This is the new category of celestial bodies for which the former planet, "Pluto" is now a major member of.
The decision to create this new category was made by the International Astronomical Union (IAU) when they met in Prague in the Czech Republic in August 2006. They were meeting to determine the fate of Pluto. Recent evidence had been revealed to prove Pluto was unlike all the other planets of the Solar System, but more like the minor planets beyond the orbit of Neptune (see "Kuiper Belts Objects" topic page).
More and more of these Kuiper Belt Objects (KBO's) were being discovered. With the majority smaller than Pluto and, some rivalling its size. It was the discovery of "Eris" (See "Sedna, Eris and Buffy" topic page) which cast Pluto's future into doubt. The team behind the discovery wanted Eris to be officially named as the tenth planet. But the discoveries of "Buffy", "Santa", etc., would mean there would have to be many more planets than the usual nine, or, on the other hand, strip Pluto of its title.
Which is exactly what the IAU decided to do. They decided to put Pluto into a new category, "Dwarf Planets".
The IAU have issued a new set of guidelines for what constitutes a Dwarf Planet. They are:
* It is in orbit around the Sun
* It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes an almost round shape (hydrostatic equilibrium)
* Has not "cleared the neighbourhood" around it's orbit
* Is not a satellite of a planet or other nonstellar body
This above definition is what reclassified Pluto from being a Planet to becoming a Dwarf Planet, because it has not cleared its orbit.
Dwarf Planets are traditionally to be known as Minor Planets -like the Minor Planets between the orbits of Mars and Jupiter (the Asteroid Belt), and the Minor Planets beyond the orbit of Neptune (the Kuiper Belt).
So far, the IAU has officially identified three celestial bodies as being Dwarf Planets. They are, Ceres, Pluto and Eris.
See also, "Ceres", "Pluto" and, "Sedna, Eris and Buffy" topic pages.
