'Plutons' push planet total up to 12

A NEW kind of planet, the "pluton", could soon be taking its place in the Solar System.

Astronomers have agreed on a draft proposal for redefining what constitutes a planet.

If approved at a meeting underway in the Czech capital, Prague, school science text books will have to be re-written.

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The new definition would mean there are 12, not nine planets, and more could be added to the list in the future.

They include eight "classic" planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - Ceres, currently considered an asteroid, and three "plutons," one of which is Pluto.

The other plutons are Charon, currently described as a moon of Pluto, and the newly-discovered object 2003 UB313, which has not been named officially, but is nicknamed Xena.

Ceres is the largest object in the asteroid belt between Mars and Jupiter, and like a planet is spherical in shape.

A resolution to accept the new planet definition will be voted on by members of the International Astronomical Union (IAU) next Thursday, 24 August. If passed, the days of simply learning the names of the nine planets will be over for the world's schoolchildren. In future, many more planets could join the Sun's family as other plutons are discovered.

A dozen "candidate planets" are already on the IAUs "watchlist". They include Varuna, Quaor and Sedna, all Pluto-like objects residing within a region on the fringe of the Solar System known as the "Kuiper Belt".

Plutons differ from classical planets in that they have orbits round the Sun that take longer than 200 years to complete, and their orbits are highly-tilted and non-circular.

All these characteristics suggest that they have an origin different from that of classical planets.

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The IAU has taken two years working out the differences between planets and smaller Solar System bodies such as comets and asteroids.

IAU president Professor Ron Ekers said: "Modern science provides much more knowledge than the simple fact that objects orbiting the Sun appear to move with respect to the background of fixed stars.

"For example, 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.

"These discoveries have rightfully called into question whether or not they should be considered as new 'planets'."

According to the new draft definition, two conditions must be satisfied for an object to be called a "planet".

First, the object must be in orbit around a star, while not itself being a star. Second, and most importantly, it must be massive enough for its own gravity to pull it into a nearly spherical shape.

The IAU, responsible for the naming of planets and moons since 1919, set up a Planet Definition Committee (PDC) to consider the problem. Committee member Professor 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."

Mnemonic needed

IF ASTRONOMERS decide to change the number of planets in our solar system then piles of science textbooks will have to be rewritten.

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Generations of children have learned the names of the planets using mnemonics, listing the celestial bodies in their order from the Sun.

"My Very Eager Mother Just Sent Us Nine Pies" is one popular aide memoir, helping students to remember Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

Other useful phrases include "Make Very Easy Mash - Just Squash Up New Potatoes" and "My Very Easy Method Just Showed Us Nine Planets".