For decades little Pluto was considered to be the ninth planet in our solar system, though everything about it just didn't fit in with the others. All the rocky planets reside in the inner solar system while the gas giants, Jupiter, Uranus and Neptune orbit in the outer regions.
The 2006 BBC Horizon documentary narrated by English actor Ian Holm (who played Ash in Alien (1979)) Bye Bye Planet Pluto, tells the story how Pluto lost its status as the ninth planet.
Scientifically it was inevitable because new large bodies in the Kuiper belt were being discovered more and more frequently due to advances in tracking techniques. Pluto was nothing unusual and made predominantly from ice and was smaller than the moon.
Pluto, 2003 UB313, and any other objects passing the "roundness test", would be reclassified as a third type of planet - perhaps "icy dwarfs".