Pluto

Pluto is usually the outermost planet in the solar system, and is the only planet that has not been visited by a spacecraft from Earth. This mysterious planet is really a double planet system, since its moon, Charon, is very close to Pluto and about the same size.

Pluto's unique orbit causes it to periodically come within the orbit of Neptune. Pluto was the eighth planet from the Sun from February 7th, 1979 through February 11, 1999. Now Pluto will remain the ninth planet through the 23rd century.