Pluto

Pluto (minor planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered and is the largest known plutoid (or ice dwarf).

Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc which is 27% more massive than Pluto, was discovered. This led the International Astronomical Union (IAU) to define the term "planet" formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet.

Pluto is the largest and second-most-massive (after Eris) known dwarf planet in the Solar System, and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris. Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU (4.4–7.4 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.5 AU).

Pluto has five known moons: Charon (the largest, with a diameter 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.

The New Horizons spacecraft performed a flyby of Pluto on July 14, 2015, becoming the first ever spacecraft to do so. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, and produced from methane, nitrogen and other gases released from the atmosphere of Pluto and transferred about 19,000 km (12,000 mi) to the orbiting moon.

Pluto 2nd Astronomical symbol for Pluto
Pluto in True Color - High-Res
Northern hemisphere of Pluto in true color[a]
Discovery
Discovered byClyde W. Tombaugh
Discovery dateFebruary 18, 1930
Designations
MPC designation(134340) Pluto
Pronunciation/ˈpluːtoʊ/ (listen)
Named after
Pluto
AdjectivesPlutonian
Orbital characteristics[3][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)[1]
  • (September 5, 1989)[2]
  • 39.48 AU
  • (5.90638 billion km)
Eccentricity0.2488
366.73 days[1]
4.67 km/s[1]
14.53 deg
Inclination
  • 17.16°
  • (11.88° to Sun's equator)
110.299°
113.834°
Known satellites5
Physical characteristics
Mean radius
Flattening<1%[6]
  • 1.779×107 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[4][6]
1.212 km/s[f]
  • 6.387230 d
  • 6 d, 9 h, 17 m, 36 s
Equatorial rotation velocity
47.18 km/h
122.53° (to orbit)[1]
North pole right ascension
132.993°[7]
North pole declination
−6.163°[7]
Albedo0.49 to 0.66 (geometric, varies by 35%)[1][8]
Surface temp. min mean max
Kelvin 33 K 44 K (−229 °C) 55 K
13.65[1] to 16.3[9]
(mean is 15.1)[1]
−0.7[10]
0.06″ to 0.11″[1][g]
Atmosphere
Surface pressure
1.0 Pa (2015)[6][12]
Composition by volumeNitrogen, methane, carbon monoxide[11]
NH-Pluto-Day1-TenImages-20150714-20151120
Mosaic of best-resolution images of Pluto from different angles

History

Discovery

Pluto discovery plates
Discovery photographs of Pluto
Clyde W. Tombaugh.jpeg
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.[13] Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune.

In 1906, Percival Lowell—a wealthy Bostonian who had founded Lowell Observatory in Flagstaff, Arizona, in 1894—started an extensive project in search of a possible ninth planet, which he termed "Planet X".[14] By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet.[15] 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.[15][16] There are fourteen other known precovery observations, with the earliest made by the Yerkes Observatory on August 20, 1909.[17]

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.[18] 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.[18]

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.[19] After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.[15] Pluto has yet to complete a full orbit of the Sun since its discovery because one Plutonian year is 247.68 years long.[20]

Name

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 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 every vote.[25] The name was announced on May 1, 1930.[24][26] Upon the announcement, Madan gave Venetia £5 (equivalent to 300 GBP, or 450 USD in 2014)[27] 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 astronomical symbol (Pluto symbol.svg, Unicode U+2647, ♇) was then created as a monogram constructed from the letters "PL".[28] Pluto's astrological symbol resembles that of Neptune (Neptune symbol.svg), but has a circle in place of the middle prong of the trident (Pluto's astrological symbol.svg).

The name 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.[29] 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.[30]

Most languages use the name "Pluto" in various transliterations.[h] In Japanese, Houei Nojiri suggested the translation Meiōsei (冥王星, "Star of the King (God) of the Underworld"), and this was borrowed into Chinese, Korean, and Vietnamese (which instead uses "Sao Diêm Vương", which was derived from the Chinese term 閻王 (Yánwáng), as "minh" is a homophone for the Sino-Vietnamese words for "dark" (冥) and "bright" (明)).[31][32][33] Some Indian languages use the name Pluto, but others, such as Hindi, use the name of Yama, the God of Death in Hindu and Buddhist mythology.[32] Polynesian languages also tend to use the indigenous god of the underworld, as in Māori Whiro.[32]

Planet X disproved

Once Pluto was found, its faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell's Planet X.[14] Estimates of Pluto's mass were revised downward throughout the 20th century.[34]

Mass estimates for Pluto
Year Mass Estimate by
1915 7 Earth Lowell (prediction for Planet X)[14]
1931 1 Earth Nicholson & Mayall[35][36][37]
1948 0.1 (1/10) Earth Kuiper[38]
1976 0.01 (1/100) Earth Cruikshank, Pilcher, & Morrison[39]
1978 0.0015 (1/650) Earth Christy & Harrington[40]
2006 0.00218 (1/459) Earth Buie et al.[41]

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.[36][38] 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.[39] (Pluto's albedo is 1.4–1.9 times that of Earth.[1])

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,[42] 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.[43] Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist.[44] 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,[45] a view still held today.[43]

Classification

EarthMoonCharonCharonNixNixKerberosStyxHydraHydraPlutoPlutoDysnomiaDysnomiaErisErisNamakaNamakaHi'iakaHi'iakaHaumeaHaumea2007 OR102007 OR10S/(225088) 1S/(225088) 1MakemakeMakemakeMK2MK2WeywotWeywotQuaoarQuaoarSednaSedna2002 MS42002 MS4VanthVanthOrcusOrcusActaeaActaeaSalaciaSalaciaFile:EightTNOs.png
Artistic comparison of Pluto, Eris, Haumea, 2007 OR10, Makemake, Quaoar, Sedna, 2002 MS4, Orcus, Salacia, 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. The Hayden Planetarium reopened—in February 2000, after renovation—with a model of only eight planets, which made headlines almost a year later.[46]

As objects increasingly closer in size to Pluto were discovered in the region, it was argued that Pluto should be reclassified as one of the Kuiper belt objects, just as Ceres, Pallas, Juno and Vesta lost their planet status after the discovery of many other asteroids. 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.[47] Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.[48]

IAU classification

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:

  1. The object must be in orbit around the Sun.
  2. 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.
  3. It must have cleared the neighborhood around its orbit.[49][50]

Pluto fails to meet the third condition.[51] 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).[52][50] 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".[53] 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.[54]

There has been some resistance within the astronomical community toward the reclassification.[55][56][57] Alan Stern, principal investigator with NASA's New Horizons mission to Pluto, derided the IAU resolution, stating that "the definition stinks, for technical reasons".[58] 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.[59] He argued that all big spherical moons, including the Moon, should likewise be considered planets.[60] He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.[59] Marc W. Buie, then at the Lowell Observatory petitioned against the definition.[61] 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."[62]

Public reception to the IAU decision was mixed. Many accepted the reclassification, but some sought to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California State Assembly facetiously called the IAU decision a "scientific heresy".[63] 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.[64][65] 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."[66] 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.[67]

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".[68]

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.[69] Entitled "The Great Planet Debate",[70] the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of planet.[71] 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 objects that have an orbital semi-major axis greater than that of Neptune and enough mass to be of near-spherical shape.[72][73][74]

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.
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 orbit
Animation of Pluto's orbit from 1900 to 2100
   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,[2][i] and was last closer to the Sun than Neptune between February 7, 1979, and February 11, 1999.[75]

In the long term, Pluto's orbit is chaotic. Computer simulations can be used to predict its position for several million years (both forward and backward in time), but after intervals longer than the Lyapunov time of 10–20 million years, calculations become speculative: 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.[76][77]

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.[78]

Plutoorbit1.5sideview
Orbit of Pluto – ecliptic view. This "side view" of Pluto's orbit (in red) shows its large inclination to the ecliptic.
TheKuiperBelt Orbits Pluto Polar
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 are aligned so that they can never collide or even approach closely.

The two orbits do not intersect. When Pluto is closest to the Sun, and hence closest 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.[79][80][81]

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. 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).[81] The minimum separation between Pluto and Neptune actually occurs near the time of Pluto's aphelion.[78]

The 2:3 resonance between the two bodies is highly stable and has been preserved over millions of years.[82] 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.[81] The long term stability of the mean-motion resonance is due to phase protection. If 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 strong gravitational pull between the two 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, and Neptune sufficiently sped up, that Pluto orbit relative to Neptune drifts in the opposite direction until the process is reversed. The whole process takes about 20,000 years to complete.[81][82][83]

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.[79][78] 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°.[78] 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,[79] 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.[82]

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.[79]

Quasi-satellite

In 2012, it was hypothesized that 15810 Arawn could be a quasi-satellite of Pluto, a specific type of co-orbital configuration.[84] 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.[84][85] Measurements made by the New Horizons spacecraft in 2015 made it possible to calculate the orbit of Arawn more accurately.[86] These calculations confirm the overall dynamics described in the hypothesis.[87] 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.[88][86][87]

Rotation

Pluto's rotation period, its day, is equal to 6.39 Earth days.[89] 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.[90] 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.[91] 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 if the Antarctic ice sheet was several times larger.[92]

Geology

Pluto-01 Stern 03 Pluto Color TXT
High-resolution MVIC image of Pluto in enhanced color to bring out differences in surface composition
NH-Pluto-WaterIceDetected-BlueRegions-Released-20151008
Regions where water ice has been detected (blue regions)

Surface

The plains on Pluto's surface are composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.[93] 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.[94] The mountains are made of water ice.[95] Pluto's surface is quite varied, with large differences in both brightness and color.[96] Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn's moon Iapetus.[97] The color varies from charcoal black, to dark orange and white.[98] Pluto's color is more similar to that of Io with slightly more orange and significantly less red than Mars.[99] Notable geographical features include Tombaugh Regio, or the "Heart" (a large bright area on the side opposite Charon), Cthulhu Macula,[4] 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;[100][101][102] there are obvious signs of glacial flows both into and out of the basin.[103][104] It has no craters that were visible to New Horizons, indicating that its surface is less than 10 million years old.[105] Latest studies have shown that the surface has an age of 180000+90000
−40000
years.[106] 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."[6]

PIA20154-Pluto-MapOfOver1000Craters-20151110
Distribution of over 1000 craters of all ages on Pluto. The variation in density (with none found in Sputnik Planitia) indicates a long history of varying geological activity.
Pluto's Sputnik Planum geologic map (cropped)
Geologic map of Sputnik Planitia and surroundings (context), with convection cell margins outlined in black
Pluto’s Heart - Like a Cosmic Lava Lamp
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.[107]

Internal structure

Internal Structure of Pluto
Internal structure of Pluto[108]
  • 1. Frozen nitrogen[93]
  • 2. Water ice
  • 3. Rock

Pluto's density is 1.860±0.013 g/cm3.[6] 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 diameter of the core is hypothesized to be approximately 1700 km, 70% of Pluto's diameter.[108] 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.[108][109][110] 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.[111] Pluto has no magnetic field.[112]

Mass and size

Selected size estimates for Pluto
Year Radius Notes
1993 1195 km Millis, et al.[113] (if no haze)[114]
1993 1180 km Millis, et al. (surface & haze)[114]
1994 1164 km Young & Binzel[115]
2006 1153 km Buie, et al.[41]
2007 1161 km Young, Young, & Buie[116]
2011 1180 km Zalucha, et al.[117]
2014 1184 km Lellouch, et al.[118]
2015 1187 km New Horizons measurement (from optical data)[119]
2017 1188.3 km New Horizons measurement (from radio occultation data)[5][4]

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).[120] Its surface area is 1.779×107 km2, or roughly the same surface area as Russia. Its surface gravity is 0.063 g (compared to 1 g for Earth).

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.[121]

Pluto, Earth & Moon size comparison
Size comparisons: Earth, the Moon, and Pluto
GanymedeTitanCallistoIoMoonEuropaTritonPlutoFile:Pluto compared2.jpg
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

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.

Pluto is more than twice the diameter and a dozen times the mass of the dwarf planet 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 2376.6 km[5] compared to Eris's approximate diameter of 2326 km.[122]

Determinations of Pluto's size had been complicated by its atmosphere,[116] and hydrocarbon haze.[114] 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 2360 km, with a "best guess" of 2368 km.[118] 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),[122][123] which was later revised to be 2,372 km (1,474 mi) on July 24,[119] and later to 2374±8 km.[6] Using radio occultation data from the New Horizons Radio Science Experiment (REX), the diameter was found to be 2376.6±3.2 km.[5]

Atmosphere

PIA21590 – Blue Rays, New Horizons' High-Res Farewell to 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.
PIA21061-Pluto-DwarfPlanet-XRays-20160914
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.[124][125] According to the measurements by New Horizons, the surface pressure is about 1 Pa (10 μbar),[6] 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.[126][127] New Horizons observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected.[127] 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.[127] Just like sweat cools the body as it evaporates from the skin, the sublimation of Pluto's atmosphere cools its surface.[128] The presence of atmospheric gases was traced up to 1670 kilometers high; the atmosphere does not have a sharp upper boundary.

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,[129] 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).[127] Pluto's atmosphere is divided into roughly 20 regularly spaced haze layers up to 150 km high,[6] thought to be the result of pressure waves created by airflow across Pluto's mountains.[127]

Satellites

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 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,[130] Kerberos was discovered in 2011,[131] and Styx was discovered in 2012.[132] The satellites' orbits are circular (eccentricity < 0.006) and coplanar with Pluto's equator (inclination < 1°),[133][134] 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.[135]

The orbital periods of all Pluto's moons are linked in a system of orbital resonances and near resonances.[134][136] When precession is accounted for, the orbital periods of Styx, Nix, and Hydra are in an exact 18:22:33 ratio.[134] 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.[134][137]

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

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.[138] The similarity in size of Charon and Pluto has prompted some astronomers to call it a double dwarf planet.[139] 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. From any position on either body, the other is always at the same position in the sky, or always obscured.[140] This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its barycenter.[89]

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.[141]

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.[142]

1. The Pluto system: Pluto, Charon, Styx, Nix, Kerberos, and Hydra, imaged by the Hubble Space Telescope in July 2012. 2. Pluto and Charon, to scale. Image acquired by New Horizons on July 8, 2015. 3. Family portrait of the five moons of Pluto, to scale.[143] 4. Pluto's moon Charon as viewed by New Horizons on July 13, 2015

Pluto moon P5 discovery with moons' orbits
Pluto charon 150709 color final
Nh-pluto moons family portrait-truecolor
Charon in True Color - High-Res

Origin

Outersolarsystem objectpositions labels comp
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,[144] knocked out of orbit by its 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.[145]

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,[j] 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.[146] Like other Kuiper-belt objects (KBOs), Pluto shares features with comets; for example, the solar wind is gradually blowing Pluto's surface into space.[147] It has been claimed that if Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.[148] This claim has been disputed with the argument that Pluto's escape velocity is too high for this to happen.[149] Nonetheless, it has also been claimed that Pluto may have formed as a result of the agglomeration of numerous comets and related Kuiper belt objects.[150][151]

Though Pluto is the largest Kuiper belt object discovered,[114] Neptune's moon Triton, which is slightly larger than Pluto, is similar to it both geologically and atmospherically, and is thought to be a captured Kuiper belt object.[152] 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.

A large number of Kuiper belt objects, like Pluto, are in a 2:3 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto.[153]

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.[154] 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.[155] 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.[156] The Nice model requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk, which included Triton and Eris.[155]

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.[157]

Observation

Pluto animiert 200px
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.[1] To see it, a telescope is required; around 30 cm (12 in) aperture being desirable.[158] It looks star-like and without a visible disk even in large telescopes, because its angular diameter is only 0.11".

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.[159][160]

Better maps were produced from images taken by the Hubble Space Telescope (HST), which offered higher resolution, and showed considerably more detail,[97] resolving variations several hundred kilometers across, including polar regions and large bright spots.[99] These maps were produced by complex computer processing, which finds the best-fit projected maps for the few pixels of the Hubble images.[161] 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.[161]

Exploration

Pluto-Map-Annotated
The portions of Pluto's surface mapped by New Horizons (annotated)

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.[162] 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.[163][164][165][166]

Since the New Horizons flyby, scientists have advocated for an orbiter mission that would return to Pluto to fulfill new science objectives.[167] They include mapping the surface at 30 feet per pixel, observations of Pluto's smaller satellites, observations of how Pluto changes as it rotates on its axis, 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.[168] 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.[169][170]

Videos

(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. (No Audio – 1080p 60fps)

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. ^ 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.

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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.

External links

Charon (moon)

Charon, also known as (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).

With half the diameter and one eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Plutonian system lies outside Pluto.

The reddish-brown cap of the north pole of Charon is composed of tholins; organic macromolecules that may be essential ingredients of life. These tholins were produced from methane, nitrogen and related gases released from the atmosphere of Pluto and transferred over 19,000 km (12,000 mi) to the orbiting moon.The New Horizons spacecraft is the only probe that has visited the Pluto system. It approached Charon to within 27,000 km (17,000 mi) in 2015.

Dwarf planet

A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite. That is, it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit.The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of objects farther away from the Sun than Neptune that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized.As of July 2008 the International Astronomical Union (IAU) recognizes five dwarf planets: Ceres in the asteroid belt, and Pluto, Haumea, Makemake, and Eris in the outer Solar System.Only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they actually fit the IAU's definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto. They subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1) are to be named under the assumption that they are dwarf planets. At the time (and still as of 2019), the only additional bodies to meet this secondary criterion were Haumea and Makemake. However, doubts have since been raised about Haumea.

Eris (dwarf planet)

Eris (minor-planet designation 136199 Eris) is the most massive and second-largest (by volume) dwarf planet (and plutoid) known in the Solar System. Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its discovery was verified later that year. In September 2006 it was named after Eris, the Greek goddess of strife and discord. Eris is the ninth most massive object directly orbiting the Sun, and the 16th most massive overall, because seven moons are more massive than all known dwarf planets. It is also the largest which has not yet been visited by a spacecraft. Eris was measured to be 2,326 ± 12 kilometers (1,445.3 ± 7.5 mi) in diameter. Eris's mass is about 0.27% of the Earth mass, about 27% more than dwarf planet Pluto, although Pluto is slightly larger by volume.Eris is a trans-Neptunian object (TNO) and a member of a high-eccentricity population known as the scattered disk. It has one known moon, Dysnomia. As of February 2016, its distance from the Sun was 96.3 astronomical units (1.441×1010 km; 8.95×109 mi), roughly three times that of Pluto. With the exception of some long-period comets, until 2018 VG18 was discovered on December 17, 2018, Eris and Dysnomia were the most distant known natural objects in the Solar System.Because Eris appeared to be larger than Pluto, NASA initially described it as the Solar System's tenth planet. This, along with the prospect of other objects of similar size being discovered in the future, motivated the International Astronomical Union (IAU) to define the term planet for the first time. Under the IAU definition approved on August 24, 2006, Eris is a "dwarf planet", along with objects such as Pluto, Ceres, Haumea and Makemake, thereby reducing the number of known planets in the Solar System to eight, the same as before Pluto's discovery in 1930. Observations of a stellar occultation by Eris in 2010 showed that its diameter was 2,326 ± 12 kilometers (1,445.3 ± 7.5 mi), very slightly less than Pluto, which was measured by New Horizons as 2,372 ± 4 kilometers (1,473.9 ± 2.5 mi) in July 2015.

Exploration of Pluto

The exploration of Pluto began with the arrival of the New Horizons probe in July 2015, though proposals for such a mission had been studied for many decades. There are no plans as yet for a follow-up mission, though follow-up concepts have been studied.

Hades

Hades (; Greek: ᾍδης Hádēs; Ἅιδης Háidēs), in the ancient Greek religion and myth, is the god of the dead and the king of the underworld, with which his name became synonymous. Hades was the eldest son of Cronus and Rhea, although the last son regurgitated by his father. He and his brothers, Zeus and Poseidon, defeated their father's generation of gods, the Titans, and claimed rulership over the cosmos. Hades received the underworld, Zeus the sky, and Poseidon the sea, with the solid earth, long the province of Gaia, available to all three concurrently. Hades was often portrayed with his three-headed guard dog Cerberus.

The Etruscan god Aita and the Roman gods Dis Pater and Orcus were eventually taken as equivalent to Hades and merged into Pluto, a Latinization of Plouton (Greek: Πλούτων, Ploútōn), itself a euphemistic title often given to Hades.

Hydra (moon)

Hydra is a natural satellite of Pluto, with a diameter of approximately 51 km (32 mi) across its longest dimension. It is the second largest moon of Pluto, being slightly larger than Nix. Hydra was discovered along with Nix by the Pluto Companion Search Team in June 2005. It was named after the Hydra, the nine-headed underworld serpent in Greek mythology. By distance, Hydra is the fifth and outermost moon of Pluto, orbiting beyond Pluto's fourth moon Kerberos.Hydra has a highly reflective surface caused by the presence of water ice, similar to other Plutonian moons. Hydra's reflectivity is intermediate, in between those of Pluto and Charon. The New Horizons spacecraft imaged Pluto and its moons in July 2015 and had returned multiple images of Hydra.

IAU definition of planet

The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body which:

is in orbit around the Sun,

has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and

has "cleared the neighborhood" around its orbit.Among other things, this definition caused Pluto to no longer be a planet, a change from how it had been widely considered until that point.

A non-satellite body fulfilling only the first two of these criteria (such as Pluto) is classified as a "dwarf planet". According to the IAU, "planets and dwarf planets are two distinct classes of objects". A non-satellite body fulfilling only the first criterion is termed a "small Solar System body" (SSSB). An alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The definition was a controversial one, and has drawn both support and criticism from different astronomers, but has remained in use.

According to this definition, there are eight known planets in the Solar System. The definition distinguishes planets from smaller bodies and is not applicable outside the Solar System. To date, there is no accepted definition of extrasolar planets, or exoplanets. In 2007, an IAU working group issued a position statement that proposes to distinguish exoplanets from brown dwarfs on the basis of mass, but there has been no IAU-wide resolution or vote associated with this position statement. A separate proposal to extend the IAU definition to exoplanets has not been formally reviewed by the IAU.

Kerberos (moon)

Kerberos is a small natural satellite of Pluto, about 19 km (12 mi) in its longest dimension. It was the fourth moon of Pluto to be discovered and its existence was announced on 20 July 2011. It was imaged, along with Pluto and its four other moons, by the New Horizons spacecraft in July 2015. The first image of Kerberos was released to the public on 22 October 2015.

Kuiper belt

The Kuiper belt (), occasionally called the Edgeworth–Kuiper belt, is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. The Kuiper belt is home to three officially recognized dwarf planets: Pluto, Haumea and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.The Kuiper belt was named after Dutch-American astronomer Gerard Kuiper, though he did not predict its existence. In 1992, Albion was discovered, the first Kuiper belt object (KBO) since Pluto and Charon. Since its discovery, the number of known KBOs has increased to over a thousand, and more than 100,000 KBOs over 100 km (62 mi) in diameter are thought to exist. The Kuiper belt was initially thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. Studies since the mid-1990s have shown that the belt is dynamically stable and that comets' true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago; scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.The Kuiper belt is distinct from the theoretical Oort cloud, which is a thousand times more distant and is mostly spherical. The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects (TNOs). Pluto is the largest and most massive member of the Kuiper belt, and the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc. Originally considered a planet, Pluto's status as part of the Kuiper belt caused it to be reclassified as a dwarf planet in 2006. It is compositionally similar to many other objects of the Kuiper belt and its orbital period is characteristic of a class of KBOs, known as "plutinos", that share the same 2:3 resonance with Neptune.

List of Walt Disney Animation Studios short films

This is a list of animated short films produced by Walt Disney and Walt Disney Animation Studios from 1921 to the present. This includes films produced at the Laugh-O-Gram Studio which Disney founded in 1921 as well as the animation studio now owned by The Walt Disney Company, previously called the Disney Brothers Cartoon Studio (1923), The Walt Disney Studio (1926), Walt Disney Productions (1929), and Walt Disney Feature Animation (1986).

This list does not include:

Segments of feature-length package films later released individually (see List of Disney theatrical animated features)

Animated cartoon segments originally made for television (e.g. Disney's House of Mouse or the Mickey Mouse TV series)

Short films which contain animation but are primarily live-action (see List of Disney live-action shorts)

Short films which contain no new animation (i.e., films re-edited from other films)

Pixar short filmsA gold star () indicates an Academy Award for Best Animated Short Film, while a silver star () indicates a nomination.

Moons of Pluto

The dwarf planet Pluto has five moons down to a detection limit of about 1 km in diameter. In order of distance from Pluto, they are Charon, Styx, Nix, Kerberos, and Hydra. Charon, the largest of the five moons, is mutually tidally locked with Pluto, and is massive enough that Pluto–Charon is sometimes considered a double dwarf planet.

New Horizons

New Horizons is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a team led by S. Alan Stern, the spacecraft was launched in 2006 with the primary mission to perform a flyby study of the Pluto system in 2015, and a secondary mission to fly by and study one or more other Kuiper belt objects (KBOs) in the decade to follow, which as of 2019 includes 2014 MU69. It is the fifth space probe to achieve the escape velocity needed to leave the Solar System.

On January 19, 2006, New Horizons was launched from Cape Canaveral Air Force Station by an Atlas V rocket directly into an Earth-and-solar escape trajectory with a speed of about 16.26 km/s (10.10 mi/s; 58,500 km/h; 36,400 mph). It was the fastest man-made object ever launched from Earth. After a brief encounter with asteroid 132524 APL, New Horizons proceeded to Jupiter, making its closest approach on February 28, 2007, at a distance of 2.3 million kilometers (1.4 million miles). The Jupiter flyby provided a gravity assist that increased New Horizons' speed; the flyby also enabled a general test of New Horizons' scientific capabilities, returning data about the planet's atmosphere, moons, and magnetosphere.

Most of the post-Jupiter voyage was spent in hibernation mode to preserve on-board systems, except for brief annual checkouts. On December 6, 2014, New Horizons was brought back online for the Pluto encounter, and instrument check-out began. On January 15, 2015, the spacecraft began its approach phase to Pluto.

On July 14, 2015, at 11:49 UTC, it flew 12,500 km (7,800 mi) above the surface of Pluto, making it the first spacecraft to explore the dwarf planet. On October 25, 2016, at 21:48 UTC, the last of the recorded data from the Pluto flyby was received from New Horizons. Having completed its flyby of Pluto, New Horizons then maneuvered for a flyby of Kuiper belt object (486958) 2014 MU69 "Ultima Thule", which occurred on January 1, 2019, when it was 43.4 AU from the Sun. In August 2018, NASA cited results by Alice on New Horizons to confirm the existence of a "hydrogen wall" at the outer edges of the Solar System. This "wall" was first detected in 1992 by the two Voyager spacecraft.

Nix (moon)

Nix is a natural satellite of Pluto, with a diameter of 49.8 km (30.9 mi) across its longest dimension. It was discovered along with Pluto's outermost moon Hydra in June 2005 by the Pluto Companion Search Team. It was named after Nyx, the Greek goddess of the night. Nix is the third moon of Pluto by distance, orbiting between the moons Styx and Kerberos.Nix was imaged along with Pluto and its other moons by the New Horizons spacecraft as it flew by the Pluto system in July 2015. Images from the New Horizons spacecraft reveal a large reddish area on Nix that is likely an impact crater.

Planets beyond Neptune

Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.Clyde Tombaugh's discovery of Pluto in 1930 appeared to validate Lowell's hypothesis, and Pluto was officially named the ninth planet. In 1978, Pluto was conclusively determined to be too small for its gravity to affect the giant planets, resulting in a brief search for a tenth planet. The search was largely abandoned in the early 1990s, when a study of measurements made by the Voyager 2 spacecraft found that the irregularities observed in Uranus's orbit were due to a slight overestimation of Neptune's mass. After 1992, the discovery of numerous small icy objects with similar or even wider orbits than Pluto led to a debate over whether Pluto should remain a planet, or whether it and its neighbours should, like the asteroids, be given their own separate classification. Although a number of the larger members of this group were initially described as planets, in 2006 the International Astronomical Union (IAU) reclassified Pluto and its largest neighbours as dwarf planets, leaving Neptune the farthest known planet in the Solar System.While the astronomical community widely agrees that Planet X, as originally envisioned, does not exist, the concept of an as-yet-unobserved planet has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System. As of March 2014, observations with the WISE telescope have ruled out the possibility of a Saturn-sized object (95 Earth masses) out to 10,000 AU, and a Jupiter-sized (≈318 Earth masses) or larger object out to 26,000 AU.In 2014, based on similarities of the orbits of a group of recently discovered extreme trans-Neptunian objects, astronomers hypothesized the existence of a super-Earth planet, 2 to 15 times the mass of the Earth and beyond 200 AU with possibly a high inclined orbit at some 1,500 AU. In 2016, further work showed this unknown distant planet is likely on an inclined, eccentric orbit that goes no closer than about 200 AU and no farther than about 1,200 AU from the Sun. The orbit is predicted to be anti-aligned to the clustered extreme trans-Neptunian objects. Because Pluto is no longer considered a planet by the IAU, this new hypothetical object has become known as Planet Nine.

Pluto (Disney)

Pluto, also called Pluto the Pup, is a cartoon dog created in 1930 at Walt Disney Productions. He is a yellow-orange color, medium-sized, short-haired dog with black ears. Unlike most Disney characters, Pluto is not anthropomorphic beyond some characteristics such as facial expression. He is Mickey Mouse's pet. Officially a mixed-breed dog, he made his debut as a bloodhound in the Mickey Mouse cartoon The Chain Gang. Together with Mickey Mouse, Minnie Mouse, Donald Duck, Daisy Duck, and Goofy, Pluto is one of the "Sensational Six"—the biggest stars in the Disney universe. Though all six are non-human animals, Pluto alone is not dressed as a human.Pluto debuted in animated cartoons and appeared in 24 Mickey Mouse films before receiving his own series in 1937. All together Pluto appeared in 89 short films between 1930 and 1953. Several of these were nominated for an Academy Award, including The Pointer (1939), Squatter's Rights (1946), Pluto's Blue Note (1947), and Mickey and the Seal (1948). One of his films, Lend a Paw (1941), won the award in 1942. Because Pluto does not speak, his films generally rely on physical humor. This made Pluto a pioneering figure in character animation, by expressing personality through animation rather than dialogue.Like all of Pluto's co-stars, the dog has appeared extensively in comics over the years, first making an appearance in 1931. He returned to theatrical animation in 1990 with The Prince and the Pauper and has also appeared in several direct-to-video films. Pluto also appears in the television series Mickey Mouse Works (1999–2000), House of Mouse (2001–2003), Mickey Mouse Clubhouse (2006–2016), and the new Mickey Mouse shorts (2013–present).

In 1998, Disney's copyright on Pluto, set to expire in several years, was extended by the passage of the Sonny Bono Copyright Term Extension Act. Disney, along with other studios, lobbied for passage of the act to preserve their copyrights on characters such as Pluto for 20 additional years.

Pluto (Marvel Comics)

Pluto is a fictional deity appearing in American comic books published by Marvel Comics. The character is based on the Greco-Roman god of the same name.

Pluto (mythology)

Pluto (Latin: Plūtō; Greek: Πλούτων, Ploutōn) was the ruler of the underworld in classical mythology. The earlier name for the god was Hades, which became more common as the name of the underworld itself. In ancient Greek religion and mythology, Pluto represents a more positive concept of the god who presides over the afterlife. Ploutōn was frequently conflated with Ploutos (Πλοῦτος, Plutus), a god of wealth, because mineral wealth was found underground, and because as a chthonic god Pluto ruled the deep earth that contained the seeds necessary for a bountiful harvest. The name Ploutōn came into widespread usage with the Eleusinian Mysteries, in which Pluto was venerated as a stern ruler but the loving husband of Persephone. The couple received souls in the afterlife, and are invoked together in religious inscriptions. Hades, by contrast, had few temples and religious practices associated with him, and he is portrayed as the dark and violent abductor of Persephone.

Pluto and Hades differ in character, but they are not distinct figures and share two dominant myths. In Greek cosmogony, the god received the rule of the underworld in a three-way division of sovereignty over the world, with his brother Zeus ruling the Sky and his other brother Poseidon sovereign over the Sea. His central narrative is the abduction of Persephone to be his wife and the queen of his realm. Plouton as the name of the ruler of the underworld first appears in Greek literature of the Classical period, in the works of the Athenian playwrights and of the philosopher Plato, who is the major Greek source on its significance. Under the name Pluto, the god appears in other myths in a secondary role, mostly as the possessor of a quest-object, and especially in the descent of Orpheus or other heroes to the underworld.Plūtō ([ˈpluːtoː]; genitive Plūtōnis) is the Latinized form of the Greek Plouton. Pluto's Roman equivalent is Dis Pater, whose name is most often taken to mean "Rich Father" and is perhaps a direct translation of Plouton. Pluto was also identified with the obscure Roman Orcus, like Hades the name of both a god of the underworld and the underworld as a place. The borrowed Greek name Pluto is sometimes used for the ruler of the dead in Latin literature, leading some mythology handbooks to assert misleadingly that Pluto was the Roman counterpart of Hades. Pluto (Pluton in French and German, Plutone in Italian) becomes the most common name for the classical ruler of the underworld in subsequent Western literature and other art forms.

Pluto TV

Pluto TV is an Internet-based TV platform owned by Viacom. The company was founded in 2013 and is based in Los Angeles, California. Pluto TV runs as an app on 14 platforms, including Apple TV, Android TV, Amazon Fire TV, Chromecast, Roku, and PlayStation consoles. Live and on-demand Pluto TV content is also available via their website. The company currently has more than 75 content partners, over 100 free channels, and reaches over 15 million monthly active users. Pluto TV generates revenue from video advertisements between programming, with plans to remain a free television provider. Pluto TV licenses its content directly from providers.

Styx (moon)

Styx is a small natural satellite of Pluto whose discovery was announced on 11 July 2012. It was imaged along with Pluto and Pluto's other moons by the New Horizons spacecraft in July 2015. A single image was returned.Styx is the second satellite of Pluto by distance and the fifth discovered. It was discovered one year after Kerberos. Styx is approximately 19 km (12 mi) across its longest dimension, and its orbital period is 20.2 days.

Pluto
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Moons
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Asteroid belt
Centaurs
Centaurs (extended)
Plutinos
Twotinos
Other resonances/
unknown resonances:
Cubewanos:
Scattered disc
Area uncertain
Detached objects
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Dwarf planets (moons)
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