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Moon of Eris
Dysnomia
The dark little moon that told us how heavy Eris really is
Key facts
- Diameter
- About 382 miles / 615 km
- Distance from Eris
- About 23,200 miles / 37,300 km
- One orbit
- About 15.8 Earth days
- Discovered
- 10 September 2005, by Mike Brown and the Keck Observatory adaptive optics team
- Named after
- Dysnomia, Greek goddess of lawlessness and daughter of Eris
- Nickname before naming
- Gabrielle, after the companion of the TV warrior Xena
- Surface
- Very dark, reflecting only about 5% of sunlight; probably mostly ice
- Rotation
- Keeps the same face towards Eris (tidally locked)
- Parent body
- Eris, a dwarf planet about 68 AU from the Sun
- Known moons of Eris
- 1 (as of August 2026)
- Visited by
- No spacecraft; studied by Keck, Hubble and ALMA
About Dysnomia
Dysnomia is a tiny moon. It goes round a world called Eris. Eris is very far from the Sun. It is much further out than Pluto. Out there the Sun looks like a bright star. Dysnomia is dark, like coal. It is hard to see. Big telescopes found it not long ago. It takes about 16 days to go round Eris. Eris and Dysnomia are both very cold. They are made mostly of ice. No spaceship has been there yet. Maybe one day you will send one.
Dysnomia is the only known moon of the dwarf planet Eris. Eris orbits the Sun far beyond Pluto, about 68 times further than Earth. Sunlight takes more than nine hours to reach this distant pair. Astronomers found Dysnomia in September 2005 using the Keck telescope in Hawaii.
A laser made an artificial star so the telescope could sharpen its view. The moon is about 615 kilometres across, roughly a quarter the width of Eris. It orbits Eris once every 16 days at a distance of about 37,000 kilometres. Watching this orbit let scientists measure the gravity of Eris.
That is how they learned Eris is about 27 per cent more massive than Pluto. Dysnomia is one of the darkest moons known, reflecting about five per cent of sunlight. It is probably an icy sphere, but nobody has seen its surface closely. Dysnomia keeps the same face turned towards Eris as it travels round.
Its name comes from a Greek goddess of lawlessness, the daughter of Eris. No spacecraft has ever visited Eris or Dysnomia. Everything we know comes from telescopes on Earth and in space.
Dysnomia is the only known moon of Eris, the most massive dwarf planet in the solar system. It was discovered on 10 September 2005 by Mike Brown of Caltech and the adaptive optics team at the W. M. Keck Observatory on Mauna Kea, Hawaii. Keck's new laser guide star system created an artificial star beside the target, letting the telescope correct for atmospheric blurring and reveal a faint companion that earlier observations had missed. While Eris was still nicknamed Xena, the moon was called Gabrielle after Xena's companion; both received official names in September 2006. Dysnomia is the Greek goddess of lawlessness and daughter of Eris, a choice that also nods to Lucy Lawless, the actress who played Xena.
Dysnomia orbits Eris at about 37,300 km in a nearly circular path every 15.79 days. That orbit is scientifically valuable. By applying Newton's laws to its period and distance, Brown's team measured the mass of Eris and found it about 27 per cent greater than Pluto's. This is often misreported as Eris being larger than Pluto. In fact Eris is slightly smaller in diameter, about 2,326 km by the best stellar occultation measurement (NASA rounds this to about 2,400 km) against Pluto's 2,377 km, but it is denser and therefore more massive.
ALMA observations reported in 2018 refined Dysnomia's diameter to roughly 615 km, about a quarter of the width of Eris, making it the second-largest known moon of a dwarf planet after Charon. Its albedo of about 0.05 means it reflects only around 5 per cent of sunlight, far darker than bright, icy Eris. A density estimate of about 0.7 g/cm³, with large uncertainty, suggests a mostly icy body. The leading hypothesis is that it formed from a giant collision, much as Earth's Moon did.
Recent work indicates that Eris and Dysnomia are mutually tidally locked, like Pluto and Charon, with Eris rotating once per Dysnomia orbit. NASA's fact page still lists an older rotation period of 25.9 hours for Eris, so sources disagree; the tidally locked value is the current best estimate.
No spacecraft has visited the Eris system. Everything known about Dysnomia comes from the Keck Observatory, the Hubble Space Telescope and ALMA. As of August 2026, Dysnomia remains the only known moon of Eris.
Did you know?
- Dysnomia is so dark that it reflects only about 5 per cent of the sunlight that reaches it, far less than bright, icy Eris.
- Timing Dysnomia's 16-day orbit let astronomers weigh Eris, showing it is about 27 per cent more massive than Pluto.
- Its name means lawlessness, a playful nod to Lucy Lawless, the actress who played Xena, the nickname Eris had before it was named.
Neighbours around Eris
Dysnomia is the only known moon of Eris.
Questions children ask
How did astronomers find Dysnomia?
In September 2005 the Keck Observatory in Hawaii used a laser to make an artificial star in the sky. This let its adaptive optics cancel out the blurring of Earth's air, and a faint dot appeared beside Eris.
How big is Dysnomia?
It is about 615 kilometres across, roughly a quarter of the width of Eris. That makes it the second-largest known moon of any dwarf planet, after Pluto's moon Charon.
Why does a moon matter so much to scientists?
A moon lets you weigh its parent. By measuring how far Dysnomia is from Eris and how long its orbit takes, astronomers worked out the mass of Eris using Newton's laws of gravity.
Has any spacecraft been to Dysnomia?
No. Eris and Dysnomia are so far away that sunlight takes more than nine hours to reach them. Everything we know comes from the Keck telescopes, the Hubble Space Telescope and the ALMA radio telescope array.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- Which dwarf planet does Dysnomia orbit?
- About how long does Dysnomia take to orbit Eris?
- What did Dysnomia's orbit help scientists measure?
Charon
Eris
MK2 (S/2015 (136472) 1)