In 2006, the International Astronomical Union created a new category of object in the solar system. The first object to be listed in that category was Pluto, and several more have been discovered and included since then.
The category is "Dwarf Planet" .
(D is the answer and here is why) "Aug. 24, 2006: Pluto Deplanetized. 2006: Pluto, once the ninth planet from the sun, is downgraded to a mere "dwarf planet." Our solar system loses a favorite kid brother and now has, officially, only eight planets."
