Eris

• Region: Scattered Disk
  
  

• Diameter: ~2,326 km
  
  

• Mass: 1.66 × 10²² kg
  
  

• Orbit: 558 Earth years, semi-major axis ~67.7 AU, eccentricity 0.44
  
  

• Rotation: Unknown precisely; estimates suggest 25–26 hours
  
  

Surface & Geology:
Eris has an extremely bright surface with an albedo of ~0.96, likely coated in methane ice. Spectroscopic studies suggest frozen nitrogen and minor carbon monoxide. No surface features have been directly imaged in detail due to distance.

Atmosphere:
Any atmosphere is likely extremely thin or temporary, sublimating when Eris approaches perihelion.

Moons:

• Dysnomia: ~700 km from Eris, discovered 2005. Its orbit allows calculation of Eris’s mass and density (~2.52 g/cm³), suggesting a rocky composition.
  
  

Internal Structure:
Eris is expected to be differentiated with a rocky core surrounded by an icy mantle. High density indicates a larger rock fraction than Pluto.

Temperature:
Surface temperatures are ~–243°C, extremely cold due to its distance from the Sun.

Discovery:
Discovered in 2005 by Mike Brown et al., its size and mass prompted the redefinition of “planet” by the IAU, leading to Pluto’s reclassification.