Brief Information of Pluto
information about Pluto
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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. It is the largest and second-most-massive 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, a dwarf planet in the scattered disc. Like other Kuiper belt objects, Pluto is primarily made of ice and rock[14] 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 (4.4–7.3 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun thanNeptune, but a stable orbital resonance with Neptune prevents them from colliding. In 2014, Pluto was 32.6 AU from the Sun. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.4 AU).[15]
Pluto was discovered in 1930 by Clyde Tombaugh, and was originally considered the ninth planet from the Sun. After 1992, its status as aplanet fell into question following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered, which led the International Astronomical Union (IAU) to define the term "planet" formally for the first time the following year.[16] This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as a plutoid).[17] Some astronomers think that Pluto, as well as the other dwarf planets, should be considered planets.[18][19][20]
Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra.[21] Pluto and Charon are sometimes considered a binary system because thebarycenter of their orbits does not lie within either body.[22] The IAU has not formalized a definition for binary dwarf planets, and Charon is officially classified as a moon of Pluto.[23]
On 14 July 2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto.[24][25][26] During its brief flyby, New Horizonsmade detailed measurements and observations of Pluto and its moons.[27]
| Discovery | |||||||||
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| Discovered by | |||||||||
| Discovery date |
18 February 1930
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| Designations | |||||||||
| MPC designation |
134340 Pluto
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| Pronunciation | |||||||||
Named after
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| Adjectives |
Plutonian
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| Orbital characteristics[4][a] | |||||||||
| Epoch J2000 | |||||||||
| Aphelion |
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| Perihelion |
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| Eccentricity |
0.24897
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366.73 days[2]
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Average orbital speed
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4.67 km/s[2]
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14.85 deg
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| Inclination |
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110.301°
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113.777°
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| Known satellites | |||||||||
| Physical characteristics | |||||||||
Mean radius
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| Volume |
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| Mass | |||||||||
Mean density
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1.88 g/cm3[7]
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1.212 km/s[e]
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Sidereal rotation period
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Equatorial rotation velocity
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47.18 km/h
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North poleright ascension
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132.993°[8]
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North poledeclination
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−6.163°[8]
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| Albedo | |||||||||
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(mean is 15.1)[2]
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−0.7[11]
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| Atmosphere | |||||||||
Surface pressure
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| Composition by volume | |||||||||
Planet X disproved
Once found, Pluto's faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell's Planet X.[29]Estimates of Pluto's mass were revised downward throughout the 20th century.[48]
| Year | Mass | Estimate by |
|---|---|---|
| 1915 | 7 Earth | Lowell (prediction for Planet X)[29] |
| 1931 | 1 Earth | Nicholson & Mayall[49][50][51] |
| 1948 | 0.1 (1/10) Earth | Kuiper[52] |
| 1976 | 0.01 (1/100) Earth | Cruikshank, Pilcher, & Morrison[53] |
| 1978 | 0.0015 (1/650) Earth | Christy & Harrington[54] |
| 2006 | 0.00218 (1/459) Earth | Buie et al.[6] |
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.[50][52] In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaiicalculated 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.[53] (Pluto's albedo is 1.4–1.9 times greater than 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,[55] 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.[56] Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist.[57] 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,[58] a view still held today.[56]
Classification
Further information: Definition of planet
From 1992 onward, many bodies were discovered orbiting in the same area 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.[59]
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 Vestaeventually lost their planet status after the discovery of many other asteroids. On 29 July 2005, astronomers at Cal Techannounced 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.[60] Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.[61]
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