From Mount Olympus to Pluto's Demotion: a Timeline of Eris Shaking the Heavens
By Sophia Al-Mansoor
For eight decades following Clyde Tombaugh’s 1930 discovery of Pluto, our mental model of the solar system remained tidy. School curricula drilled nine core planetary names into children worldwide. Yet starting in the 1990s, telescope technology advanced rapidly, revealing that the distant outskirts beyond Neptune, a region known as the Kuiper Belt and the scattered disc, were not empty voids. They swarmed with thousands of frozen relics left behind from planetary formation.
On January 5, 2005, Mike Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale reviewed automated images captured months earlier by the 48-inch Samuel Oschin telescope at Palomar Observatory. They flagged a slow-moving, luminous point of light roughly 96 astronomical units from the Sun, three times further away than Pluto.
The team initially tagged the object with the provisional designation 2003 UB313, code-naming it "Xena" after the television warrior princess. Early optical data indicated that 2003 UB313 was roughly the size of Pluto, and subsequent observations confirmed it was approximately 27% more massive.
The media exploded. NASA initially issued a press statement hailing 2003 UB313 as the "tenth planet." Front-page headlines around the globe announced the expansion of our cosmic family. But inside planetary science circles, panic set in. If this new icy body was a planet, what about Quaoar, Sedna, and the dozens of similar heavy bodies predicted to inhabit the Kuiper Belt? Astronomers faced an escalating dilemma: either teach school children an endless list of fifty or more newly minted planets, or systematically redefine what a planet actually was.
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