Dysnomia (moon)
On the 10th of September 2005, astronomers at the W. M. Keck Observatory in Hawaii spotted a faint point of light orbiting the distant dwarf planet Eris. Mike Brown and his Laser Guide Star Adaptive Optics team had been studying four bright Kuiper belt objects when this new moon appeared in their data. The discovery carried the provisional designation until official naming occurred in September 2006. Brown chose the name Dysnomia from Ancient Greek words meaning anarchy or lawlessness. This choice honored the daughter of the Greek goddess Eris while echoing Lucy Lawless, the actress who played Xena on television. Before receiving their formal names, the pair went by the nicknames Xena and Gabrielle. Brown later stated that the connection to the TV show was accidental.
Submillimeter observations taken by the Atacama Large Millimeter Array in 2018 refined Dysnomia's diameter to approximately 870 kilometers. This measurement makes it span between 24% and 29% of Eris's total diameter. Despite its size, the moon has a very dark surface that reflects only 5% of incoming visible light. This low albedo contrasts sharply with Eris's highly reflective icy surface which reflects 96% of light. Astronomers describe Dysnomia's surface as darker than coal. Such darkness is typical for trans-Neptunian objects of similar size. Density calculations suggest the moon consists mainly of ice rather than rock. Its shape remains unknown but likely lacks hydrostatic equilibrium due to low density.
Dysnomia completes one orbit around Eris every 15.786 days or roughly half a month. The average distance between the two bodies spans approximately 37,000 kilometers. Observations from Hubble Space Telescope reveal a nearly circular path with an eccentricity near zero. Yet slight variations cause the distance to fluctuate by about 1,000 kilometers over time. The system exhibits mutual tidal locking like Pluto and Charon do together. Astrometric data shows no detectable barycentric wobble in Eris's position. This implies Dysnomia's mass stays below 1/40th of Eris's total mass. The orbital inclination relative to Eris's heliocentric path measures approximately 78 degrees. Since this angle falls under 90 degrees, the motion counts as prograde.
Eight out of ten largest known trans-Neptunian objects possess at least one satellite. Only about 10% of fainter members in that population show similar companionship. Scientists believe frequent collisions between large Kuiper belt objects created these moons. Impacts involving bodies thousands of kilometers across ejected massive amounts of debris. That material eventually coalesced into satellites orbiting their parent planets. A comparable mechanism likely formed Earth's Moon when a giant impactor struck early in Solar System history. Simulations suggest Dysnomia's orbit should have fully circularized within five to seventeen million years through tidal interactions. The current non-zero eccentricity might indicate perturbation from an unseen inner satellite. Alternatively measurement errors or albedo features could explain the observed irregularities.
Hubble Space Telescope observations spanning from 2005 to 2018 tracked changes in brightness and position. These records helped determine Eris's mass using Kepler's third law of planetary motion. Keck telescope data showed Dysnomia appears roughly 60 times fainter than Eris near-infrared wavelengths. Visible light measurements reveal a difference of 500 times fainter with a magnitude gap of 6.70. Submillimeter observations by ALMA confirmed thermal emissions indicating size and low reflectivity. Future predictions point toward mutual events beginning in the year 2239. During that period, Dysnomia's orbital plane will align edge-on to the Sun. This alignment allows both bodies to eclipse each other as they travel along their paths.
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Common questions
When was Dysnomia discovered by astronomers?
Astronomers at the W. M. Keck Observatory in Hawaii spotted Dysnomia on the 10th of September 2005. Mike Brown and his Laser Guide Star Adaptive Optics team identified this faint point of light orbiting Eris during their study of four bright Kuiper belt objects.
What is the origin of the name Dysnomia for the moon of Eris?
Mike Brown chose the name Dysnomia from Ancient Greek words meaning anarchy or lawlessness to honor the daughter of the Greek goddess Eris. This choice also echoed Lucy Lawless, the actress who played Xena on television before the pair received formal names as Xena and Gabrielle.
How large is Dysnomia compared to its parent planet Eris?
Submillimeter observations taken by the Atacama Large Millimeter Array in 2018 refined Dysnomia's diameter to approximately 870 kilometers. This measurement makes it span between 24% and 29% of Eris's total diameter while maintaining a very dark surface that reflects only 5% of incoming visible light.
How long does it take Dysnomia to complete one orbit around Eris?
Dysnomia completes one orbit around Eris every 15.786 days which equals roughly half a month. The average distance between the two bodies spans approximately 37,000 kilometers with an orbital inclination relative to Eris's heliocentric path measuring approximately 78 degrees.
What evidence suggests how Dysnomia formed after collisions in the Kuiper belt?
Scientists believe frequent collisions between large Kuiper belt objects created these moons when impacts involving bodies thousands of kilometers across ejected massive amounts of debris. That material eventually coalesced into satellites orbiting their parent planets similar to the mechanism likely forming Earth's Moon when a giant impactor struck early in Solar System history.