‘Plutoid’?

Jun 11, 2008 5:59pm

Two years ago the International Astronomical Union did Pluto the indignity of demoting it from the list of planets in the solar system.  Now it’s rearranged the language. For the last two years Pluto and other like bodies have been labeled "dwarf planets."  Now, says the IAU, they’ll be "Plutoids." Plutoid…Plutoid?  Roll it over on your tongue a little bit.  "Plutoids," says the IAU, "are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit."  That’s a long way of saying they’re far, and big enough to be spheres, but not big enough that their gravity has drawn in other objects that may have passed nearby. For now there are only two Plutoids — Pluto, and Eris, a little world far beyond that was discovered in 2004. Ceres, the largest of the asteroids between Mars and Jupiter, spent two years as a "dwarf planet," but it won’t be a Plutoid.  No doubt the Ceresians will be insulted.  There are reasons, astronomers have argued, for this bit of sophistry.  The goal of science is to understand and categorize the universe, and new categories are needed as our understanding of the universe changes. But…Plutoid?

(Above: Pluto with its three known moons, Charon, Hydra and Nix, which will presumably continue to be known as "moons.")

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