Eris
What is Eris?
Dwarf planets· Diameter 2,326 km. More massive than Pluto — its 2005 discovery forced the "what is a planet?" debate. Aptly named after the goddess of discord.
The orbit of Eris at this moment: https://nowsky.org/solar/orbit?fw=eris&sp=200 — replayed live, no login.
Orbit and size
| Diameter | 2,326 km |
|---|---|
| Distance | 38~97 AU |
| Year | 558 yr |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Frequently asked
- How big is Eris?
- 2,326 km.
- How far is Eris from Earth?
- 38~97 AU.
- In one line, what is Eris?
- Dwarf planets — More massive than Pluto — its 2005 discovery forced the "what is a planet?" debate.
What Dysnomia told us about Eris's mass
Eris's mass is known precisely thanks to its moon Dysnomia. When the Keck telescope found the moon in 2005, its 15.8-day period and distance gave the mass by Kepler's law: 27 percent more than Pluto. That number was the direct basis for the 2006 debate over the definition of a planet. The diameter, measured by stellar occultation in 2010 at 2,326 km, turned out slightly smaller than Pluto's 2,377 km.
Smaller but heavier means denser — Eris is 2.5 times the density of water, rockier than Pluto at 1.85. Dysnomia, about 700 km across, is large for a moon but very dark, and is named for Eris's daughter, the goddess of lawlessness. In 2023 Webb measured the isotope ratios of methane on Eris's surface, raising the possibility of geological activity inside.
The dwarf planet in depth
Eris is the world that cost Pluto its planethood. When its 2005 discovery proved it more massive than Pluto, astronomy faced a choice: crown a tenth planet, or redefine 'planet'. In 2006 the IAU chose the latter — and the dwarf planet category was born.
It is named for the goddess of discord — the one whose golden apple sparked the Trojan War. No name ever fit better; even its moon Dysnomia is named for lawlessness.
Eris rides a 558-year orbit swinging from 38 to 97 AU, its methane-frosted surface among the most reflective in the solar system. Toggle dwarf planets in the Orbit view to trace it.
Eris is named for the goddess of discord — fitting, for the world that forced Pluto's demotion. Out near 96 AU, more than twice Pluto's distance, its entire atmosphere lies frozen as frost on the ground. Recent work even suggests its interior stays warm enough for a buried ocean — the solar system's rim may not be dead after all.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.