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Pluto

January 07, 2023

This article is about the dwarf planet. For the deity, see Pluto (mythology). For other uses, see Pluto (disambiguation).

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 slightly 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. Compared to Earth's moon, Pluto has only one sixth its mass and one third its volume.

134340 Pluto or
Northern hemisphere of Pluto in true color, taken by NASA's New Horizons probe in 2015[a]
Discovery
Discovered byClyde W. Tombaugh
Discovery siteLowell Observatory
Discovery dateFebruary 18, 1930
Designations
Designation
(134340) Pluto
Pronunciation/ˈpluːtoʊ/ (listen)
Named after
Pluto
AdjectivesPlutonian /pluːˈtoʊniən/[1]
Orbital characteristics[4][b]
Epoch J2000
Earliest precovery dateAugust 20, 1909
Aphelion
  • 49.305 AU
  • (7.37593 billion km)
  • February 2114
Perihelion
  • 29.658 AU
  • (4.43682 billion km)[2]
  • (September 5, 1989)[3]
  • 39.482 AU
  • (5.90638 billion km)
Eccentricity0.2488
366.73 days[2]
4.743 km/s[2]
14.53 deg
Inclination
  • 17.16°
  • (11.88° to Sun's equator)
110.299°
113.834°
Known satellites5
Physical characteristics
Dimensions2,376.6±1.6 km (observations consistent with a sphere, predicted deviations too small to be observed)[5]
Mean radius
Flattening<1%[7]
  • 1.774443×107 km2[c]
  • 0.035 Earths
Volume
  • (7.057±0.004)×109 km3[d]
  • 0.00651 Earths
Mass
Mean density
1.854±0.006 g/cm3[6][7]
1.212 km/s[f]
  • −6.38680 d
  • −6 d, 9 h, 17 m, 00 s
[8]
  • −6.387230 d
  • −6 d, 9 h, 17 m, 36 s
Equatorial rotation velocity
47.18 km/h
122.53° (to orbit)[2]
North pole right ascension
132.993°[9]
North pole declination
−6.163°[9]
Albedo0.52 geometric[2]
0.72 Bond[2]
Surface temp. min mean max
Kelvin 33 K 44 K (−229 °C) 55 K
13.65[2] to 16.3[10]
(mean is 15.1)[2]
−0.44[11]
0.06″ to 0.11″[2][g]
Atmosphere
Surface pressure
1.0 Pa (2015)[7][13]
Composition by volumeNitrogen, methane, carbon monoxide[12]
Pluto compared in size to the Earth and Moon

Pluto has a moderately eccentric and inclined orbit, ranging from 30 to 49 astronomical units (4.5 to 7.3 billion kilometers; 2.8 to 4.6 billion miles) from the Sun. Light from the Sun takes 5.5 hours to reach Pluto at its average distance (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 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. The New Horizons mission 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, the first object in the Kuiper belt. It was immediately hailed as the ninth planet, but its planetary status was questioned when it 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.

History

Discovery

Further information: Planets beyond Neptune
 
Discovery photographs of Pluto
 
Clyde Tombaugh, in Kansas

In 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.[14]

In 1906, Percival Lowell—a wealthy Bostonian who had founded Lowell Observatory in Flagstaff, Arizona, in 1894—started an extensive project in search of a possible ninth planet, which he termed "Planet X".[15] By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet.[16] Lowell and his observatory conducted his search 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.[16][17] There are fourteen other known precovery observations, with the earliest made by the Yerkes Observatory on August 20, 1909.[18]

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.[19] 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 a sample of his astronomical drawings.[19]

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 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.[20] After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.[16]

As one Plutonian year corresponds to 247.94 Earth years,[2] Pluto will complete its first orbit since its discovery in 2178.

Name and symbol

 
Mosaic of best-resolution images of Pluto from different angles

The discovery made headlines around the globe.[21] Lowell Observatory, which had the right to name the new object, received more than 1,000 suggestions from all over the world, ranging from Atlas to Zymal.[22] Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.[22] Constance Lowell proposed Zeus, then Percival and finally Constance. These suggestions were disregarded.[23]

The name Pluto, after the Greek/Roman god of the underworld, was proposed by Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford, England, who was interested in classical mythology.[24] She suggested it in a conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford's Bodleian Library, who passed the name to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States.[24]

Each member of the Lowell Observatory was allowed to vote on a short-list of three potential names: Minerva (which was already the name for an asteroid), Cronus (which had lost reputation through being proposed by the unpopular astronomer Thomas Jefferson Jackson See), and Pluto. Pluto received a unanimous vote.[25] The name was published on May 1, 1930.[24][26] Upon the announcement, Madan gave Venetia £5 (equivalent to £336 in 2021[27], or US$394 in 2021[28][29]) as a reward.[24]

The final choice of name was helped in part by the fact that the first two letters of Pluto are the initials of Percival Lowell. Pluto's planetary symbol ⟨ ⟩ was then created as a monogram of the letters "PL" (in Unicode: U+2647 ♇ PLUTO),[30] though it is rarely used in astronomy today. For example, ⟨♇⟩ occurs in a table of the planets identified by their symbols in a 2004 article written before the 2006 IAU definition,[31] but not in a graph of planets, dwarf planets and moons from 2016, where only the eight IAU planets are identified by their symbols.[32] (Planetary symbols in general are uncommon in astronomy, and are discouraged by the IAU.)[33] The ♇ monogram is also used in astrology, but the most-common astrological symbol for Pluto, at least in English-language sources, is an orb over Pluto's bident ⟨ ⟩ (U+2BD3 ⯓ PLUTO FORM TWO). The bident symbol has seen some astronomical use as well since the IAU decision on dwarf planets, for example in a public-education poster on dwarf planets published by the NASA/JPL Dawn mission in 2015, in which each of the five dwarf planets announced by the IAU receives a symbol.[34] There are in addition several other symbols for Pluto found in European astrological sources, including three accepted by Unicode:  , U+2BD4 ⯔ PLUTO FORM THREE;  , U+2BD5 ⯕ PLUTO FORM FOUR, used in Uranian astrology; and  / , U+2BD6 ⯖ PLUTO FORM FIVE, found in various orientations, showing Pluto's orbit cutting across that of Neptune.[35]

The name 'Pluto' 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.[36] 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 was named after Uranus, and neptunium, which was named after Neptune.[37]

Most languages use the name "Pluto" in various transliterations.[h] 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, the God of Death in Hinduism.[38] Polynesian languages also tend to use the indigenous god of the underworld, as in Māori Whiro.[38] 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".[39][38][40]

Planet X disproved

Once Pluto was found, its faintness and lack of a viewable disc cast doubt on the idea that it was Lowell's Planet X.[15] Estimates of Pluto's mass were revised downward throughout the 20th century.[41]

Mass estimates for Pluto
Year Mass Estimate by
1915 7 Earths Lowell (prediction for Planet X)[15]
1931 1 Earth Nicholson & Mayall[42][43][44]
1948 0.1 (1/10) Earth Kuiper[45]
1976 0.01 (1/100) Earth Cruikshank, Pilcher, & Morrison[46]
1978 0.0015 (1/650) Earth Christy & Harrington[47]
2006 0.00218 (1/459) Earth Buie et al.[48]

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.[43][45] In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii 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.[46] (Pluto's albedo is 1.4–1.9 times that of Earth.[2])

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 the discrepancies in the orbit of Uranus. Subsequent searches for an alternative Planet X, notably by Robert Sutton Harrington,[49] 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, the discrepancies, and with them the need for a Planet X, vanished.[50] Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist.[51] 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.[52]

Classification

Further information: Definition of planet
EarthMoonCharonCharonNixNixKerberosKerberosStyxStyxHydraHydraPlutoPlutoDysnomiaDysnomiaErisErisNamakaNamakaHi'iakaHi'iakaHaumeaHaumeaMakemakeMakemakeMK2MK2XiangliuXiangliuGonggongGonggongWeywotWeywotQuaoarQuaoarSednaSednaVanthVanthOrcusOrcusActaeaActaeaSalaciaSalacia2002 MS42002 MS4 
Artistic comparison of Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Orcus, Salacia, 2002 MS4, and Earth along with the Moon

From 1992 onward, many bodies were discovered orbiting in 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 the Hayden Planetarium in New York City displayed a Solar System model of only eight planets, which made headlines almost a year later.[53]

Ceres, Pallas, Juno and Vesta lost their planet status after the discovery of many other asteroids. Similarly, objects increasingly closer in size to Pluto were discovered in the Kuiper belt region. On July 29, 2005, astronomers at Caltech announced the discovery of a new trans-Neptunian object, Eris, which was substantially more massive than Pluto and 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.[54] Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.[55]

IAU classification

Main article: IAU definition of planet

The debate came to a head in August 2006, with an IAU resolution that created an official definition for 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.[58] 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).[59][57] 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, 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".[60] 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.[61]

There has been some resistance within the astronomical community toward the reclassification.[62][63][64] Alan Stern, principal investigator with NASA's New Horizons mission to Pluto, derided the IAU resolution, stating that "the definition stinks, for technical reasons".[65] Stern contended that, by the terms of the new definition, Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded[66] (even though he had himself previously suggested a criterion for clearing the neighbourhood, which considered all four of them to have done so).[67] He argued that all big spherical moons, including the Moon, should likewise be considered planets.[68] He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.[66] Marc W. Buie, then at the Lowell Observatory, petitioned against the definition.[69] Others have supported the IAU. Mike Brown, the astronomer who discovered Eris, said "through this whole crazy, circus-like procedure, somehow the right answer was stumbled on. It's been a long time coming. Science is self-correcting eventually, even when strong emotions are involved."[70]

Public reception to the IAU decision was mixed. A resolution introduced in the California State Assembly facetiously called the IAU decision a "scientific heresy".[71] The New Mexico House of Representatives passed a resolution in honor of Tombaugh, 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.[72][73] The Illinois Senate passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois. The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU."[74] 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.[75]

In 2006, in its 17th annual words-of-the-year vote, the American Dialect Society voted plutoed as the word of the year. To "pluto" is to "demote or devalue someone or something".[76]

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 current IAU definition of a planet.[77] Entitled "The Great Planet Debate",[78] the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of planet.[79] 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 the term has not seen significant use.[80][81][82]

Orbit

 
Pluto was discovered in 1930 near the star δ Geminorum, and merely coincidentally crossing the ecliptic at this time of discovery. Pluto moves about 7 degrees east per decade with small apparent retrograde motion as seen from Earth. Pluto was closer to the Sun than Neptune between 1979 and 1999.
 
Animation of Pluto's orbit from 1850 to 2097
   Sun ·    Saturn ·    Uranus ·    Neptune ·    Pluto

Pluto's orbital period is currently 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 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,[3][i] and was last closer to the Sun than Neptune between February 7, 1979, and February 11, 1999.[83]

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 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.[84][85]

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.[86]

 
Orbit of Pluto – ecliptic view. This "side view" of Pluto's orbit (in red) shows its large inclination to the ecliptic.
 
Orbit of Pluto – polar view. This "view from above" shows how Pluto's orbit (in red) is less circular than Neptune's (in blue), and how Pluto is sometimes closer to the Sun than Neptune. The darker sections of both orbits show where they pass below the plane of the ecliptic.

Relationship with Neptune

Despite Pluto's orbit appearing to cross that of Neptune when viewed from directly above, the two objects' orbits do not intersect. When Pluto is closest to the Sun, and close to Neptune's orbit as viewed from above, it is also the farthest above Neptune's path. Pluto's orbit passes about 8 AU above that of Neptune, preventing a collision.[87][88][89][j]

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 a collision could be possible. 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. Each cycle lasts about 495 years. (There are many other objects in this same resonance, called plutinos.) This pattern is such that, in each 495-year cycle, the first time Pluto is near perihelion, Neptune is over 50° behind Pluto. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be nearly 130° ahead of Pluto. Pluto and Neptune's minimum separation is over 17 AU, which is greater than Pluto's minimum separation from Uranus (11 AU).[89] The minimum separation between Pluto and Neptune actually occurs near the time of Pluto's aphelion.[86]

The 2:3 resonance between the two bodies is highly stable and has been preserved over millions of years.[91] This prevents their orbits from changing relative to one another, and so the two bodies can never pass near each other. Even if Pluto's orbit were not inclined, the two bodies could never collide.[89] The long term stability of the mean-motion resonance is due to phase protection. When Pluto's period is slightly shorter than 3/2 of Neptune, its orbit relative to Neptune will drift, causing it to make closer approaches behind Neptune's orbit. The gravitational pull between the two then causes angular momentum to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger orbit, where it travels slightly more slowly, according to Kepler's third law. After many such repetitions, Pluto is sufficiently slowed that Pluto's orbit relative to Neptune drifts in the opposite direction until the process is reversed. The whole process takes about 20,000 years to complete.[89][91][92]

Other factors

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.[87][86] There are several other resonances and interactions that enhance Pluto's stability. These arise principally from two additional mechanisms (besides the 2:3 mean-motion resonance).

First, Pluto's argument of perihelion, the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, librates around 90°.[86] This means that when Pluto is closest to the Sun, it is at its farthest above the plane of the Solar System, preventing encounters with Neptune. This is a consequence of the Kozai mechanism,[87] 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.[91]

Second, the longitudes of ascending nodes of the two bodies—the points where they cross the ecliptic—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 highest above Neptune's orbit. This is known as the 1:1 superresonance. All the Jovian planets, particularly Jupiter, play a role in the creation of the superresonance.[87]

Quasi-satellite

In 2012, it was hypothesized that 15810 Arawn could be a quasi-satellite of Pluto, a specific type of co-orbital configuration.[93] According to the hypothesis, the object would be a quasi-satellite of Pluto for about 350,000 years out of every two-million-year period.[93][94] Measurements made by the New Horizons spacecraft in 2015 made it possible to calculate the orbit of Arawn more accurately.[95] These calculations confirm the overall dynamics described in the hypothesis.[96] However, it is not agreed upon among astronomers whether Arawn should be classified as a quasi-satellite of Pluto based on this motion, since its orbit is primarily controlled by Neptune with only occasional smaller perturbations caused by Pluto.[97][95][96]

Rotation

Pluto's rotation period, its day, is equal to 6.387 Earth days.[2][98] Like Uranus, 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.[99] The reason for this unusual orientation has been debated. Research from the University of Arizona has suggested that it may be due to the way that a body's spin will always adjust to minimise 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.[100] 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 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 effect seen on Pluto would be observed on Earth were the Antarctic ice sheet is several times larger.[101]

Geology

Main articles: Geology of Pluto and Geography of Pluto

Surface

 
High-resolution MVIC image of Pluto in enhanced color to bring out differences in surface composition
 
Regions where water ice has been detected (blue regions)

The plains on Pluto's surface are composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.[102] Nitrogen and carbon monoxide are most abundant on 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.[103] The mountains are made of water ice.[104] Pluto's surface is quite varied, with large differences in both brightness and color.[105] Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn's moon Iapetus.[106] The color varies from charcoal black, to dark orange and white.[107] Pluto's color is more similar to that of Io with slightly more orange and significantly less red than Mars.[108] Notable geographical features include Tombaugh Regio, or the "Heart" (a large bright area on the side opposite Charon), Cthulhu Macula,[6] 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, 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;[109][110][111] there are obvious signs of glacial flows both into and out of the basin.[112][113] It has no craters that were visible to New Horizons, indicating that its surface is less than 10 million years old.[114] Latest studies have shown that the surface has an age of 180000+90000
−40000 years.[115] 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."[7]

 
Distribution of over 1000 craters of all ages in the northern anti-Charon quadrant of Pluto. The variation in density (with none found in Sputnik Planitia) indicates a long history of varying geological activity. The lack of crater on the left and right of the map is due to low-resolution coverage of those sub-Charon regions.
 
Geologic map of Sputnik Planitia and surroundings (context), with convection cell margins outlined in black
 
Sputnik Planitia is covered with churning nitrogen ice "cells" that are geologically young and turning over due to convection.

In Western parts of Sputnik Planitia there are fields of transverse dunes formed by 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 they are likely consists of methane particles 200–300 μm in size.[116]

Internal structure

 
Model of the internal structure of Pluto[117]
  • Water ice crust
  • Liquid water ocean
  • Silicate core

Pluto's density is 1.860±0.013 g/cm3.[7] 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 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.[117] Pluto has no magnetic field.[118]

It is possible that such heating continues today, creating a subsurface ocean of liquid water 100 to 180 km thick at the core–mantle boundary.[117][119][120] In September 2016, scientists at Brown University simulated the impact thought to have formed Sputnik Planitia, and showed that it might have been the result of liquid water upwelling from below after the collision, implying the existence of a subsurface ocean at least 100 km deep.[121] 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.[122][123] In March 2022, they concluded that peaks on Pluto are actually a merger of "ice volcanoes", suggesting a source of heat on the body at levels previously thought not possible.[124]

Mass and size

 
Pluto (bottom right) compared in size to the largest satellites in the solar system (from left to right and top to bottom): Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton

Pluto's diameter is 2376.6±3.2 km[5] and its mass is (1.303±0.003)×1022 kg, 17.7% that of the Moon (0.22% that of Earth).[125] Its surface area is 1.774443×107 km2, or just slightly bigger than Russia. Its surface gravity is 0.063 g (compared to 1 g for Earth and 0.17 g for the Moon).[2]

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.[126]

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 is much less than thought before Charon was discovered.[127]

Pluto is more than twice the diameter and a dozen times the mass of Ceres, the largest object in the asteroid belt. It 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[5] compared to Eris's approximate diameter of 2,326 km.[128]

Determinations of Pluto's size have been complicated by its atmosphere[129] and hydrocarbon haze.[130] In March 2014, Lellouch, de Bergh et al. published findings regarding methane mixing ratios in Pluto's atmosphere consistent with a Plutonian diameter greater than 2,360 km, with a "best guess" of 2,368 km.[131] 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 (1,470 mi),[128][132] which was later revised to be 2,372 km (1,474 mi) on July 24,[133] and later to 2374±8 km.[7] Using radio occultation data from the New Horizons Radio Science Experiment (REX), the diameter was found to be 2376.6±3.2 km.[5]

The masses of Pluto and Charon compared to other dwarf planets (Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, Ceres) and to the icy moons Triton (Neptune I), Titania (Uranus III), Oberon (Uranus IV), Rhea (Saturn V) and Iapetus (Saturn VIII). The unit of mass is ×1021 kg.
Selected size estimates for Pluto
Year Radius Notes
1993 1195 km Millis, et al.[134] (if no haze)[130]
1993 1180 km Millis, et al. (surface & haze)[130]
1994 1164 km Young & Binzel[135]
2006 1153 km Buie, et al.[48]
2007 1161 km Young, Young, & Buie[129]
2011 1180 km Zalucha, et al.[136]
2014 1184 km Lellouch, et al.[131]
2015 1187 km New Horizons measurement (from optical data)[133]
2017 1188.3 km New Horizons measurement (from radio occultation data)[5][6]

Atmosphere

Main article: Atmosphere of Pluto
 
A near-true-color image taken by New Horizons after its flyby. Numerous layers of blue haze float in Pluto's atmosphere. Along and near the limb, mountains and their shadows are visible.
 
Image of Pluto in X-rays by Chandra X-ray Observatory (blue spot). The X-rays are probably created by interaction of the gases surrounding Pluto with solar wind, although details of their origin are not clear.

Pluto has a tenuous atmosphere consisting of nitrogen (N2), methane (CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto's surface.[137][138] According to the measurements by New Horizons, the surface pressure is about 1 Pa (10 μbar),[7] roughly one million to 100,000 times less than Earth's atmospheric pressure. It 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.[139][140] New Horizons observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected.[140] Alan Stern has contended that even 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.[140] Just like sweat cools the body as it evaporates from the skin, the sublimation of Pluto's atmosphere cools its surface.[141] Pluto has no or almost no troposphere; observations by New Horizons suggest only 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.[142]

In July 2019, an occultation by Pluto showed that its atmospheric pressure, against expectations, had fallen by 20% since 2016.[143] In 2021, astronomers at the Southwest Research Institute confirmed the result using data from an occultation in 2018, which showed that light was appearing less gradually from behind Pluto's disc, indicating a thinning atmosphere.[144]

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,[145] 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).[140] Pluto's atmosphere is divided into roughly 20 regularly spaced haze layers up to 150 km high,[7] thought to be the result of pressure waves created by airflow across Pluto's mountains.[140]

Satellites

 
An oblique view of the Pluto–Charon system showing that Pluto orbits a point outside itself. The two bodies are mutually tidally locked.
Main article: Moons of Pluto

Pluto has five known natural satellites. The 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 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,[146] Kerberos was discovered in 2011,[147] and Styx was discovered in 2012.[148] The satellites' orbits are circular (eccentricity < 0.006) and coplanar with Pluto's equator (inclination < 1°),[149][150] and therefore tilted approximately 120° relative to Pluto's orbit. The Plutonian system is highly compact: the five known satellites orbit within the inner 3% of the region where prograde orbits would be stable.[151]

The orbital periods of all Pluto's moons are linked in a system of orbital resonances and near resonances.[150][152] When precession is accounted for, the orbital periods of Styx, Nix, and Hydra are in an exact 18:22:33 ratio.[150] There is a sequence of approximate ratios, 3:4:5:6, between 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.[150][153]

The Pluto–Charon system is one of the few in the Solar System whose barycenter lies outside the primary body; the Patroclus–Menoetius system is a smaller example, and the Sun–Jupiter system is the only larger one.[154] The similarity in size of Charon and Pluto has prompted some astronomers to call it a double dwarf planet.[155] The system is also unusual among planetary systems in that each is tidally locked to the other, which means that Pluto and Charon always have the same hemisphere facing each other — a property shared by only one other known system, Eris and Dysnomia.[156] From any position on either body, the other is always at the same position in the sky, or always obscured.[157] This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its barycenter.[98]

In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.[158]

Pluto's moons are hypothesized to have been formed by a collision between Pluto and a similar-sized body, early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto.[159]

  •  

    The Pluto system: Pluto, Charon, Styx, Nix, Kerberos, and Hydra, imaged by the Hubble Space Telescope in July 2012.

  •  

    Pluto and Charon, to scale. Image acquired by New Horizons on July 8, 2015.

  •  

    Family portrait of the five moons of Pluto, to scale.[160]

  •  

    Pluto's moon Charon as viewed by New Horizons on July 13, 2015

Origin

Further information: Kuiper belt and Nice model
 
Plot of the known Kuiper belt objects, set against the four giant planets

Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune[161] knocked out of orbit by Neptune's largest current moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit.[162]

Pluto's true place in the Solar System began to reveal itself only in 1992, when astronomers began to find small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This trans-Neptunian population is thought to be the source of many short-period comets. Pluto is now known to be the largest member of the Kuiper belt,[k] a stable belt of objects located between 30 and 50 AU from the Sun. As of 2011, surveys of the Kuiper belt to magnitude 21 were nearly complete and any remaining Pluto-sized objects are expected to be beyond 100 AU from the Sun.[163] Like other Kuiper-belt objects (KBOs), Pluto shares features with comets; for example, the solar wind is gradually blowing Pluto's surface into space.[164] It has been claimed that if Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.[165] This claim has been disputed with the argument that Pluto's escape velocity is too high for this to happen.[166] It has been proposed that Pluto may have formed as a result of the agglomeration of numerous comets and Kuiper-belt objects.[167][168]

Though Pluto is the largest Kuiper belt object discovered,[130] Neptune's moon Triton, which is larger than Pluto, is similar to it both geologically and atmospherically, and is thought to be a captured Kuiper belt object.[169] Eris (see above) is about the same size as Pluto (though more massive) but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the scattered disc.[170]

Many Kuiper belt objects, like Pluto, are in a 2:3 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto.[171]

Like other members of the Kuiper belt, Pluto is thought to be a residual planetesimal; a component of the original protoplanetary disc around the Sun that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its current position to a sudden migration undergone by Neptune early in the Solar System's formation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit around itself (Triton), locking others into resonances, and knocking others into chaotic orbits. The objects in the scattered disc, a dynamically unstable region overlapping the Kuiper belt, are thought to have been placed in their current positions by interactions with Neptune's migrating resonances.[172] A computer model created in 2004 by Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice suggested that the migration of Neptune into the Kuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and Saturn, which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places, ultimately doubling Neptune's distance from the Sun. The resultant expulsion of objects from the proto-Kuiper belt could also explain the Late Heavy Bombardment 600 million years after the Solar System's formation and the origin of the Jupiter trojans.[173] It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration perturbed it into a resonant capture.[174] The Nice model requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk, which included Triton and Eris.[173]

Observation and exploration

Pluto's distance from Earth makes its in-depth study and exploration difficult. On July 14, 2015, NASA's New Horizons space probe flew through the Pluto system, providing much information about it.[175]

Observation

 
Computer-generated rotating image of Pluto based on observations by the Hubble Space Telescope in 2002–2003

Pluto's visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion.[2] To see it, a telescope is required; around 30 cm (12 in) aperture being desirable.[176] It looks star-like and without a visible disk even in large telescopes,[177] because its angular diameter is maximum 0.11".[2]

The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto–Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time.[178][179]

Better maps were produced from images taken by the Hubble Space Telescope (HST), which offered higher resolution, and showed considerably more detail,[106] resolving variations several hundred kilometers across, including polar regions and large bright spots.[108] These maps were produced by complex computer processing, which finds the best-fit projected maps for the few pixels of the Hubble images.[180] These remained the most detailed maps of Pluto until the flyby of New Horizons in July 2015, because the two cameras on the HST used for these maps were no longer in service.[180]

Exploration

Main articles: Exploration of Pluto and New Horizons
 
The portions of Pluto's surface mapped by New Horizons (annotated)
 
Panoramic view of Pluto's icy mountains and flat ice plains, imaged by New Horizons 15 minutes after its closest approach to Pluto. Distinct haze layers in Pluto's atmosphere can be seen backlit by the Sun.

The New Horizons spacecraft, which flew by Pluto in July 2015, is the first and so far only attempt to explore Pluto directly. Launched in 2006, it captured its first (distant) images of Pluto in late September 2006 during a test of the Long Range Reconnaissance Imager.[181] The images, taken from a distance of approximately 4.2 billion kilometers, confirmed the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper belt objects. In early 2007 the craft made use of a gravity assist from Jupiter.

New Horizons made its closest approach to Pluto on July 14, 2015, after a 3,462-day journey across the Solar System. Scientific observations of Pluto began five months before the closest approach and continued for at least a month after the encounter. Observations were conducted using a remote sensing package that included imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments. The scientific goals of New Horizons were to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition, and analyze Pluto's neutral atmosphere and its escape rate. On October 25, 2016, at 05:48 pm ET, the last bit of data (of a total of 50 billion bits of data; or 6.25 gigabytes) was received from New Horizons from its close encounter with Pluto.[182][183][184][185]

Since the New Horizons flyby, scientists have advocated for an orbiter mission that would return to Pluto to fulfill new science objectives.[186][187][188] They include mapping the surface at 9.1 m (30 ft) per pixel, observations of Pluto's smaller satellites, observations of how Pluto changes as it rotates on its axis, investigations of a possible subsurface ocean, and topographic mapping of Pluto's regions that are covered in long-term darkness due to its axial tilt. The last objective could be accomplished using laser pulses to generate a complete topographic map of Pluto. New Horizons principal investigator Alan Stern has advocated for a Cassini-style orbiter that would launch around 2030 (the 100th anniversary of Pluto's discovery) and use Charon's gravity to adjust its orbit as needed to fulfill science objectives after arriving at the Pluto system.[189] The orbiter could then use Charon's gravity to leave the Pluto system and study more KBOs after all Pluto science objectives are completed. A conceptual study funded by the NASA Innovative Advanced Concepts (NIAC) program describes a fusion-enabled Pluto orbiter and lander based on the Princeton field-reversed configuration reactor.[190][191]

Sub-Charon hemisphere

The equatorial region of the sub-Charon hemisphere of Pluto has only been imaged at low resolution, as New Horizons made its closest approach to the anti-Charon hemisphere.[192]

  • Composite image maps of Pluto from July 14, 2015 (updated 2019)
  •  

    A composite image of the sub-Charon hemisphere of Pluto. The region inside/below the white line was on the far side of Pluto when New Horizons made its closest approach, and was only imaged (at lower resolution) in the early days of the flyby. Black regions were not imaged at all.

  •  

    The low-resolution area, with named features labeled

  •  

    The low-resolution area, with features classified by geological type

Sources:[193][194]

Southern hemisphere

New Horizons imaged all of Pluto's northern hemisphere, and the equatorial regions down to about 30° South. Higher southern latitudes have only been observed, at very low resolution, from Earth.[195] Images from the Hubble Space Telescope in 1996 cover 85% of Pluto and show large albedo features down to about 75° South.[196][197] This is enough to show the extent of the temperate-zone maculae. Later images had slightly better resolution, due to minor improvements in Hubble instrumentation.[198]

Some albedo variations in the higher southern latitudes could be detected by New Horizons using Charon-shine (light reflected off Charon). The south polar region seems to be darker than the north polar region, but there is a high-albedo region in the southern hemisphere that may be a regional nitrogen or methane ice deposit.[199]

  •  

    A map of Pluto based on Hubble images from 1996, centered on the anti-Charon hemisphere (Sputnik Planitia), covering the southern hemisphere down to 75°S

  •  

    (a) Synthesized HST map of Pluto from Buie et al. (2010).
    (b) Colorized New Horizons MVIC and LORRI mosaic.
    (c–d) Destreaked Charon-illuminated image stack, shown to approximately the same stretch. The green line is the limit of the sub-Charon hemisphere.[199]

Videos

Pluto flyover animated (July 14, 2015)
(00:30; released September 18, 2015)
(00:50; released December 5, 2015)
This mosaic strip – extending across the hemisphere that faced the New Horizons spacecraft as it flew past Pluto.

See also

Notes

  1. ^ This photograph was taken by the Ralph telescope aboard New Horizons on July 14, 2015 from a distance of 35,445 km (22,025 mi). The most prominent feature in the image, the bright, youthful plains of Tombaugh Regio and Sputnik Planitia, can be seen at right. It contrasts the darker, more cratered terrain of Cthulhu Macula at lower left. Because of Pluto's 119.591° tilt at its axis, the southern hemisphere is barely visible in this image; the equator runs through Cthulhu Macula and the southern parts of Sputnik Planitia.
  2. ^ The mean elements here are from the Theory of the Outer Planets (TOP2013) solution by the Institut de mécanique céleste et de calcul des éphémérides (IMCCE). They refer to the standard equinox J2000, the barycenter of the Solar System, and the epoch J2000.
  3. ^ Surface area derived from the radius r:  .
  4. ^ Volume v derived from the radius r:  .
  5. ^ Surface gravity derived from the mass M, the gravitational constant G and the radius r:  .
  6. ^ Escape velocity derived from the mass M, the gravitational constant G and the radius r:  .
  7. ^ Based on geometry of minimum and maximum distance from Earth and Pluto radius in the factsheet
  8. ^ The equivalence is less close in languages whose phonology differs widely from Greek's, such as Somali Buluuto and Navajo Tłóotoo.
  9. ^ The discovery of Charon in 1978 allowed astronomers to accurately calculate the mass of the Plutonian system. But it did not indicate the two bodies' individual masses, which could only be estimated after other moons of Pluto were discovered in late 2005. As a result, because Pluto came to perihelion in 1989, most Pluto perihelion date estimates are based on the Pluto–Charon barycenter. Charon came to perihelion 4 September 1989. The Pluto–Charon barycenter came to perihelion 5 September 1989. Pluto came to perihelion 8 September 1989.
  10. ^ Because of the eccentricity of Pluto's orbit, some have theorized that it was once a satellite of Neptune.[90]
  11. ^ The dwarf planet Eris is roughly the same size as Pluto, about 2330 km; Eris is 28% more massive than Pluto. Eris is a scattered-disc object, often considered a distinct population from Kuiper-belt objects like Pluto; Pluto is the largest body in the Kuiper belt proper, which excludes the scattered-disc objects.

References

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Further reading

  • Codex Regius (2016), Pluto & Charon, CreateSpace Independent Publishing Platform ISBN 978-1534960749
  • Stern, S A and Tholen, D J (1997), Pluto and Charon, University of Arizona Press ISBN 978-0816518401
  • Stern, Alan; Grinspoon, David (2018). Chasing New Horizons: Inside the Epic First Mission to Pluto. Picador. ISBN 978-125009896-2.
  • Stern, S. Alan (August 10, 2021). The Pluto System After New Horizons. University of Arizona Press. p. 688. ISBN 978-0816540945.

External links

  • New Horizons homepage
  • at NASA's Solar System Exploration site
  • NASA Pluto factsheet
  • Website of the observatory that discovered Pluto
  • Keck infrared with AO of Pluto system
  • Gray, Meghan (2009). "Pluto". Sixty Symbols. Brady Haran for the University of Nottingham.
  • Video – Pluto – viewed through the years (GIF) (NASA; animation; July 15, 2015).
  • Video – Pluto – "FlyThrough" (00:22; MP4) (YouTube) (NASA; animation; August 31, 2015).
  • "A Day on Pluto Video made from July 2015 New Horizon Images" Scientific American
  • NASA CGI video of Pluto flyover (July 14, 2017)
  • CGI video simulation of rotating Pluto by Seán Doran (see album for more)
  • Google Pluto 3D, interactive map of the dwarf planet
  • . Archived from the original on June 11, 2020.

January 07, 2023
pluto, this, article, about, dwarf, planet, deity, mythology, other, uses, disambiguation, minor, planet, designation, 134340, dwarf, planet, kuiper, belt, ring, bodies, beyond, orbit, neptune, ninth, largest, tenth, most, massive, known, object, directly, orb. This article is about the dwarf planet For the deity see Pluto mythology For other uses see Pluto disambiguation 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 slightly 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 Compared to Earth s moon Pluto has only one sixth its mass and one third its volume 134340 Pluto orNorthern hemisphere of Pluto in true color taken by NASA s New Horizons probe in 2015 a DiscoveryDiscovered byClyde W TombaughDiscovery siteLowell ObservatoryDiscovery dateFebruary 18 1930DesignationsDesignation 134340 PlutoPronunciation ˈ p l uː t oʊ listen Named afterPlutoMinor planet categoryDwarf planet Trans Neptunian object Kuiper belt object PlutinoAdjectivesPlutonian p l uː ˈ t oʊ n i e n 1 Orbital characteristics 4 b Epoch J2000Earliest precovery dateAugust 20 1909Aphelion49 305 AU 7 37593 billion km February 2114Perihelion29 658 AU 4 43682 billion km 2 September 5 1989 3 Semi major axis39 482 AU 5 90638 billion km Eccentricity0 2488Orbital period sidereal 247 94 years 2 90 560 d 2 Orbital period synodic 366 73 days 2 Average orbital speed4 743 km s 2 Mean anomaly14 53 degInclination17 16 11 88 to Sun s equator Longitude of ascending node110 299 Argument of perihelion113 834 Known satellites5Physical characteristicsDimensions2 376 6 1 6 km observations consistent with a sphere predicted deviations too small to be observed 5 Mean radius1 188 3 0 8 km 6 5 0 1868 EarthsFlattening lt 1 7 Surface area1 774443 107 km2 c 0 035 EarthsVolume 7 057 0 004 109 km3 d 0 00651 EarthsMass 1 303 0 003 1022 kg 7 0 00218 Earths 0 177 MoonsMean density1 854 0 006 g cm3 6 7 Surface gravity0 620 m s2 e 0 063 gEscape velocity1 212 km s f Synodic rotation period 6 38680 d 6 d 9 h 17 m 00 s 8 Sidereal rotation period 6 387230 d 6 d 9 h 17 m 36 sEquatorial rotation velocity47 18 km hAxial tilt122 53 to orbit 2 North pole right ascension132 993 9 North pole declination 6 163 9 Albedo0 52 geometric 2 0 72 Bond 2 Surface temp min mean maxKelvin 33 K 44 K 229 C 55 KApparent magnitude13 65 2 to 16 3 10 mean is 15 1 2 Absolute magnitude H 0 44 11 Angular diameter0 06 to 0 11 2 g AtmosphereSurface pressure1 0 Pa 2015 7 13 Composition by volumeNitrogen methane carbon monoxide 12 Pluto compared in size to the Earth and Moon Pluto has a moderately eccentric and inclined orbit ranging from 30 to 49 astronomical units 4 5 to 7 3 billion kilometers 2 8 to 4 6 billion miles from the Sun Light from the Sun takes 5 5 hours to reach Pluto at its average distance 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 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 The New Horizons mission 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 the first object in the Kuiper belt It was immediately hailed as the ninth planet but its planetary status was questioned when it 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 Contents 1 History 1 1 Discovery 1 2 Name and symbol 1 3 Planet X disproved 1 4 Classification 1 4 1 IAU classification 2 Orbit 2 1 Relationship with Neptune 2 1 1 Other factors 2 2 Quasi satellite 3 Rotation 4 Geology 4 1 Surface 4 2 Internal structure 5 Mass and size 6 Atmosphere 7 Satellites 8 Origin 9 Observation and exploration 9 1 Observation 9 2 Exploration 9 3 Sub Charon hemisphere 9 4 Southern hemisphere 9 5 Videos 10 See also 11 Notes 12 References 13 Further reading 14 External linksHistoryDiscovery Further information Planets beyond Neptune Discovery photographs of Pluto Clyde Tombaugh in Kansas In 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 14 In 1906 Percival Lowell a wealthy Bostonian who had founded Lowell Observatory in Flagstaff Arizona in 1894 started an extensive project in search of a possible ninth planet which he termed Planet X 15 By 1909 Lowell and William H Pickering had suggested several possible celestial coordinates for such a planet 16 Lowell and his observatory conducted his search 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 16 17 There are fourteen other known precovery observations with the earliest made by the Yerkes Observatory on August 20 1909 18 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 19 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 a sample of his astronomical drawings 19 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 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 20 After the observatory obtained further confirmatory photographs news of the discovery was telegraphed to the Harvard College Observatory on March 13 1930 16 As one Plutonian year corresponds to 247 94 Earth years 2 Pluto will complete its first orbit since its discovery in 2178 Name and symbol Mosaic of best resolution images of Pluto from different angles The discovery made headlines around the globe 21 Lowell Observatory which had the right to name the new object received more than 1 000 suggestions from all over the world ranging from Atlas to Zymal 22 Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did 22 Constance Lowell proposed Zeus then Percival and finally Constance These suggestions were disregarded 23 The name Pluto after the Greek Roman god of the underworld was proposed by Venetia Burney 1918 2009 an eleven year old schoolgirl in Oxford England who was interested in classical mythology 24 She suggested it in a conversation with her grandfather Falconer Madan a former librarian at the University of Oxford s Bodleian Library who passed the name to astronomy professor Herbert Hall Turner who cabled it to colleagues in the United States 24 Each member of the Lowell Observatory was allowed to vote on a short list of three potential names Minerva which was already the name for an asteroid Cronus which had lost reputation through being proposed by the unpopular astronomer Thomas Jefferson Jackson See and Pluto Pluto received a unanimous vote 25 The name was published on May 1 1930 24 26 Upon the announcement Madan gave Venetia 5 equivalent to 336 in 2021 27 or US 394 in 2021 28 29 as a reward 24 The final choice of name was helped in part by the fact that the first two letters of Pluto are the initials of Percival Lowell Pluto s planetary symbol was then created as a monogram of the letters PL in Unicode U 2647 PLUTO 30 though it is rarely used in astronomy today For example occurs in a table of the planets identified by their symbols in a 2004 article written before the 2006 IAU definition 31 but not in a graph of planets dwarf planets and moons from 2016 where only the eight IAU planets are identified by their symbols 32 Planetary symbols in general are uncommon in astronomy and are discouraged by the IAU 33 The monogram is also used in astrology but the most common astrological symbol for Pluto at least in English language sources is an orb over Pluto s bident U 2BD3 PLUTO FORM TWO The bident symbol has seen some astronomical use as well since the IAU decision on dwarf planets for example in a public education poster on dwarf planets published by the NASA JPL Dawn mission in 2015 in which each of the five dwarf planets announced by the IAU receives a symbol 34 There are in addition several other symbols for Pluto found in European astrological sources including three accepted by Unicode U 2BD4 PLUTO FORM THREE U 2BD5 PLUTO FORM FOUR used in Uranian astrology and U 2BD6 PLUTO FORM FIVE found in various orientations showing Pluto s orbit cutting across that of Neptune 35 The name Pluto 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 36 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 was named after Uranus and neptunium which was named after Neptune 37 Most languages use the name Pluto in various transliterations h 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 the God of Death in Hinduism 38 Polynesian languages also tend to use the indigenous god of the underworld as in Maori Whiro 38 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 Diem Vương 星閻王 Yama s Star derived from Chinese 閻王 Yan Wang Yihm Wohng King Yama 39 38 40 Planet X disproved Once Pluto was found its faintness and lack of a viewable disc cast doubt on the idea that it was Lowell s Planet X 15 Estimates of Pluto s mass were revised downward throughout the 20th century 41 Mass estimates for Pluto Year Mass Estimate by1915 7 Earths Lowell prediction for Planet X 15 1931 1 Earth Nicholson amp Mayall 42 43 44 1948 0 1 1 10 Earth Kuiper 45 1976 0 01 1 100 Earth Cruikshank Pilcher amp Morrison 46 1978 0 0015 1 650 Earth Christy amp Harrington 47 2006 0 00218 1 459 Earth Buie et al 48 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 43 45 In 1976 Dale Cruikshank Carl Pilcher and David Morrison of the University of Hawaii 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 46 Pluto s albedo is 1 4 1 9 times that of Earth 2 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 the discrepancies in the orbit of Uranus Subsequent searches for an alternative Planet X notably by Robert Sutton Harrington 49 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 the discrepancies and with them the need for a Planet X vanished 50 Today the majority of scientists agree that Planet X as Lowell defined it does not exist 51 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 52 Classification Further information Definition of planet Artistic comparison of Pluto Eris Haumea Makemake Gonggong Quaoar Sedna Orcus Salacia 2002 MS4 and Earth along with the Moon vte From 1992 onward many bodies were discovered orbiting in 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 the Hayden Planetarium in New York City displayed a Solar System model of only eight planets which made headlines almost a year later 53 Ceres Pallas Juno and Vesta lost their planet status after the discovery of many other asteroids Similarly objects increasingly closer in size to Pluto were discovered in the Kuiper belt region On July 29 2005 astronomers at Caltech announced the discovery of a new trans Neptunian object Eris which was substantially more massive than Pluto and 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 54 Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet 55 IAU classification Main article IAU definition of planet The debate came to a head in August 2006 with an IAU resolution that created an official definition for the term planet According to this resolution there are three conditions for an object in the Solar System to be considered a planet The object must be in orbit around the Sun The object must be massive enough to be rounded by its own gravity More specifically its own gravity should pull it into a shape defined by hydrostatic equilibrium It must have cleared the neighborhood around its orbit 56 57 Pluto fails to meet the third condition 58 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 59 57 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 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 60 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 61 There has been some resistance within the astronomical community toward the reclassification 62 63 64 Alan Stern principal investigator with NASA s New Horizons mission to Pluto derided the IAU resolution stating that the definition stinks for technical reasons 65 Stern contended that by the terms of the new definition Earth Mars Jupiter and Neptune all of which share their orbits with asteroids would be excluded 66 even though he had himself previously suggested a criterion for clearing the neighbourhood which considered all four of them to have done so 67 He argued that all big spherical moons including the Moon should likewise be considered planets 68 He also stated that because less than five percent of astronomers voted for it the decision was not representative of the entire astronomical community 66 Marc W Buie then at the Lowell Observatory petitioned against the definition 69 Others have supported the IAU Mike Brown the astronomer who discovered Eris said through this whole crazy circus like procedure somehow the right answer was stumbled on It s been a long time coming Science is self correcting eventually even when strong emotions are involved 70 Public reception to the IAU decision was mixed A resolution introduced in the California State Assembly facetiously called the IAU decision a scientific heresy 71 The New Mexico House of Representatives passed a resolution in honor of Tombaugh 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 72 73 The Illinois Senate passed a similar resolution in 2009 on the basis that Clyde Tombaugh the discoverer of Pluto was born in Illinois The resolution asserted that Pluto was unfairly downgraded to a dwarf planet by the IAU 74 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 75 In 2006 in its 17th annual words of the year vote the American Dialect Society voted plutoed as the word of the year To pluto is to demote or devalue someone or something 76 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 current IAU definition of a planet 77 Entitled The Great Planet Debate 78 the conference published a post conference press release indicating that scientists could not come to a consensus about the definition of planet 79 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 the term has not seen significant use 80 81 82 Orbit Pluto was discovered in 1930 near the star d Geminorum and merely coincidentally crossing the ecliptic at this time of discovery Pluto moves about 7 degrees east per decade with small apparent retrograde motion as seen from Earth Pluto was closer to the Sun than Neptune between 1979 and 1999 Animation of Pluto s orbit from 1850 to 2097 Sun Saturn Uranus Neptune Pluto Pluto s orbital period is currently 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 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 3 i and was last closer to the Sun than Neptune between February 7 1979 and February 11 1999 83 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 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 84 85 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 86 Orbit of Pluto ecliptic view This side view of Pluto s orbit in red shows its large inclination to the ecliptic Orbit of Pluto polar view This view from above shows how Pluto s orbit in red is less circular than Neptune s in blue and how Pluto is sometimes closer to the Sun than Neptune The darker sections of both orbits show where they pass below the plane of the ecliptic Relationship with Neptune Despite Pluto s orbit appearing to cross that of Neptune when viewed from directly above the two objects orbits do not intersect When Pluto is closest to the Sun and close to Neptune s orbit as viewed from above it is also the farthest above Neptune s path Pluto s orbit passes about 8 AU above that of Neptune preventing a collision 87 88 89 j 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 a collision could be possible 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 Each cycle lasts about 495 years There are many other objects in this same resonance called plutinos This pattern is such that in each 495 year cycle the first time Pluto is near perihelion Neptune is over 50 behind Pluto By Pluto s second perihelion Neptune will have completed a further one and a half of its own orbits and so will be nearly 130 ahead of Pluto Pluto and Neptune s minimum separation is over 17 AU which is greater than Pluto s minimum separation from Uranus 11 AU 89 The minimum separation between Pluto and Neptune actually occurs near the time of Pluto s aphelion 86 The 2 3 resonance between the two bodies is highly stable and has been preserved over millions of years 91 This prevents their orbits from changing relative to one another and so the two bodies can never pass near each other Even if Pluto s orbit were not inclined the two bodies could never collide 89 The long term stability of the mean motion resonance is due to phase protection When Pluto s period is slightly shorter than 3 2 of Neptune its orbit relative to Neptune will drift causing it to make closer approaches behind Neptune s orbit The gravitational pull between the two then causes angular momentum to be transferred to Pluto at Neptune s expense This moves Pluto into a slightly larger orbit where it travels slightly more slowly according to Kepler s third law After many such repetitions Pluto is sufficiently slowed that Pluto s orbit relative to Neptune drifts in the opposite direction until the process is reversed The whole process takes about 20 000 years to complete 89 91 92 Other factors 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 87 86 There are several other resonances and interactions that enhance Pluto s stability These arise principally from two additional mechanisms besides the 2 3 mean motion resonance First Pluto s argument of perihelion the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun librates around 90 86 This means that when Pluto is closest to the Sun it is at its farthest above the plane of the Solar System preventing encounters with Neptune This is a consequence of the Kozai mechanism 87 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 91 Second the longitudes of ascending nodes of the two bodies the points where they cross the ecliptic 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 highest above Neptune s orbit This is known as the 1 1 superresonance All the Jovian planets particularly Jupiter play a role in the creation of the superresonance 87 Quasi satellite In 2012 it was hypothesized that 15810 Arawn could be a quasi satellite of Pluto a specific type of co orbital configuration 93 According to the hypothesis the object would be a quasi satellite of Pluto for about 350 000 years out of every two million year period 93 94 Measurements made by the New Horizons spacecraft in 2015 made it possible to calculate the orbit of Arawn more accurately 95 These calculations confirm the overall dynamics described in the hypothesis 96 However it is not agreed upon among astronomers whether Arawn should be classified as a quasi satellite of Pluto based on this motion since its orbit is primarily controlled by Neptune with only occasional smaller perturbations caused by Pluto 97 95 96 RotationPluto s rotation period its day is equal to 6 387 Earth days 2 98 Like Uranus 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 99 The reason for this unusual orientation has been debated Research from the University of Arizona has suggested that it may be due to the way that a body s spin will always adjust to minimise 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 100 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 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 effect seen on Pluto would be observed on Earth were the Antarctic ice sheet is several times larger 101 GeologyMain articles Geology of Pluto and Geography of Pluto Surface High resolution MVIC image of Pluto in enhanced color to bring out differences in surface composition Regions where water ice has been detected blue regions The plains on Pluto s surface are composed of more than 98 percent nitrogen ice with traces of methane and carbon monoxide 102 Nitrogen and carbon monoxide are most abundant on 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 103 The mountains are made of water ice 104 Pluto s surface is quite varied with large differences in both brightness and color 105 Pluto is one of the most contrastive bodies in the Solar System with as much contrast as Saturn s moon Iapetus 106 The color varies from charcoal black to dark orange and white 107 Pluto s color is more similar to that of Io with slightly more orange and significantly less red than Mars 108 Notable geographical features include Tombaugh Regio or the Heart a large bright area on the side opposite Charon Cthulhu Macula 6 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 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 109 110 111 there are obvious signs of glacial flows both into and out of the basin 112 113 It has no craters that were visible to New Horizons indicating that its surface is less than 10 million years old 114 Latest studies have shown that the surface has an age of 180000 90000 40000 years 115 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 7 Distribution of over 1000 craters of all ages in the northern anti Charon quadrant of Pluto The variation in density with none found in Sputnik Planitia indicates a long history of varying geological activity The lack of crater on the left and right of the map is due to low resolution coverage of those sub Charon regions Geologic map of Sputnik Planitia and surroundings context with convection cell margins outlined in black Sputnik Planitia is covered with churning nitrogen ice cells that are geologically young and turning over due to convection In Western parts of Sputnik Planitia there are fields of transverse dunes formed by 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 they are likely consists of methane particles 200 300 mm in size 116 Internal structure Model of the internal structure of Pluto 117 Water ice crustLiquid water oceanSilicate core Pluto s density is 1 860 0 013 g cm3 7 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 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 117 Pluto has no magnetic field 118 It is possible that such heating continues today creating a subsurface ocean of liquid water 100 to 180 km thick at the core mantle boundary 117 119 120 In September 2016 scientists at Brown University simulated the impact thought to have formed Sputnik Planitia and showed that it might have been the result of liquid water upwelling from below after the collision implying the existence of a subsurface ocean at least 100 km deep 121 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 122 123 In March 2022 they concluded that peaks on Pluto are actually a merger of ice volcanoes suggesting a source of heat on the body at levels previously thought not possible 124 Mass and size Pluto bottom right compared in size to the largest satellites in the solar system from left to right and top to bottom Ganymede Titan Callisto Io the Moon Europa and Triton Pluto s diameter is 2376 6 3 2 km 5 and its mass is 1 303 0 003 1022 kg 17 7 that of the Moon 0 22 that of Earth 125 Its surface area is 1 774443 107 km2 or just slightly bigger than Russia Its surface gravity is 0 063 g compared to 1 g for Earth and 0 17 g for the Moon 2 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 126 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 is much less than thought before Charon was discovered 127 Pluto is more than twice the diameter and a dozen times the mass of Ceres the largest object in the asteroid belt It 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 5 compared to Eris s approximate diameter of 2 326 km 128 Determinations of Pluto s size have been complicated by its atmosphere 129 and hydrocarbon haze 130 In March 2014 Lellouch de Bergh et al published findings regarding methane mixing ratios in Pluto s atmosphere consistent with a Plutonian diameter greater than 2 360 km with a best guess of 2 368 km 131 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 1 470 mi 128 132 which was later revised to be 2 372 km 1 474 mi on July 24 133 and later to 2374 8 km 7 Using radio occultation data from the New Horizons Radio Science Experiment REX the diameter was found to be 2376 6 3 2 km 5 The masses of Pluto and Charon compared to other dwarf planets Eris Haumea Makemake Gonggong Quaoar Orcus Ceres and to the icy moons Triton Neptune I Titania Uranus III Oberon Uranus IV Rhea Saturn V and Iapetus Saturn VIII The unit of mass is 1021 kg Selected size estimates for Pluto Year Radius Notes1993 1195 km Millis et al 134 if no haze 130 1993 1180 km Millis et al surface amp haze 130 1994 1164 km Young amp Binzel 135 2006 1153 km Buie et al 48 2007 1161 km Young Young amp Buie 129 2011 1180 km Zalucha et al 136 2014 1184 km Lellouch et al 131 2015 1187 km New Horizons measurement from optical data 133 2017 1188 3 km New Horizons measurement from radio occultation data 5 6 AtmosphereMain article Atmosphere of Pluto A near true color image taken by New Horizons after its flyby Numerous layers of blue haze float in Pluto s atmosphere Along and near the limb mountains and their shadows are visible Image of Pluto in X rays by Chandra X ray Observatory blue spot The X rays are probably created by interaction of the gases surrounding Pluto with solar wind although details of their origin are not clear Pluto has a tenuous atmosphere consisting of nitrogen N2 methane CH4 and carbon monoxide CO which are in equilibrium with their ices on Pluto s surface 137 138 According to the measurements by New Horizons the surface pressure is about 1 Pa 10 mbar 7 roughly one million to 100 000 times less than Earth s atmospheric pressure It 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 139 140 New Horizons observations showed that atmospheric escape of nitrogen to be 10 000 times less than expected 140 Alan Stern has contended that even 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 140 Just like sweat cools the body as it evaporates from the skin the sublimation of Pluto s atmosphere cools its surface 141 Pluto has no or almost no troposphere observations by New Horizons suggest only 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 142 In July 2019 an occultation by Pluto showed that its atmospheric pressure against expectations had fallen by 20 since 2016 143 In 2021 astronomers at the Southwest Research Institute confirmed the result using data from an occultation in 2018 which showed that light was appearing less gradually from behind Pluto s disc indicating a thinning atmosphere 144 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 145 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 140 Pluto s atmosphere is divided into roughly 20 regularly spaced haze layers up to 150 km high 7 thought to be the result of pressure waves created by airflow across Pluto s mountains 140 Satellites An oblique view of the Pluto Charon system showing that Pluto orbits a point outside itself The two bodies are mutually tidally locked Main article Moons of Pluto Pluto has five known natural satellites The 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 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 146 Kerberos was discovered in 2011 147 and Styx was discovered in 2012 148 The satellites orbits are circular eccentricity lt 0 006 and coplanar with Pluto s equator inclination lt 1 149 150 and therefore tilted approximately 120 relative to Pluto s orbit The Plutonian system is highly compact the five known satellites orbit within the inner 3 of the region where prograde orbits would be stable 151 The orbital periods of all Pluto s moons are linked in a system of orbital resonances and near resonances 150 152 When precession is accounted for the orbital periods of Styx Nix and Hydra are in an exact 18 22 33 ratio 150 There is a sequence of approximate ratios 3 4 5 6 between 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 150 153 The Pluto Charon system is one of the few in the Solar System whose barycenter lies outside the primary body the Patroclus Menoetius system is a smaller example and the Sun Jupiter system is the only larger one 154 The similarity in size of Charon and Pluto has prompted some astronomers to call it a double dwarf planet 155 The system is also unusual among planetary systems in that each is tidally locked to the other which means that Pluto and Charon always have the same hemisphere facing each other a property shared by only one other known system Eris and Dysnomia 156 From any position on either body the other is always at the same position in the sky or always obscured 157 This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its barycenter 98 In 2007 observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo geysers 158 Pluto s moons are hypothesized to have been formed by a collision between Pluto and a similar sized body early in the history of the Solar System The collision released material that consolidated into the moons around Pluto 159 The Pluto system Pluto Charon Styx Nix Kerberos and Hydra imaged by the Hubble Space Telescope in July 2012 Pluto and Charon to scale Image acquired by New Horizons on July 8 2015 Family portrait of the five moons of Pluto to scale 160 Pluto s moon Charon as viewed by New Horizons on July 13 2015OriginFurther information Kuiper belt and Nice model Plot of the known Kuiper belt objects set against the four giant planets Pluto s origin and identity had long puzzled astronomers One early hypothesis was that Pluto was an escaped moon of Neptune 161 knocked out of orbit by Neptune s largest current moon Triton This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit 162 Pluto s true place in the Solar System began to reveal itself only in 1992 when astronomers began to find small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition This trans Neptunian population is thought to be the source of many short period comets Pluto is now known to be the largest member of the Kuiper belt k a stable belt of objects located between 30 and 50 AU from the Sun As of 2011 surveys of the Kuiper belt to magnitude 21 were nearly complete and any remaining Pluto sized objects are expected to be beyond 100 AU from the Sun 163 Like other Kuiper belt objects KBOs Pluto shares features with comets for example the solar wind is gradually blowing Pluto s surface into space 164 It has been claimed that if Pluto were placed as near to the Sun as Earth it would develop a tail as comets do 165 This claim has been disputed with the argument that Pluto s escape velocity is too high for this to happen 166 It has been proposed that Pluto may have formed as a result of the agglomeration of numerous comets and Kuiper belt objects 167 168 Though Pluto is the largest Kuiper belt object discovered 130 Neptune s moon Triton which is larger than Pluto is similar to it both geologically and atmospherically and is thought to be a captured Kuiper belt object 169 Eris see above is about the same size as Pluto though more massive but is not strictly considered a member of the Kuiper belt population Rather it is considered a member of a linked population called the scattered disc 170 Many Kuiper belt objects like Pluto are in a 2 3 orbital resonance with Neptune KBOs with this orbital resonance are called plutinos after Pluto 171 Like other members of the Kuiper belt Pluto is thought to be a residual planetesimal a component of the original protoplanetary disc around the Sun that failed to fully coalesce into a full fledged planet Most astronomers agree that Pluto owes its current position to a sudden migration undergone by Neptune early in the Solar System s formation As Neptune migrated outward it approached the objects in the proto Kuiper belt setting one in orbit around itself Triton locking others into resonances and knocking others into chaotic orbits The objects in the scattered disc a dynamically unstable region overlapping the Kuiper belt are thought to have been placed in their current positions by interactions with Neptune s migrating resonances 172 A computer model created in 2004 by Alessandro Morbidelli of the Observatoire de la Cote d Azur in Nice suggested that the migration of Neptune into the Kuiper belt may have been triggered by the formation of a 1 2 resonance between Jupiter and Saturn which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places ultimately doubling Neptune s distance from the Sun The resultant expulsion of objects from the proto Kuiper belt could also explain the Late Heavy Bombardment 600 million years after the Solar System s formation and the origin of the Jupiter trojans 173 It is possible that Pluto had a near circular orbit about 33 AU from the Sun before Neptune s migration perturbed it into a resonant capture 174 The Nice model requires that there were about a thousand Pluto sized bodies in the original planetesimal disk which included Triton and Eris 173 Observation and explorationPluto s distance from Earth makes its in depth study and exploration difficult On July 14 2015 NASA s New Horizons space probe flew through the Pluto system providing much information about it 175 Observation Computer generated rotating image of Pluto based on observations by the Hubble Space Telescope in 2002 2003 Pluto s visual apparent magnitude averages 15 1 brightening to 13 65 at perihelion 2 To see it a telescope is required around 30 cm 12 in aperture being desirable 176 It looks star like and without a visible disk even in large telescopes 177 because its angular diameter is maximum 0 11 2 The earliest maps of Pluto made in the late 1980s were brightness maps created from close observations of eclipses by its largest moon Charon Observations were made of the change in the total average brightness of the Pluto Charon system during the eclipses For example eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot Computer processing of many such observations can be used to create a brightness map This method can also track changes in brightness over time 178 179 Better maps were produced from images taken by the Hubble Space Telescope HST which offered higher resolution and showed considerably more detail 106 resolving variations several hundred kilometers across including polar regions and large bright spots 108 These maps were produced by complex computer processing which finds the best fit projected maps for the few pixels of the Hubble images 180 These remained the most detailed maps of Pluto until the flyby of New Horizons in July 2015 because the two cameras on the HST used for these maps were no longer in service 180 Exploration Main articles Exploration of Pluto and New Horizons The portions of Pluto s surface mapped by New Horizons annotated Panoramic view of Pluto s icy mountains and flat ice plains imaged by New Horizons 15 minutes after its closest approach to Pluto Distinct haze layers in Pluto s atmosphere can be seen backlit by the Sun The New Horizons spacecraft which flew by Pluto in July 2015 is the first and so far only attempt to explore Pluto directly Launched in 2006 it captured its first distant images of Pluto in late September 2006 during a test of the Long Range Reconnaissance Imager 181 The images taken from a distance of approximately 4 2 billion kilometers confirmed the spacecraft s ability to track distant targets critical for maneuvering toward Pluto and other Kuiper belt objects In early 2007 the craft made use of a gravity assist from Jupiter New Horizons made its closest approach to Pluto on July 14 2015 after a 3 462 day journey across the Solar System Scientific observations of Pluto began five months before the closest approach and continued for at least a month after the encounter Observations were conducted using a remote sensing package that included imaging instruments and a radio science investigation tool as well as spectroscopic and other experiments The scientific goals of New Horizons were to characterize the global geology and morphology of Pluto and its moon Charon map their surface composition and analyze Pluto s neutral atmosphere and its escape rate On October 25 2016 at 05 48 pm ET the last bit of data of a total of 50 billion bits of data or 6 25 gigabytes was received from New Horizons from its close encounter with Pluto 182 183 184 185 Since the New Horizons flyby scientists have advocated for an orbiter mission that would return to Pluto to fulfill new science objectives 186 187 188 They include mapping the surface at 9 1 m 30 ft per pixel observations of Pluto s smaller satellites observations of how Pluto changes as it rotates on its axis investigations of a possible subsurface ocean and topographic mapping of Pluto s regions that are covered in long term darkness due to its axial tilt The last objective could be accomplished using laser pulses to generate a complete topographic map of Pluto New Horizons principal investigator Alan Stern has advocated for a Cassini style orbiter that would launch around 2030 the 100th anniversary of Pluto s discovery and use Charon s gravity to adjust its orbit as needed to fulfill science objectives after arriving at the Pluto system 189 The orbiter could then use Charon s gravity to leave the Pluto system and study more KBOs after all Pluto science objectives are completed A conceptual study funded by the NASA Innovative Advanced Concepts NIAC program describes a fusion enabled Pluto orbiter and lander based on the Princeton field reversed configuration reactor 190 191 Sub Charon hemisphere The equatorial region of the sub Charon hemisphere of Pluto has only been imaged at low resolution as New Horizons made its closest approach to the anti Charon hemisphere 192 Composite image maps of Pluto from July 14 2015 updated 2019 A composite image of the sub Charon hemisphere of Pluto The region inside below the white line was on the far side of Pluto when New Horizons made its closest approach and was only imaged at lower resolution in the early days of the flyby Black regions were not imaged at all The low resolution area with named features labeled The low resolution area with features classified by geological typeSources 193 194 Southern hemisphere New Horizons imaged all of Pluto s northern hemisphere and the equatorial regions down to about 30 South Higher southern latitudes have only been observed at very low resolution from Earth 195 Images from the Hubble Space Telescope in 1996 cover 85 of Pluto and show large albedo features down to about 75 South 196 197 This is enough to show the extent of the temperate zone maculae Later images had slightly better resolution due to minor improvements in Hubble instrumentation 198 Some albedo variations in the higher southern latitudes could be detected by New Horizons using Charon shine light reflected off Charon The south polar region seems to be darker than the north polar region but there is a high albedo region in the southern hemisphere that may be a regional nitrogen or methane ice deposit 199 A map of Pluto based on Hubble images from 1996 centered on the anti Charon hemisphere Sputnik Planitia covering the southern hemisphere down to 75 S a Synthesized HST map of Pluto from Buie et al 2010 b Colorized New Horizons MVIC and LORRI mosaic c d Destreaked Charon illuminated image stack shown to approximately the same stretch The green line is the limit of the sub Charon hemisphere 199 Videos Pluto flyover animated July 14 2015 source source source source source source source source source source source source source source 00 30 released September 18 2015 source source source source source source source source source source source source 00 50 released December 5 2015 source source source source source source source source source source source source This mosaic strip extending across the hemisphere that faced the New Horizons spacecraft as it flew past Pluto See also Solar System portal Outer space portal Astronomy portalHow I Killed Pluto and Why It Had It Coming List of geological features on Pluto Pluto in astrology Pluto in fiction Stats of planets in the Solar SystemNotes This photograph was taken by the Ralph telescope aboard New Horizons on July 14 2015 from a distance of 35 445 km 22 025 mi The most prominent feature in the image the bright youthful plains of Tombaugh Regio and Sputnik Planitia can be seen at right It contrasts the darker more cratered terrain of Cthulhu Macula at lower left Because of Pluto s 119 591 tilt at its axis the southern hemisphere is barely visible in this image the equator runs through Cthulhu Macula and the southern parts of Sputnik Planitia The mean elements here are from the Theory of the Outer Planets TOP2013 solution by the Institut de mecanique celeste et de calcul des ephemerides IMCCE They refer to the standard equinox J2000 the barycenter of the Solar System and the epoch J2000 Surface area derived from the radius r 4 p r 2 displaystyle 4 pi r 2 Volume v derived from the radius r 4 p r 3 3 displaystyle 4 pi r 3 3 Surface gravity derived from the mass M the gravitational constant G and the radius r G M r 2 displaystyle GM r 2 Escape velocity derived from the mass M the gravitational constant G and the radius r 2 G M r displaystyle sqrt 2GM r Based on geometry of minimum and maximum distance from Earth and Pluto radius in the factsheet The equivalence is less close in languages whose phonology differs widely from Greek s such as Somali Buluuto and Navajo Tlootoo The discovery of Charon in 1978 allowed astronomers to accurately calculate the mass of the Plutonian system But it did not indicate the two bodies individual masses which could only be estimated after other moons of Pluto were discovered in late 2005 As a result because Pluto came to perihelion in 1989 most Pluto perihelion date estimates are based on the Pluto Charon barycenter Charon came to perihelion 4 September 1989 The Pluto Charon barycenter came to perihelion 5 September 1989 Pluto came to perihelion 8 September 1989 Because of the eccentricity of Pluto s orbit some have theorized that it was once a satellite of Neptune 90 The dwarf planet Eris is roughly the same size as Pluto about 2330 km Eris is 28 more massive than Pluto Eris is a scattered disc object often considered a distinct population from Kuiper belt objects like Pluto Pluto is the largest body in the Kuiper belt proper which excludes the 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1910 08833 Bibcode 2019LPICo2133 7024S Nadia Drake July 14 2016 5 Amazing Things We ve Learned a Year After Visiting Pluto National Geographic Retrieved August 19 2021 HUBBLE REVEALS SURFACE OF PLUTO FOR FIRST TIME HubbleSite org Space Telescope Science Institute March 7 1996 MAP OF PLUTO S SURFACE HubbleSite org Space Telescope Science Institute March 7 1996 A S Ganesh March 7 2021 Seeing Pluto like never before The Hindu Retrieved August 19 2021 a b Lauer Todd R Spencer John R Bertrand Tanguy Beyer Ross A Runyon Kirby D White Oliver L Young Leslie A Ennico Kimberly MacKinnon William B Moore Jeffrey M Olkin Catherine B Stern S Alan Weaver Harold A October 20 2021 The Dark Side of Pluto The Planetary Science Journal 2 214 214 Bibcode 2021PSJ 2 214L doi 10 3847 PSJ ac2743 S2CID 239047659 Retrieved February 5 2022 Further readingCodex Regius 2016 Pluto amp Charon CreateSpace Independent Publishing Platform ISBN 978 1534960749 Stern S A and Tholen D J 1997 Pluto and Charon University of Arizona Press ISBN 978 0816518401 Stern Alan Grinspoon David 2018 Chasing New Horizons Inside the Epic First Mission to Pluto Picador ISBN 978 125009896 2 Stern S Alan August 10 2021 The Pluto System After New Horizons University of Arizona Press p 688 ISBN 978 0816540945 External linksPluto at Wikipedia s sister projects Definitions from Wiktionary Media from Commons News from Wikinews Quotations from Wikiquote Texts from Wikisource Textbooks from Wikibooks Resources from Wikiversity New Horizons homepage Pluto Profile at NASA s Solar System Exploration site NASA Pluto factsheet Website of the observatory that discovered Pluto Earth telescope image of Pluto system Keck infrared with AO of Pluto system Gray Meghan 2009 Pluto Sixty Symbols Brady Haran for the University of Nottingham Video Pluto viewed through the years GIF NASA animation July 15 2015 Video Pluto FlyThrough 00 22 MP4 YouTube NASA animation August 31 2015 A Day on Pluto Video made from July 2015 New Horizon Images Scientific American NASA CGI video of Pluto flyover July 14 2017 CGI video simulation of rotating Pluto by Sean Doran see album for more Google Pluto 3D interactive map of the dwarf planet Interactive 3D gravity simulation of the Plutonian system Archived from the original on June 11 2020 Portals Stars Spaceflight Outer space Retrieved from https en wikipedia org w index php title Pluto amp oldid 1131461954, wikipedia, wiki, book, books, library,

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