Illustration by Adam Billyeald
At first, it seemed like Pluto might pull through. On August 15, when word leaked out that the Planet Definition Committee had proposed letting Pluto persist as a bona fide planet (and not just as a "dwarf planet"), it looked like a victory for the cosmic underdog. Pluto, the runt of our solar system, was going to be okay.
The astronomers admitted that this act of generosity wasn't particularly scientific. After all, if Pluto were discovered today, it would be classified as just another frozen rock trapped within the orbit of Neptune. But the committee decided to overrule the cosmological facts: Pluto should remain a planet because everybody already thinks it's a planet. As one scientist lamented, "There could be a public relations disaster if we just throw out Pluto, especially if we don't even give it a tip of the hat."
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For the most part, astronomers were simply a victim of their own success. Their old model of the solar system had become a sturdy cultural icon. The nine planets, all of them anthropomorphized and named for Roman and Greek gods, were now affixed to T-shirts, posters, screensavers and mobiles. Odd mnemonics were invented so that their order could be remembered ("My Very Efficient Metal Jaguar Sometimes Uses No Petrol"). Astrologers used the planets to forecast the future. (Pluto, for example, rules Scorpio.) In a universe full of dark matter and hungry black holes, our genteel neighborhood had become a source of reassurance, a suburb of space that seemed ordinary and safe. Beyond Pluto lay nothing but interstellar emptiness, the nameless sprawl of the Milky Way.
Unfortunately, all it takes is a single new observation—a faint orangeish dot slowly moving against a quilt of stars—and a scientific model taught to generations of schoolchildren can come crashing down. In July 2005 Mike Brown, an astronomer at Cal-Tech, announced that he had found a "planet-like object" that was bigger than Pluto. This object resided in the Kuiper Belt, a swath of icy debris left over from the formation of the solar system. Brown called this new object Eris, after the Greek goddess of discord and strife.
The name was prophetic. The discovery of Eris (its technical name was 2003 UB313) started an astronomical brawl. If Eris isn't a planet, then why is Pluto? After all, Pluto is smaller than Eris. Either the solar system had to be expanded, or Pluto had to go.
For astronomers, the debate was embarrassing. Why was there no rigorous way to distinguish Pluto from its Kuiper Belt cousins? Shouldn't astronomers know what a planet is? Just where does our solar system end? This dispute led the International Astronomical Union (IAU) to form the Planet Definition Committee, which decided that Pluto and Eris, in addition to more than 40 other Kuiper Belt objects, were genuine planets, since they were "spherical objects that orbit the sun." Mike Brown called it the
"No Ice Ball Left Behind" policy.
Alas, most astronomers didn't agree with the committee. At the 2006 IAU meeting held in Prague this past summer, the scientists voted that every planet must also have "cleared the neighborhood around its orbit." Since only spheres with a large mass can achieve such orbital dominance, Pluto was no longer a planet. The scientific bureaucracy had spoken; our solar system had shrunk.