Short answers, pulled from the story.
Eris, designated minor-planet number 136199, is the most massive known dwarf planet in the Solar System and the second-largest by volume. It is a trans-Neptunian object in the scattered disk, orbiting the Sun at distances ranging from 38.4 AU at closest approach to 97.7 AU at its farthest.
Eris was discovered by Mike Brown, Chad Trujillo, and David Rabinowitz on the 5th of January 2005, from images originally taken on the 21st of October 2003, using the 1.2 m Samuel Oschin Schmidt telescope at Palomar Observatory in California. The discovery was publicly announced on the 29th of July 2005.
Eris was initially thought to be larger than Pluto, prompting NASA to describe it as a tenth planet. The International Astronomical Union responded by formally defining the word planet for the first time, adopting the definition on the 24th of August 2006. Both Eris and Pluto failed the new criterion of clearing their orbital neighborhoods and were reclassified as dwarf planets.
Eris has a diameter of 2326 km, making it slightly smaller than Pluto, which the New Horizons mission measured at a mean diameter of 2376.6 km in July 2015. Despite being smaller by volume, Eris is 27% more massive than Pluto and has a substantially higher density of 2.52 g/cm3.
Dysnomia is the only known moon of Eris, discovered on the 10th of September 2005, using the Keck telescopes in Hawaii. It is named after the Greek goddess of lawlessness and daughter of Eris. Dysnomia has a diameter of approximately 615 km and is tidally locked to Eris, with both bodies rotating in synchrony over a period of 15.78 Earth days.
Eris is composed largely of rocky material, as indicated by its high density of 2.52 g/cm3. Its surface is covered with methane ice and nitrogen ice, detected by the James Webb Space Telescope in 2022. Because methane and nitrogen are volatile, some surface ices may sublimate into a tenuous atmosphere as Eris approaches the Sun during its 558-year orbit.