When is a Planet not a Planet?

There has been a debate raging as to whether Pluto should be considered a planet at all - and what should define a planet.

The International Astronomical Union is a 19 member committee that is set to deliver a verdict as to the definition of a planet.

There are five main ideas, each of which is fiercely contended:

1 Planets Are Spherical

Over a certain size any object will be pulled into a sphere by its gravity, that size is about 800km in diameter. Alan Stern is a leading proponent of this argument. Although it would include Pluto it would also include Ceres the asteroid and several Kuiper Belt Objects.

2 Planets are Loners
This idea states that a planet dominates its zone in the solar system, orbiting alone. This would exclude the asteroid belt and the Kuiper belt objects including Pluto. Unfortunately we would have only eight planets in the solar system.

3 Planets of a certain size
Anything with a radius over 1000km would be considered a planet. This system sets a clear physical limit and would include Pluto as well as UB313 discovered in 2003, also does not rule out future discoveries.

4 By History
Although its orbit would normally rule out Pluto this group say we should include it as historically it has been considered a planet for 75 years. But future Kuiper Belt objects would be excluded.

5 No Planets?
We would not use the word planet alone to describe anything, but precede it with adjectives to distinguish four types of planets: terrestrial planets, minor planets, gas giant planets, and trans-Neptunian planets.