Originally discovered in 1930, Pluto was at that time classed as a planet, and named for the Roman god of the Underworld. However, as the years went by, evidence mounted that it was not truly a major planet. Although it did have moons of its own, it also had an eccentric orbit (which crosses that of Neptune, the next furthest out planet) and a lower mass than any other planet.
The discovery that Pluto was just one of a number of bodies in the Kuiper Belt, many of them with comparable size and mass, also weakened the arguments for considering it a planet. Finally, a new definition of what a planet issued by the International Astronomical Union on August 24, 2006, excluded Pluto. On September 13, Pluto was named a Dwarf Planet, alongside Ceres, Haumea, Makemake, and Eris – all of which, other than Ceres, are also Kuiper Belt objects.
By NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker – http://pluto.jhuapl.edu/Galleries/Featured-Images/image.php?page=1&gallery_id=2&image_id=543, Public Domain, Link
As mentioned in: