On the afternoon of 24 August 2006, in a conference hall in Prague, a few hundred astronomers raised yellow cards and voted the solar system down from nine planets to eight. It was the closing session of the International Astronomical Union's General Assembly; most of the roughly 2,500 delegates had already flown home, leaving around 400 to decide what, officially, a planet is. Textbook publishers, planetarium designers and generations drilled in nine-planet mnemonics woke the next morning to find that Pluto — a planet since 1930 — had been reclassified as a "dwarf planet". No scientific demotion has ever caused such public grief.
Pluto's planethood had been an accident of optimism from the start. Percival Lowell spent his fortune hunting a "Planet X" he believed was tugging at Neptune's orbit, and in 1930 Clyde Tombaugh — a 24-year-old Kansas farm boy who had taught himself astronomy by grinding telescope mirrors — spotted a faint dot shifting between photographic plates at Lowell Observatory in Flagstaff, Arizona. The name came from Venetia Burney, an 11-year-old in Oxford who proposed the Roman god of the underworld over breakfast; her grandfather cabled the suggestion onwards, and it helped that Pluto's first two letters were Percival Lowell's initials. But the new planet never fitted. It was far too faint and small to be anyone's Planet X; its orbit is tilted seventeen degrees and so stretched that from 1979 to 1999 it sat closer to the Sun than Neptune; and when James Christy discovered its moon Charon in 1978, the resulting arithmetic showed Pluto to be a featherweight, smaller than Earth's own Moon.
The real trouble began in 1992, when astronomers found a second small icy body beyond Neptune, then hundreds more: the Kuiper Belt, a vast ring of frozen leftovers in which Pluto looked less like a lonely planet and more like the biggest fish in a very crowded pond. The breaking point came in 2005, when Mike Brown's team at Palomar announced an object — catalogued 2003 UB313, nicknamed Xena, later formally named Eris after the goddess of discord — that appeared to be larger than Pluto. NASA's press release hailed a tenth planet. The IAU faced a fork: wave Eris in, along with who knew how many future discoveries, or define the word planet properly for the first time.
What "clearing your orbit" means
The Prague definition sets three tests. A planet must orbit the Sun; it must be massive enough for its own gravity to pull it into a rounded shape; and it must have "cleared the neighbourhood around its orbit". The third clause did the demoting, and it is the most misunderstood. It does not demand an empty orbit — Earth coexists with thousands of nearby asteroids. It means the body gravitationally dominates its zone: over billions of years it has swallowed, flung away or enslaved everything of comparable size near its path. Earth outweighs the rest of its orbital neighbourhood millions of times over. Pluto, by contrast, is one modest member of a swarm, and even its orbit is chaperoned by Neptune, which locks it in a 3:2 resonance — three Neptune laps for every two of Pluto's. Worlds that pass the first two tests but fail the third became dwarf planets: Pluto, Eris and, promoted from the asteroids, Ceres.
History had run this experiment before. Ceres, found by Giuseppe Piazzi in 1801, was celebrated as a planet for decades and printed in textbooks as such — until astronomers realised it was merely the largest of a swarm of asteroids, and quietly struck it off the list. Pluto's fate was Ceres's fate, with better publicity.
The aftermath was gloriously noisy. The American Dialect Society crowned "plutoed" — to be demoted or devalued — its 2006 Word of the Year. Legislatures in New Mexico and Illinois, both with Tombaugh connections, passed symbolic resolutions insisting Pluto remains a planet in their skies. Mike Brown embraced the villainy with a memoir titled How I Killed Pluto and Why It Had It Coming. And in a twist of timing, NASA's New Horizons probe had launched in January 2006, seven months before the vote, carrying a portion of Clyde Tombaugh's ashes towards a world that would be an ex-planet by arrival.
When New Horizons flashed past in July 2015, it found anything but a dull iceball: mountains of water ice, a vast heart-shaped nitrogen glacier in a region named Tombaugh Regio, and a thin, hazy, blue-tinged atmosphere. Eris, for its part, proved marginally smaller than Pluto in diameter, though more massive. The vote in Prague never diminished the world itself; it simply moved it to a truer shelf — no longer the runt of the planets, but the king of the Kuiper Belt.
Quiz nuggets
- Clyde Tombaugh discovered Pluto at Lowell Observatory in 1930; 11-year-old Venetia Burney of Oxford suggested its name.
- The 2005 discovery of Eris — nicknamed Xena before its formal naming — forced the IAU to define the word planet.
- On 24 August 2006 in Prague, roughly 400 astronomers voted to reclassify Pluto as a dwarf planet.
- Pluto fails the third planetary test, clearing the neighbourhood around its orbit, and sits in a 3:2 resonance with Neptune.
- "Plutoed" was the American Dialect Society's 2006 Word of the Year, and New Horizons flew past Pluto in July 2015.