Pluto's Demotion

The trouble started when astronomer Mike Brown and his team, working from images taken in October 2003, identified a distant icy world in January 2005. Announced that July as 2003 UB313 — and nicknamed Xena — the new object appeared to be even bigger than Pluto, and NASA initially hailed it as the solar system's tenth planet. That forced the question astronomy had dodged for decades: if Pluto is a planet, this thing has to be one too, and so does whatever turns up next. The International Astronomical Union decided to define the word 'planet' for the first time ever.

The vote came on August 24, 2006, at the IAU General Assembly in Prague — 424 astronomers raising yellow cards, a tiny fraction of the world's astronomers deciding for everyone. Resolution 5A set three tests: a planet orbits the Sun, is massive enough to be round, and has cleared the neighborhood around its orbit. Pluto passes two, but its orbit is crowded with fellow Kuiper Belt objects, so it failed the third and was reclassified a dwarf planet — alongside the troublemaker itself, eventually named Eris after the Greek goddess of discord. In a fitting twist, a 2010 measurement showed Eris is actually slightly smaller than Pluto.

The public reaction was pure heartbreak-as-comedy: tribute songs, mock outrage, and the American Dialect Society choosing 'plutoed' — to demote or devalue something — as its 2006 Word of the Year. Every 'My Very Educated Mother' mnemonic lost its ninth word overnight. And just months before the vote, in January 2006, NASA had launched the New Horizons probe toward Pluto; when it finally arrived in 2015 and sent back pictures of a little world with a heart on its surface, the demoted planet won the sympathy vote all over again.