Eris (dwarf planet)

Lore & Background

The object was informally called 'Xena' by the discovery team, inspired by the television series Xena: Warrior Princess. The team initially considered the name 'Lila' but dropped it after a breach of protocol. Brown also considered Persephone, but that name was already used for a minor planet. The name refers to the goddess of strife and discord, which Brown noted was fitting because the object caused discord among astronomers. Eris has two competing pronunciations, with a 'long' or 'short' e, and the adjective form is 'Eridian'.

Reader's Guide

Eris's significance lies in its role as a catalyst for redefining what constitutes a planet. When it appeared larger than Pluto, NASA initially described it as the Solar System's tenth planet. This, along with the prospect of other similar objects, motivated the International Astronomical Union to define the term 'planet' for the first time. Eris is the ninth-most massive known object orbiting the Sun and the sixteenth-most massive in the Solar System (counting moons). It is also the largest-known object in the Solar System that has not been visited by a spacecraft. With the exception of long-period comets, Eris and its moon Dysnomia were the most distant known natural objects in the Solar System until the discovery of 2018 AG37 and 2018 VG18 in 2018.

Did You Know?

• Eris has a surface area comparable to that of Russia or South America.
• Eris's mass is 27% greater than Pluto's, though Pluto is slightly larger by volume.
• Eris was informally called 'Xena' by the discovery team before its official naming.

The Slow-Motion Discovery

In the autumn of 2003, a team at Palomar Observatory in California—Mike Brown, Chad Trujillo, and David Rabinowitz—routinely pointed the 1.2-metre Samuel Oschin Schmidt telescope at the night sky, capturing images that would sit unremarked in their archives for over a year. Their automated software, designed to filter out false positives, discarded anything drifting slower than 1.5 arcseconds per hour against the starfield. Eris, moving at a glacial pace, slipped right through that net. It was only when Sedna's discovery in 2003 revealed an object at 1.75 arcseconds per hour that the team lowered their threshold and re-examined old frames by eye. In January 2005, Eris's faint, slow trail finally emerged. The announcement came on July 29, 2005—coinciding with Makemake's reveal and landing just two days after a rival Spanish team claimed Haumea. Precovery images trace Eris's path back to September 3, 1954, a half-century before anyone recognized it.

The Planet That Broke the Rules

When Eris first appeared in the public eye, its apparent size—seemingly exceeding that of Pluto—prompted NASA to label it the Solar System's tenth planet. That single designation sent shockwaves through the astronomical community. The prospect of yet more comparable objects lurking in the outer dark made the existing, informal notion of a planet untenable. The International Astronomical Union was forced to craft a formal definition for the first time, and on August 24, 2006, they voted to classify Eris, Pluto, and Ceres as dwarf planets, shrinking the official roster back to eight worlds—the same count that had held before Pluto's 1930 discovery. The irony was sharp: the very object that seemed to dethrone Pluto ended up demoting it alongside itself. A 2010 stellar occultation confirmed Eris was in fact slightly smaller than Pluto, and New Horizons' July 2015 flyby pinned Pluto's mean diameter at 2,377 ± 4 kilometres, compared with Eris's 2,326 ± 12.

A Distant World and Its Companion

Eris orbits the Sun in a highly eccentric path that carries it far beyond the familiar planetary neighborhood. Classified as a scattered-disk trans-Neptunian object, it reached 96.3 astronomical units from the Sun in February 2016—more than triple the distance of Neptune or Pluto. For a time, Eris and its confirmed moon Dysnomia held the record as the most distant known natural objects in the Solar System, a title only relinquished in 2018 when 2018 AG37 and 2018 VG18 were identified. Despite its remoteness, Eris is no minor body: it is the most massive known dwarf planet, weighing in at 0.28 percent of Earth's mass and roughly 27 percent heavier than Pluto, even though Pluto edges it out slightly in volume. Its surface area rivals that of Russia or South America. Scientists suspect a second, potentially larger moon may orbit the system. To date, Eris remains the largest known object in the Solar System that no spacecraft has ever visited.

From Xena to the Goddess of Strife

For nearly two years, the object carried the bland provisional label 2003 UB313 while the astronomical community debated its classification. Behind the scenes, the discovery team had a more colorful nickname. Brown has explained that Xena—borrowed from the television warrior-princess series—was reserved for the first body they found that outshone Pluto. It began with X, evoked mythology, and fit their desire to feature more female deities alongside names like Sedna. The name was accidentally leaked when a New York Times reporter, a college friend of Brown's, asked what the team called it among themselves. Once the IAU's August 2006 ruling settled the planet question, the Caltech team formally proposed the Greek goddess Eris, personification of strife and discord, on September 6, 2006; the IAU assigned it on September 13. Pronunciation remains contested—some favor a short e, as Brown and his students do, while others prefer a long one. The English adjective, drawn from the Greek and Latin oblique stem, is Eridian.

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