Quick: What's the largest object in the solar system beyond Neptune? Pluto was what you learned long ago, and now there's fresh evidence to indicate that old answer was right all along. Pluto belongs to the Edgeworth-Kuiper belt, which boasts more than a thousand known objects revolving around the sun beyond Neptune's orbit. Most are much smaller than Pluto, but in 2005 astronomers discovered its biggest rival: Eris, which they claimed definitely surpassed Pluto in size. In 2010, however, Eris staged an underwhelming performance when it moved in front of a distant star. The short duration of this so-called occultation revealed that Eris is just 2326 kilometers across—possibly smaller than Pluto, whose diameter is somewhere between 2300 and 2400 kilometers. The uncertainty arises because Pluto, unlike Eris, has air that complicates the interpretation of observational data. Now, as astronomers will report in Icarus, analysis of methane gas in Pluto's atmosphere suggests that Pluto is about 2368 kilometers across, in which case it's larger than Eris and thus the champ of the Edgeworth-Kuiper belt. We'll get a final verdict in July 2015, when NASA's New Horizons spacecraft (artist's conception shown) flies past Pluto and measures its exact size.

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