The dwarf planets - Ceres, Pluto, & Eris

Pluto, a Kuiper object, was demoted to the status of dwarf planet in 2006 when the International Astronomical Union (IAU) reviewed Pluto's status in light of the recently discovered Eris, also a Kuiper object, but somewhat larger than Pluto.  At the same time the former asteroid Ceres was promoted to dwarf planet. The IAU defines a dwarf plant as a celestial body that, within the Solar System:

1. is in orbit around the Sun;

2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (near-spherical) shape;

3. has not cleared the neighborhood around its orbit; and

4. is not a satellite

http://en.wikipedia.org/wiki/Dwarf_planet

Point 3 is what distinguishes the dwarf planets from the remaining eight planets.

In this image, from left to right are the dwarf planets Ceres, Pluto, and Eris. Eris was discovered in 2003 and is now the largest of the known dwarf planets. It is believed to be slightly more massive than Pluto and follows a highly eccentric orbit that alternately brings it as close as the orbits of Neptune and Pluto and as far as over twice Pluto's furthest distance from the Sun.