August 21, 2006

Piling on "pluton"

As we wait nervously for the General Assembly of the International Astronomical Union to decree how many planets are in the solar system, we are also kept on the edge of our seats about the status of the term pluton. The IAU is considering the creation of the category pluton to cover any planet beyond Neptune, taking more than 200 Earth years to complete its orbit around the Sun. (Or as phrased more pungently by Geoff Pullum, a "planet that is really nothing more than a God-forsaken frozen slushball way out beyond Neptune drifting around in the Kuiper belt and damn lucky to be called a planet at all.")

Pluton has been kicking around astronomical circles since at least 1991, when it showed up in a Science News cover story ("Plutos galore: ice dwarfs may dominate the solar system's planetary population," 9/21/91). Though the article generally refers to Pluto-like objects as Plutos, one quoted source, George W. Wetherill of the Carnegie Institution, called them Plutons instead, using criteria a bit looser than the IAU's:

There's really no adequate theory for the formation of Uranus and Neptune, and I really can't see how one can speak too intelligently about Triton and Pluto and these 1,000 'Plutons' without some framework for the whole origin of that part of the solar system.

(Triton, Neptune's largest moon, would not make the grade for the IAU definition of pluton, nor would the thousand or so ice dwarfs discussed in the article. So far the only transneptunian qualifiers are Pluto; Charon, formerly considered a moon of Pluto; and 2003 UB313, soon to be officially named but now traveling under the moniker Xena.)

There are scattered uses of pluton or Pluton in the relevant sense throughout the 1990s. For instance, the astronomer Tom Burns wrote a column for the Columbus Dispatch on June 8, 1997 in which he stated his preference for the label Pluton. And it has entered science fiction as well: according to a 1996 discussion on the Usenet newsgroup sci.astro, Fred Pohl used pluton as a generic term for Pluto-type objects in his 1992 novel Mining the Oort. (The word has had other sci-fi uses, though. Michael Quinion observes that Robert Heinlein used it as the name for a plutonium-based Earth currency in his novellas Gulf [1949] and Tunnel in the Sky [1955].)

One scientific group is not too pleased with the proposed designation: geologists, who already use the term pluton to refer to igneous rock intrusions. On SciAm Observations, the blog of Scientific American, George Musser considers the "appropriation" of pluton to be "a remarkable violation of professional courtesy." In another post, he continues his gripe:

It's bad enough when popular culture pilfers scientific terms, such as "quantum leap", "epicenter", and "light-year", and twists their meaning. But it's all the more galling when one scientific discipline appropriates the terminology of another.

Musser reports that University of North Carolina-Chapel Hill geologist Allen Glazner has petitioned the Geological Society of America and the American Geophysical Union to draft a cease-and-desist order of sorts, telling the IAU to lay off pluton. Glazner has asked his colleagues to join him in "trying to get the IAU to come up with a new word, instead of hijacking our perfectly good one and causing endless confusion."

Is polysemy among scientific disciplines really such a terrible offense? Glazner wonders, "What happens when we find a pluton on a pluton?" I think context should be more than enough to obviate the "endless confusion" that Glazner foresees. Do geologists also worry about the fact that veins can refer to both rock fractures and blood vessels, or that a delta can be either a sedimentary deposit or a finite mathematical increment?

A more damaging objection about pluton, to my mind, is that it's a bit too similar to the root form Pluto, especially when you consider that the French term for Pluto is... Pluton. Similarly, Spanish uses Plutón and Italian has Plutone. How would we able to say in such languages, "Pluto is a pluton"? Linguablogger Gheuf suggests that the plutons would be better served with some other suffix:

But if the Plutons are given a suffix, what suffix should it be? I am partial to the diminutives, but it would be hard to choose between the contending charms of Plutitos, Plütchen, and Plutoncini; the last has a certain gastronomic appeal. Or we could give up the Roman gods, and name them "Planettes". But to please the sober tastes of the Scientists, I suggest "Plutonoids" (Pluto-like), which has a learned air, and is not too badly formed either.

I do like plutitos, plütchen, and plutoncini, but there's a problem with diminutivizing Pluto: astronomers have already done that with plutinos, a term they use to refer to various small objects in the inner part of the Kuiper Belt up to and including Pluto. (If I understand the distinction correctly, Pluto itself is the only plutino that would also fit the IAU definition of pluton.) Plutonoid sounds good though, and I wonder why the IAU hasn't considered it (or the similar plutoid). Perhaps it was deemed inappropriate because the -oid suffix implies that such objects only resemble Pluto, which might make it hard to justify including Pluto itself in the same category.

Whatever the IAU's decision ends up being, we'll probably all learn to live with it, regardless of current terminological grumbling. And we can always fall back on "God-forsaken frozen slushballs" (GFFSBs for short).

[Update, 8/23/06: It looks like the astronomers are backing off pluton. Details here.]

Posted by Benjamin Zimmer at August 21, 2006 12:11 PM