Most of the time, Pluto is the most distant planet of the solar system. It's also the smallest planet with a diameter of 2,237 miles. Because it is so small and far away, we don't know that much about the planet.

Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Pluto rotates in the opposite direction from most of the other planets.

Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous the 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.

Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.

After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no tenth planet.

Pluto's composition is unknown, but its density (about 2 gm/cm3) 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, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays.

Pluto is the only planet that has not yet been visited by a spacecraft. 

Pluto has a moon, which is about half pluto's size. The moon is called Charon. Because Charon is so big (and Pluto so small) they're almost like a double-planet.

Pluto lies 3,667 million miles from the sun. The rotation around the sun takes 247.7 years and a rotation around the planet's own axis takes 6 days and 9 hours. The planets mass is 1.27e22 kg.

Pictures of Pluto (click to see the large picture):