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Definition: Pluto (astronomy) from The Macmillan Encyclopedia

The smallest and usually outermost planet, orbiting the sun every 248 years at a mean distance of 5900 million km. Its eccentric orbit has brought it inside Neptune's orbit for the period 1979-99. Pluto was discovered in 1930 but little was known about it until its satellite was discovered in 1978. It is a very cold low-density body about 3000 km in diameter.


Summary Article: Pluto from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Dwarf planet. Until its reclassification in 2006 it was considered to be the smallest and, usually, outermost planet of the Solar System. The existence of Pluto was predicted by calculation by US astronomer Percival Lowell and located by US astronomer Clyde Tombaugh in 1930. Its highly elliptical orbit occasionally takes it within the orbit of Neptune, as in 1979–99. Pluto has a mass about 0.002 of that of Earth.

Mean distance from the Sun 5.8 billion km/3.6 billion mi

Equatorial diameter 2,300 km/1,438 mi

Rotation period 6.39 Earth days

Year 248.5 Earth years

Atmosphere thin atmosphere with small amounts of methane gas

Surface low density, composed of rock and ice, primarily frozen methane; there is an ice cap at Pluto's north pole

Satellites one moon, Charon

Charon, Pluto's largest moon, was discovered in 1978 by US astronomer James Christy. It is about 1,200 km/750 mi in diameter, half the size of Pluto. It orbits about 20,000 km/12,500 mi from the planet's centre every 6.39 days – the same time that Pluto takes to spin on its axis. Charon is composed mainly of ice. Two smaller moons were identified in 2005. Some astronomers have suggested that Pluto was a former moon of Neptune that escaped, but it is more likely that it was an independent body that was captured. Pluto has not yet been visited by a space probe, but the New Horizons probe, launched in 2006, is due to fly by Pluto in mid-2015.

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