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(♇) The smallest of the nine ‘traditional’ planets in the Solar System, and the farthest from the Sun. In 2006 the International Astronomical Union introduced a new definition which categorizes Pluto as a dwarf planet rather than one of the major planets; Pluto is now assigned the minor planet number (134340). This reclassification recognizes the fact that Pluto has many characteristics which distinguish it from the eight planets from Mercury to Neptune. It is far smaller than any of those, with a diameter of only 2390 km, less than that of our Moon. Its orbit has a greater inclination than any of the major planets, 17°.1 to the ecliptic, and its orbit is also the most elliptical (eccentricity 0.25). At aphelion Pluto lies 7375 million km from the Sun, but only 4425 million km at perihelion, inside the orbit of Neptune; it last reached perihelion in 1989. Its mean opposition magnitude is +15. Pluto was discovered in 1930 by C. W. Tombaugh. Its rotation axis is tilted at 122°.5 to its orbital plane, so that its rotation is retrograde, and it presents its poles and its equator alternately towards the Sun and the Earth as it moves around its orbit. Its axial rotation period, 6.387 days, is the same as the orbital period of its largest satellite, Charon, so that Pluto always keeps the same face towards Charon. Two much smaller and more distant moons, Hydra and Nix, were discovered by the Hubble Space Telescope in 2005, and a fourth in 2011.

Pluto has an extremely thin atmosphere with a surface pressure of about 10 μbar, composed of methane, possibly with some nitrogen and carbon monoxide. This atmosphere may be seasonal, forming when the planet heats up and releases surface volatiles around the time of perihelion. Methane may escape from the atmosphere near perihelion, so that Pluto behaves somewhat like a comet. Its mean surface temperature is estimated at −220°C. Pluto is thought to have a large rocky core, probably surrounded by a layer of frozen water and other icy materials, and a surface layer of methane. Pluto is now regarded as simply the largest of the sub-group of trans-Neptunian objects known as Plutinos.



Physical data



Inclination of equator to orbit

Axial rotation period (sidereal)

2390 km



6.387 days

Mean density

Mass (Earth = 1)

Volume (Earth = 1)

Mean albedo (geometric)

Escape velocity

1.8 g/cm3





Orbital data

Mean distance from Sun

106 km


Eccentricity of orbit

Inclination of orbit to ecliptic

Orbital period (sidereal)





247.9 years

Subjects: Astronomy and Astrophysics.

Reference entries

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