Planet status looking shaky for Pluto

And then there were eight

By Lucy Sherriff • Get more from this author

Posted in Space, 23rd August 2006 14:50 GMT

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Just when things were looking up for Pluto, astronomers have come up with a new definition of a planet that would leave the poor little mite out in the cold.

After last week's proposed definition of a planet, Pluto, along with the asteroid Ceres and two other large Kuiper belt objects, looked set to be officially classed as a planet. Or at least a pluton*, a sort of minor planetish thing, but still a planet.

However, scientists being scientists, a counter definition was almost immediately put forward at the International Astronomers Union in Prague, according to New Scientist.

Following a bit of a scuffle behind closed doors, accusations of ignoring the democratic process, and much bad language (we're just guessing on that last bit) a compromise definition has been written.

The original draft said a body could be classed as a planet if it met the following conditions: (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.

The new draft includes a third quality: the object must be the dominant body in its orbital area. Pluto would be ejected from the planetary brotherhood under this definition, as its orbit crosses that of Neptune, which is much larger (Sadly, Beavis, any jokes about Uranus will have to be filed for another time).

Planet-ish objects that meet the earlier definition, but fail to make the grade because of the new criterion could be called dwarf planets, or planetoids. Naturally, this is still a controversial point.

The definition would also only apply to planets in our solar system, leaving astronomers plenty to argue about when they find other almost spherical objects orbiting other stars.

The assembled professional skywatchers are set to vote on the definition tomorrow. Stay tuned for the result. ®

*Bootnote: Geologists have objected to the word Pluton, as it already has a meaning in Geology. It is a term used to describe a body of igneous rock formed beneath the surface of the earth by consolidation of magma.

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