 Map of Pluto based on Charon eclipses, approximately true color and among the highest resolutions possible with current technology | |||||||
| Discovery | |||||||
|---|---|---|---|---|---|---|---|
| Discovered by | Clyde W. Tombaugh | ||||||
| Discovery date | February 18, 1930 | ||||||
| Designations | |||||||
| MPC designation | 134340 Pluto | ||||||
| Pronunciation | /ˈpluːtoʊ/ ( listen),[1] | ||||||
| Minor planet category | dwarf planet, TNO, plutoid, KBO, plutino | ||||||
| Adjective | Plutonian | ||||||
| Epoch J2000 | |||||||
| Aphelion | 7,375,927,931 km 49.305 032 87 AU | ||||||
| Perihelion | 4,436,824,613 km 29.658 340 67 AU | ||||||
| Semi-major axis | 5,906,376,272 km 39.481 686 77 AU | ||||||
| Eccentricity | 0.248 807 66 | ||||||
| Orbital period | 90,613.305 days 248.09 years 14,164.4 Pluto solar days[2] | ||||||
| Synodic period | 366.73 days | ||||||
| Average orbital speed | 4.666 km/s | ||||||
| Inclination | 17.141 75° 11.88° to Sun's equator | ||||||
| Longitude of ascending node | 110.303 47° | ||||||
| Argument of perihelion | 113.763 29° | ||||||
| Satellites | 3 | ||||||
| Physical characteristics | |||||||
| Mean radius | 1,151 km[3] 0.18 Earths | ||||||
| Surface area | 1.665 × 107 km²[3] 0.033 Earths | ||||||
| Volume | 6,39 × 109 km³[3] 0.0059 Earths | ||||||
| Mass | (1.305 ± 0.007) × 1022 kg[4] 0.002 1 Earths 0.178 moon | ||||||
| Mean density | 2.03 ± 0.06 g/cm³[4] | ||||||
| Equatorial surface gravity | 0.81 m/s²[3] 0.083 g | ||||||
| Escape velocity | 1.27 km/s[3] | ||||||
| Sidereal rotation period | −6.387 230 day 6 d 9 h 17 m 36 s | ||||||
| Equatorial rotation velocity | 47.18 km/h | ||||||
| Axial tilt | 119.591 ± 0.014° (to orbit)[4][5] | ||||||
| North pole right ascension | 133.046 ± 0.014°[4] | ||||||
| North pole declination | -6.145 ± 0.014°[4] | ||||||
| Albedo | 0.49–0.66 (varies by 35%)[6][7] | ||||||
| Surface temp. Kelvin |
| ||||||
| Apparent magnitude | up to 13.65 (mean is 15.1)[7] | ||||||
| Absolute magnitude (H) | −0.7[8] | ||||||
| Angular diameter | 0.065" to 0.115"[7][9] | ||||||
| Atmosphere | |||||||
| Surface pressure | 0.30 Pa (summer maximum) | ||||||
| Composition | nitrogen, methane | ||||||
| | |||||||
Pluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest member of a distinct population called the Kuiper belt.[10]
Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth's Moon and a third its volume. It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto periodically to come closer to the Sun than Neptune.
Pluto and its largest moon, Charon, are sometimes treated together as a binary system because the barycentre of their orbits does not lie within either body.[11] The International Astronomical Union (IAU) has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, Charon is classified as a moon of Pluto.[12] Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.[13] Like Uranus, Pluto rotates on its "side" relative to its orbital plane, and the Pluto-Charon system does also.[14]
From its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's very low mass, its status as a major planet began to be questioned.[15] Later, in the early 21st century, many objects similar to Pluto were discovered in the outer solar system, notably the scattered disc object Eris, which is 27% more massive than Pluto.[16] On August 24, 2006, the IAU defined the term "planet" for the first time. This definition excluded Pluto as a planet, and added it as a member of the new category "dwarf planet" along with Eris and Ceres.[17] After the reclassification, Pluto was added to the list of minor planets and given the number 134340.[18][19] A number of scientists continue to hold that Pluto should be classified as a planet.[20]
Contents[hide] |
Discovery
In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus.[21] Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet in addition to Neptune. In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X".[22] By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet.[23] Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unbeknownst to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but did not recognise them for what they were.[23][24]
Due to a ten-year legal battle with Constance Lowell, Percival's widow, who attempted to wrest the observatory's million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929,[25] when its director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde Tombaugh, a 23-year-old Kansas man who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.[25]
Tombaugh's task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called 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 January 29 of that year. A lesser-quality photograph taken on January 21 helped confirm the movement.[26] After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.[23]
Name
The discovery made front page news around the world. The Lowell Observatory, who had the right to name the new object, received over 1000 suggestions, from "Atlas" to "Zymal".[22] Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.[22] Name suggestions poured in from all over the world. Constance Lowell proposed Zeus, then Lowell, and finally her own first name. These suggestions were disregarded.[27]
The name "Pluto" was proposed by Venetia Burney (later Venetia Phair), an eleven-year-old schoolgirl in Oxford, England.[28] Venetia was interested in classical mythology as well as astronomy, and considered the name, one of the alternate names of Hades, the Greek god of the Underworld, appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfather Falconer Madan, a former librarian of Oxford University's Bodleian Library. Madan passed the name to Professor Herbert Hall Turner, who then cabled it to colleagues in America.[29]
The object was officially named on March 24, 1930.[30] Each member of the Lowell Observatory was allowed to vote on a short-list of three: "Minerva" (which was already the name for an asteroid), "Cronus" (which had garnered a bad reputation after being suggested by an unpopular astronomer named Thomas Jefferson Jackson See), and Pluto. Pluto received every vote.[31] The name was announced on May 1, 1930.[28] Upon the announcement, Madan gave Venetia five pounds as a reward.[28]
The name was soon embraced by wider culture. The Disney character Pluto, introduced in 1930, was named in the object's honour.[32] 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, such as uranium, which was named after Uranus, and neptunium which was named after Neptune.[33]
Demise of Planet X
| Year | Mass | Notes |
|---|---|---|
| 1931 | 1 Earth | Nicholson & Mayall[34] |
| 1948 | .1 (1/10 Earth) | Kuiper [35] |
| 1976 | .01 (1/100 Earth) | Cruikshank, Pilcher, & Morrison [36] |
| 1978 | .002 (2/1,000 Earth) | Christy & Harrington [37] |
Once found, Pluto's faintness and lack of a resolvable disc cast doubt on the idea that it could be Lowell's Planet X. Throughout the mid-20th century, estimates of Pluto's mass were often revised downward. In 1978, the discovery of Pluto's moon Charon allowed the measurement of Pluto's mass for the first time. Its mass, roughly 0.2 percent that of the Earth, was far too small to account for the discrepancies in Uranus. Subsequent searches for an alternate Planet X, notably by Robert Sutton Harrington,[38] failed. In 1992, Myles Standish used data from Voyager 2's 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5 percent, 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.[39] Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist.[40] Lowell had made a prediction of Planet X's position in 1915 that was fairly close to Pluto's actual position at that time; however, Ernest W. Brown concluded almost immediately that this was a coincidence, a view still held today.[41]
Nomenclature
The name Pluto was intended to evoke the initials of the astronomer Percival Lowell, a desire echoed in the P-L monogram that is Pluto's astronomical symbol (
).[42] Pluto's astrological symbol resembles that of Neptune (
), but has a circle in place of the middle prong of the trident (
).
In Chinese, Japanese and Korean the name was translated as underworld king star (冥王星), [43][44] as suggested by Houei Nojiri in 1930.[45] Many other non-European languages use a transliteration of "Pluto" as their name for the object; however, some Indian languages may use a form of Yama, the Guardian of Hell in Hindu mythology, such as the Gujarati Yamdev.[43]
Physical characteristics
Pluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until 2015, when the New Horizons spacecraft is expected to arrive there.[46]
Appearance
Pluto's apparent magnitude averages 15.1, brightening to 13.65 at perihelion.[7] To see it, a telescope is required; around 30 cm (12 in) aperture being desirable.[47] It looks indistinct and star-like even in very large telescopes because its angular diameter is only 0.11". Its surface is light brown with a very slight tint of yellow.[48]
Spectroscopic analysis of Pluto's surface reveals it to be composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.[49][50] Distance and current limits on telescope technology make it impossible to directly photograph surface details on Pluto. Images from the Hubble Space Telescope barely show any distinguishable surface definitions or markings.[51]
Some images of Pluto are derived from brightness maps created from close observations of eclipses by its largest moon, Charon. Using computer processing, observations are made in brightness factors as Pluto is eclipsed by Charon. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Using this technique, one can measure the total average brightness of the Pluto-Charon system and track changes in brightness over time.[52] Maps composed by the Hubble Space Telescope reveal that Pluto's surface is remarkably heterogeneous, a fact also evidenced by its lightcurve and by periodic variations in its infrared spectra. The face of Pluto oriented toward Charon contains more methane ice, while the opposite face contains more nitrogen and carbon monoxide ice.[53]
Structure
Observations by the Hubble Space Telescope place Pluto's density at between 1.8 and 2.1 g/cm³, suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass.[50] Because decay of radioactive minerals 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 ice. The diameter of the core should be around 1,700 km, 70% of Pluto's diameter.[54] It is possible that such heating continues today, creating a subsurface ocean layer of liquid water some 100 to 180 km thick at the core–mantle boundary.[54][55] The DLR Institute of Planetary Research calculated that Pluto's density-to-radius ratio lies in a transition zone, along with Neptune's moon Triton, between icy satellites like the mid-sized moons of Uranus and Saturn, and rocky satellites such as Jupiter's Europa.[56]
Mass and size
Pluto's mass is 1.31×1022 kg; less than 0.24 percent that of the Earth,[57] while its diameter is roughly 2,390 km, or roughly 70% that of the Moon.[58] Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its mass on the basis of its presumed effect on Neptune and Uranus. In 1955 Pluto was calculated to be roughly the mass of the Earth, with further calculations in 1971 bringing the mass down to roughly that of Mars.[59] However, 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 the Earth.[59][60]
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. Once Charon's gravitational effect was measured, Pluto's true mass could be determined. Observations of Pluto in occultation with Charon allowed scientists to establish Pluto's diameter, while the invention of adaptive optics allowed them to determine its shape accurately.[61]
Among the objects of the Solar System, Pluto is smaller and much less massive than the terrestrial planets, and at less than 0.2 lunar masses it is also less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. 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. However, it is smaller than the dwarf planet Eris, a trans-Neptunian object discovered in 2005.
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