A classification introduced by the International Astronomical Union (IAU) in 2006 to describe celestial objects that don't quite reach planetary status. This new class currently includes Pluto, Ceres, Eris, Haumea and Makemake. Of these, Pluto was formerly a planet, Ceres is the largest object in the main asteroid belt and Eris, Haumea and Makemake are trans-Neptunian objects.
The classification was introduced after the discovery of Eris, which is larger in size than Pluto. With the discovery of numerous objects further than Neptune it was decided a new classification of object was required otherwise the number of planets in the Solar System would dramatically increase.