Moons of Pluto moons orbit Pluto at two to four times the distance of Charon, ranging from Styx at 42,700 kilometres to Hydra at 64,800 kilometres from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon. All are much smaller than Charon. Nix and Hydra, the two larger, are roughly 42 and 55 kilometers on their longest axis respectively, and Styx and Kerberos are 7 and 12 kilometers respectively. All four are irregularly shaped. The Pluto system is highly compact and largely empty. Moons could potentially orbit Pluto at up to 53% (or 69%,
Eris (dwarf planet) decide the issue. This was announced as the IAU's "Definition of a Planet in the Solar System," adopted on 24 August 2006. At this time, both Eris and Pluto were classified as "dwarf planets," a category distinct from the new definition of "planet." Brown has since stated his approval of this classification. The IAU subsequently added Eris to its Minor Planet Catalogue, designating it "(136199) Eris." Eris has an orbital period of 558 years. Its maximum possible distance from the Sun (aphelion) is 97.65 AU, and its closest (perihelion) is 37.91 AU. It came to perihelion between 1698 and 1699,