During 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest Kuiper belt objects, using the newly commissioned laser guide star adaptive optics system. Observations taken on September 10, 2005, revealed a moon in orbit around Eris, provisionally designated. In keeping with the Xena nickname that was already in use for Eris, the moon was nicknamed "Gabrielle" by its discoverers, after Xena's sidekick.
Properties
Dysnomia has an estimated diameter of with an albedo of. The estimate was obtained using radiometric observation by ALMA observatory in submillimeter spectral region. The same study suggested a wide range of potential masses for the satellite, depending on its actual density, with a system mass ratio anywhere from 37:1 to 115:1. Although the shape of Dysnomia is not known, it is presumed to have a spherical shape due to its large dimensions; it is larger than the three smallest ellipsoidal moons of Saturn and Uranus. In its discovery images Dysnomia was ~60 times fainter than Eris in the K band, and later observations with the Hubble Space Telescope found it to be 500 times fainter in the visible band. This indicates a very different, and quite redder, spectrum, indicating a significantly darker surface. Its diameter reveals it to be a rather large trans-Neptunian object. Of the known moons of minor planets, only Charon is larger than Dysnomia. Combining Keck and Hubble observations, the satellite was used to determine the mass of Eris, and orbital parameters were estimated. Its orbital period is calculated to be. These observations indicate that Dysnomia has a circular orbit around Eris, with a radius of. This shows that the mass of Eris is 1.27 times that of Pluto.
Formation
Astronomers now know that the six brightest Kuiper belt objects have satellites. Among the fainter members of the belt only about 10% are known to have satellites. This is thought to imply that collisions between large KBOs have been frequent in the past. Impacts between bodies of the order of across would throw off large amounts of material that would coalesce into a moon. A similar mechanism is thought to have led to the formation of the Moon when Earth was struck by a giant impactor early in the history of the Solar System.
Name
Mike Brown, the moon's discoverer, chose the name Dysnomia for the moon. As the daughter of Eris, the mythlogical Dysnomia fit the established pattern of naming moons after gods associated with the primary body. Also, the English translation of "Dysnomia", "lawlessness", echoes Lucy Lawless, the actress who played Xena in on television. Before receiving their official names, Eris and Dysnomia had been nicknamed "Xena" and "Gabrielle", though Brown states that the connection was accidental. A primary reason for the name was its similarity to the name of Brown's wife, Diane, following a pattern established with Pluto. Pluto owes its name in part to its first two letters, which form the initials of Percival Lowell, the founder of the observatory where its discoverer, Clyde Tombaugh, was working, and the person who inspired the search for "Planet X". James Christy, who discovered Charon, did something similar by choosing a name which shared its first four letters with his wife's name, Charlene. "Dysnomia", similarly, has the same first letter as Brown's wife, Diane, and Brown uses the nickname "Dy" for the moon, which he pronounces the same as his wife's nickname, Di. Because of this, Brown pronounces the full name, with a long "y" sound.
