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 (dwarf planet)

Dwarf planet

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…

Palomar Observatory

Astronomical observatory in Southern California

The Palomar Observatory is an astronomical research observatory in the Palomar Mountains of San Diego County, California, United States. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observatory is granted to Caltech and its…

International Astronomical Union

Association of professional astronomers

The International Astronomical Union is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and development through global cooperation. It was founded on…

Definition of planet

The definition of the term planet has changed several times since the word was coined by the ancient Greeks. Greek astronomers employed the term ἀστέρες πλανῆται, 'wandering stars', for star-like objects which apparently moved over the sky. Over the millennia, the term has…

January 5

Day of the year

January 5 is the fifth day of the year in the Gregorian calendar; 360 days remain until the end of the year.