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Pluto's rival gets downsized

A. Schaller / STScI

New observations suggest that the dwarf planet Eris, shown in this artist's conception, isn't as big as scientists thought it was.

Everyone knows that the solar system is no longer seen as a nine-planet set. When you count the dwarfs, there could be scores or hundreds of planets out there -- as I write in my book, "The Case for Pluto." But a case could be made that Pluto is once again the ninth-largest planet orbiting the sun, based on observations reported over the weekend.

Pluto was ousted from the No. 9 spot five years ago, after the discovery of another dwarf planet on the solar system's icy frontier, known today as Eris. It was Eris' apparent status as an object slightly bigger than Pluto that brought the controversy over the definition of a planet to a head. If Pluto was an honest-to-goodness planet, shouldn't Eris be one as well?


In 2006, the International Astronomical Union approved a definition that established a new class of objects, called dwarf planets, which were big enough to be basically round but not gravitationally dominant enough to "clear out the neighborhood of their orbit." What's more, the IAU ruled that dwarf planets were not really planets.

My book delves into the questions raised about that definition, particularly in light of what we've been learning about planetary systems since then. Now there's a new question: Is Eris bigger than Pluto after all? Based on observations of Eris' occultation of a faraway star in the constellation Cetus, the answer could well be no.

Sky & Telescope's Kelly Beatty reports that the latest observations suggest Eris is actually slightly smaller than Pluto. He quotes the Paris Observatory's Bruno Sicardy as saying Eris is "almost certainly" no wider than 1,454 miles (2,340 kilometers), compared with Pluto's estimated width of 1,456.5 miles, plus or minus 6.5 miles (2,344 kilometers, plus or minus 10 kilometers).

"If the early results hold up, this time it's the dwarf planet Eris' turn to be demoted, and Pluto might have just regained its status as the largest object in the Kuiper Belt," Beatty writes. The Kuiper Belt is the broad zone of icy objects that lie beyond Neptune's orbit.

Gathering the data for the measurements was a grand astronomical feat: Three teams of scientists watched the distant star disappear when Eris crossed in front of it. By analyzing how long the star was covered over, as seen from three vantage points in Chile, the astronomers could calculate how wide Eris' round disk was. Previous estimates were based on indirect data, such as Eris' brightness.

Further observations will be required to reduce the uncertainties surrounding the two worlds' widths. The current estimates are so close that Caltech astronomer Mike Brown, whose team discovered Eris and two other dwarf planets, can justifiably say Eris and Pluto are "more or less the same size." And when you rank the two by mass rather than size, Eris clearly comes out on top. That implies that Eris' interior is denser and thus rockier than Pluto's.

"How could Eris and Pluto look so similar in size and exterior composition yet be totally unalike on the inside?" Brown writes. "As of today I have absolutely no idea. ... Something is going on in the outer solar system, and I don’t know what."

Whether Pluto is bigger or smaller than Eris really doesn't affect its status as a dwarf planet. But it does illustrate that small celestial objects can deliver some big scientific surprises -- and that it's a huge mistake to write off the little guys of the solar system.

Correction for 6:50 a.m. ET Nov. 8: I originally wrote that Pluto might be the ninth-widest object in the solar system, neglecting to take into account that some moons are wider than Pluto (and in fact wider than Mercury). Thanks to Stevesliva for pointing that out.

More on dwarf planets:


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