About the image. Drawn as a bright white icy world with its smaller moon Dysnomia. Eris has never been seen as more than a point of light; its brightness follows measurements, its surface is imagined, and the moon is drawn closer than its real orbit.
On 5 January 2005 Mike Brown, Chad Trujillo and David Rabinowitz spotted a slow-moving object in images taken in 2003 at Palomar Observatory. When they worked out its distance and brightness, they realised it was at least as large as Pluto. Announced in July 2005, it was briefly hailed in the press as the tenth planet.
The end of nine planets
Eris forced a decision. If it was a planet, other similar worlds beyond Neptune would soon follow, and the Solar System might have dozens of planets. If it was not, then neither, arguably, was Pluto. On 24 August 2006 the International Astronomical Union adopted its first formal definition of a planet. A planet must have cleared its orbit of other bodies; Eris and Pluto have not. Both, along with Ceres, became dwarf planets.
The name chosen that September fitted the controversy: Eris is the Greek goddess of strife and discord. Its moon, found in 2005, was named Dysnomia, her daughter, the spirit of lawlessness.
Smaller but heavier
Eris turned out to be very slightly smaller than Pluto: 2 326 kilometres across, against Pluto’s 2 377. But by tracking Dysnomia’s orbit, astronomers found that Eris is about 27% more massive. It must be denser, with more rock inside.
Its surface reflects 96% of the light that falls on it, making it one of the brightest objects in the Solar System. It is probably coated in fresh frost, including methane ice. In 2023 Szakáts and colleagues showed that Eris always keeps the same face towards Dysnomia, turning once every 15.8 days, as Pluto does with Charon.
Far out
Eris travels on a long, steeply tilted orbit, between about 38 and 97 AU from the Sun, and takes about 560 years to go around once. It was near its farthest point in the 1970s; in 2016 it was still about 96 AU from the Sun. Its surface temperature is between 30 and 56 K (-217 °C).
How far out it would be
Eris goes around once every 558 years, at 67.69 AU from its star. Dashed circles show Jupiter, Saturn and Neptune around the Sun, on the same scale.
Earth days passing: 0
Your weight on Eris
On Eris
Gravity there is about 0.08× Earth's. Calculated from the measured mass and radius.
Gallery
A bright white, frost-smooth world lit from the left by the Sun, which is only a small bright star, with a tiny moon beside it, an impression of the dwarf planet Eris and its moon Dysnomia. AI-generated painting.A white frozen plain with a block of ice casting a long shadow, a small moon in the black sky and the Sun as a bright star on the horizon, an impression of the surface of Eris. AI-generated painting.Eris, 3D artist's impression, rendered from the interactive model.Eris, 3D artist's impression, from another angle, rendered from the interactive model.Eris, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
2003Eris is photographed on 21 October, unrecognised.
2005Discovered in January and announced in July; Dysnomia found in September.
2006The IAU defines a planet; Eris and Pluto become dwarf planets.
2023Eris is shown to keep one face towards Dysnomia.
Questions people ask
Is Eris bigger than Pluto?
No. It is slightly smaller in size, 2 326 km against 2 377, but about 27% more massive.
Why was Pluto demoted because of Eris?
Eris, at least as big as Pluto, forced astronomers to define a planet. Under the 2006 definition, neither has cleared its orbit, so both are dwarf planets.