Today in history: On July 29, 2005 – Astronomers announce their discovery of the dwarf planet Eris.

 

On July 29, 2005, a team of astronomers led by Mike Brown from the California Institute of Technology announced the discovery of a distant celestial object later named Eris. Initially identified in images taken in 2003, Eris was found in the scattered disc region beyond the Kuiper Belt, a zone of icy bodies orbiting the Sun past Neptune. Eris appeared to be slightly larger than Pluto, which led to considerable excitement in the astronomical community. For a time, Eris was even considered the “tenth planet” of the solar system. Its discovery challenged long-held assumptions about what defines a planet and sparked a major debate over planetary classification.

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The discovery of Eris had a profound impact on astronomy. In 2006, partly in response to Eris, the International Astronomical Union (IAU) redefined the criteria for planethood, which led to Pluto being reclassified as a “dwarf planet” alongside Eris. This decision stirred controversy among scientists and the public alike. Eris was named after the Greek goddess of strife and discord, fitting, given the turmoil its discovery caused in the astronomical world. It has a moon named Dysnomia and follows an elongated orbit that sometimes takes it farther from the Sun than any other known large object in the Solar System. Today, Eris remains one of the most massive known dwarf planets and a symbol of how scientific discovery can challenge and reshape our understanding of the universe.

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