Quick Facts Mythology Orbit Composition Atmosphere Charon

Ruler of the Dead

Quick Facts:
Pluto is the farthest planet from the Sun and the smallest.
Pluto is even smaller than the Earth's Moon.
Distance from the Sun: 5,913,520,000 km (3,664,000,000 miles or 39.5 AU)
Period of Revolution: 247.7 years
Diameter: 2,274 km (3,500 miles)
Mass: 1.27e22 kg
Satellites: Charon

Mythology:
In Roman mythology, Pluto (Greek equivalent Hades) is the god of the underworld. The planet received this name perhaps because it's so far from the Sun that it is in perpetual darkness.
Pluto's satellite, Charon, is named for the mythological figure who ferried the dead across the River Styx into Hades.

Orbit:
Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (it has been so since January 1979 and will continue until February 1999). Pluto rotates in the opposite direction from most of the other planets.

Composition:
Pluto's composition is unknown, but its density indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of solid methane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown.

Atmosphere:
Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It has extremely tenuous surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. The Pluto Express mission planners want to arrive at Pluto while the atmosphere is unfrozen.

Charon:
Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). Some prefer to think of Pluto and Charon as a double planet rather than a planet and a moon. Pluto and Charon are also unique in that not only does Charon rotate  synchronously but Pluto does, too (they both keep the same face toward one another). It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon.