Pluto is being downgraded to a dwarf planet at 24 August 2006 is because Pluto failed to apply to one of the characteristic of becoming a planet.
1. A planet is able to produce its gravity that attract particle into a sphere.
2. A planet must be able to orbit around the sun every time.
3. A planet's orbiting according to it's orbit must clean out the debris such as rock around the orbital region.
The third characteristic failed to apply to Pluto thus to become a dwarf planet just like more than 1,000 dwaft planet outside.