Will Pluto make it ?

Astronomers are gathering in the Czech capital, Prague, hoping to define exactly what counts as a planet.

The International Astronomical Union (IAU) hopes to settle the question of Pluto, which was first spotted in 1930.

Experts are divided over whether Pluto - further away and considerably smaller than the eight other planets in our Solar System - deserves the title.

The stakes were raised when a bigger planet-type body, known as 2003 UB313, was discovered by a US astronomer.

Any decision to downgrade Pluto would send shockwaves through the scientific community, instantly outdate textbooks, and change how the basics of the Solar System are taught in schools.

There are suggestions the scientists could decide to include Pluto in a new classification system that marks it out as different to the eight larger planets.

Since the discovery of the ninth planet, astronomers have become aware of a vast population of small, icy bodies resembling Pluto that orbit the Sun beyond Neptune, in a region called the Kuiper Belt.

This led some astronomers to argue that Pluto belonged with this population of "icy dwarfs", not with the objects we call planets.

Allowances could once be made for Pluto on account of its size. At just 2,360km (1,467 miles) across, Pluto is significantly smaller than the other planets. But until recently, it was still the biggest known Kuiper Belt object.

That changed with the discovery of 2003 UB313 by Professor Mike Brown and colleagues at the California Institute of Technology (Caltech).

After being measured with the Hubble Space Telescope, it was shown to be some 3,000km (1,864 miles) in diameter, making it larger than the ninth planet.