In addition to the eight planets in our Solar System, there are a number of Dwarf Planets that do not meet all the criteria required to be a full-blown planet, in the sense that they are not large enough to have cleared their orbits of other similar objects.
The term "Dwarf Planet" came about after disagreements among astronomers as to whether Pluto should still be considered a planet following the discovery of other objects, such as Eris, which were similar in size to Pluto. Finally in 2006, an official definition of a planet was made, and this led to a downgrading in the status of Pluto into the dwarf planet category.
To be classed as a planet the object must:
Currently there are 5 official dwarf planets, Ceres (in the asteroid belt), Pluto, Haumea, Makemake and Eris. In truth there are many similar objects orbiting our Sun well beyond the orbit of Pluto and no doubt these will be considered for addition to the list of dwarf planets in the future. More commonly known as Kuiper Belt objects, they exist in what is believed to be a huge shell of icy and rocky objects that live at the very edge of our Solar System.