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Eris and Dysnomia: artist's impression
Discoverer M. E. Brown,
M. A. van Dam,
A. H. Bouchez,
D. Le Mignant
(using the Keck Observatory)
Discovery date 10 September 2005
Orbital characteristics
Semi-major axis (a) 30,000 - 36,000 km
Eccentricity 0?
Orbital period (P) ~14 d
Inclination unknown
Parent body Eris
Physical characteristics
Mean diameter 300 - 400 km

Dysnomia (IPA: [dɪsˈnoʊ.mi.ə]), officially designated (136199) Eris I Dysnomia, is a moon of the dwarf planet Eris. It was discovered in 2005 by Mike Brown and the laser guide star adaptive optics team at the W. M. Keck Observatory, and carried the provisional designation of S/2005 (2003 UB313) 1 until officially named Dysnomia[1] (from the Greek word Δυσνομία meaning "lawlessness") after the daughter of the Greek goddess Eris.

In keeping with the Xena nickname that was formerly in use for Eris, the moon was nicknamed "Gabrielle" by its discoverers, after the television warrior princess's sidekick. Also, Xena's character was played by actress Lucy Lawless, whereas Dysnomia is the Greek goddess of lawlessness.


During 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest Kuiper belt objects (Pluto, 2005 FY9, 2003 EL61, and Eris), using the newly commissioned laser guide star adaptive optics system. Observations taken on 10 September revealed a moon in orbit around Eris, it was provisionally designated S/2005 (2003 UB313) 1.


The satellite is about 60 times fainter than Eris, and its diameter is estimated to be approximately eight times smaller.[2] With only a single observation, the satellite cannot yet be used to measure or constrain the mass of Eris, but likely orbital parameters were nevertheless estimated. One of these is its orbital period, thought to be about two weeks (14 days). Further observations made with the Keck telescopes in August and September 2006 have led to a much better measurement of the period. Once astronomers refine the period and the semimajor axis of the satellite's orbit (currently estimated at 36,000 km[3]), they will be able to determine the mass of the system.


Astronomers now know that three of the four brightest Kuiper belt objects (KBOs) have satellites, while among the fainter members of the belt only about 10% are known to have satellites. This is believed to imply that collisions between large KBOs have been frequent in the past. Impacts between bodies of the order of 1000 km across would throw off large amounts of material which would coalesce into a moon. A similar mechanism is believed to have led to the formation of Earth's own Moon when the Earth was struck by a giant impactor early in the history of the solar system.


The name Dysnomia (Greek Δυσνομία) was chosen due to its multiple resonances for its discoverer, Mike Brown. Dysnomia, as daughter of Eris, fits the general historically established pattern of naming moons after lesser gods associated with that for which the primary is named. Also, the English translation of "Dysnomia", "lawlessness," echoes Lucy Lawless, the actress famous for starring in Xena: Warrior Princess on television. Before receiving their official names, Eris and Dysnomia were known informally as "Xena" and "Gabrielle" respectively (Gabrielle being Xena's sidekick), and Brown decided to retain that honor. Brown also notes that James Christy, who discovered Charon, followed the principle established with Pluto (the name of which was chosen in part because its first two letters reflected the initials of its supposed discoverer, Percival Lowell) by choosing a name which started with the same first four letters as his wife's name, Charlene. "Dysnomia" has the same first letter as Brown's wife, Diane.[4]

In addition, both Eris and Dysnomia, representing conflict, reflect the effect their existence had in the disputation on the definition of a planet, and specifically on Pluto's status as such.


  1. IAU Circular 8747 - Official publication of the IAU reporting the naming of Eris and Dysnomia (PDF file)
  2. Eris' moon — includes section on Dysnomia's size.
  3. Michael E. Brown, et al (2006). "Satellites of the largest Kuiper belt objects" (PDF). Astrophys.J. 639 (L43). Template:Arxiv.
  4. Tytell, David, "All Hail Eris and Dysnomia", Sky & Telescope, September 14, 2006. Retrieved on 2006-12-30.

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