Pluto

The ninth (9th) planet from the sun … well … Pluto is unlike other planets in many respects. It is smaller than our moon. Its orbit carries inside the orbit of Neptune and the way out beyond that orbit. From 1979 until early 1999, Pluto had actually been the eighth planet from the sun. Then, on Feb. 11, 1999, it crossed Neptune's path and once again became the solar system's most distant planet — until it was demoted to dwarf planet status. Pluto will stay beyond Neptune for 228 years. Pluto’s orbit is tilted to the main plane of the solar system — where the other planets orbit — by 17.1 degrees. It’s a cold, rocky world with only a very ephemeral atmosphere.

    Discovery: 1930 by Clyde Tombaugh

    Named for: Roman god of the underworld, Hades

    Diameter: 1,430 miles (2,301 kilometers)

    Orbit: 248 Earth years

    Day: 6.4 Earth day