Eris sits 68 astronomical units (AU) from the Sun on average—about 10.2 billion kilometers—making it the most distant known object in the solar system that still orbits our star directly.
Where does Eris live—and why should we care?
Eris drifts in the scattered disk, a distant zone of icy bodies beyond Neptune’s orbit. Its extreme distance from the Sun helps scientists probe the solar system’s far frontier and challenges what it means to be a planet. Because Eris is so far away, its surface temperature hovers around –230 °C. That’s cold enough to flash-freeze methane—(no surprise, given how little sunlight reaches it).
What are the key numbers for Eris?
| Property | Details |
|---|---|
| Average distance from Sun | 68 AU (10.2 billion km) |
| Orbital period | 559 Earth years |
| Diameter | 2,326 km ± 12 km |
| Surface composition | Methane ice, rocky silicates |
| Mass | 0.0028 Earth masses |
How was Eris discovered—and why did it spark a debate?
Eris was spotted in 2005 by Mike Brown’s team at Palomar Observatory. Its discovery forced astronomers to define “planet” more strictly; Eris failed the new criterion of clearing its orbit, so the International Astronomical Union reclassified Pluto—and Eris—as dwarf planets in 2006. Named after the Greek goddess of strife, Eris still stirs discussion almost two decades later. (Honestly, this is one of the most interesting debates in modern astronomy.)
Can you actually see Eris from Earth?
Because Eris is so faint—magnitude 18.7 at best—you’ll need a telescope with an aperture of 12 inches or greater and a CCD camera to catch its slow drift across the constellation Cetus. The best time to look is when Earth is between the Sun and Eris, which happens every 279 years. Sky & Telescope’s ephemeris tool can plot its position for any date in 2026.
