The dwarf planets - Ceres, Pluto, & Eris

Cere's was elevated from asteroid to the status of dwarf planet in 2006 when the International Astronomical Union (IAU) reviewed Pluto's status as the solar system's 9th planet. Pluto was subsequently demoted 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. It was the discovery of Eris that prompted the re-evaluation of Pluto as a planet.