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Last Updated: Wednesday, 17 March, 2004, 01:32 GMT
'New planet' forces rethink
By Helen Briggs
BBC News Online

Pluto-Kuiper Express, Nasa
A probe will be sent to investigate Pluto - a planet or not
Astronomers are to rethink the system for classifying planets following the discovery of what some claim is the 10th in the Solar System.

A working group of the International Astronomy Union (IAU) will consider whether objects such as Sedna should be classed as planets.

The IAU says the group will consider the definition of a minimum size for a planet.

But in the meantime Sedna will not be considered one.

The outcome could lead to a demotion for Pluto, which some astronomers argue is too small to be called a planet.

"If we were starting anew, undoubtedly Pluto wouldn't be labelled a planet," Professor Iwan Williams, of the IAU, told BBC News Online.

"But we have almost a 100 years of culture that says Pluto's a planet. So the IAU will set up a working group to try to ponder the imponderable."

Icy worlds

Sedna, named unofficially after the Inuit goddess of the sea, is the latest in a string of icy objects approaching the size of Pluto discovered in the outer reaches of the Solar System.

Sedna is believed to be about three-quarters of the size of Pluto, based on measurements of light reflected from its surface detected by telescopes on Earth.

Sedna, Nasa
Distant and faint, Sedna is causing a stir
Many astronomers, including Mike Brown of the California Institute of Technology, who led the team that discovered Sedna, admit it is not a true planet, preferring to describe it as a planetoid - somewhere between a planet and an asteroid.

But like other objects found in recent months that inhabit the band of cosmic debris beyond the Inner Solar System, the Kuiper Belt, it is much bigger than a typical asteroid.

Some believe it is only a matter of time before another such body is found which dwarfs Pluto.

Reclassifying Pluto is one way to solve the dilemma. When it was discovered in 1930 it was thought to be much bigger - and thus more planet-like - than it really is.

However, the astronomical community will not take kindly to the idea of downgrading Pluto's status. The last time it was suggested, in 1999, it caused an uproar.

'New planet' may have a moon
15 Mar 04  |  Science/Nature
How do planets get their names?
16 Mar 04  |  Magazine
Strange events on distant Pluto
09 Jul 03  |  Science/Nature

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