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Dwarf planet

Eris

The planet-definition trigger

Eris is one of the largest known dwarf planets, orbiting far beyond Pluto in the scattered disc region of the outer solar system. Its discovery in 2005 directly triggered the 2006 debate that led to the official definition of "planet" and Pluto's reclassification — Eris is about the same size as Pluto but more massive, so astronomers had to decide whether both were planets or neither. It is named after the Greek goddess of discord, fittingly for the argument it caused.

Quick facts

Distance from Sun
10.1 billion km (68 AU average)
Diameter
2,326 km
Day length
25.9 hours
Year length
557 Earth years
Moons
1 (Dysnomia)
Avg. temperature
-231 C

What's it like there?

Eris is one of the coldest sizable worlds known, at around -231 C. It is so far from the Sun that from its surface, the Sun would look like an extremely bright star rather than a disk. Its surface is coated in bright methane ice, making Eris one of the most reflective objects in the solar system. When its 557-year orbit brings it closer to the Sun, its frozen atmosphere may partially thaw into a thin gas layer.

Notable moons

  • Dysnomia

    Eris's only known moon, named after the daughter of Eris.

Fun facts

  • Eris is more massive than Pluto, despite being almost the same size.

  • Its discovery forced astronomers to define what a planet actually is.

  • One year on Eris lasts 557 Earth years.

  • Eris is one of the most reflective bodies in the solar system due to methane ice.

  • It orbits nearly three times farther from the Sun than Pluto on average.

Exploration history

  1. None yet —

    No spacecraft has visited Eris; it is studied by telescopes like Hubble and JWST.

Eris vs Earth

PropertyEarthEris
Diameter12,742 km2,326 km
Year length365 days557 years
Moons11
Average temperature15 C-231 C

Scientific Sources & Provenance

All data is verified against official space agency records (NASA, ESA, and IAU).