dwarf planet

(redirected from Plutonic planet)
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Related to Plutonic planet: Hydrostatic equilibrium

dwarf planet,

a nonluminous body of rock or gas that orbits the sun and has a rounded shape due to its gravity. Unlike a planetplanet
[Gr.,=wanderer], a large nonluminous body of rock, gas, or ice that orbits the sun or another star, has a rounded shape due to gravity, and has cleared its orbit of smaller objects.
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, a dwarf planet is not capable of clearing its orbit of smaller objects by collision, capture, or other means. The known dwarf planets in the solar systemsolar system,
the sun and the surrounding planets, natural satellites, dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass.
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 include CeresCeres
, 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 year later
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, PlutoPluto,
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.
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, and ErisEris,
in astronomy, the largest known dwarf planet. Eris, whose highly eccentric elliptical orbit ranges from 38 AU to 97 AU and is inclined more than 44°, is the largest known object of the Kuiper belt (see comet), with a diameter (1,445 mi/2,317 km) slightly larger than
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. All of these are significantly smaller in size and mass than Mercury. Eris, which is slightly larger than Pluto, is the largest known dwarf planet. Except for Ceres, all known dwarf planets follow orbits that lie or take them beyond Neptune into the outer reaches of the solar system.

Ceres and Pluto have been considered planets at time in the past. The discovery of other bodies in the asteroidasteroid,
or minor planet,
small body orbiting the sun. More than 300,000 asteroids have been identified and cataloged; more than a million are believed to exist in the main belt between Mars and Jupiter, with many more in the Kuiper belt
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 belt led to the reclassification of Ceres in the mid-19th cent. as an asteroid. The discovery of other bodies beyond the orbit of Neptune that were similar in size to Pluto led to the declassification of Pluto as a planet in 2006, when the International Astronomical Union adopted the classification of dwarf planet.

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dwarf planet

Any of the four terrestrial planets.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006