Wednesday, August 16, 2006

So, I was wrong. But 12 planets instead of 9?

This news release from the General Assembly of the International Astronomical Union surprised me in some ways. It is the official proposal on the definition of a "planet." Instead of demoting Pluto to a "minor planet," it actually saves Pluto and re-promotes the asteroid Ceres to planet status (see my last post for that story).

The proposed definition actually makes some sense, scientifically. It is a definition based on measurable properties -- the shape of an object and its "dynamical state," (e.g., does it orbit another, much larger planet?).

In other ways, this definition is lacking. It says that planets are "not stars," which just means that the group did not want to develop a definition for separating stars, brown dwarfs, and planets. It also ignores the fact that, while Ceres and Pluto may be round, there are many, many non-round objects that are extremely similar (asteroids and the icy Kuiper belt objects). However, this definition may be the best possible one.

The new definition is not yet "accepted." That requires a vote of the full assembly of the IAU, which will not happen for a couple of weeks yet.

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