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Dysnomia sounds like an illness of some kind – actually it does refer to a condition that affects memory – however, it is also the name of a moon. Dynomia was the name given to the only moon of the dwarf planet Eris. The object was discovered in 2005 by the astronomer Mike Brown and a team of scientists soon after its planet was discovered. Eris’ moon is only about 100 to 250 kilometers in diameter according to scientists’ estimation. In comparison, it is smaller than the state of Massachusetts, which is 182 km by 295 km. Despite its rather small size, it is actually the third largest object in the Kuiper Belt – Pluto and Eris are bigger. In addition to being a satellite of a dwarf planet, Dysnomia is also a Kuiper Belt Object (KBO) as is Eris. KBOs are celestial objects that have formed in a region of space beyond Neptune. Scientists cannot be sure what Dysnomia is made of. However, they believe that it is composed of almost pure frozen water.
Because Eris was nicknamed “Xena” by scientists before it received its official name, its moon was termed “Gabrielle,” fittingly referring to the television character’s sidekick. However, after Eris was given its official name, the name of its moon obviously needed to be changed as well. Mike Brown named the moon Dysnomia because in Greek mythology, it is the daughter of Eris who is the goddess of discord and strife. Eris received that name in part because of the uproar it caused with its discovery – because of it, the IAU defined what a planet which resulted in Pluto losing its status as a planet. According to Greek mythology, Dysnomia is the daemon spirit of lawlessness. The moon also has the first letter as Brown’s wife’s name, which is Diane.
Eris lived up to its namesake by stirring things up; however, astronomers have also used its moon, Dysnomia, to learn things about Eris and Pluto. Astronomers knew that Eris was bigger than Pluto, but they did not know whether it was more massive. Using an object’s moon though, they can determine how massive the object it is. They do this by measuring the distance between the object and its moon and how long it takes the moon to orbit the planet. Using this method, astronomers were able to discover that Eris is 27% more massive than Pluto is.
Astronomy Cast has an episode on Pluto’s planetary identity crisis.