Pluto is certainly the most famous (and beloved) object among the group that astronomers call dwarf planets, but for years it's appeared to rank a distant second in terms of size. Eris, a dwarf planet discovered in 2005, has been estimated to be as much as 700 kilometers larger than Pluto in diameter.

But a new look at Eris has cut the dwarf planet down to size. New data presented last week at a joint meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress in Nantes, France, show that Eris is smaller than had been estimated. The new estimate for its diameter means that Eris is almost exactly the same size as Pluto—and possibly even a bit smaller.

Bruno Sicardy of the Paris Observatory and the University of Pierre and Marie Curie in France and his colleagues derived the smaller size for Eris from a 2010 celestial alignment called an occultation. On November 6 of that year, the dwarf planet temporarily blotted out the light of a background star and cast a small shadow on Earth. By comparing the shadow's size at two different sites in Chile, the researchers estimated that the dwarf planet is 2,326 kilometers in diameter. In 2009 Sicardy and his colleagues had calculated that Pluto's diameter is at least 2,338 kilometers, although some earlier estimates for dwarf planet's size ran a bit lower. Regardless of which dwarf planet holds the slight edge in terms of diameter, it now appears that Pluto and Eris are near-equals in terms of size. The findings have been submitted for publication in Nature. (Scientific American is part of Nature Publishing Group.)

There is little question that Eris is substantially more massive than Pluto, however, meaning that it is noticeably denser than its fellow dwarf planet. Eris's high density, according to Sicardy and his colleagues (pdf), "indicates that Eris is mainly composed of rocky material, with a relatively thin ice mantle." If it formed in the outer solar system where it now resides, Eris would be expected to host quite a bit of ice. So its assumed dearth today "might imply an impact that scooped away the ice mantle, leaving a mainly rocky body," the researchers ventured.

Given its brightness in astronomers' telescopes and its newly shrunken size estimate, Eris also appears to be brighter—that is, more reflective—than had been thought before. Sicardy and his colleagues say that Eris ranks alongside the Saturnian moons Tethys and Enceladus as one of the brightest objects in the solar system. But that might be a transient feature relating to the dwarf planet's current orbital position. Eris is now nearly 15 billion kilometers from the sun (roughly 100 times the Earth–sun distance), which is near aphelion, the most distant point it reaches on its elliptical orbit. At perihelion, its closest point, Eris is only 5.8 billion kilometers from the sun. Its bright surface, then, "could be due to the collapse of a nitrogen or methane atmosphere that is activated at perihelion through sublimation" and that condenses again as temperature drops, the researchers report.