Eris is a dwarf planet far beyond Neptune, and one of the largest dwarf planets in the Solar System. It is 2,326 kilometers (1,445 miles) across, almost exactly the width of Pluto, but it has about 27 percent more mass. Its discovery in 2005 helped start the debate that led astronomers to define what a planet is.

Size

Eris's size was measured on November 6, 2010, when it passed in front of a faint star, an event called an occultation. Telescopes at two sites in Chile recorded the star's light dropping out as Eris blocked it. Timing how long the star disappeared at each site gave the size of Eris.

The result was a radius, the distance from the center to the surface, of 1,163 kilometers, give or take 6 kilometers. That makes Eris 2,326 kilometers across, and the measurements fit a round shape. Earlier estimates, made by other methods, had put Eris at about 3,000 kilometers, about 25 percent wider than Pluto.

The orbit of Eris's small moon, Dysnomia, shows how strongly Eris pulls on it. That gives the mass of Eris, the amount of matter it contains: 1.66 × 10²² kilograms. Because Eris packs more mass than Pluto into a slightly smaller ball, it is denser, and astronomers think it is mostly rock.

Size comparisons

Pluto is 2,377 kilometers across, about 50 kilometers wider than Eris. Their widths differ by about 2 percent.
Eris is about two-thirds as wide as the Moon, which is 3,475 kilometers across, and a little less than one-fifth as wide as Earth.
Neptune's largest moon, Triton, is 2,707 kilometers across, about 380 kilometers wider than Eris.
If the Sun were the size of a basketball, Earth would be a grain about 2 millimeters across, 26 meters (85 feet) away. Eris would be a speck 0.4 millimeters across, between 1 and 2.5 kilometers (0.6 and 1.6 miles) away.

Discovery and name

Mike Brown of the California Institute of Technology, Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University discovered Eris on January 5, 2005. They found it in images taken on October 21, 2003, during a survey of the outer Solar System at Palomar Observatory. The team nicknamed it Xena.

A world that seemed bigger than Pluto raised the question of what should count as a planet. In August 2006 the International Astronomical Union voted on a new definition, which made Pluto, Eris and Ceres dwarf planets. A dwarf planet orbits the Sun and is big enough to pull itself into a nearly round shape, but has not cleared other objects from around its orbit.

In September 2006, Eris received its name, after the Greek goddess of discord and strife. Its moon became Dysnomia, after Eris's daughter, the goddess of lawlessness.

A distant, tilted orbit

Eris follows a long, oval orbit, tilted about 44 degrees to the plane of Earth's orbit. Its distance from the Sun ranges from 38 to 98 astronomical units, where one astronomical unit is the average distance from Earth to the Sun, about 150 million kilometers. On average it is 68 astronomical units away, about 10 billion kilometers, and one orbit takes about 560 years.

At its average distance, sunlight takes more than nine hours to reach Eris. Its surface temperature is estimated to be between −217 °C and −243 °C (−359 °F and −405 °F). Its surface reflects light very well. Astronomers think it is covered by a thin layer of ice, probably its atmosphere frozen onto the ground as frost.

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