Observation 2.1 Astronomy Cast Ep. 194 (Dwarf Planets)

Midway through the decade, in 2006, beloved Pluto was reclassified as a Dwarf planet, a "demotion" of sorts from it's former classification as a standard planet. This reclassification was forced by the discovery of  Eris, which was classified as a dwarf planet despite being larger than Pluto. The International Astronomical Union thus defined a planet as 1) The planet must have a stable round shape, as the result of it's own gravity; 2) The planet must  orbit the sun, and be greater in diameter than 2000 km, and 3) The Planet must clear it's own orbit in it's immediate neighborhood.