Dysnomia (moon)

• 1. Introduction

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  Dysnomia orbits a world so far from the Sun that sunlight takes more than five hours to reach it. It is the only known moon of the dwarf planet Eris, and its discovery in September 2005 turned a solitary object at the solar system's edge into something stranger and more compelling: a double world. The team that found it was working at the W. M. Keck Observatory in Hawaii, using a laser guide star adaptive optics system so new it had only just been commissioned. What they captured in their data was a faint companion tucked close to Eris, a moon with a surface so dark it reflects just 5% of incoming sunlight. That darkness stands in almost absurd contrast to Eris, whose icy surface bounces back 96% of the light that falls on it. How did two objects that share an orbit end up so different from each other? And why does the name Dysnomia carry so many layers of meaning, from ancient mythology to a New Zealand actress to a deliberate nod toward the discoverer's wife?

• 2. A Name Loaded with Meaning

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  Mike Brown, who led the discovery, chose Dysnomia from Ancient Greek: Dysnomia means anarchy or lawlessness. In Greek mythology, Dysnomia is the daughter of Eris, the goddess of discord, so naming the moon after Eris's mythological daughter followed the established pattern by which moons are named after figures associated with their parent body. Jupiter's largest moons carry the names of Jupiter's lovers; Saturn's major moons are named after his fellow Titans.

  The name also carried a quieter echo. Before the official designations were announced, Brown's team had nicknamed Eris "Xena" and its moon "Gabrielle", after characters from the television series Xena: Warrior Princess. The English meaning of Dysnomia, lawlessness, is a nod to Lucy Lawless, the actress who played Xena. Brown says the connection was accidental, but it became part of the name's appeal.

  There was a third layer, more personal. Brown followed a tradition of discoverers encoding a private reference into an official astronomical name. James Christy, who discovered Charon, built his name from Char, the nickname of his wife Charlene, then added a Greek suffix. Christy was not aware at the time that Charon happened to be a genuine figure in Greek mythology. Pluto's name was chosen in part because its first two letters form the initials of Percival Lowell, the founder of the observatory where Clyde Tombaugh worked. Brown drew from the same tradition: Dysnomia shares its first letter with the name of his wife, Diane.

• 3. The Discovery at Keck

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  On the 10th of September 2005, the adaptive optics team at the Keck telescopes turned their attention to the four brightest Kuiper belt objects, which at that time were Pluto and three others including Eris. The laser guide star adaptive optics system they were using had only recently been put into service. It works by firing a laser into the upper atmosphere to create an artificial reference star, which allows ground-based telescopes to compensate for the blurring effect of Earth's atmosphere in real time.

  In the data from that night, a moon appeared in orbit around Eris. It was given a provisional designation before receiving its permanent name the following year, in September 2006. The discoverers, keeping with the Xena nickname already attached to Eris, called the new moon Gabrielle after Xena's sidekick. That informal name persisted for about a year before the formal mythological name replaced it.

• 4. A Dark and Icy World

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  Submillimeter-wavelength observations by the Atacama Large Millimeter Array, known as ALMA, produced the first reliable measurements of Dysnomia's size. Initial ALMA data from 2015 suggested a diameter of around 700 kilometers. Refined ALMA observations in 2018 settled on an estimated diameter of 615 kilometers, placing Dysnomia at between 24% and 29% of Eris's own diameter.

  Dysnomia's albedo, a measure of how much light a surface reflects, is just 0.05. Its surface has been described as darker than coal, a characteristic typical of trans-Neptunian objects at Dysnomia's size. Eris itself has an albedo of 0.96, one of the highest measured for any body in the solar system. Hubble Space Telescope data show that Dysnomia is 500 times fainter than Eris in visible light, a difference of 6.70 magnitudes. At near-infrared wavelengths captured by Keck, the gap narrows: Dysnomia is about 60 times fainter than Eris, a 4.43-magnitude difference. That shift tells astronomers that Dysnomia's surface is not just darker but also significantly redder than its host, which points to a very different surface composition.

  ALMA measurements of how Dysnomia moves through space allowed a calculation of its mass. The data show it does not induce detectable wobbling in Eris's position, placing an upper limit on Dysnomia's mass at less than 1.4 times ten to the twentieth kilograms, a mass ratio below 0.0050. That upper limit corresponds to a maximum density of about 1.2 grams per cubic centimeter, consistent with a body made mostly of ice rather than rock. Whether Dysnomia is massive enough to pull itself into a spherical shape under its own gravity remains unknown.

• 5. An Orbit That Raises Questions

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  Dysnomia travels around Eris at an average distance of approximately 37,300 kilometers, completing each orbit in 15.786 days, roughly half a month. Extensive Hubble observations confirm the orbit is nearly circular, with an eccentricity of just 0.0062. Eris and Dysnomia are mutually tidally locked, meaning each body always shows the same face to the other, a configuration shared by Pluto and Charon.

  Dynamical simulations suggest that if Dysnomia started with a more elongated orbit, mutual tidal forces between Eris and Dysnomia should have pulled it into a circular orbit within 5 to 17 million years, regardless of Dysnomia's density. The fact that a small but non-zero eccentricity is still measurable is puzzling. One possible explanation is that an undiscovered inner satellite of Eris is nudging Dysnomia's path. Another is that the measured eccentricity is not real, and instead reflects interference from albedo features on one of the bodies or systematic errors in the measurements.

  Hubble observations spanning 2005 to 2018 place Dysnomia's orbital inclination relative to Eris's path around the Sun at approximately 78 degrees. Because that figure is below 90 degrees, the orbit is prograde. Combining the Keck and Hubble data with Kepler's third law of planetary motion, astronomers used Dysnomia's orbit to calculate that Eris has 1.27 times the mass of Pluto. In 2239, Eris and Dysnomia will enter a period of mutual eclipses, when their orbital plane aligns edge-on to the Sun and the two bodies will take turns passing in front of each other.

• 6. How Dysnomia Formed

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  Eight of the ten largest trans-Neptunian objects are known to have at least one moon. Among the smaller members of that distant population, only about 10% have known satellites. The high rate of moons around the largest Kuiper belt objects suggests that large collisions between bodies in that region were common in the solar system's past.

  Dysnomia's physical properties point toward a specific formation mechanism: a giant impact. A large impactor striking Eris could have thrown off enormous amounts of material, which then coalesced in orbit to form Dysnomia. The scenario mirrors the Giant impact hypothesis for how Earth's Moon formed, and the Pluto-Charon system likely arose the same way. The contrast between Dysnomia's dark, icy surface and Eris's bright, reflective one is consistent with this picture. Eris's crust is thought to be a frost of methane or nitrogen, which would give it its high reflectivity. Material excavated from deeper layers by an impact could be far darker in composition, which would explain why Dysnomia looks so different from the world it circles. The tidal locking between the two bodies, combined with Eris's apparently high dissipative properties, gives future researchers a way to probe the internal structure of a dwarf planet from the outside in.