Two years ago, the International Astronomical Union voted to demote Pluto from planetary status, deciding that it was only a "dwarf planet." There was great uproar among fans of Pluto, even spawning a group calling themselves The Society for the Preservation of Pluto as a Planet.
The IAU held firm to its decision, though, and moved on to other nomenclatural issues. A term was needed to encompass Pluto and all Pluto-like objects on the fringes of the solar system out beyond Neptune.
This week the IAU finally came up with an official term: plutoid. It's not the prettiest word, but it does the trick.
At the 2006 IAU meeting where Pluto was demoted, astronomers were deadlocked on a suitable term for Pluto and its icy peers. The original suggestion was pluton, but critics raised a number of objections. The chief criticism was that pluton is already in use as a term in geology. Owen Gingerich, chairman of the IAU's Planet Definition Committee, said at the time that he didn't think this was a problem. "Since the term is not in the MS Word or the WordPerfect spell checkers, we thought it was not that common," Gingerich told Nature. If Gingerich had only looked up pluton in our dictionary, he would have found it defined as "a large mass of intrusive igneous rock believed to have solidified deep within the earth," synonymous with batholith, batholite, and plutonic rock.
Just because geologists had been using pluton to mean something else, that doesn't mean astronomers couldn't use it for their own purposes, of course. Gingerich argued that the two meanings could coexist happily: "We think words can (and frequently do) have alternative meanings — for example, is there mercury on Mercury?" Linguists refer to this ability of a word to express more than one thing as polysemy, and it's a very common feature of the English lexicon (as any trip through the Visual Thesaurus readily shows). Nonetheless, the opponents of pluton won the day, and the suggestion was voted down. The IAU also considered plutonian object, or just plain plutonian. That came close to passing, but in the end it came up just a few votes short.
So it was back to the drawing board, with the IAU's Committee on Small Body Nomenclature charged with finding a more appropriate name. And here we are two years later with their final decision of plutoid. As committee secretary Brian Marsden explained, "We wanted a rather clear name that related to Pluto. Plutoid fit the bill. And I am happy enough with it. It means Pluto-like." Indeed, the -oid suffix shows up in many scientific terms to indicate that something has a resemblance to something else: a spheroid is like a sphere, a meteoroid is like a meteor, and so forth. Plutoid is a little tricky because it's a classification that includes Pluto itself, not just objects that are like Pluto.
At the moment there is just one other officially recognized plutoid to keep Pluto company: Eris, a dwarf planet first spotted in 2003 that is actually larger than Pluto. (The discovery of Eris helped kickstart the reclassification of the planets: either the planetary club had to include Pluto and Eris, or both of them had to be kicked out.) But astronomers are expecting to classify many more plutoids in the future. Two more celestial bodies are already lined up for recognition, though they don't have official names yet. Until they're named, astronomers are calling them "Santa" and "Easterbunny." Clearly, the unofficial nicknames in astronomical circles are a lot more fun than the official designations. Eris was originally known as Xena, because the discoverers were fans of the television show Xena: Warrior Princess! Then again, Eris is a fitting name, since it's the Greek goddess of discord. Ever since Eris was discovered, astronomers have been a pretty discordant lot.
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