In 2003, Palomar Observatory astronomers were in the midst of their search for planet-sized objects beyond Neptune when they imaged Eris – but because they’d limited their search based on the speed that objects were moving, and Eris was quite sedate, it went undetected. It wasn’t until they discovered the likewise pokey Sedna the following month that the researchers realized they needed to go back through their data and search for slower-moving objects. On Jan. 5, 2005, Mike Brown of the California Institute of Technology, Chad Trujillo, then of the Gemini Observatory, and David Rabinowitz of Yale University zeroed in on Eris.
The object spelled doom for Pluto, however. The then-ninth planet’s status had been disputed for years, and the discovery of Eris and other bodies of a similar size called the question. The International Astronomical Union (IAU) formally defined a planet as an object circling the Sun, sufficiently large and gravitationally massive to attain a spherical shape, and has cleared its region of space. Eris failed the third criteria – but so did Pluto.
“We knew that second that we found it that our concept of the solar system was going to have to change,” Brown told writer Ben Evans in January 2025, for an Astronomy piece on the 20th anniversary of the find. “We were either going to have to add new planets or subtract one. I would have predicted the former, but even though it means I lost my chance to be called the discoverer of a few new planets, I am delighted that astronomers had the nerve to make the right choice and realize that Pluto should never have been called a planet in the first place.”
