Pluto a Planet Again -- On Friday the 13th, in Illinois

Victoria Jaggard
National Geographic News
March 11, 2009
It took about three minutes for members of the Illinois state senate to make the unanimous vote: "that March 13, 2009, be declared 'Pluto Day' in the State of Illinois in honor of the date its discovery was announced in 1930."

Quietly adopted on February 26, the state resolution is meant to honor Pluto discoverer Clyde Tombaugh, who was born and raised in the farming village of Streator.

"This is one of those things that the village is very proud of," said Illinois State Senator Gary Dahl, who sponsored the resolution.

"I don't think we are changing the status of the planet. We're simply asking that March 13 be declared Pluto Day and that, for the day, Pluto is a planet."

National Pride

Despite these seemingly humble intentions, the bill has reignited heated debate over what exactly a planet is.

"Go, Illinois!" said planetary scientist Alan Stern, principle investigator for NASA's New Horizons mission to Pluto.

"I think it's wonderful, in the sense that, as an American, I'm proud that Clyde Tombaugh made one of the biggest finds in 20th-century astronomy."

Mike Brown, an astronomer at the California Institute of Technology, agreed that Tombaugh deserves accolades.

"I am amazed at what he did," Brown said.

Even with advanced computers and larger telescopes, "it took 77 years until we found something bigger than he did"—the distant object now called Eris, which Brown discovered in 2005.

Still, Brown calls the Illinois resolution "very silly," noting that such legislation can be dangerous to public understanding of science.

"The impression that it gives is that there's still a vigorous scientific debate going on, and there's just not."

NASA's Stern counters that plenty of scientists still want Pluto classified as a planet.

"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."

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