Science and Space

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How to define a planet.

Astronomers are arguing over how to define a planet.

The International Astronomical Union (IAU), which is responsible for settling such issues, assembled a special working group to come up with a new definition about 18 months ago.

Then a large object was discovered in July 2005 in a ring of rocky bodies beyond Neptune called the Kuiper Belt. It was dubbed the "tenth planet" by its discoverers because it is larger than Pluto. But the 19-member working group has been unable to arrive at a consensus.

Some submitted a proposals based simply on an object’s size.

The second proposal says a planet is the dominant body in its immediate neighbourhood - a title based on its size relative to its neighbours and the dynamics of its orbit. Objects in the asteroid belt beyond Mars, for example, would not be planets because there are so many of them in the same region, while Pluto would not count because it crosses the orbit of its more massive neighbour Neptune.

But a third proposal has been discussed that does away entirely with the term "planet". It uses qualifying adjectives to define subclasses of planets based on characteristics like location, composition or culture. In this scheme, Earth might be a "terrestrial planet" and Pluto a "historic planet".

The working group may vote on the proposals within the next two weeks.

posted by: kyawoo at 11:11 | link | comments |
planets

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