Today is the fifth anniversary of the demotion of Pluto from "planet" to "dwarf planet."
Pluto was first discovered in 1930. Part of the reason it was accepted as a planet was due to the fact that despite some behavior not fitting a proper planet it was assumed to be larger than Mercury unti l1978, when its moon, Charon, was discovered, revealing that the mass of Pluto was much smaller than had been thought... roughly a twentieth the mass of Mercury. Two more orbiting objects, Nix and Hydra, were discovered in 2005. S/2011 P 1 (aka P4) was discovered in 2011.
Reaction to Pluto's re-designation was controversial, especially among young nerds who failed to see how going from the smallest planet in the solar system to largest known object in the Kuiper Belt could be viewed as a positive move. The New Mexico House of Representatives and Illinois State Senate passed ridiculous anti-scientific resolutions to continue recognizing Pluto as a planet.
Of the Trans-Neptunian Dwarf Objects, Pluto remains the most popular, if not the largest. In animation it's appeared in Cowboy Bebop, Futurama, Galaxy Express 999, Roughnecks -Starship Troopers Chronicles, Space Battleship Yamato, Science Ninja Team Gatchaman, Super Dimensional Fortress Macross and The Magic School Bus.