A ‘planet’ is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”
- from IAU Resolution 5A
The IAU came up with the definition to resolve astronomers’ long-running debate over whether or not Pluto was a planet. In early August 2006, a group of astronomers appointed by the IAU to determine the definition of a planet decided on this definition as well as the definition of a dwarf planet. On August 24, 2006, the IAU passed one resolution that declared this definition of a planet and defined all other objects that orbit the sun as dwarf planets or small solar system bodies. The second resolution declares Pluto a dwarf planet, based on the definitions in Resolution 5A.
Head over to Curiosity.com for a simple explanation of the controversy over Pluto and why it was demoted. Or check out The New York Times’s August 24, 2006 coverage of Pluto’s demotion for an in-depth look at the controversy and how the decision to demote Pluto came to be.