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The Sciences

#97: All-Powerful Astronomers Turn "Dwarf Planets" Into "Plutoids"

Faced with an outcry over ungainly titles, the IAU comes up with a better alternative.

By Karen WrightDecember 4, 2008 6:00 AM
Image credit: R. Hurt (SSC-Caltech), JPL-Caltech, NASA | NULL


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Since 1992 astronomers have discovered more than 1,000 diminutive objects circling the sun beyond Neptune’s orbit. But no one knew what to call them, and this spelled mayhem for the International Astronomical Union (IAU), which is charged with naming such objects. In 2006, IAU members officially adopted the term “dwarf planet” for certain minor orbiting bodies. The choice of nomenclature gave modest outlier Pluto and some of the newfound worlds beyond Neptune the ungainly title of “transneptunian dwarf planet”—a designation that did not prove popular.

“It was a horrible phrase, a real mouthful,” says Edward Bowell, an IAU division president at the Lowell Observatory in Flagstaff, Arizona, where Pluto was first spotted in 1930. “Just a plain nuisance.”

After two years of debate, the IAU an­nounced in June that some bodies in this distant realm would be called plutoids, in honor of their prototype. The group defined a plutoid as an object orbiting the sun at an average distance greater than Neptune’s, massive enough to assume a nearly spherical shape (as planets do) but not massive enough to clear its orbital path of other bodies (as planets also do). In addition to Pluto itself, the remote dwarf planets Eris and Makemake qualify as plutoids, as does the recently named Haumea. According to the IAU, dozens more plutoids may yet be identified, but hundreds of smaller bodies seem likely to remain burdened with the designation “transneptunian object.”

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