Pluto, in astronomy

Pluto, in astronomy, a dwarf planet and the first Kuiper belt, or transneptunian, object (see comet comet [Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun. Comets visible from the earth can be seen for periods ranging from a few days to several months.
..... Click the link for more information. ) to be discovered (1930) by astronomers. Pluto has an elliptical orbit usually lying beyond that of Neptune Neptune, in astronomy, 8th planet from the sun at a mean distance of about 2.8 billion mi (4.5 billion km) with an orbit lying between those of Uranus and the dwarf planet Pluto; its period of revolution is about 165 years.
..... Click the link for more information. . Although Pluto was long regarded as a planet, since the discovery (beginning in 1992) of other Kuiper belt objects, including one with a diameter larger than that of Pluto, astronomers have recognized the need to reclassify Pluto, and in 2006 the International Astronomical Union (IAU) ended official recognition of Pluto as a planet.

Pluto's mean distance from the sun is 3.67 billion mi (5.91 billion km), and its period of revolution is about 248 years. Since Pluto has an orbit that is more elliptical and tilted than those of the planets (eccentricity .250, inclination 17°), at its closest point to the sun it passes inside the orbit of Neptune; between 1979 and 1999 it was closer to the sun than Neptune was. It will remain farther from the sun for 220 years, when it will again pass inside Neptune's orbit. Its surface consists largely of frozen nitrogen. It is thought to have a rocky, silicate core; its thin atmosphere probably contains nitrogen, carbon monoxide, and methane. Its surface temperature is estimated to be about −360°F; (−218°C;), a temperature at which most gases exist in the frozen state.

The existence of an unknown planet beyond the orbit of Neptune was first proposed by Percival Lowell Lowell, Percival, 1855–1916, American astronomer, b. Boston, grad. Harvard, 1876; brother of Abbott Lawrence Lowell and Amy Lowell. He visited Korea and Japan, where he acted as counselor and foreign secretary to the Korean Special Mission to the United States
..... Click the link for more information. on the basis of observed perturbations of the orbits of Uranus Uranus , in astronomy, 7th planet from the sun, at a mean distance of 1.78 billion mi (2.87 billion km), with an orbit lying between those of Saturn and Neptune; its period of revolution is slightly more than 84 years.
..... Click the link for more information. and Neptune. He began searching for such a planet in 1905, although he did not publish his calculations of its predicted position until 1914. Independent calculations were published by W. H. Pickering Pickering, William Henry, 1858–1938, American astronomer, b. Boston, grad. Massachusetts Institute of Technology (B.S., 1879). He taught at the Massachusetts Institute of Technology (1880–87) and at Harvard Observatory.
..... Click the link for more information. and others. In 1929, the search for a ninth planet was resumed at Lowell Observatory Lowell Observatory, astronomical observatory located in Flagstaff, Ariz.; it was founded in 1894 by Percival Lowell, the American astronomer who popularized the idea that Mars might support intelligent life. Its original telescope, still in operation, is a 24-in.
..... Click the link for more information. , and on Feb. 18, 1930, using photographic plates and a blink microscope blink microscope, in astronomy, device for determining a change in position or magnitude (brightness) of a star relative to other stars in the background. Two photographs of the same field or area of the sky are projected so that they precisely coincide.
..... Click the link for more information. , Clyde W. Tombaugh Tombaugh, Clyde William , 1906–97, American astronomer, b. Streator, Ill. Although lacking formal training or a college degree, he was hired in 1929 as an assistant by the Lowell Observatory to continue the search for a planet beyond Neptune, which had been
..... Click the link for more information. discovered an object whose motion was consistent with that of a transneptunian planet.

In 1978, American astronomers James Christy and Robert Harrington discovered the moon Charon, and two smaller, more distant moons, Hydra and Nix, were reported in 2005 by American astronomers Hal Weaver and S. Alan Stern. Pluto's diameter is c.1,400 mi (2,300 km), Charon's is c.748 mi (1,203 km), and the radius of Charon's orbit is 12,200 mi (19,640 km); Charon completes one orbit in about 6.4 earth days. Hydra and Nix have diameters of less than 100 mi (160 km). Pluto and Charon both keep the same side facing one another at all times because they rotate synchronously as Charon orbits Pluto. No spacecraft has yet visited Pluto, and it and its moons are too distant for precise telescopic observation, so little is known for certain about their size, composition, surface, and other aspects.

An increasing number of Kuiper belt objects were discovered after 1992, many astronomers came to believe that Pluto, rather than being a planet, was really an unusually large and close Kuiper belt object. In 1999, however, the International Astronomical Union (IAU) reaffirmed that Pluto was a planet because of its size and its satellite, something no other transneptunian object was then known to have, but subsequent discoveries brought Pluto's status into question once again. One Kuiper belt object, now named Eris (and originally nicknamed Xena), whose orbit extends to roughly three times the distance of Pluto's, has an estimated diameter (1,500 mi/2,400 km) slightly larger than that of Pluto and also has a moon. It was the discovery of Eris in particular that ultimately led to Pluto's classification (2006), along with Eris and Ceres Ceres , in astronomy, a dwarf planet, the first asteroid to be discovered. It was found on Jan. 1, 1801, by G. Piazzi. He took three distinct observations; on the basis of these the mathematician Gauss calculated Ceres' orbit with such accuracy that it was found one
..... Click the link for more information. , as a dwarf planet.

Bibliography

See W. Hoyt, Planets X and Pluto (1980); S. A. Stern and J. Mitton, Pluto and Charon (1999).

Pluto, in Greek religion and mythology

Pluto, in Greek religion and mythology, god of the underworld, son of Kronos and Rhea; also called Hades. After the fall of the Titans Titan, in Greek religion and mythology, one of 12 primeval deities. The female Titan is also called Titaness. The Titans—six sons and six daughters—were the children of Uranus and Gaea.
..... Click the link for more information. , Pluto and his brothers Zeus and Poseidon divided the universe, and Pluto was awarded everything underground. There, with Persephone as his queen, he ruled over Hades. Not only a god of the dead, he is identified as a god of the earth's fertility. The Romans derived their god of the dead—Orcus, Dis, or Dis Pater—from Pluto.

Pluto

Solar system body, regarded as the ninth planet from the Sun until struck from the list of planets and reclassified as a dwarf planet in August 2006. It was discovered in 1930 by Clyde W. Tombaugh (1906–97) and named after the Greek god of the underworld. Its average distance from the Sun is about 3.7 billion mi (5.9 billion km)—it is located within the Kuiper belt—but its eccentric orbit brings it closer to the Sun than Neptune for 22 years during its 248-year orbit. Its axis is tipped 120°, so it rotates nearly on its side and “backward” (see retrograde motion) once every 6.39 days, locked synchronously with the orbit of its largest moon, Charon, discovered in 1978. Two additional, small moons were discovered in 2005. Pluto has a diameter of about 1,455 mi (2,340 km), less than 1% of Earth's mass, and only about 6% of Earth's surface gravity. Its estimated average surface temperature is near −390 °F (−235 °C). Its thin atmosphere contains nitrogen, methane, and perhaps other heavier gases. Pluto is thought to be made of frozen gases with a significant fraction of rocky material. Its size, composition, and orbital location in the Kuiper belt sparked a long debate over its classification as a major planet, which culminated in a decision by the International Astronomical Union to drop it from the planetary ranks.

Pluto¹

Classical myth the god of the underworld; Hades

Pluto²

the smallest planet and the farthest known from the sun. Discovered in 1930 by Clyde Tombaugh (1906--97), it has one known satellite, Charon. Mean distance from sun: 5907 million km; period of revolution around sun: 248.6 years; period of axial rotation: 6.4 days; diameter and mass: 18 and 0.3 per cent that of earth respectively

Pluto [′plüd·ō]

(astronomy)

The most distant planet in the solar system; mean distance to the sun is about 3.7 × 10⁹miles (5.9 × 10⁹kilometers); it has no known satellite, and its sidereal revolution period is 248 years.

Pluto

pet of a brutal alcoholic who mutilates and hangs it, with dire consequences to himself. [Am. Lit.: Poe “The Black Cat”]

See : Cats

Pluto

god of underworld. [Gk. Myth.: Howe, 224 ]

See : Underworld

the ninth planet from the sun in the solar system; its astronomical sign is FE. Pluto was discovered in 1930 by the amateur astronomer C. Tombaugh in photographs taken at an observatory in Flagstaff, Ariz., as a star of the 15th stellar magnitude moving among the other stars. Tombaugh was guided by the theoretical predictions of P. Lowell, whose initials make up the astronomical sign of Pluto and who in 1915 calculated the motion of the as yet unknown planet on the basis of perturbations in the motion of Uranus.

Pluto’s orbit differs in many respects from the orbits of neighboring planets, which are closer to the sun. Of all the planetary orbits, it has the largest eccentricity (e = 0.253) and the highest inclination to the ecliptic (i = 17°8’). Pluto’s distance from the sun ranges from 49 to 29 astronomical units (AU), with a mean distance of 39.75 AU. From 1979 until nearly the end of the 20th century, Pluto will be closer to the sun than Neptune. It revolves around the sun in 250.6 years with an average velocity of 4.7 km/sec. Its synodic period of rotation is 366.8 days. All these characteristics, except the last, are subject to large variations owing to the strong perturbative effects of Neptune and Uranus on Pluto’s motion.

At mean opposition, Pluto’s angular diameter as seen by a terrestrial observer does not exceed 1/4”, so that even through average-size telescopes Pluto is indistinguishable from the stars. Its disk can be detected only in the largest instruments when the atmosphere is exceptionally calm, but even then, of course, no surface details are visible. The value for Pluto’s linear diameter (5,500–6,000 km) obtained on the basis of such observations is unreliable. To some extent it is corroborated by photometric measurements of brightness, on the basis of which Pluto’s diameter is estimated to be between 2,200 and 10,000 km, corresponding to the maximum and minimum possible values of the albedo —0.8 to 0.04. However, the upper limit of the possible values of the diameter has been reduced—on the grounds that while passing through the stellar sky at a distance less than 0.143” from a star, Pluto did not occult it. It follows from this that Pluto’s angular diameter is less than 0.29” (when the distance from earth is 32 AU), and its linear diameter less than 6,800 km. Adopting 6,000 km as the probable value of the diameter, we obtain a value for Pluto’s albedo of 0.11, which is similar to that of the moon and asteroids, which lack atmospheres. The mass of Pluto is determined from the small perturbations Pluto induces in the motions of Neptune and Uranus. Various determinations give values ranging from 0.18 to 0.11 times the earth’s mass. The first value yields an unlikely average density of 10.3 g/cm³, while the second yields a more likely average density of 6.3 g/cm³. The mass of Pluto may be still smaller.

Pluto’s brightness varies regularly with an amplitude of ten percent and a period of 6 days 9 hr 17 min. This appears to be Pluto’s period of rotation about its axis. The direction of rotation and the attitude of the axis in space are unknown. Pluto’s color differs little from that of the sun, which illuminates the planet. Pluto’s calculated temperature is about — 230°C. Pluto is not known to have any satellites.

Pluto’s low mass, high density, slow rotation, absence of an atmosphere, and peculiar orbit make it entirely dissimilar to the superior giant planets. Some astronomers hold the view that Pluto was a satellite of one of these planets, possibly of Neptune. However, the large mass of Pluto, which is four times greater than that of the most massive satellite in the solar system— Ganymede—and which is comparable to the mass of Mars, argues against this.

REFERENCES

See references under PLANET.

D. IA. MARTYNOV

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Pluto, in astronomy

Bibliography

Pluto, in Greek religion and mythology

Pluto

REFERENCES

Pluto
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