Dysnomia (formally (136199) Eris I Dysnomia) is the only known moon of the dwarf planet Eris and is the second-largest known moon of a dwarf planet, after Pluto I Charon. It was discovered in September 2005 by Mike Brown and the Laser Guide Star Adaptive Optics (LGSAO) team at the W. M. Keck Observatory. It carried the provisional designation of until it was officially named Dysnomia (from the Ancient Greek word meaning anarchy/lawlessness) in September 2006, after the daughter of the Greek goddess Eris.
With an estimated diameter of , Dysnomia spans 24% to 29% of Eris's diameter. It is significantly less massive than Eris, with a density consistent with it being mainly composed of ice. In stark contrast to Eris's highly-reflective icy surface, Dysnomia has a very dark surface that reflects 5% of incoming visible light, resembling typical trans-Neptunian objects around Dysnomia's size. These physical properties indicate Dysnomia likely formed from a large impact on Eris, in a similar manner to other binary dwarf planet systems like Pluto and , and as well as the Earth–Moon system.
Discovery
During 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest Kuiper belt objects (Pluto, , , and ), using the newly commissioned laser guide star adaptive optics system. Observations taken on 10 September 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.
Physical characteristics
Submillimeter-wavelength observations of the Eris–Dysnomia system's thermal emissions by the Atacama Large Millimeter Array (ALMA) in 2015 first showed that Dysnomia had a large diameter and a very low albedo, with the initial estimate being . Further observations by ALMA in 2018 refined Dysnomia's diameter to (24% to 29% of Eris's diameter) and an albedo of . Of the known moons of dwarf planets, only Charon is larger, making Dysnomia the second-largest moon of a dwarf planet. Dysnomia's low albedo significantly contrasts with Eris's extremely high albedo of 0.96; its surface has been described to be darker than coal, which is a typical characteristic seen in trans-Neptunian objects around Dysnomia's size.
Eris and Dysnomia are mutually tidally locked. Astrometric observations of the Eris–Dysnomia system by ALMA show that Dysnomia does not induce detectable barycentric wobbling in Eris's position, implying its mass must be less than (mass ratio ). This is below the estimated mass range of (mass ratio 0.01–0.03) that would normally allow Eris to be tidally locked within the range of the Solar System, suggesting that Eris must therefore be unusually dissipative. ALMA's upper-limit mass estimate for Dysnomia corresponds to an upper-limit density of , implying a mostly icy composition. The shape of Dysnomia is not known, but its low density suggests that it should not be in hydrostatic equilibrium.
The brightness difference between Dysnomia and Eris decreases with longer and redder wavelengths; Hubble Space Telescope observations show that Dysnomia is 500 times fainter than Eris (6.70-magnitude difference) in visible light, whereas near-infrared Keck telescope observations show that Dysnomia is ~60 times fainter (4.43-magnitude difference) than Eris. This indicates Dysnomia has a very different spectrum and redder color than Eris, indicating a significantly darker surface, something that has been proven by submillimeter observations.
Orbit
Combining Keck and Hubble observations, the orbit of Dysnomia was used to determine the mass of Eris through Kepler's third law of planetary motion. Dysnomia's average orbital distance from Eris is approximately , with a calculated orbital period of 15.786 days, or approximately half a month. This shows that the mass of Eris is 1.27 times that of Pluto. Extensive observations by Hubble indicate that Dysnomia has a nearly circular orbit around Eris, with a low orbital eccentricity of . Over the course of Dysnomia's orbit, its distance from Eris varies by due to its slightly eccentric orbit.
Dynamical simulations of Dysnomia suggest that its orbit should have completely circularized through mutual tidal interactions with Eris within timescales of 5–17 million years, regardless of the moon's density. A non-zero eccentricity would thus mean that Dysnomia's orbit is being perturbed, possibly due to the presence of an additional inner satellite of Eris. However, it is possible that the measured eccentricity is not real, but due to interference of the measurements by albedo features, or to systematic errors.