On August 24, 2006, astronomers at the General Assembly of the International Astronomical Union approved a definition of a planet.
"Planets" and other bodies in our Solar System, except satellites, belong into three distinct categories in the following way:
(1) A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
The first members of the "dwarf planet" category are Ceres, Pluto and Eris. More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are listed on IAU's "dwarf planet" watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.
(3) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies". These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
IAU 2006 General Assembly: Result of the IAU Resolution votes
The official site of IAU XXVIth General Assembly
In the news release on June 11, 2008, the International Astronomical Union announced that at a meeting of the IAU's Executive Committee in Oslo, it has decided on the term plutoid as a name for dwarf planets like Pluto:
Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity.
The two known and named plutoids are Pluto and Eris. The dwarf planet Ceres is not a plutoid as it is located in the asteroid belt between Mars and Jupiter. Current scientific knowledge lends credence to the belief that Ceres is the only object of its kind. Therefore, a separate category of Ceres-like dwarf planets will not be proposed at this time.
For naming purposes, any Solar System body having (a) a semimajor axis greater than that of Neptune, and (b) an absolute magnitude brighter than H = +1 magnitude will, for the purpose of naming, be considered to be a plutoid, and be named. If further investigations show that the object is not massive enough and does not qualify as a plutoid, it will keep its name but change category.
News Release - IAU0804: Plutoid chosen as name for Solar System objects like Pluto
In July, 2008, the dwarf planet/Plutoid formerly known as 2005 FY9 was given a proper name:
Mike Brown's Planets
News Release - IAU0806: Fourth dwarf planet named Makemake
On September 17, 2008, the International Astronomical Union (the IAU) announced that the object previously known as 2003 EL61 was classified as the fifth dwarf planet in the Solar System and named Haumea. The two moons of Haumea were named Hi'iaka and Namaka.