Cross-posted from Scientific American’s Observations blog, on behalf of John Matson.

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 kilometres 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 6 November 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 kilometres in diameter. In 2009, Sicardy and his colleagues had calculated that Pluto’s diameter is at least 2,338 kilometres, although some earlier estimates for the 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.

Read the rest of this blog over at Scientific American.

Image: Eris and its moon from the Hubble Space Telescope. Credit: NASA, ESA, and M. Brown (California Institute of Technology)