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Dwarf planet's downsizing confirmed

NASA / JPL-Caltech

It turns out that Eris, shown in this artist's conception, may be Pluto's denser twin.

It's been almost a year since astronomers suggested that Eris, the icy world whose discovery prompted Pluto's controversial reclassification in 2006, wasn't as big as they originally thought. Now the official word has leaked out unofficially: Pluto just might be the largest dwarf planet after all — although Eris is still seen as more massive.

The latest measurements were reported last week in Nantes, France, at a joint meeting of the American Astronomical Society's Division of Planetary Sciences and the European Planetary Science Congress. But as the Planetary Society's Emily Lakdawalla explains, it took a while for the report to become public, due to worries about the journal Nature's rules on embargoes and confidentiality.

Here are the statistics: Based on measurements made last November during the dwarf planet's occultation of a faraway star, Eris' diameter is estimated at 2,326 kilometers (1,445 miles). A similar set of measurements, published in 2009. estimated that Pluto was at least 2,338 kilometers (1,453 miles). When you include the margin of error, Pluto is essentially Eris' equal in size.

"It could be smaller, it could be larger; basically, it is a twin," Lakdawalla quoted Paris Observatory astronomer Bruno Sicardy, the lead researcher for the Eris measurements, as saying at the conference.

Lakdawalla held back from reporting what Sicardy said because she was asked to. The research paper about the measurements is under consideration for publication in Nature, and Sicardy said the journal's editors told him he could discuss the results only if he instructed his audience not to report them publicly. The implication was that Sicardy's paper would be tossed out if his team's findings appeared in the press.

The audience was all abuzz about the findings, of course, but Lakdawalla said she wouldn't "break anything until somebody else breaks it."

She did, however, refer to the zipped-lip situation in a Twitter message to Embargo Watch's Ivan Oransky. Long story short, Oransky checked with Nature and was told that "researchers with papers in submission at a Nature journal can certainly present at a scientific meeting but shouldn't court the press." Oransky blogs about the back-and-forth today on Embargo Watch, but the bottom line is that Sicardy needn't have feared having his paper rejected, as long as he confined his public remarks to the presentation.

If Nature sticks to the reported publication plan, the paper will be published on Oct. 26. Today, a lot of the details came out not only on Lakdawalla's blog, but also on Scientific American's Observations blog — which is interesting, because Scientific American is part of the Nature Publishing Group. (SciAm's John Matson helpfully included a link to Sicardy's conference report.)

So what else do Sicardy and his colleagues say? Although Pluto and Eris are roughly the same size, Eris is more massive, which implies it's "mainly composed of rocky material, with a relatively thin ice mantle," the astronomers say. They suggest that Eris once had a thicker layer of ice, most of which was "blasted away" as the result of a catastrophic cosmic collision.

Sicardy and his colleagues also note that when you factor in Eris' distance, its observed brightness and its relatively small size, the dwarf planet stands out as one of the brightest bodies in the solar system, after the Saturnian moons Tethys and Enceladus. They suggest that the dwarf planet is so bright because it has a surface layer of nitrogen or methane frost, due to the freezing-out of its atmosphere.

A similar freeze-out might well happen on Pluto as it heads out to the farthest point of its orbit around the sun. Eris, meanwhile, is coming closer to the sun — and at some point the nitrogen or methane might thaw back into the atmosphere.

The two worlds seem destined to stand in the planetary pantheon as separated twins — in possession of moons, seasons, their own distinctive geologies and potentially some kind of cryovolcanic activity. Should they really be regarded as non-planets, or is it better to see them as a different class of planets? I argue for the latter in my book, "The Case for Pluto," but I'd love to hear what you think. Please feel free to add your comments below.

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