Pluto

Pluto is the most distant planet from the Sun, usually orbiting beyond the orbit of Neptune. 

Pluto is the Roman name for Hades, the Greek god of the Underworld. The planet received it's name because of it's distance from the Sun and because it is in perpetual darkness.

It is much smaller than any of the official planets and now classified as a "dwarf planet".

See Also: Venus, Sun, Moon, Mercury