The real facts about Pluto

Pluto orbits away from the orbit of Neptune (usually). It is greatly smaller than any of the official planets and currently classified as a "dwarf planet". Pluto is lesser than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton). Pluto has been given number 134340 in the minor planet catalog. Pluto's orbit is extremely eccentric. At times it is nearer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Pluto rotates in the conflicting direction from most of the other planets.

Pluto is safe in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is just 1.5 times longer than Neptune's. Its orbital inclination is as well much higher than the other planets'. Thus though it looks that Pluto's orbit crosses Neptune's, it actually doesn't and they will never collide. Pluto's composition is unidentified, but its density (about 2 gm/cm3) indicates that it is almost certainly a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface appear to be covered with ices of nitrogen with lesser amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unidentified but may be due to primordial organic material or photochemical reactions driven by cosmic rays.

Little is recognized about Pluto's atmosphere, but it perhaps consists primarily of nitrogen with some carbon monoxide and methane. It is very tenuous, the surface pressure being only some microbars. Pluto's atmosphere can exist as a gas only when Pluto is near its perihelion; for the greater part of Pluto's long year, the atmospheric gases are frozen into ice. Close to perihelion, it is likely that some of the atmosphere escapes to space possibly even interacting with Charon. NASA mission planners want to appear at Pluto while the atmosphere is still unfrozen.