Eris (dwarf planet)

Eris is a massive dwarf planet located in the far reaches of our Solar System.

It is classified as a trans-Neptunian object (TNO) within the scattered disk. This means it orbits the Sun beyond the planet Neptune. Eris is the most massive dwarf planet known to astronomers.

While Pluto has a slightly larger volume, Eris is 27% more massive than Pluto. It is the ninth-most massive object orbiting the Sun. It also holds the title of the sixteenth-most massive object in the entire Solar System when counting moons. Eris is a significant world because it is the largest known object that no spacecraft has ever visited.

The movement of Eris through space is quite unusual compared to the major planets. It follows a high-eccentricity orbit, meaning its path is a stretched-out oval rather than a circle.

Its orbit is also highly inclined, tilted at an angle of about 44 degrees relative to the ecliptic. The ecliptic is the flat plane where most planets orbit. Eris also travels a vast distance from the Sun. In February 2016, its distance was more than three times that of Neptune or Pluto. Its orbital period lasts 561 years. This long journey takes it from a closest point of 38.4 AU to a furthest point of 97.7 AU.

Eris is accompanied by one large known moon named Dysnomia.

Scientists were able to determine the mass of Eris by observing the orbit of this moon. In June 2007, calculations showed Eris had a mass of approximately 1.66 × 10^22 kg. This mass is 0.28% that of the Earth. The surface area of Eris is quite large. It is comparable in size to the continents of Russia or South America. The two bodies, Eris and its moon, were the most distant known natural objects in the Solar System until 2018. This distinction remained until the discovery of Sedna and 2012 VP113.

The discovery of Eris began with images taken on October 21, 2003.

A team led by Mike Brown, Chad Trujillo, and David Rabinowitz used the 1.2 m Samuel Oschin Schmidt telescope at Palomar Observatory. They initially missed Eris because it moved very slowly across the sky. Their automatic software excluded objects moving at less than 1.5 arcseconds per hour to avoid false positives. After the discovery of Sedna, the team re-analyzed old data by eye with a lower motion limit. This re-analysis revealed Eris in January 2005. The discovery was officially announced on July 29, 2005, alongside the discovery of the dwarf planet Makemake.

This discovery led to a major change in how we define space objects. Because Eris appeared larger than Pluto, NASA initially called it the tenth planet.

This created a debate about what should be called a planet. In response, the International Astronomical Union (IAU) met to create a formal definition. On August 24, 2006, the IAU approved a new rule. They classified Eris, Pluto, and Ceres as "dwarf planets." This decision reduced the number of recognized planets in our Solar System back to eight. This was the same number that existed before Pluto was discovered in 1930. The discovery team expressed approval of this new scientific classification.

The naming of Eris was a long and careful process.

Before it had an official name, the discovery team used the informal nickname "Xena." This was inspired by the character from the television series *Xena: Warrior Princess*. The team wanted a name that reflected the object's importance. They eventually proposed the name Eris, after the Greek goddess of strife and discord. This name was chosen because the discovery had caused much debate among astronomers. The IAU officially assigned the name on September 13, 2006. The name has two common pronunciations in English: one with a "long" e and one with a "short" e.

Eris connects to broader studies of how our Solar System formed. It is a scattered-disk object, which is a type of TNO. These objects were likely scattered from the inner edge of the Kuiper belt into distant orbits. This happened due to gravitational interactions with Neptune during the formation of the Solar System. Studying Eris helps scientists understand the history of these distant, icy worlds. It also helps us understand the complex gravitational dance that shapes our cosmic neighborhood.