Atlas · Eris
The moon of Eris
Eris has one known moon, Dysnomia, found in 2005. Its path is very nearly a circle. Timing it gave the mass of Eris, which turned out to be greater than Pluto’s.
Eris and its moon
Eris and the orbit of its moon, to scale, seen from above the orbit.
The edge of the drawing is 46,000 km from the centre of Eris.
How to read it
You are looking straight down on the orbit of Dysnomia, from its northern side. Which way Eris itself is tipped is not well known, so its moon’s path is the surest thing to look down on. Eris is drawn to scale with the orbits. The moons are not: at this scale most would be far too small to see, so each is a dot.
- Solid dot. A spacecraft has mapped the whole surface.
- Half dot. A spacecraft has seen part of it.
- Ring. Never seen as more than a point of light.
- Pale line. Goes round the usual way, the way the planets go round the Sun: anticlockwise on these drawings.
The orbit is a slowly turning ellipse fitted to positions from NASA’s Jet Propulsion Laboratory for the years 2000 to 2030. The list gives how far out each fit can be. Touch a row to pick out its orbit.
The moon
1 moon
Dysnomia’s width comes from the heat it gives off, measured with the ALMA radio telescope, and is uncertain by about a sixth either way.
| Moon | Orbit size | One orbit | Tilt | Width | Found | See it in 3D |
|---|---|---|---|---|---|---|
| Dysnomia. Seen only as a point of light.drawn to within 0.01° | 37,300 km | 15.79 days | 62° | 700 km | 2005 | Fly there |
Where this comes from
- NASA/JPL Horizons: orbit solutions for the moons of trans-Neptunian objects, fitted here over 2000 to 2030
- Sicardy et al. 2011, Nature 478, 493 (stellar occultation)
- Brown & Butler 2018, Astronomical Journal 156, 164 (the sizes of Dysnomia and Vanth)
- NASA/JPL Small-Body Database
Orbit size is the average distance from the centre of Eris. Tilt is measured from the plane of the planets, and above 90° means the moon goes round backwards compared with the planets.