welcome to our new neighbours in space; Ceres, UB 313 and Charon

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The headline news in "The Star " today, has been the addition of three new planets into the solar system. The International Astronamical Union (IAU) which has been an arbiter of planetary nomencluture since 1919, has received a new definition of a planet from a special committee of seven experts that has been set up two years ago.

The new recommended definition of a planet is : A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite.

The ancient description of a planet was " an object that wanders against a backdrop of fixed stars" and is no longer valid in a modern age of advanced telescopes.

According to the panel of experts that were given the task of defining what a planet was, many factors had to be taken into consideration. However, after two years of confering ,their main goal being a new definition of the word 'planet'. They have come to a unanimous agreement that in future , a planet should be defined as a celestial body that is big enough for its gravity field to form a near-spherical shape.

The object must also be in orbit around the sun, or another star, but not as a satellite of another planet, which rules out the Moon and the larger moons of the other planets.They have thus chosen gravity as the determining factor.

Recent new discoveries have been made of objects in the outer regions of our solar system that have sizes comparable to and larger than Pluto. Thus, calling into question whether or not they should be considered planets.

The three new planets are Charon, once considered a moon of Pluto, but now described as its double planet. Ceres, formerly known as an asteroid or mino planet, and UB 313 (it has to be formally named, although it has been nicknamed Xena) was only identified last year.

There are now eight "classical" planets, three 'plutons" - those planets that are similar in size to Pluto with extremely wid solar orbits- and the asteroid -like Ceres.

The new definition means that there are possibly another two dozen known objects in the solar system that may be considered a planets.