What Makes a Planet?

As just about everyone in the solar system knows by now, members of the International Astronomical Union, meeting in Prague, Czech Republic, in August, came up with a new definition of planets that leaves Pluto out in the cold.

The problem was that the term "planet" had no firm definition, just as in geology, "continent" remains a somewhat arbitrary concept. (Europe and Asia form a continuous landmass, yet they are separate continents.) Size alone doesn't bestow planethood. Moons of Jupiter and Saturn are wider than Mercury, and Pluto is smaller than Earth's moon. For years, astronomers tried to define a planet by shape, orbit around a star and its influence on other bodies. But they couldn't agree.

In 2000, an exhibit at the American Museum of Natural History in New York City downgraded Pluto to merely the largest object in the Kuiper Belt, a vast swarm of icy bodies beyond Neptune. The resulting uproar over the demotion made it clear that Earthlings loved our familiar family of nine planets. Some astronomers noted that the controversy would really take off if more sensitive telescopes started to spot objects larger than Pluto. What would we call them?

That very quandary presented itself late last year when a team led by Michael Brown of the California Institute of Technology in Pasadena examined an object in the Kuiper Belt called 2003 UB313, informally known as Xena. Analysis revealed that Xena—more than twice as far from the sun as Pluto—is about 1,500 miles in diameter, compared with Pluto's 1,430 miles. And solar system experts predict that surveys will reveal Xena's even bigger siblings in distant orbits. "It's absurd that dozens of Pluto-size objects should all be planets," said planet-hunter Geoff Marcy of the University of California at Berkeley. "It demotes them all by sheer numbers."

Many astronomers argued that a planet should have sufficient gravity to sweep up or expel the debris in its orbit, clearing its own path around its star. The eight main planets have largely done so, but not Pluto or Xena. Ultimately, that was the key factor in Prague. "It is scientifically the right thing to do," Brown said after the vote, gracefully giving up the right to call Xena—his team’s discovery—the tenth planet.

Still, some researchers think the orbit-clearing rule is too fuzzy. (For example, Neptune hasn't purged its orbit of large bodies—including Pluto, which crosses Neptune's path.) "It's a sloppy definition," says Alan Stern of the Southwest Research Institute in Boulder, Colorado, director of NASA's New Horizons mission. "It's bad science. It ain't over." He and others intend to fight for a new definition; they're organizing a conference to that end for this winter.

Pluto will always be special. We'll see it up close when the New Horizons spacecraft, launched in January, flies past it in 2015. But Pluto is almost certainly one of scores or even hundreds of impressively sized icy bodies in the Kuiper Belt. So, like Xena and Ceres, the largest asteroid in a band between Mars and Jupiter, it is now classified as a "dwarf planet." It’s cruel logic, perhaps, but it makes sense.