In 2006, the IAU decided to create a new classification category for solar system objects called "dwarf planets" that is distinct from "planets". Pluto was reclassified as a dwarf planet. At present there are only four other members of this category, Ceres, Eris, Makemake and Haumea but it is likely that there will be many more in the future as the Kuiper Belt is more fully explored.
The actual definition of "dwarf planet" is kind of technical: 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,
c. has not cleared the neighbourhood around its orbit, and
d. is not a satellite.
In short, something that looks like a planet but isn't a planet.
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