Dysnomia, formal designation(136199) Eris I, is the only knownmoon of thedwarf planetEris and is the second-largest known moon of a dwarf planet, afterPluto I Charon. It was discovered in September 2005 byMike Brown and the Laser Guide StarAdaptive Optics (LGSAO) team at theW. M. Keck Observatory. It carried theprovisional designation ofS/2005 (2003 UB313) 1 until it was officially namedDysnomia (from the Ancient Greek wordΔυσνομία (Dysnomía) meaning anarchy/lawlessness) in September 2006, after thedaughter of the Greek goddessEris.[6]
With an estimated diameter of615+60 −50 km, 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.[3]: 8 In stark contrast to Eris's highly-reflective icy surface, Dysnomia has a very dark surface that reflects 5% of incoming visible light,[3] resembling typicaltrans-Neptunian objects around Dysnomia's size.[7] 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 andOrcus, and the Earth–Moon system.
In 2005, theadaptive optics team at theKeck telescopes inHawaii carried out observations of the four brightestKuiper belt objects (Pluto,Makemake,Haumea, andEris), using the newly commissionedlaser guide star adaptive optics system. Observations taken on 10 September 2005, revealed amoon in orbit around Eris, provisionally designatedS/2005 (2003 UB313) 1. In keeping with theXena nickname that was already in use for Eris, the moon was nicknamed "Gabrielle" by its discoverers, after Xena's sidekick.[8][9]
Artist's conception of Dysnomia's dark surface, with dwarf planet Eris in the backgroundEris and Dysnomia (lower left) imaged by theJames Webb Space TelescopeNIRCam in August 2024
Submillimeter-wavelength observations of the Eris–Dysnomia system'sthermal emissions by theAtacama Large Millimeter Array (ALMA) in 2015 first showed that Dysnomia had a large diameter and a very low albedo, with the initial estimate being700±115 km.[7] Further observations by ALMA in 2018 refined Dysnomia's diameter to615+60 −50 km (24% to 29% of Eris's diameter) and analbedo of0.05±0.01.[3] Of the known moons of dwarf planets, onlyCharon is larger, making Dysnomia the second-largest moon of a dwarf planet.[10] Dysnomia's low albedo significantly contrasts with Eris's extremely high albedo of 0.96; its surface has been described to be darker thancoal,[10] which is a typical characteristic seen in trans-Neptunian objects around Dysnomia's size.[7]
Eris and Dysnomia are mutuallytidally locked, like Pluto and Charon.Astrometric observations of the Eris–Dysnomia system by ALMA show that Dysnomia does not induce detectablebarycentric wobbling in Eris's position, implying its mass must be less than1.4×1020 kg (mass ratio0.0050±0.0035).[3] This is below the estimated mass range of(2–5)×1020 kg (mass ratio 0.01–0.03) that would normally allow Eris to be tidally locked within the range of the Solar System,[4] suggesting that Eris must therefore be unusually dissipative.[3] ALMA's upper-limit mass estimate for Dysnomia corresponds to an upper-limit density of< 1.2 g/cm3, implying a mostly icy composition.[3] The shape of Dysnomia is not known, but its low density suggests that it is unlikely to be in hydrostatic equilibrium.[11]
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,[12][5] whereasnear-infrared Keck telescope observations show that Dysnomia is ~60 times fainter (4.43-magnitude difference) than Eris.[13] 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.[14][7]
Diagram of three binary trans-Neptunian dwarf planets and their satellites with true colors, diameters, and distances to scale. Each system's barycenter position marked is in red crosshairs.
Combining Keck and Hubble observations, the orbit of Dysnomia was used to determine the mass of Eris throughKepler'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.[2] This shows that the mass of Eris is 1.27 times that of Pluto.[15][16] Extensive observations by Hubble indicate that Dysnomia has a nearly circular orbit around Eris, with a loworbital eccentricity of0.0062±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.[2]
Dynamical simulations of Dysnomia suggest that its orbit should have completelycircularized 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 systematic errors.[2]
From Hubble observations from 2005 to 2018, the inclination of Dysnomia's orbit with respect to Eris'sheliocentric orbit is calculated to be approximately 78°. Since the inclination is less than 90°, Dysnomia's orbit is thereforeprograde 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 turnseclipsing each other.[2]
Eight of the ten largest trans-Neptunian objects are known to have at least one satellite. Among the fainter members of the trans-Neptunian population, only about 10% are known to have satellites.[3] This is thought to imply that collisions between large KBOs have been frequent in the past. Impacts between bodies of the order of1000 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 theMoon whenEarth wasstruck bya giant impactor (seeGiant impact hypothesis) early in the history of theSolar System.[3]
Mike Brown, the moon's discoverer, chose the nameDysnomia for the moon. As the daughter of Eris, the mythologicalDysnomia fit the established pattern of naming moons after gods associated with the primary body (hence,Jupiter's largest moons are named after lovers ofJupiter, whileSaturn's are named after his fellow Titans). The English translation of Dysnomia, "lawlessness", also echoesLucy Lawless, the actress who played Xena inXena: 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.[17]
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 ofPercival 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 discoveredCharon, did something similar by adding the Greek ending-on toChar, 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.[18]
^Full list of discoverers areMichael E. Brown, M. A. van Dam, A. H. Bouchez, D. Le Mignant, R. D. Campbell, J. C. Y. Chin, A. Conrad, S. K. Hartman, E. M. Johansson, R. E. Lafon,D. L. Rabinowitz, P. J. Stomski Jr., D. M. Summers,C. A. Trujillo, and P. L. Wizinowich.
^The orbital period (P) is15.774 d. The orbital circumference (C) is 2π*semi-major axis. Dividing these (P/C) using the correct units gives0.172 km/s.
^Given Dysnomia's orbit poleecliptic latitude of +28.41°±0.16°,[2]: 5 subtracting this angle from the ecliptic north pole of +90° gives the inclination with respect to the ecliptic:i = +90° – (+28.41°) = +61.59°.
^Holler et al. (2021) only give thelongitude of periapsis for Dysnomia's orbit, which is307°±12°.[2]: 5 The longitude of periapsis is the sum of theascending node andargument of periapsis, so subtracting Dysnomia's ascending node of126.17°±0.26° from its longitude of periapsis gives180.83° for its argument of periapsis.
^abDysnomia's brightness is 1/500 of Eris in the visible band. WithH = −1.19 for Eris and a magnitude difference ofΔm = 6.70, this givesH ≈ 5.6 for Dysnomia.[5]
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