Eris passed in front of a faint background star in November 2010 in an event called an occultation, which allowed observatories on Earth to make calculations of the dwarf planet's size, a release from the Munich headquarters of the European Southern Observatory said Wednesday.
New analysis of the observations show Eris is almost the exact same size as Pluto, astronomers said.
The discovery of Eris was one of the reasons for the creation of a new class of objects called dwarf planets and the reclassification of Pluto from planet to dwarf planet in 2006.
"It is extraordinary how much we can find out about a small and distant object such as Eris by watching it pass in front of a faint star, using relatively small telescopes," astronomer Bruno Sicardy said. "Five years after the creation of the new class of dwarf planets, we are finally really getting to know one of its founding members."
Eris is currently three times further from the sun than Pluto.