Dysnomia (moon)

Dysnomia is the only known moon orbiting the dwarf planet Eris. It is a significant object in our solar system because it is the second-largest moon of a dwarf planet ever discovered. Only Charon, the moon of Pluto, is larger in comparison. Dysnomia is quite large relative to its parent body, spanning between 24% and 29% of Eris's diameter.

This large size makes the Eris-Dysnomia system an important subject for astronomers studying the outer reaches of our solar system.

Astronomers discovered Dysnomia in September 2005. A team at the W. M. Keck Observatory in Hawaii used a system called Laser Guide Star Adaptive Optics (LGSAO) to find it. This technology helps telescopes see more clearly by correcting for atmospheric blurring. During their observations of the brightest Kuiper belt objects, the team spotted a moon orbiting Eris on September 10, 2005.

Before it received an official name, the discoverers gave it the nickname "Gabrielle" after a character from a television show.

The official name, Dysnomia, was chosen in September 2006. The name comes from the daughter of the Greek goddess Eris. In Greek mythology, Dysnomia represents lawlessness or anarchy. The discoverer, Mike Brown, chose this name to follow the pattern of naming moons after figures associated with their primary body. This follows traditions seen with Jupiter and Saturn. Brown also noted the name shared the same first letter as his wife, Diane, similar to how the name Pluto was partially inspired by the initials of Percival Lowell.

Dysnomia has physical properties that are very different from Eris. Eris has an extremely bright, highly reflective icy surface with an albedo of 0.96. In contrast, Dysnomia has a very dark surface with an albedo of only 0.05. This means it reflects only 5% of the visible light that hits it. Some descriptions suggest its surface is darker than coal.

This dark appearance is a common characteristic for trans-Neptunian objects of this size. Observations from the Atacama Large Millimeter Array (ALMA) helped refine these measurements of its diameter and darkness.

Scientists believe Dysnomia formed through a massive impact. It likely resulted from a collision between Eris and another large Kuiper belt object. Such an impact would throw large amounts of material into space, which then coalesced to form a moon. This mechanism is similar to how the Earth's Moon formed. It is also how other binary dwarf planet systems, like Pluto and Charon, likely began.

Such collisions were likely frequent in the history of the Kuiper belt.

The relationship between Eris and Dysnomia is defined by their orbit and gravity. Dysnomia orbits Eris at an average distance of approximately 37,000 kilometers. It completes one full orbit every 15.786 days, which is about half of a month. The orbit is nearly circular, with a low orbital eccentricity of 0.082.

Eris and Dysnomia are also mutually tidally locked. This means they always show the same side to one another as they orbit. In the year 2239, they will experience a period of mutual events where they take turns eclipsing each other from the perspective of the Sun.

Research into the mass and composition of these bodies provides deeper insights into the outer solar system. By using Kepler's third law of planetary motion, scientists used Dysnomia's orbit to calculate the mass of Eris. They determined that Eris is 1.27 times more massive than Pluto. While the exact shape of Dysnomia is unknown, its low density suggests it is likely composed mostly of ice. This density is consistent with the mass limits found by ALMA observations.

These details help astronomers understand how gravity and collisions shape the dwarf planets of our solar system.