PLUTO A Planet Again -- On Friday The 13th, In Illinois

"Crazy" Process

The Illinois resolution comes more than two years after the International Astronomical Union (IAU) "demoted" Pluto, reclassifying it as a dwarf planet and reducing the solar system's official planetary count to eight.

The organization's main role is to name objects and features in space. It was the struggle over who got to name Eris—the committee in charge of asteroids or the one that names planets—that led to reassessment of Pluto's status.

In August 2006 the IAU announced that from then on a body can only be called a planet if it orbits the sun, is large enough to have become round due to the force of its own gravity, and has swept its orbital neighborhood clean of large objects.

Pluto was demoted because it violates the last criterion: Charon, one of its moons, is about half the size of Pluto.

According to Stern, the ruling was unrepresentative because only a handful of IAU members were present to vote. What's more, he said, the IAU's scientific purview is mostly astrophysics, not planetary science.

"It's as if a bunch of thoracic [chest cavity] surgeons declared brain cancer not a cancer—it's not their field of expertise," he said.

Brown counters that the IAU ruling was sound, and that only a handful of scientists continue to lobby for Pluto's reinstatement. Most astronomers, he said, have moved on.

"There are all these arguments made about the process, which was crazy!" Brown said. "But I think even if we did it over, we would still come to the same conclusions."

"It really is quite intuitively obvious when you are presented with the data."

What Stern and Brown do agree on is that the IAU's current definition of a planet only works for bodies orbiting our sun.

When it comes to discoveries made in recent years about planetary bodies outside our solar system, "we just keep being amazed at the variety of things we never expected," Stern said.

"What we finally end up calling planets will take some time to sort out," he added. "We have to finish looking around."