|Discovered by||Brown et al.|
|Discovery date||September 10, 2005|
|S/2005 (2003 UB313) 1|
Dy // (nickname)
|Epoch 31 August 2006 (JD 2453979.0)|
Average orbital speed
|Inclination||45.49°±0.15° (to geocentric equator)|
78.29°±0.65° (to Eris's orbit)
|Mass||estimated 1⁄115 system mass, or 0.143×1021 kg, at density 0.8 g/cm3, to 1⁄37 system mass, or 0.445×1021 kg, if same density as Eris|
Dysnomia (formally (136199) Eris I Dysnomia) is the only known moon of the dwarf planet Eris and likely the second-largest known moon of a dwarf planet, after Pluto I Charon. 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 (from the Ancient Greek word Δυσνομία meaning anarchy/lawlessness) after the daughter of the Greek goddess Eris.
Dysnomia has an estimated diameter of 700±115 km (25% to 35% of Eris's diameter), and is among the dozen or so largest objects in the trans-Neptunian region.
During 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest Kuiper belt objects (Pluto, Makemake, Haumea, and Eris), 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 S/2005 (2003 UB313) 1. 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.
Dysnomia has an estimated diameter of 700 ± 115 km with an albedo of 0.04+0.02
−0.01. The estimate was obtained using radiometric observation by ALMA observatory in submillimeter spectral region. A wide range of potential masses for the satellite is possible, depending on its density, with a system mass ratio of anywhere from 37:1 to 115:1. Thus Dysnomia's mass could be anywhere from 0.143×1021 kg with a minimal 0.8 g/cm3 density, to 0.445×1021 kg if it has the same density as Eris (2.43 g/cm3, three times more). However, given the extreme difference in albedo, it is unlikely that Dysnomia is a solid body like Eris, and something toward the low end of that range is more likely.
The shape of Dysnomia is not known. It might have a spherical shape due to its large dimensions; it is larger than the three smallest ellipsoidal moons of Saturn and Uranus (Enceladus, Mimas, and Miranda). It is the 17th-largest known moon in the solar system. In its discovery images Dysnomia was ~60 times fainter (or 4.43 magnitudes) 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. Of the known moons of dwarf planets, only Charon is larger than Dysnomia.
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 37,300 km (23,200 mi), 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 0.0062±0.0010. Over the course of Dysnomia's orbit, its distance from Eris varies by 462 ± 105 km (287 ± 65 mi) 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.
From Hubble observations from 2005 to 2018, the inclination of Dysnomia's orbit with respect to Eris's heliocentric orbit is calculated to be approximately 78°. Since the inclination is less than 90°, Dysnomia's orbit is therefore prograde relative to Eris's orbit. In 2239, Eris and Dysnomia will enter a period of mutual events in which Dysnomia's orbital plane is aligned edge-on to the Sun, allowing for Eris and Dysnomia to take turns eclipsing each other.
Astronomers now know that the six brightest Kuiper belt objects (KBOs) 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 1000 km 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.
Mike Brown, the moon's discoverer, chose the name Dysnomia for the moon. As the daughter of Eris, the mythological Dysnomia fit the established pattern of naming moons after gods associated with the primary body (hence, Jupiter's largest moons are named after lovers of Jupiter, while Saturn's are named after his fellow Titans). Also, the English translation of "Dysnomia", "lawlessness", echoes Lucy Lawless, the actress who played Xena in Xena: Warrior Princess 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 adding the Greek ending -on to Char, the nickname of his wife Charlene. (Christy wasn't aware that the resulting 'Charon' was a figure in Greek mythology.) "Dysnomia", similarly, has the same first letter as Brown's wife, Diane, and Brown uses the nickname "Dy" /ˈdaɪ/ 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.