Wednesday, August 16, 2006

The definition of a planet: IAU conference and the issue of 2003 UB313

It's been expected for a long time now; the IAU meeting has attempted to produce the definition of what a planet is.

Pluto was long too small to really match into the classical planets without letting other bodies like 2003 UB313 in, and now a committee has suggested simply the "spherical body not orbiting another object" suggestion, which includes some other minor planets in it including Pluto. You can't have Pluto in a real definition as a planet and not Ceres and some of the big Kuiper Belt objects without putting an arbitrarily size limit on the definition and this drove astronomers to impasse.

The "Spherical bodies" idea was agreed by some as a radical definition that would bestow planet status on many objects; some argued it would dilute the idea of what a planet was. Keeping the old 9 planets and restricting 2003 UB313 was cruel and decidely the wrong thing to do. Putting a higher size limit to keep 2003 UB313 out would have been physically arbitrary and would have forced Pluto out of major planet status.

Brian Marsden, who just retired, long argued that Pluto really was nothing more than a large Kuiper Belt Object. It was unfortunate in my opinion that public sentiment drove the arguments about was a planet was more than science. He makes the point that we will likely have a lot more planets in the near future when we prove the sphericity of the bigger minor planets and KBOs, and if the public doesn't like that, they chose to live with it by forcing Pluto into the issue.

UPDATE: Mike Brown's thoughts. And his NYTimes Op-ed.

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UPDATE: Looks like they've come to their senses.

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