Pluto lost its designation as a planet at the 2006 General Assembly of the International Astronomical Union (IAU). Pluto was placed in the new category of dwarf planet, and declared to be the prototype of a new class of trans-Neptunian objects (see Astronomical Terms, p. 381–382).
Pluto's mean distance from the Sun is 3,687.5 million miles (5,900 million kilometers), and it takes 248 years to orbit the Sun. Appropriately named for the Roman god of the underworld, it must be frozen, dark, and dead.
In 1978, light-curve studies gave evidence of a moon revolving around Pluto within the same period as Pluto's rotation; therefore, it stays over the same point on Pluto's surface. In addition, it keeps the same face toward the planet. The satellite was later named Charon and is estimated to be about 789 mi (1,262.4 km) in diameter. Recent estimates indicate Pluto's diameter is about 1,441.6 mi (2,306.56 km), making the pair more like a double planet than any other in the solar system. Previously, the Earth–Moon system had held this distinction. Some astronomers have called for Charon to be given equal standing with Pluto as a planet or dwarf planet. The Hubble Space Telescope sighted two more Pluto moons in Oct. 2005.
The density of Pluto is slightly greater than that of water. There is evidence that Pluto has an atmosphere containing methane and polar ice caps that increase and decrease in size with its seasons. It is not known to have water. The Hubble Space Telescope's faint-object camera revealed light and dark regions on Pluto indicating an ice cap at the planet's north pole. It is not known if there is an ice cap at Pluto's south pole.
Pluto was predicted by calculation when Percival Lowell (1855–1916) noticed irregularities in the orbits of Uranus and Neptune. Clyde Tombaugh (1906–1997) discovered the dwarf planet in 1930, precisely where Lowell predicted it would be. The name was originally suggested by an 11-year-old British girl named Venetia Burney.
Pluto's eccentric orbit brings it at times closer to the Sun than Neptune. Pluto approached the perihelion of its orbit on Sept. 5, 1989, and until Feb. 1999 was closer to the Sun than Neptune. Even then, it could be seen only with a large telescope.
The New Horizons mission was launched in Jan. 2006 and is scheduled to reach Pluto in 2015. It would be the first mission to study Pluto, Charon, and other objects in the Kuiper Belt, a region beyond Neptune's orbit containing comets and what is believed to be detritus from the formation of the solar system.