A dwarf planet, as is defined by the IAU (International Astronomical Union), is defined as a celestial body that orbits a sun and has a big enough mass so that gravity shapes the planet into a spherical shape by overcoming its compressive strength to reach hydrostatic equilibrium. It must not have such a strong gravity as to clear the area of planetesimals (solid objects in protoplanetary disks/debris disks) or be a satellite.
The term ‘dwarf planet’ comes only from 2006 after a big increase of Trans-Neptunian (any object that currently orbits the sun but at a further distance from it than Neptune) objects that were rivalling Pluto in terms of size, then a planet. Before that there was no need for another classification as Pluto was accepted as a planet. Also, accurate estimates of Pluto’s size were not possible before the discovery of its satellite, Charon. Before it was believed to be bigger than Mercury but after the accurate measurements, far smaller – 1/20th actually. Other factors such as its elliptical shape and high orbital inclination were also very different to the rest of the planets in our solar system. The final straw was Eris a Trans-Neptunian object but it was far bigger than Pluto this led to a re-classification and ultimately the term ‘dwarf planet’. Another outcome was Pluto being re-classified as a dwarf planet and not as a planet. The term being accepted by the IAU has been applauded, criticised and disputed. In the foremost of this argument, Alan Stern who strongly disputes the term being used as he claims that the earth, Mars, Jupiter, Neptune have all not completely cleared their own orbital fields.
The IAU has only 5 dwarf planets officially on its list (but 40 other known objects are suspected of for-filling the definition of a dwarf planet, and there are up to 2,000 dwarf planets in our solar system many scientists believe); Ceres, Pluto, Haumea, Makemake and Eris. However only 2 of these planets: Pluto and Ceres have actually been studied and analysed enough to solidly prove that they do in dead fit the definition of a dwarf planet. The IAU has defined the classification of a newly found planet in simpler terms though. If the planet has a diameter of 838km or bigger it will be accepted as a dwarf planet and therefore be given a name under the assumption that it is one.
http://en.wikipedia.org/wiki/File:Pluto-map-hs-2010-06-d270.jpg
http://en.wikipedia.org/wiki/File:Ceres_optimized.jpg
http://en.wikipedia.org/wiki/File:Eris_and_dysnomia2.jpg
