Pluto was discovered in 1930 by American astronomer Clyde W Tombaugh and at the time it was thought to be large enough to be dubbed a planet. Over the last 76 years our understanding of the Solar System has improved dramatically, and we now know there could be dozens of Pluto-sized (or larger) objects orbiting in the outer Solar System.

The debate, which has been raging for years, came to a head with the discovery of 2003 UB313 � an object in the outer Solar System, slightly larger than Pluto.

Some scientists argue that Pluto�s planet status should remain for historical reasons, whereas others believe that we should use our improved knowledge of the Solar System�s formation to classify objects more systematically. There are several viewpoints about what the exact definition of the word �planet� should be, but the International Astronomical Union (IAU) have narrowed their choice down to three:

A planet is any object in orbit round the Sun with a diameter greater than 2000 kilometres. This would mean Pluto would remain a planet, but we would also gain 2003 UB313 as a planet � the Solar System would have ten planets!

A planet is any object in orbit round the Sun whose shape is stable due to its own gravity. This would allow many more objects to be classed as planets, but would exclude some larger objects with non-stable shapes, such as Pallas, in favour of other smaller objects.

A planet is any object in orbit round the Sun that is dominant in its immediate neighbourhood (this means the object has a mass greater than the sum of all the smaller objects in the same orbit). This would mean the Solar System would have eight planets because Pluto would be downgraded.

The IAU are due to announce their decision on Thursday.