It's now been a week since the International Astronomical Union (IAU) stripped Pluto of its planethood. In choosing the more stringent of two competing definitions of the term planet, the IAU has booted Pluto into a new underclass of "dwarf planets", and seemingly capped the solar system's planet total at eight. Many scientists aren't pleased with the new solar-system order, saying it's imprecise and too restrictive. Meanwhile, Pluto continues to orbit out in the cold (literally, and now figuratively), oblivious to its demotion.

The ancient Greeks called the points of light that roved along the zodiac planetes: wanderers. But despite the discovery of several new planets since the invention of the telescope, astronomers never defined what a planet actually was. When Ceres, Pallas, Juno, and Vesta were discovered in the early 1800s, they were originally considered planets, but later reclassified as asteroids. Pluto, discovered by Clyde Tombaugh of Lowell Observatory in 1930, proved to be much smaller than originally thought, smaller than Earth's moon, leading some astronomers to call for its demotion. But it was Caltech astronomer Mike Brown's discovery of an ice-world slightly larger than Pluto, which he nicknamed "Xena", that brought the issue to a head—would Xena be classified as a planet, or a mere asteroid? And however Xena went, Pluto was likely to follow.

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