How round is your planet?

When is a planet not a planet? That is the question. At least according to the International Astronomical Union (IAU). For the first time, the organisation will be officially defining the word "planet", and it is causing much debate in the world of astronomy. There is only one thing that everyone seems to agree on: there are no longer nine planets in the Solar System.
The whole debate was sparked off by the discovery in January of last year of a potential 10th planet, temporarily named 2003 UB313, which is bigger than Pluto.
Pluto is already an unusual planet. It is made predominantly of ice, and is smaller even than the Earth's Moon.
In 1992, scientists at the University of Hawaii discovered a new collection of objects beyond Neptune called the Kuiper Belt. Some suggest Pluto should no longer be considered a planet, but a Kuiper Belt Object. One researcher has come up with a clear planetary definition he would like to see the IAU adopt. In a nutshell – is it round?
This definition could lead to a Solar System with as many as 20 planets, including Pluto, 2003 UB313, and many objects previously classified as moons or asteroids.
New categories of planet could be introduced. Mercury, Venus, the Earth and Mars would be "rocky planets". The gas-giants Jupiter, Saturn, Uranus and Neptune would be a second category. Pluto, 2003 UB313, and any other objects passing the "roundness test", would be reclassified as a third type of planet - perhaps "icy dwarfs". Whatever the final outcome, by September there will no longer be nine planets in the Solar System. What? What about My Very Easy Method Just Shows Us Nine Planets? How are we supposed to write a rhyme to remember twenty or so planets? There’s no respect…
For more information check out the BBC News website
Pluto - NASA