Eris

Dwarf planet · Dwarf planet (scattered disc) · orbits Sun

Eris is a scattered-disc world almost exactly Pluto’s size but 28% more massive, currently nearly 100 AU from the Sun on a steeply tilted 558-year orbit. Its discovery in 2005 - briefly hyped as a “tenth planet” - is what finally forced astronomers to define the word planet.

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Quick facts

At 2326 km across it is within ~2% of Pluto’s diameter, yet 28% more massive - so its interior must be rockier.
Its icy surface reflects ~96% of incoming sunlight, making it one of the most reflective bodies known.
JWST found an isotopic signature in its methane ice that hints at a warm rocky interior - perhaps even a past subsurface ocean.

The object that demoted Pluto

When Eris turned up - Pluto-sized, and more massive - astronomers faced a choice: either call it the tenth planet and accept dozens more to come, or give “planet” a real definition at last. The IAU chose the definition in 2006, and Pluto and Eris entered the new dwarf-planet class together. Eris is fittingly named for the Greek goddess of discord.

Physical characteristics

Diameter	2,326 km
Mass	1.660e+22 kg
Surface gravity	0.82 m/s²
Density	2.43 g/cm³
Temperature range	-243 °C to -217 °C
Geometric albedo	0.96
Known moons	1

Atmosphere and composition

Atmosphere	None at present - any nitrogen or methane air is frozen solid out here; a thin atmosphere may appear near perihelion.
Composition	A rock-rich interior under a brilliantly reflective veneer of frozen methane and nitrogen.

Discovery

Discovered by	Mike Brown, Chad Trujillo & David Rabinowitz
Year	2005
How	Palomar Observatory survey images (taken in 2003, identified in 2005)

Missions

James Webb Space Telescope NASA/ESA/CSA · Space telescope · 2021 · active

The most powerful space telescope ever built, JWST observes the infrared universe with a 6.5 m gold-coated mirror from a point 1.5 million km beyond Earth. Alongside the distant cosmos it studies Solar System targets, from Jupiter’s auroras to the surfaces of Kuiper belt worlds.

What is still uncertain

Eris’ rotation is uncertain: recent measurements suggest it is tidally locked to its moon Dysnomia, turning once per ~15.8-day orbit.