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90-520: Pluto ( minor-planet designation : 134340 Pluto ) 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

180-532: 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

270-440: 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

360-494: 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,

450-476: A million minor planets that received a number, only about 20 thousand (or 4%) have received a name . In addition, approximately 700,000 minor planets have not been numbered , as of November 2023. The convention for satellites of minor planets , such as the formal designation (87) Sylvia I Romulus for the asteroid moon Romulus , is an extension of the Roman numeral convention that had been used, on and off, for

540-424: 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

630-534: 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

720-523: 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

810-464: 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,

900-446: 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

990-596: 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

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1080-529: 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

1170-541: 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

1260-704: A temporary assistant. Slipher worked as an assistant from 1901 to 1915 when Lowell finally named him the assistant director of the observatory. One year later Percival Lowell died and Vesto became the acting director for the next ten years. In 1926, 25 years after arriving in Flagstaff, Slipher was named director of the Lowell Observatory. He remained in charge for 28 more years when he retired from professional life. Slipher spent his years there studying many things, but most notably, spectroscopy and redshifts of spiral nebulae . The first major task Slipher

1350-511: 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

1440-589: 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

1530-425: 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

1620-403: 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

1710-498: 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

1800-410: 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

1890-424: 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

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1980-524: 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

2070-408: 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,

2160-731: 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

2250-512: Is much smaller than the inner planets . Pluto has roughly one-sixth 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

2340-547: 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

2430-533: 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

2520-474: 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

2610-526: 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

2700-633: 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

2790-468: 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

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2880-696: 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

2970-568: 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

3060-472: 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

3150-544: The sodium layer in 1929. He was responsible for hiring Clyde Tombaugh and supervised the work that led to the discovery of Pluto in 1930. By 1917, Slipher had measured the radial velocities of 25 "spiral nebulae," and found that all but three of those galaxies were moving away from us, at substantial speeds. Slipher himself speculated that this might be due to the motion of our own galaxy – as in his sample, those galaxies moving towards us and those moving away from us were roughly in opposite directions. In hindsight, this

3240-619: 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

3330-401: 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

3420-694: 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

3510-539: The IAU Decision of October 2018 recommends the use of a new name ), was formulated by Hubble and Humason in 1929 and became the basis for the modern model of the expanding universe . Slipher married Emma R. Munger in 1904 in Frankfort, IN. Vesto and Emma had two children together, David Clark and Marcia Frances. In 1901, Vesto Slipher moved to Flagstaff, Arizona and began work at Lowell Observatory. He spent

3600-408: 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

3690-581: 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 was disfavored because it

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3780-478: 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

3870-409: 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

3960-492: 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

4050-490: 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,

4140-594: 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

4230-544: 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

4320-532: The discoverer of galactic redshifts . Using the Doppler effect and noting subtle changes, he measured the speeds in which spiral nebulae traveled during his research from 1912 and onward. These subtle changes in the speeds of the nebulae led Slipher to conclude that the nebulae were not within the Milky Way galaxy. In 1914, Slipher also made the first discovery of the rotation of spiral galaxies. He discovered

4410-410: 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,

4500-457: 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

4590-566: 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

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4680-421: The early twentieth century, Vesto Slipher elongated the spectrum to include the red and infrared wavelengths and showed that the major planets display strong absorption lines at many different wavelengths. Slipher used spectroscopy to investigate the rotation periods of planets and the composition of planetary atmospheres. In 1912, he was the first to observe the shift of spectral lines of galaxies , making him

4770-421: 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

4860-442: 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

4950-409: 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] ⟩

5040-403: 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,

5130-644: The main-belt asteroid 274301 Misplaced Pages was discovered in August 2008, it was provisionally designated 2008 QH 24 , before it received a number and was then written as (274301) 2008 QH 24 . On 27 January 2013, it was named Misplaced Pages after being published in the Minor Planet Circulars . According to the preference of the astronomer and publishing date of the journal, 274301 Misplaced Pages may be referred to as 2008 QH 24 , or simply as (274301) . In practice, for any reasonably well-known object

5220-452: The moons of the planets since Galileo 's time. Comets are also managed by the MPC, but use a different cataloguing system . A formal designation consists of two parts: a catalog number , historically assigned in approximate order of discovery, and either a name , typically assigned by the discoverer, or, the minor planet's provisional designation. The permanent syntax is: For example,

5310-470: 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

5400-659: The nebulae were moving. His discoveries were confirmed ten years later when Edwin Hubble used the Mount Wilson Observatory reflector to view the galaxies much more clearly. Slipher introduced as early as 1909 that the infrared spectrum could be recorded using photographic emulsions , and used those to record the absorption lines of sunlight and major planets. He found that the planets showed different absorption lines that were not present in sunlight, and identified those bands with ammonia and methane . In

5490-517: The number is mostly a catalogue entry, and the name or provisional designation is generally used in place of the formal designation. So Pluto is rarely written as 134340 Pluto, and 2002 TX 300 is more commonly used than the longer version (55636) 2002 TX 300 . By 1851 there were 15 known asteroids, all but one with their own symbol . The symbols grew increasingly complex as the number of objects grew, and, as they had to be drawn by hand, astronomers found some of them difficult. This difficulty

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5580-435: 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. Minor-planet designation Currently, a number is assigned only after the orbit has been secured by four well-observed oppositions . For unusual objects, such as near-Earth asteroids , numbering might already occur after three, maybe even only two, oppositions. Among more than half

5670-503: The same program. At the age of 33, Vesto graduated with his Ph.D. in Mechanics and Astronomy from Indiana University. While at school at Indiana University, Slipher formed a personal bond with one of his professors, William Cogshall. Cogshall was one of the main reasons Slipher became interested in astronomy in the first place. Cogshall convinced Percival Lowell , director of the Lowell Observatory in Flagstaff, AZ, to take Vesto in as

5760-486: 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

5850-426: 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

5940-547: 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,

6030-427: 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

6120-507: 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

6210-565: 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

6300-858: The universe. He was also the first to relate these redshifts to velocity . Vesto Melvin Slipher was born in Mulberry, Indiana , to Daniel Clark and Hannah App Slipher. He spent his early years working on his family farm in Mulberry. Vesto had a younger brother, Earl C. Slipher , who was also an astronomer at Lowell Observatory . Slipher went to high school in Frankfort, IN . He then attended Indiana University in Bloomington, IN and earned his Bachelor's Degree in Mechanics and Astronomy in June 1901. Two years later, Slipher earned his Master's Degree in

6390-495: The unnamed minor planet (388188) 2006 DP 14 has its number always written in parentheses, while for named minor planets such as (274301) Misplaced Pages, the parentheses may be dropped as in 274301 Misplaced Pages . Parentheses are now often omitted in prominent databases such as the JPL Small-Body Database . Since minor-planet designations change over time, different versions may be used in astronomy journals . When

6480-476: 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

6570-429: 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

6660-430: 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

6750-520: 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

6840-536: Was addressed by Benjamin Apthorp Gould in 1851, who suggested numbering asteroids in their order of discovery, and placing this number in a circle as the symbol for the asteroid, such as ④ for the fourth asteroid, Vesta . This practice was soon coupled with the name itself into an official number–name designation, "④ Vesta", as the number of minor planets increased. By the late 1850s, the circle had been simplified to parentheses, "(4)" and "(4) Vesta", which

6930-461: 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

7020-427: Was easier to typeset. Other punctuation such as "4) Vesta" and "4, Vesta" was also used, but had more or less completely died out by 1949. The major exception to the convention that the number tracks the order of discovery or determination of orbit is the case of Pluto. Since Pluto was initially classified as a planet, it was not given a number until a 2006 redefinition of "planet" that excluded it. At that point, Pluto

7110-609: 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

7200-493: 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

7290-490: Was given the formal designation (134340) Pluto. Vesto Melvin Slipher Vesto Melvin Slipher ( / ˈ s l aɪ f ər / ; November 11, 1875 – November 8, 1969) was an American astronomer who performed the first measurements of radial velocities for galaxies. He was the first to discover that distant galaxies are redshifted , thus providing the first empirical basis for the expansion of

7380-456: Was given was to measure our solar system's planets' rotation interval. He was one of the first astronomers to show that Uranus has a much faster rotation that Earth , similar to the other giant planets in our solar system. What Vesto is most known for though is his work with spiral nebulae, or, spiral galaxies, like the Milky Way and Andromeda . His initial goal was to measure how fast

7470-434: 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

7560-515: 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 ,

7650-563: 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

7740-476: 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

7830-441: 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

7920-419: Was the first data supporting models of an expanding universe . Later, Slipher's and additional spectroscopic measurements of radial velocities were combined by Edwin Hubble with Hubble's own determinations of galaxy distances, leading Hubble to discover the (at that time, rough) proportionality between galaxies' distances and redshifts, which is today termed Hubble–Lemaître's law (formerly named as Hubble's law,

8010-406: 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

8100-408: 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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