How the Solar System may appear to future generations, if the IAU declare three new planets.

Credit: The International Astronomical Union/Martin Kornmesser

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SYDNEY, 16 August 2006: The Solar System may gain an extra three planets after today's meeting of the International Astronomical Union (IAU) in Prague, upsetting text-books and science-classroom mnemonics world-wide.

With the advent of ever-more powerful telescopes, objects orbiting our Sun out beyond Pluto have been discovered. Some of these, Xena being the most famous example, are actually larger than Pluto and, its discoverers declare, should be considered as much of as planet as the others.

Other astronomers contest that these far distant objects are merely bits of rubble in an area of space filled with other bits of rubble, the Kuiper Belt.

The discoveries and the associated debate has thrown into question the definition of a ‘planet' (see Cosmos, Issue 9 p 74) and at its general meeting this week, the governing body of such matters, the International Astronomical Union has sat down to nut out the problem.

The solution they propose is to describe all the objects as planets. Eight of them will be the ‘classical' planets we have come to know and love. Ceres, an asteroid floating in the asteroid belt between Mars and Jupiter will become a planet. And a kind of sub-planet category known as 'plutons' will be invented for the remaining three planets. They will be Pluto, it's moon Charon and 2003-UB313, nick-named Xena. Xena's real name will be determined at a future date.

The part of "IAU Resolution 5 for GA-XXVI" that describes the planet definition, states: "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 of a planet."

A recent poll of Cosmos Online readers found that a slim majority (28 per cent) preferred Pluto remaining a planet. Twenty per cent of respondents thought Pluto should be a Kuiper Belt Object, and 26 per cent wanted a new category for Pluto and the Kuiper Belt Objects. Some 25 per cent of readers playfully suggested that Pluto was a Disney dog.

Plutons are to be distinguished from classical planets in that their orbits around the Sun that take longer than 200 years to complete and their orbits are highly tilted with respect to the classical planets. Plutons also typically have orbits that are far from being perfectly circular.

The IAU said that all of these distinguishing characteristics for plutons are scientifically interesting in that they suggest a different origin from the classical planets.

Member of the Planet Definition Committee, Richard Binzel said, "Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet."

The draft "Planet Definition" resolution will be discussed and refined during the General Assembly and then it will be presented for voting on 24 August 2006.