Why The Former Planet Pluto Got Demoted
In 2006 the former ninth planet in the solar system, Pluto, got demoted to a mere Kuiper Belt object. The man who was in large part responsible for that demotion, Caltech planetary scientist Mike Brown, discusses the status of Pluto and why it doesn't qualify as a planet.
Source: http://www.npr.org/2011/02/04/133498152/why-the-former-planet-pluto-got-demoted?ft=1&f=1007