The International Astronomical Union (IAU) redefined the definition of a planet in 2006, leading to Pluto's demotion. The new definition requires a planet to:
1. Orbit the Sun
2. Have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
3. Have cleared the neighborhood around its orbit.
Pluto failed to meet the third criteria, as its orbit is shared with other objects in the Kuiper Belt.
