Pluto was discovered in 1930 by a fortunate accident by Clyde W. Tombaugh at Lowell Observatory in Arizona.

Due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249-year orbit. Pluto made its closest approach during 1989 and moved beyond the orbit of Neptune on March 14, 1999.

Pluto, the outermost of the planets, is icy, has a satellite companion, and a tenuous atmosphere. No images of this planet have been made by any spacecraft. Pluto and Charon form a double -planet system with an elliptical orbit.

Pluto has a methane-ice surface and an atmosphere that contains methane, perhaps like that of Neptune's Triton. Its overall composition is probably a mixture of 70% rock and 30% water ice.

Pluto/Charon have a density higher than those of the moons of Uranus, suggesting that it has a greater proportion of rocky materials. This may suggest that the outer solar nebula was water-poor and dominated instead by carbon monoxide, which did not condense to form solids.