Eris was the catalyst for Pluto's demotion last summer, when the International Astronomical Union redefined the term "planet" and created the category of dwarf planet for objects such as Eris and Pluto.
Using the Keck Observatory and the Hubble Space Telescope, Michael E. Brown and Emily Schaller at the California Institute of Technology in Pasadena have now put the mass of Eris at a third more than Pluto's.
June 14, 2007
Eris is bigger than Pluto
Eris—formerly known as 2003 UB313 and then Xena—is the largest so-called dwarf planet in the solar system.