Pluto: The Dwarfs Planet

Orbital characteristics

Pluto's highly elliptical orbit can take it more than 49 times as far out from the sun as Earth. It actually gets closer to the sun than Neptune for 20 years out of Pluto's 248-Earth-years-long orbit, providing astronomers a rare chance to study this small, cold, distant world. So after 20 years as the eighth planet (in order going out from the sun), in 1999, Pluto crossed Neptune's orbit to become the farthest planet from the sun (until it was demoted to the status of dwarf planet).

Composition & structure

Atmospheric composition: Methane, nitrogen
Magnetic field: It remains unknown whether Pluto has a magnetic field, but its small size and slow rotation suggest it has little to none.
Chemical composition: Probably a mixture of 70 percent rock and 30 percent water ice.
Internal structure: Probably a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane and nitrogen frost coating its surface.

Orbit & rotation

Average distance from the sun: 3,670,050,000 miles (5,906,380,000 km) — 39.482 times that of Earth
Perihelion (closest approach to the sun): 2,756,902,000 miles (4,436,820,000 km) — 30.171 times that of Earth
Aphelion (farthest distance from the sun): 4,583,190,000 miles (7,375,930,000 km) — 48.481 times that of Earth

Pluto's moons

In 1978, astronomers discovered Pluto had a very large moon nearly half its size, dubbed Charon, named for the mythological demon who ferried souls to the underworld in Greek mythology. The huge size of Charon sometimes leads scientists to refer to Pluto and Charon as a double dwarf planet or binary system.
Pluto and Charon are just 12,200 miles (19,640 km) apart, less than the distance by flight between London and Sydney. Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation — a Pluto day — also takes 6.4 Earth days. This is because Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking.
While Pluto appears reddish, Charon seems grayish. Scientists suggest Pluto is covered with nitrogen and methane while Charon is covered with ordinary water ice. In its early days, the moon may have contained a subsurface ocean, though it probably can’t support one today.
Compared with most of solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun. Also, Pluto's rotation is retrograde compared to the other worlds — it spins backward, from east to west.
In 2005, as scientists photographed Pluto with the Hubble Space Telescope in preparation for the New Horizons mission — the first spacecraft to visit Pluto and the Kuiper Belt — they discovered two other tiny moons of Pluto, now dubbed Nix and Hydra. These are two to three times farther away from Pluto than Charon, and they are thought to be just 31 to 62 miles (50 to 100 km) wide.
Scientists using Hubble discovered a fourth moon, Kerberos, in 2011. This moon is estimated to be 8 to 21 miles (13 to 34 km) in diameter. P4's orbit is between the orbits of Nix and Hydra. On July 11, 2012, a fifth moon Styx, was discovered, fueling the debate about Pluto’s status as a planet.
The four newly spotted moons may have formed from the collision that created Charon, hurled away from Pluto by the gravity of the massive moon.

source: space.com