linguistic:paraphrasing
Size, mass and density

In November 2010, Eris was the subject of one of the most distant stellar occultations yet from Earth. Preliminary data from this event cast doubt on previous size estimates. The teams announced their final results from the occultation in October 2011, with an estimated diameter of 2326 ± 12 km.

This makes Eris a little smaller than Pluto by area and diameter, which is 2372 ± 4 km across, although Eris is more massive. It also indicates a geometric albedo of 0.96. It is speculated that the high albedo is due to the surface ices being replenished because of temperature fluctuations as Eris's eccentric orbit takes it closer and farther from the Sun.

The mass of Eris can be calculated with much greater precision. Based on the accepted value for Dysnomia's period at the time—15.774 days —Eris is 27% more massive than Pluto. Using the 2011 occultation results, Eris has a density of 2.52 ± 0.07 g/cm, substantially denser than Pluto, and thus must be composed largely of rocky materials.

Models of internal heating via radioactive decay suggest that Eris could have an subsurface ocean of liquid water at the mantle–core boundary. Tidal heating of Eris by its moon Dysnomia may additionally contribute to the preservation of its possible subsurface ocean. More research concluded that Eris, Pluto and Makemake could harbor active subsurface oceans and show active geothermal activity.

In July 2015, after nearly a decade of Eris being thought to be the ninth-largest known object to directly orbit the Sun, close-up imagery from the New Horizons mission determined the volume of Pluto to be slightly larger than that of Eris. Eris is now understood to be the tenth-largest known object to directly orbit the Sun by volume, but remains the ninth-largest by mass.