Eris: The Dwarf Planet That Reopened the Debate

How Eris was found

NASA credits the discovery to Mike Brown of Caltech, Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University, who were running a survey of the outer Solar System at Palomar. The object carried the provisional designation 2003 UB313, and the team's nickname for it was Xena.

• 2003: three images are taken on October 21 at Palomar Observatory.
• 2005: the object is found in the archived data on January 5. It is announced in the last week of July.
• 2006: the IAU defines "planet" on August 24 and names Eris on September 13.
• 2007: Eris's mass is measured, using the orbit of its moon (June).
• 2010: Eris passes in front of a distant star, which gives a precise width (November).
• 2015: NASA publishes Pluto's New Horizons size, about 2,370 km.

The team's own account says early reports gave a discovery date of January 8, which the team admits was a mistake. January 5 is the date NASA uses.

Why a "tenth planet" forced a decision

The discovery of Eris, which was thought to be larger than Pluto, made the question of what counts as a planet impossible to ignore. NASA JPL wrote in September 2006 that the 2005 discovery sparked debate over whether the object should be a planet, since it was "larger than Pluto," and that it revived questions about Pluto's own status.

Early size estimates mattered. Brown's page lists a measurement with the IRAM radio telescope of about 3,000 km (1,860 miles), with a large error range, and a Hubble Space Telescope measurement of about 2,400 km (1,490 miles). Both put Eris at or above Pluto's size, so Eris looked like a serious tenth-planet candidate.

The IAU chose a different path. Instead of adding planets, it wrote a rule built on three tests, described in what a dwarf planet is, and it placed Eris and Pluto in a new category. The IAU's naming release says Eris is the second dwarf planet in a subcategory of objects of which Pluto is the prototype. The full account of that vote is in why Pluto is a dwarf planet.

Eris compared with Pluto

The two worlds are close in width, and the order has flipped as measurements improved. In 2010, an event called a stellar occultation, in which a body passes in front of a faraway star and blocks its light, let observers at two sites in Chile time the star's disappearance. The result, published in Nature in October 2011, was a width of 2,326 km (1,445 miles) with an accuracy of 12 km. NASA's July 2015 report on New Horizons gives Pluto a width of about 2,370 km (1,473 miles) and says Pluto is larger than all other known objects beyond Neptune. More about that measurement is in what New Horizons found at Pluto.

How can Eris be narrower and heavier? It packs more material into a similar volume. The 2011 study found a density of 2.52 g/cm³, which the team says suggests mostly rock with a thin ice mantle.

A bright, cold world with one moon

NASA lists Eris's average distance from the Sun as about 68 AU, or 10 billion km (6.3 billion miles). An AU is the distance from Earth to the Sun, about 150 million km. Eris takes 557 Earth years to orbit once, and sunlight needs more than nine hours to reach it. Its day lasts about 25.9 hours. NASA says surface temperatures are believed to range from about -217 °C to -243 °C (-359 °F to -405 °F).

The surface is very bright. The 2011 study found that Eris reflects 96 percent of the light that hits it, which is brighter than fresh snow on Earth. The authors think the surface is a thin layer of nitrogen-rich ice mixed with frozen methane, less than 1 mm thick, possibly frost that forms as Eris moves away from the Sun.

Eris has one known moon, Dysnomia, which circles in a nearly round orbit lasting about 16 days. That orbit is how astronomers weighed Eris. In June 2007, researchers at Caltech timed Dysnomia's orbit and calculated that Eris is 27 percent more massive than Pluto, at about 16.6 billion trillion kg. Their Keck and Hubble data gave a density near 2 g/cm³ at the time, and Brown said it was "second place at best" for Pluto.

Anyone who wants to look at the sky for fun should never point binoculars or a telescope at the Sun without a proper solar filter made for that purpose.

The name: Eris and Dysnomia

The IAU named the object on September 13, 2006, and announced it the next day. Eris is the Greek goddess of discord and strife, and the discovery team proposed the name. The moon's name, Dysnomia, comes from the spirit of lawlessness, a daughter of Eris. The choice fits the argument the discovery started. The name "Xena" was a placeholder used by the team while the IAU worked out its naming process. For how provisional codes such as 2003 UB313 become names, see how asteroids and comets get their names. To see where Eris sits among official and candidate worlds, read how many dwarf planets there are.

Common mistakes and myths

• Myth: Eris is bigger than Pluto. It is more massive, by about 27 percent, but Pluto is slightly wider according to the 2010 and 2015 measurements.
• Myth: Eris was officially the tenth planet. It was widely called that in 2005, but the IAU never gave it planet status. Its discovery led to the definition that keeps the count at eight.
• Myth: Eris alone is why Pluto was reclassified. The IAU vote of August 24, 2006 did that. Eris's discovery was one of the reasons the vote was held.
• Myth: Xena is the official name. Xena was the team's nickname. The IAU name is Eris.

Frequently asked questions

Why is it called Eris?

Eris is the Greek goddess of discord and strife. The discovery team proposed the name, and the IAU accepted it in September 2006.

How far is Eris from the Sun?

NASA lists an average of about 68 AU, or 10 billion km (6.3 billion miles). One trip around the Sun takes 557 Earth years.

Does Eris have a moon?

Yes, one: Dysnomia. It orbits in about 16 days, and timing that orbit let astronomers calculate Eris's mass.

How do astronomers know Eris's size?

Mostly from a stellar occultation in 2010, when observers in Chile timed how long a distant star was blocked. The result was 2,326 km, accurate to 12 km.