The dwarf planet Eris is discovered by Palomar Observatory-based astronomers, later motivating the International Astronomical Union (IAU) to define the term planet for the first time.

Eris is the most massive and second-largest-known dwarf planet in the Solar System. It is a trans-Neptunian object (TNO) in the scattered disk and has a high-eccentricity orbit. Eris was discovered in January 2005 by a Palomar Observatory–based team led by Mike Brown and verified later that year.…