40 Facts About Pluto


Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune.

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Pluto is the ninth-largest and tenth-most-massive known object to directly orbit the Sun.

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Pluto has five known moons: Charon, the largest, whose diameter is just over half that of Pluto; Styx; Nix; Kerberos; and Hydra.

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Pluto's suggested it in a conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford's Bodleian Library, who passed the name to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States.

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Once Pluto was found, its faintness and lack of a viewable disc cast doubt on the idea that it was Lowell's Planet X Estimates of Pluto's mass were revised downward throughout the 20th century.

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In 1931, Pluto was calculated to be roughly the mass of Earth, with further calculations in 1948 bringing the mass down to roughly that of Mars.

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Lowell had made a prediction of Planet X's orbit and position in 1915 that was fairly close to Pluto's actual orbit and its position at that time; Ernest W Brown concluded soon after Pluto's discovery that this was a coincidence.

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From 1992 onward, many bodies were discovered orbiting in the same volume as Pluto, showing that Pluto is part of a population of objects called the Kuiper belt.

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The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2, but do not meet criterion 3 would be called dwarf planets.

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Alan Stern, principal investigator with NASA's New Horizons mission to Pluto, derided the IAU resolution, stating that "the definition stinks, for technical reasons".

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Pluto argued that all big spherical moons, including the Moon, should likewise be considered planets.

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Pluto stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.

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The Illinois Senate passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois.

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The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU.

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In contrast, Pluto's orbit is moderately inclined relative to the ecliptic and moderately eccentric .

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When Pluto is closest to the Sun, and close to Neptune's orbit as viewed from above, it is the farthest above Neptune's path.

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Pluto's orbit passes about 8 AU above that of Neptune, preventing a collision.

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However, Pluto is protected by its 2:3 orbital resonance with Neptune: for every two orbits that Pluto makes around the Sun, Neptune makes three.

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However, it is not agreed upon among astronomers whether Arawn should be classified as a quasi-satellite of Pluto based on this motion, since its orbit is primarily controlled by Neptune with only occasional smaller perturbations caused by Pluto.

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Plains on Pluto's surface are composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.

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Pluto's surface is quite varied, with large differences in both brightness and color.

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Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn's moon Iapetus.

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Pluto's color is more similar to that of Io with slightly more orange and significantly less red than Mars.

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In June 2020, astronomers reported evidence that Pluto may have had a subsurface ocean, and consequently may have been habitable, when it was first formed.

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In March 2022, they concluded that peaks on Pluto are actually a merger of "ice volcanoes", suggesting a source of heat on the body at levels previously thought not possible.

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Pluto is more than twice the diameter and a dozen times the mass of Ceres, the largest object in the asteroid belt.

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Determinations of Pluto's size have been complicated by its atmosphere and hydrocarbon haze.

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Pluto has a tenuous atmosphere consisting of nitrogen, methane, and carbon monoxide, which are in equilibrium with their ices on Pluto's surface.

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Pluto has no or almost no troposphere; observations by New Horizons suggest only a thin tropospheric boundary layer.

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Presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, with the average temperature of its atmosphere tens of degrees warmer than its surface, though observations by New Horizons have revealed Pluto's upper atmosphere to be far colder than expected .

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Orbital periods of all Pluto's moons are linked in a system of orbital resonances and near resonances.

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Pluto–Charon system is one of the few in the Solar System whose barycenter lies outside the primary body; the Patroclus–Menoetius system is a smaller example, and the Sun–Jupiter system is the only larger one.

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The similarity in size of Charon and Pluto has prompted some astronomers to call it a double dwarf planet.

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Pluto's moons are hypothesized to have been formed by a collision between Pluto and a similar-sized body, early in the history of the Solar System.

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One early hypothesis was that Pluto was an escaped moon of Neptune knocked out of orbit by Neptune's largest current moon, Triton.

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Pluto is known to be the largest member of the Kuiper belt, a stable belt of objects located between 30 and 50 AU from the Sun.

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Eris is about the same size as Pluto but is not strictly considered a member of the Kuiper belt population.

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For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot.

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Equatorial region of the sub-Charon hemisphere of Pluto has only been imaged at low resolution, as New Horizons made its closest approach to the anti-Charon hemisphere.

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Map of Pluto based on Hubble images from 1996, centered on the anti-Charon hemisphere, covering the southern hemisphere down to 75°S.

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