Dysnomia (moon)
Only known moon of dwarf planet Eris, second-largest dwarf planet moon.
Dysnomia, officially known as (136199) Eris I, is the single moon belonging to the dwarf planet Eris. Among the moons of dwarf planets, only Pluto’s Charon is larger. The moon was spotted in September 2005 by Mike Brown and the Laser Guide Star Adaptive Optics team at the W. M. Keck Observatory. Initially given the provisional tag S/2005 (2003 UB313) 1, it received the name Dysnomia—from the Ancient Greek for “lawlessness”—in September 2006, after the daughter of the Greek goddess Eris.
The moon’s estimated diameter is 615 kilometers, with a possible range of plus 60 or minus 50 kilometers, making it between 24% and 29% as wide as Eris. It is far less massive than its parent, and its density suggests it is mostly ice. While Eris has a highly reflective icy surface, Dysnomia is very dark, reflecting only about 5% of incoming visible light—similar to other trans-Neptunian objects of comparable size. These traits point to Dysnomia having formed from a major impact on Eris, much like the binary systems of Pluto and Orcus, as well as the Earth–Moon system.
Mike Brown, the discoverer, chose the name Dysnomia following the tradition of naming moons after figures linked to the main body (for example, Jupiter’s large moons are named after his lovers, Saturn’s after his fellow Titans). The English meaning, “lawlessness,” also echoes Lucy Lawless, the actress who played Xena on television. Before their official names, Eris and Dysnomia had been nicknamed “Xena” and “Gabrielle,” though Brown says the connection was accidental. A key reason for the name was its similarity to Brown’s wife’s name, Diane, following a pattern set by Pluto (whose name starts with the initials of Percival Lowell) and Charon (whose discoverer added a Greek ending to his wife’s nickname, “Char”). Dysnomia shares its first letter with Diane.
The discovery came in 2005 when the adaptive optics team at Keck used a new laser guide star system to observe the four brightest Kuiper belt objects: Pluto, Makemake, Haumea, and Eris. Observations on September 10 revealed a moon orbiting Eris, which the team nicknamed “Gabrielle” after Xena’s sidekick.
Submillimeter observations by the Atacama Large Millimeter Array (ALMA) in 2015 first indicated Dysnomia’s large size and low albedo, with an initial diameter estimate of 700 kilometers (plus or minus 115). Further ALMA data in 2018 refined this to 615 ki
- discovered
- September 2005
- discoverer
- Mike Brown and the Laser Guide Star Adaptive Optics team at the W. M. Keck Observatory
- diameter
- 615+60−50 km
- albedo
- 0.05±0.01
- orbital period
- 15.786 days
- average orbital distance
- approximately 37,300 km
- mass upper limit
- less than 1.4×10^20 kg
Lore & Background
Dysnomia was discovered in September 2005 using the newly commissioned laser guide star adaptive optics system at the Keck telescopes in Hawaii, during observations of the four brightest Kuiper belt objects. It was provisionally designated S/2005 (2003 UB313) 1 and nicknamed 'Gabrielle' by its discoverers, after the sidekick of Xena, the nickname then used for Eris. The moon was officially named Dysnomia in September 2006, from the Ancient Greek word for lawlessness, after the daughter of the Greek goddess Eris. Mike Brown chose the name partly because its first letter matches that of his wife, Diane, following a pattern established with Pluto and Charon.
Reader's Guide
Dysnomia is significant as the only known moon of Eris, one of the largest dwarf planets, and as the second-largest moon of any dwarf planet. Its discovery helped determine the mass of Eris via Kepler's third law, showing Eris is 1.27 times as massive as Pluto. Dysnomia's very dark surface (reflecting only 5% of visible light) contrasts sharply with Eris's highly reflective icy surface, and its low density (upper limit <1.2 g/cm³) suggests a mostly icy composition. The moon is mutually tidally locked with Eris, and its nearly circular orbit has a slight eccentricity that may indicate perturbation by an additional inner satellite, though this could be due to measurement interference. Submillimeter observations by ALMA refined its diameter and albedo, and its formation is thought to result from a giant impact on Eris, a common mechanism among large trans-Neptunian objects. The name Dysnomia also carries a pop-culture echo, as the English translation 'lawlessness' alludes to Lucy Lawless, the actress who played Xena.
Did You Know?
- Dysnomia is the second-largest known moon of a dwarf planet, after Pluto's moon Charon.
- Its surface reflects only 5% of incoming visible light, making it darker than coal.
- The moon's name was chosen partly because its first letter matches that of discoverer Mike Brown's wife, Diane.
- Dysnomia's orbit has an eccentricity of 0.0062±0.0010, which may indicate the presence of an additional inner satellite of Eris.
More in Discoveries by Michael E. Brown 1-24
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