Planet X: 2003 UB313, Eris was discovered by Dr. Mike Brown on October 21, 2003. After multiple observations in July 2005, the discovery of Eris was confirmed and formally announced. Eris is about 14.5 billion kilometers from the sun and is in a very highly tilted orbit compared to the other planets. Eris is about 2,326 kilometers in diameter, whereas Pluto is 2,338 kilometers. Like Pluto, Eris is probably a very large comet or ice ball with a moon.Eris is named after the Greek Goddess of chaos and strife. A moon has been discovered orbiting Eris, named Dysnomia (Eris’ daughter who was the spirit of lawlessness). Dysnomia is about 250 kilometers in diameter.

When first discovered, Brown gave Eris the name Xena, and its moon Dysnomia the name Gabrielle. Later the International Astronomical Union adopted the names Eris and Dysnomia.

Image of The dwarf planet Eris, center, and its moon Dysnomia, right of Eris.
Public Domain | Image courtesy of NASA.