Today in the history of astronomy, a new dwarf planet causes Pluto’s demotion.
This artist’s visualization of Eris highlights its distance from the rest of the Solar System, highlighted by the distant bright spot at upper left. Credit: NASA/ESA/STScI
- Eris was initially imaged by Palomar Observatory astronomers in 2003, but was not discovered until 2005 due to initial search parameters that excluded slow-moving objects.
- Eris was formally identified by Mike Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005, following a reevaluation of the data inspired by the discovery of other faint trans-Neptunian objects such as Sedna.
- The existence of Eris, a body comparable in size to Pluto, directly led the International Astronomical Union (IAU) to establish a formal definition of a planet.
- Under the IAU’s new criteria, which requires an object to clear its orbital zone, both Eris and Pluto failed to qualify, leading to the reclassification of Pluto and a fundamental revision of the Solar System’s planet concept.
In 2003, astronomers at the Palomar Observatory were searching for planet-sized objects beyond Neptune when they imaged Eris – but because they had limited their search based on the objects’ motion, and Eris was quite cool, it went undetected. It wasn’t until they discovered the similarly pokey Sedna the following month that researchers realized they needed to go back through their data and search for slower-moving objects. On January 5, 2005, Mike Brown of the California Institute of Technology, Chad Trujillo of Gemini Observatory, and David Rabinowitz of Yale University focused on Eris.
However, this object spelled doom for Pluto. The status of the then-ninth planet had been debated for years, and the discovery of Eris and other bodies of similar size raised the question. The International Astronomical Union (IAU) formally defines a planet as an object orbiting the Sun, which is sufficiently large and gravitational to acquire a spherical shape, and has cleared its region of space. Eris failed the third criterion – but so did Pluto.
“We knew the moment we found out that our concept of the Solar System had to change,” Brown told author Ben Evans in January 2025. Astronomy Excerpt on the 20th anniversary of the discovery. “We either had to add new planets or subtract one. I would have predicted the former, but even if it meant I lost the chance to be called the discoverer of some new planets, I’m glad astronomers had the courage to make the right choice and realized that Pluto should never have been called a planet.”
