"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.
source: http://blog.calendars.com/2012/08/pluto-demoted/
further reading :
http://earthsky.org/space/this-date-in-science-pluto-gets-its-namehttp://curiosity.discovery.com/question/why-is-pluto-no-longer
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