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The · Psychohistorian


Twelve planets ... again

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The proposed new system for defining the term "planet" has a certain elegance to it. Rather than use arbitrary constants, everything is defined in clean, unitless terms. For a body to be a planet, gravity must dominate mechanical forces, limiting the definition to fairly large, spherical objects - without any explicit requirements on either size or sphericity. Charon becomes a "double planet" with Pluto, rather than a moon of Pluto, because their joint center of rotation is between the two rather than within one of them. Ceres and Xena (UB313) are also promoted to planet status, raising the number from 9 to 12.

We've had twelve planets before, though. Indeed, Ceres was one of them. The year was 1845, and the planets were Mercury, Venus, Earth, Mars, Vesta, Astraea, Juno, Ceres, Pallas, Jupiter, Saturn, and Uranus. So what happened?

Well, starting with Neptune in 1846, at least one new "planet" was discovered per year. Eventually people decided that the smaller ones - most of which were in the gap between Mars and Jupiter - should be demoted to avoid too much dilution of the term. Neptune was retained; Ceres, Vesta, Pallas, Juno, and Astraea were demoted.

It seems likely to me that history will repeat itself. We know there are so many objects in the Kuiper belt that dozens are likely to qualify for the new definition of planet. Eventually there will be too many for lay people to keep track of and there will be another winnowing.
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On August 18th, 2006 05:21 pm (UTC), enugent commented:
You might enjoy this post on the subject...
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