How big is Dysnomia?

Dysnomia is the only moon of Eris known as of 2026. Eris is a dwarf planet that orbits the Sun far beyond Neptune. Measurements of the heat Dysnomia gives off put its diameter at about 615 kilometers (380 miles), a little more than a quarter of the width of Eris.

Size

Dysnomia's size cannot be read directly from telescope images, so astronomers work it out from the sunlight it reflects and the heat it gives off. In visible light it is only about 0.2 percent as bright as Eris. Brightness alone cannot give a size, because a small, shiny moon and a large, dark one can look equally bright.

If Dysnomia were as reflective as the icy moons of some other dwarf planets, it would be less than 400 kilometers across. To find out, astronomers observed it with ALMA, an array of radio antennas, each 12 meters across, in the Andes mountains of Chile. ALMA picks up the heat that bodies give off, and a dark surface absorbs more sunlight and so is warmer. Comparing heat with reflected light gives both the size and the darkness.

A 2018 study found a diameter of 700 kilometers, give or take 115 kilometers. Because that detection was faint, the same team observed Dysnomia again in October 2018. In 2023 they reported a refined diameter of 615 kilometers, uncertain by about 50–60 kilometers either way.

Size comparisons

Eris is about 2,326 kilometers (1,445 miles) across, almost four times as wide as Dysnomia.
If Eris were shrunk to the size of a basketball, Dysnomia would be a ball a little smaller than a tennis ball, orbiting about 3.8 meters (12.6 feet) away.
Dysnomia is about half as wide as Charon, the largest moon of Pluto, which is 1,212 kilometers (753 miles) across.
It is about two-thirds the width of the dwarf planet Ceres, which is about 950 kilometers (590 miles) across.
About 15 marathons laid end to end would stretch across Dysnomia.

Discovery and name

A team led by Michael Brown of the California Institute of Technology found Dysnomia on September 10, 2005. They used the Keck Observatory's newly installed laser guide star adaptive optics system, which sharpens telescope images. At first the discoverers nicknamed Eris "Xena", after a warrior princess in a television show, and its moon after Xena's sidekick.

In 2006 the International Astronomical Union gave both their official names. Eris is the Greek goddess of discord, and Dysnomia, in Greek myth her daughter, is the goddess of lawlessness.

Orbit and makeup

Dysnomia circles Eris in a nearly round orbit about 37,300 kilometers (23,200 miles) away, once every 15.8 days. Recent observations show that Eris turns on its axis in the same time, so it always keeps the same side facing its moon.

The two bodies look very different. The surface of Eris is extremely bright, with a geometric albedo, a measure of how strongly a surface reflects light, of 0.97. Dysnomia's albedo is only about 0.05, so Eris reflects light about 20 times as strongly. Dysnomia's darkness is typical of mid-sized objects in the Kuiper belt, the zone of icy bodies beyond Neptune.

Dysnomia also seems to be much less dense than Eris. A 2023 study found its density to be less than 1.2 grams per cubic centimeter, compared with 2.4 for Eris. In the same study, astronomers described Dysnomia as the second-largest known moon of any dwarf planet.