The International Astronomical Union (IAU), defines a "dwarf planet" as a celestial body within the Solar System that satisfies these four conditions:

• Which is in orbit around the Sun.
• Which has sufficient mass for its gravity to overcome rigid body forces so that it is near-spherical shape.
• Which has not cleared the neighbourhood around its orbit.
• Which is not a satellite.

A dwarf planet is not a subcategory of 'planet', but a different class of object in its own right. The first three criteria are quite self-explanatory and need little interpretation, however there has been some confusion about the final criterion mentioned - that a dwarf planet must not have 'cleared the neighbourhood' around it's orbit. This criterion was included so as to exclude any of the 8 'traditional' planets from falling under the dwarf planet category.

1. Ceres
2. Pluto
3. Eris

Ceres
It is the smallest dwarf planet in the Solar System and the only one located in the main asteroid belt. Its name is derived from the Roman goddess Ceres. It was discovered on January 1, 1801, by Italian astronomer Giuseppe Piazzi. Ceres is 940km in diameter, and orbits the Sun at an average distance of 2.7 AUs, in the asteroid belt between Mars and Jupiter. Ceres follows an orbit between Mars and Jupiter, within the main asteroid belt, with a period of 4.6 years. Ceres is only just too faint to be seen with the naked eye, but can be seen easily with a telescope. Ceres probably has a rocky inner core surrounded by an icy mantle and with a thin, dusty crust on the outside. It is also suspected that Ceres may have a thin atmosphere, due to its relatively warm surface temperature (up to 239K). There are some indications that the surface of Ceres is relatively warm and that it may have a tenuous atmosphere and frost.

Pluto
Pluto was discovered in 1930 by astronomer Clyde Tombaugh. Pluto is named for the Greek god of wealth. Pluto's orbit takes it into the Kuiper Belt, which is a region containing many small objects. On August 24, 2006, the International Astronomical Union passed a new definition of planet that excludes Pluto and puts it in a new category of "dwarf planet." Pluto is probably composed of a mixture of rock and ice. It is suggested that Pluto is not a genuine planet, but simply a moon that somehow escaped from Neptune. It seems to have a bright layer of frozen methane ("marsh gas," chemically CH4) on its surface. Pluto seems to be about 3,000 to 3,500 kilometers (1,900 to 2,200 miles) in diameter. Pluto's surface gravity is .05 of Earth's gravity. Pluto is about the size of Earth's moon. Pluto's day is equal to 6.4 Earth days. Pluto's has one known satellite, Charon, was discovered in 1978 and is about half the size of Pluto. Pluto's year is 248 Earth years. Average distance from sun is 3.7 billion miles, or 5.9 billion kilometers.

Eris
Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005, from images taken on October 21, 2003 at Palomar Observatory's Samuel Oschin telescope. Eris was named after the Goddess of Discord in Greek mythology. Eris is the largest known dwarf planet in the Solar System. It is the ninth largest body orbiting the Sun directly. It is a trans-Neptunian object (TNO), orbiting the Sun in a region of space known as the scattered disc. Eris has an orbital period of 556.7 years, and currently lies at almost its maximum possible distance from the Sun. It is currently the most distant known solar system object from the Sun at a distance of roughly 97 astronomical units. Its diameter is about 2400km - slightly bigger than Pluto - and its average surface temperature is 30K (-243C), making Eris the coldest known dwarf planet. Eris has a moon called Dysnomia, discovered in 2005.