Eris is less massive than Pluto.
 Precise long-term monitoring of Eris's brightness indicates that it is tidally locked to its moon Dysnomia, with a rotation period synchronous with the moon's orbital period of 15
 Under the IAU definition approved on August 24, 2006, Eris, Pluto and Ceres are "dwarf planets", reducing the number of known planets in the Solar System to eight, the same as before Pluto's discovery in 1930

