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 region called the Kuiper belt.
In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percival Lowell.
Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of
Uranus and
Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.
After the discovery, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no Planet X. But that doesn't mean there aren't other objects out there, only that there isn't a relatively large and close one like Planet X was assumed to be. In fact, we now know that there are a very large number of small objects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size as Pluto.
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 a highly eccentric and highly inclined orbit.
Pluto's eccentricity takes it from 30 to 49 AU (4.4-7.4 billion km) from the Sun, causing Pluto to occasionally come closer to the Sun than
Neptune. Pluto and its largest moon, Charon, are often treated together as a binary system because the barycentre of their orbits does not lie within either body. 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. Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.
From its discovery in 1930 until 2006, Pluto was counted as the Solar System's ninth planet. In the late 20th and early 21st centuries, however, 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. On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto, which the IAU reclassified as a member of the new category of dwarf planets along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340.
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| Pluto Links & Resources
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New Horizons: NASA's Pluto/Kuiper Belt Mission
News about Pluto and the Pluto/Kuiper Belt mission that will fly by Pluto in 2015.
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Hubble Reveals Surface of Pluto for First Time
For the first time since Pluto's discovery , astronomers have at last directly seen details on the surface of the solar system's farthest known planet.
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Clyde Tombaugh
On January 17, 1997, Clyde Tombaugh, the discoverer of the planet Pluto, passed away. In his nearly 91 years Tombaugh made tremendous contributions to the field of astronomy and inspired generations of future astronomers to follow in his footsteps. This Web site is dedicated to his memory.
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Clyde Tombaugh
A Biography of Clyde Tombaugh, who discovered Pluto in 1930, by the Kansas State Historical Society.
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Clyde Tombaugh Papers
Personal and professional papers of the planetary astronomer who discovered Pluto in 1930. Consisting of approximately 150 linear feet of material, the papers include correspondence, educational records, administrative files, research files, writings, technical drawings, photographs, astronomical charts, and printed matter.
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Clyde Tombaugh Quotes
Quotes from the man who discovered Pluto.
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Pluto Rotation Movie 1
The animation was created from a global map constructed through computer image processing performed on the Hubble data. The tile pattern is an artifact of the image enhancement technique. Some of the variations across Pluto's surface may be caused by topographic features such as basins, or fresh impact craters. However, most of the surface features unveiled by Hubble, including the prominent northern polar cap, are likely produced by the complex distribution of frosts that migrate across Pluto's surface with its orbital and seasonal cycles and chemical byproducts deposited out of Pluto's nitrogen-methane atmosphere. The pictures was taken in blue light when Pluto was at a distance of 3 billion miles from Earth.
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Pluto Rotation Movie 2
This animation of Pluto is based on pictures courtesy of Marc Buie/Lowell Observatory.
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| Meteorite eBooks & Articles
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US $2.50
HOW I KILLED PLUTO And Why It Had It Coming By Mike Brown 267 pages. Spiegel & Grau. $25. The Caltech astronomy professor Mike Brown is not the first person to write about the rise of the planet Pluto (discovered in 1930 by Clyde Tombaugh) or its recent demotion (in 2006) to second-class citizenry in a newly reduced, eight-planet solar system. He...
