Dysnomia

Dysnomia, formal designation (136199) Eris I, is the only known moon of the dwarf planet Eris and is the second-largest known moon of a dwarf planet, after Pluto I Charon. It was discovered in September 2005 by Mike Brown and the Laser Guide Star Adaptive Optics (LGSAO) team at the W. M. Keck Observatory. It carried the provisional designation of S/2005 (2003 UB313) 1 until it was officially named Dysnomia (from the Ancient Greek word Δυσνομία (Dysnomía) meaning anarchy/lawlessness) in September 2006, after the daughter of the Greek goddess Eris.

With an estimated diameter of 615+60 −50 km, Dysnomia spans 24% to 29% of Eris's diameter. It is significantly less massive than Eris, with a density consistent with it being mainly composed of ice. In stark contrast to Eris's highly-reflective icy surface, Dysnomia has a very dark surface that reflects 5% of incoming visible light, resembling typical trans-Neptunian objects around Dysnomia's size. These physical properties indicate Dysnomia likely formed from a large impact on Eris, in a similar manner to other binary dwarf planet systems like Pluto and Orcus, and the Earth–Moon system.