Dysnomia, formally designated (136199) Eris I, is the only known moon of the dwarf planet Eris and the second-largest known moon of a dwarf planet, after Pluto's moon Charon. It was discovered in September 2005 by astronomer Mike Brown and the Laser Guide Star Adaptive Optics (LGSAO) team at the W. M. Keck Observatory in Hawaii.
Discovery and Naming
The moon was first observed on 10 September 2005 and given the provisional designation S/2005 (2003 UB313) 1. Prior to its official naming, the moon was nicknamed "Gabrielle" by its discoverers, after the sidekick of Xena in the television series Xena: Warrior Princess (Eris itself had been nicknamed "Xena" before its official naming). The moon was officially named Dysnomia in September 2006, from the Ancient Greek Δυσνομία (Dysnomía) meaning "lawlessness," after the daughter of the Greek goddess Eris. Mike Brown also noted that the name shares its first letter with his wife's name, Diane, following a pattern established with Pluto and its moon Charon.
Physical Characteristics
- Diameter: Approximately 615 km (with an uncertainty of +60/−50 km), spanning about 24% to 29% of Eris's diameter.
- Mass: Approximately (8.2 ± 5.7) × 10¹⁹ kg, with a mean density of about 0.7 ± 0.5 g/cm³, consistent with a mostly icy composition.
- Albedo: Very low at 0.05 ± 0.01, meaning it reflects only about 5% of incoming visible light. Its surface has been described as darker than coal, in stark contrast to Eris's extremely bright surface (albedo of 0.96).
- Rotation: Dysnomia is tidally locked to Eris, meaning the same face always points toward the dwarf planet.
- Color: Dysnomia has a redder color and significantly different spectrum than Eris, indicating a distinctly darker surface composition.
Orbital Characteristics
- Semi-major axis: Approximately 37,273 km from Eris.
- Orbital period: 15.786 days (about half a month).
- Eccentricity: Very low at 0.0062, indicating a nearly circular orbit.
- Inclination: Approximately 78° relative to Eris's heliocentric orbit (prograde).
- The orbit was used to calculate the mass of Eris via Kepler's third law, showing Eris has 1.27 times the mass of Pluto.
Formation
Dysnomia is believed to have formed from a large impact on Eris, similar to the formation of the Earth–Moon system and other binary dwarf planet systems such as Pluto–Charon and Orcus–Vanth. Collisions between large Kuiper belt objects are thought to have been frequent in the early Solar System, and impacts between bodies roughly 1,000 km across would have thrown off material that coalesced into a moon.
Significance
Dysnomia's orbit allows astronomers to determine the mass of Eris, and the system provides important insights into the formation and evolution of binary trans-Neptunian objects. In approximately the year 2239, Eris and Dysnomia will enter a period of mutual events, where Dysnomia's orbital plane aligns edge-on to the Sun, allowing the two bodies to eclipse each other.
