are covered in

8058-431: The planitia by glacial flow and were then collected into troughs by the convection. In some cases the hills seem to form chains along the entry paths of glaciers. The hills may also congregate in nonconvecting regions when they get stuck at locations where the nitrogen ice becomes too shallow. The planitia has numerous pits that are thought to result from fracturing and sublimation of nitrogen ice; these pits also collect in

8160-420: The planitia, and a maximum flow rate of about 7 cm per year. Cell margins can become pinched off and abandoned as cells evolve. Many of the cells are covered by sublimation pits. These pits grow larger by sublimation during transport from the centers to the edges of convection cells. Using their size distribution, scientists have estimated a convection velocity of 13.8 +4.8 −3.8 cm per year, implying

8262-487: 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

8364-434: The positive anomaly later disappeared. The creation of the gravity anomaly is thought to require thinning the ice crust by ~90 km below Sputnik Planitia. However, the crust must be kept cool to maintain such variations in its thickness. Modeling has suggested that this can be explained if underneath Pluto's water ice crust is a layer of methane hydrate . This clathrate has insulating properties; its thermal conductivity

8466-528: 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

8568-428: 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

8670-403: The surface with evidence of sublimation . The dark streaks are a few kilometers long and all aligned in the same direction. The planitia also contains pits apparently formed by sublimation. No craters were detectable by New Horizons , implying a surface less than 10 million years old. Modeling sublimation pit formation yields a surface age estimate of 180 000 +90 000 −40 000 years. Near

8772-550: 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,

8874-430: 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

8976-510: 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

9078-561: 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

9180-471: 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

9282-434: 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

9384-433: 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

9486-503: 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

9588-464: 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

9690-569: 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

9792-497: 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

9894-437: 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

9996-507: 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 ,

10098-477: 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

10200-442: 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

10302-403: Was still informal because it had not yet been adopted by the International Astronomical Union (IAU). On 7 September 2017, the name was officially approved together with the names of Tombaugh Regio and 12 other nearby surface features. Pluto Pluto ( minor-planet designation : 134340 Pluto ) is a dwarf planet in the Kuiper belt , a ring of bodies beyond the orbit of Neptune . It

10404-412: 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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