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, in astronomy

Pluto, in astronomy, a dwarf planet and the first Kuiper belt, or transneptunian, object (see comet) to be discovered (1930) by astronomers. Pluto has an elliptical orbit usually lying beyond that of Neptune. Although Pluto was long regarded as a planet, following the discovery (beginning in 1992) of other Kuiper belt objects, including one with a diameter larger than that of Pluto, astronomers considered reclassifying 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.

Pluto's axis is extremely tilted (122°), and its surface temperature is about −378℉ (−228℃), a temperature at which most gases exist in the frozen state. The surface, as imaged by the New Horizons space probe during its flyby in 2015, is complex, with cratered areas and smooth icy areas as well as mountains of water ice, possible ice volcanoes, and evidence of glacial and liquid (probably liquid nitrogen) activity, indicating the planet is geologically active. The main ice covering the surface is frozen nitrogen, but a large heart-shaped feature, Tombaugh Regio, is mostly methane ice on one side and largely nitrogen snow on the other; reddish low-lying areas areas likely result from tholins, particles produced by the breakdown and recombination of frozen nitrogen and methane. Pluto is thought to have a rocky, silicate core surrounded by ice (consisting of frozen water, nitrogen, methane, and carbon monoxide); the thin atmosphere contains nitrogen, carbon monoxide, and methane, and has discrete layers of haze.

The existence of an unknown planet beyond the orbit of Neptune was first proposed by Percival Lowell on the basis of observed perturbations of the orbits of Uranus 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 and others. In 1929, the search for a ninth planet was resumed at Lowell Observatory, and on Feb. 18, 1930, using photographic plates and a blink microscope, Clyde W. Tombaugh 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. Together, Pluto and Charon may be considered to form a double dwarf planet system. 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 about 12,180 mi (19,600 km). Pluto and Charon orbit a common center of mass that lies between them, above the surface of Pluto, completing one orbit in about 6.4 earth days. Both keep the same side facing one another at all times because they rotate synchronously as they orbit. Charon appears to consist mostly of water ice. Its surface is bisected by a canyon system whose extent and depth are greater than those of the Grand Canyon, and has a reddish north (and possibly south) pole whose color probably is due to tholins.

Two smaller, more distant moons, Nix and Hydra, were reported in 2005 by American astronomers Hal Weaver and S. Alan Stern, and two more small moons, Kerebos and Styx, were reported in 2011 and 2012 by American astronomer Mark Showalter. The smaller moons are generally elongated in shape and consist mainly of water ice. Hydra, the largest, is about 33 mi (54 km) along its longest axis; Nix, 27 mi (43 km); Kerebos, 7 mi (12 km); and Styx, 4 mi (7 km). The smaller moons orbit at roughly two to three times the distance of Charon, with Styx being the closest, Hydra the most distant, and Nix and Kerebos between them.

As 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 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) as a dwarf planet; transneptunian dwarf planets are now classified as plutoids.


See W. Hoyt, Planets X and Pluto (1980); S. A. Stern and J. Mitton, Pluto and Charon (1999); B. W. Jones, Pluto (2010).


, 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, 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.
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(ploo -toh) The ninth and smallest planet of the Solar System, discovered in 1930 by Clyde Tombaugh at the Lowell Observatory in Arizona. Its 248.59 year orbit has a higher inclination to the ecliptic (17.1°) and a greater eccentricity (0.25) than any other planet. Although it has the greatest mean planetary distance of about 39 AU from the Sun, it approaches within 30 AU at perihelion; between 1979 and 1999 Pluto was therefore closer to the Sun than Neptune. Orbital and physical characteristics are given in Table 1, backmatter.

Oppositions occur one or two days later each year, but Pluto's 14th-magnitude disk is too small to be measured using conventional Earth-based telescopes. Until 1978 the best estimate of its size was about 5900 km, a value based on negative observations of a predicted occultation of a star by Pluto during an appulse in 1965. However, examination of photographs of Pluto in 1978 by James W. Christy of the US Naval Observatory revealed that it has a very close satellite, called Charon, orbiting at a distance of about 20 000 km every 6.39 days – a period previously identified in the small light variation of the planet and ascribed to its axial rotation. The orbital parameters of Charon enable Kepler's third law to be used to estimate Pluto's mass as about 0.20% that of the Earth. Apparently Pluto is a low-density body, about 2320 km across, while Charon is about 1186 km in diameter; it is thus the largest satellite in proportion to the size of its planet in the Solar System – a title previously credited to our own Moon. Because of their proximity, both Pluto and Charon are in synchronous rotation about their common center of mass. If the revised estimates of Pluto's size and density are correct, then it would appear to be smaller than our Moon. This possibility has helped to fuel debate among astronomers about whether Pluto is a fully-fledged planet or should be classed as a Kuiper Belt object.

At a temperature of 50 K, Pluto is too cold and small to have an appreciable atmosphere. Infrared spectroscopy has shown nitrogen and methane to be present but it is difficult to distinguish spectroscopically between gaseous and solid forms of methane at the temperature and pressure found on Pluto. Part of the surface, at least, could have a covering of methane ice; there is a suggestion that Pluto has bright polar caps.

In October 2005, NASA announced that the Hubble Space Telescope had photographed two new objects that may be satellites of Pluto. Observations suggest that the satellite candidates, provisionally named S/2005 P1 and S/2005 P2, orbit Pluto at distances of about 49 000 km and 65 000 km respectively. Their likely diameters may be between 45 and 160 km.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
Enlarge picture
Pluto, ruler of the underworld, with the three-headed dog, Cerberus, at his feet. From Jack Bryant’s New System, 1774. Reproduced by permission of Fortean Picture Library.


(religion, spiritualism, and occult)

Pluto is the farthest known planet from the Sun and by far the smallest with a diameter of 1,444 miles and a mass only 2 percent that of the Earth. Pluto completes an orbit of the Sun every 247.69 years, meaning that it spends more than 20 years in each sign of the zodiac. Thus, an entire generation is born while Pluto is transiting each sign.

The existence of a ninth planet was suspected when astronomers detected a gravitational affect on the orbit of Neptune. Percival Lowell, an American astronomer, built a private observatory in Flagstaff, Arizona, called Lowell Observatory in order to locate the hypothetical planet, which he termed Planet X. He began his search for this planet in the early 1900s without success. In December 1929, a young amateur astronomer, Clyde Tombaugh, was hired to continue the search. His research eventually led to the first sighting on February 18, 1930 after conducting a very careful sky survey and examining hundreds of plate pairs. The official discovery was announced March 13 on Lowell’s 75th birthday.

Scientists at the Lowell Observatory requested suggestions from the public for the naming of Planet X. Many names were proposed, including Atlas, Minerva, Apollo, Zeus, Perseus, and Vulcan. However, its name came as a result of a letter from Venetia Burney, an 11-year-old schoolgirl in Oxford, England, who recommended it be named after the Disney character, Pluto. It is possible that this idea was accepted based on the fact that the planet is in perpetual darkness (from the myth of Pluto, the Roman god of the underworld) and because the first letters, “PL,” are the initials of Percival Lowell.

Pluto, also known as Hades, the brother of Zeus and Posiedon in the Greek culture, was a grim deity, yet not a totally evil one. He rarely ventured from the underworld, which he inherited following the defeat of the Titans. When he did travel the overworld, he hid himself with a helmet of invisibility. He was also called the God of Wealth from not only the precious metals hidden in the earth, but also the fact that the number of his subjects—the dead—could only increase. Over time, a gradual idea of judgment entered the myths of Hades, associating him with the idea of punishment and reward. Most souls were said to spend life after death in the dreary Meadows of Asphodel. Evil sinners were doomed to torture for eternity in Tartarus, while heros resided in Elysium enjoying feasts and games.

The myth surrounding Pluto’s wife, Persephone, is symbolic of the origins of the seasons. Persephone was the only daughter of the goddess Demeter. While Persephone was out picking flowers, a chasm opened up revealing Pluto. He quickly pulled her into the underworld and made her his wife and Queen of the Lower World. Demeter scoured the land looking for her daughter and once she discovered what had occurred, she was grief-stricken. She wandered the land alone, bringing famine to the earth in her grief. The people called out to Zeus for assistance and he answered by ordering Hades to release Persephone. However, she had already eaten a pomegranate seed. Having fed upon and drank from the goods of the underworld, she was obligated to spend a third of the year with Pluto. Thus, the winter when the Earth is cold and barren is the time Persephone resides in the underworld and Demeter travels the Earth alone and yearning for her only daughter. At Persephone’s release in the spring, the Earth begins to grow and flower again.

In ancient Mesopotamia, the ruler of the dead and the underworld was the goddess Ereshkigal, the Queen of the Great Below. Unlike her Roman and Greek counterparts, Ereshkigal was an unhappy figure who resented her exclusion from the divinities of the upper world and agonized over the fate of those souls unlucky enough to die in early childhood. Ereshkigal had a sister Ianna who was goddess of the sky and heavens. After Ereshkigal’s husband died, Ianna attended the funeral in the underworld. However, Ianna was not welcomed by her sister. Instead, Ereshkigal forced Ianna to meet the same tests that all souls encounter when they enter the underworld.

When traveling to the underworld, there are seven doors. At each door, each soul must give up a garment or jewel to pass through it. Ianna did the same thing and thus as she passes through the seventh door, she is completely naked. Upon her entrance into the underworld, she is forced to bow to Ereshkigal, who then kills Ianna and places her on a meat hook to rot. However, Ianna was prepared in case of any problems. Before leaving, she requested the help from two small men called mourners. Hearing that Ianna is in trouble, they go to the underworld to seek out Ereshkigal. When they arrive, they find the Queen of the Great Below in great pain not only from losing her husband, but also from the process of giving birth. The two men provide her with their company and the needed space to cry, moan, scream, and complain. Ereshkigal, grateful for the comfort the men offered, offered them the choice of a gift. They chose the freedom of Ianna, who was then brought back to life and permitted to return to her kingdom.

The myths of Ereshkigal and Persephone cover many aspects of Pluto—stripping away things one identifies with, forcing vulnerability, and understanding one’s shadow or dark side. It allows a transition beyond areas of habit where one becomes stuck in life. It helps to understand that the attitudes, people, or things once so strongly attached to are not part of one’s our true self. Pluto is ultimately about life-changing transformation from something old into something beautiful and new like the caterpillar transitioning into the butterfly, never to be the same again. One must die to the old to be transformed into the new.

Death and rebirth are Pluto’s primary associations. This powerful planet represents the ultimate threat to the ego because it obliterates the ego’s facade by penetrating deeply into the true self and exposing the shadows and pain that must be dealt with. Pluto characterizes the will, or a person’s inclinations or disposition in the sense of what drives them in life. Pluto’s sign in the natal chart describes how the person expresses these drives. Its house placement indicates where there is a tendency to control and dominate.

The reason for this association is it is human nature to fear life-altering changes, so there is an attempt to hold on tighter, to control the process and outcome. However, when undergoing a Plutoian experience, it becomes impossible to maintain this control because it is only by letting go that the transformation process can take place. Ultimately, Pluto’s placement in the natal chart exposes the areas of life where the individual is learning to surrender control and let go of old patterning. If a person works with this force throughout his or her life, the Plutoian experience can be renewing; by fighting it, the experience can be much more difficult and painful.

Like the mythological figure, Pluto is not evil. He governs the regenerative process which eventually leads to physical and psychological healing. In order to heal, an individual must first recognize and eliminate the toxins, whether physical, emotional, or mental. When Pluto is prominent in a chart, the individual often possesses intuitive knowledge of the healing process which can be used to help themselves and others.

Modern astrologers place Pluto as the ruler of the sign of Scorpio. Like Scorpio, Pluto represents hidden matters, power, metamorphosis, oppressiveness and extremes. Before Pluto’s discovery, Mars was considered the ruler of Scorpio. Interestingly, Pluto is considered the higher expression of the planet Mars. Mars rules sexual desire while Pluto is associated with orgasm and the conception, which brings about new life. Mars is also tied to aggression and courage; Pluto with intensity and the courage to let go.

—Tishelle Betterman


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Bloch, Douglas, and Demetra George. Astrology for Yourself: A Workbook for Personal Transformotion. Oakland, CA: Wingbow Press, 1987.

Burk, Kevin. Astrology: Understanding the Birth Chart. St. Paul, MN: Llewellyn Publications, 2001.
Campion, Nicholas. The Practical Astrologer. New York: Harry N. Abrams, 1987.
Campion, Nicholas, and Steve Eddy. The New Astrology: The Art and Science of the Stars. North Pomfret, VT: Trafalgar Square Publishing, 1999.
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McEvers, Joan. Planets: The Astrological Tools. Saint Paul, MN: Llewellyn Publications, 1989.

Valentine, Christine. Images of the Psyche: Exploring the Planets through Psychology and Myth. Shaftesbury, Dorset, UK: Element Books, 1991.
The Astrology Book, Second Edition © 2003 Visible Ink Press®. All rights reserved.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



in ancient Greek mythology, one of the names of the god of the underworld, added in the fifth century B.C. to the older name of Hades (Aïdes). A hospitable but inexorable god, Pluto gladly welcomes all to his abode but lets no one return from it. The best-known myth about Pluto concerns his abduction of Persephone. A number of myths linked Pluto to Plutus, the god of wealth, who ruled the earth’s trees, grains, and metals.



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.


See references under PLANET.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


The most distant planet in the solar system; mean distance to the sun is about 3.7 × 109 miles (5.9 × 109 kilometers); it has no known satellite, and its sidereal revolution period is 248 years.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


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


god of underworld. [Gk. Myth.: Howe, 224 ]
Allusions—Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. Copyright 2008 The Gale Group, Inc. All rights reserved.


Classical myth the god of the underworld; Hades


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
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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