Faraway Eris is Pluto's Twin - Dwarf planet sized up accurately as it blocks light of faint star

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Source:  European Southern Observatory Release eso1142
Excerpt: In November 2010, the distant dwarf planet Eris passed in front of a faint background star, an event called an occultation. …Occultations provide the most accurate, and often the only, way to measure the shape and size of a distant Solar System body. …The combined observations from the two Chilean sites indicate that Eris is close to spherical. …Eris… discovery was one of the factors that led to the creation of a new class of objects called dwarf planets and the reclassification of Pluto from planet to dwarf planet in 2006. Eris is currently three times further from the Sun than Pluto. …Eris’s newly determined diameter stands at 2326 kilometres, …This makes its size better known than that of its closer counterpart Pluto, which has a diameter estimated to be between 2300 and 2400 kilometres. …The motion of Eris’s satellite Dysnomia was used to estimate the mass of Eris …27% heavier than Pluto … Eris is probably a large rocky body covered in a relatively thin mantle of ice…The surface …reflecting 96% of the light that falls on it … brighter than fresh snow on Earth….

 www.eso.org/public/news/eso1142/

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