> The last time I looked at a science text book,
>more than a decade ago, Ceres was listed as the
>largest asteroid with a diameter of almost 500
>miles. More important is the fact that Ceres is
>round. It's almost a perfect sphere. Other
>asteroids are shaped like a potato or a grain of
>rice or a pork chop. (No, I'm not hungry. That's
>just the way my mind works.) Ignore size and a few
>other little details and it's a planet.

Yup. It was "officially" a planet for many years, and by one of the proposed definitions the IAU eventually rejected, it would have been a planet again. That definition would have also officially kept Pluto as a planet, but added its erstwhile "moon" Charon as another planet, and also UB2003, bringing the total number of official planets to 12. As wintermute pointed out, the major problem with that definition, which didn't include the provision that the object must have "cleared its orbit" of other objects, was that there are untold number of Kuiper Belt objects that probably fit this definition (I saw the number 53 thrown around a lot) that we know of *right now*, not to mention others we may find in the future. Nobody wants to have to remember 53 planets.

Honestly, I agree with Sam that the definition of "planet" has more cultural significance than it does scientific significance. Scientists are still going to want to study Pluto regardless of whether it's listed as the ninth planet or just one of the larger Kuiper Belt objects. However, the part of my brain that loves to categorize things craves a consistent definition of the word based on some objective standard. And I also don't want to memorize 44 new names. So in that respect, I like the IAU's new definition.

The funny thing is, the whole thing smacks of trying to come up with an "objective" definition that fits a pre-concieved idea of what a planet should be. You decide what things you think should be planets, then pick out characteristics that they share that objects you'd rather not list as planets do not have. Voila, an objective definition that fits what you already decided! It reminds me of an essay on baseball statistics I read once, that claimed that when baseball statisticians fiddle with new stats that try to objectively measure performance, the "Ruth test" is applied to them. You work out your new method of statistically ranking sluggers, and if Babe Ruth doesn't come out on top, well, your method must be wrong!

-- Dave