Constellation Consternation

What’s in a name? If your name is “Pluto,” you’re in trouble.

• Disney’s Pluto is an anomalous animal character that doesn’t get to talk or wear clothes.
• Roman mythology’s Pluto was the god of the underworld, and his closest associate, Charon, was the boatman over the River Acheron to the underworld.
• Pluto the planet never knew its place, either. It’s been called the planet farthest from the sun — except during those years in which its elliptical orbit brings it inside Neptune’s orbit. So it’s not necessarily the farthest.

And after Aug. 24, 2006, it might no longer be a planet, either.

From Aug. 15 to 25, a constellation of about 2,500 astronomers has gathered in Prague, Czech Republic, for the tri-annual assembly of the International Astronomical Union (IAU). The IAU is the world’s largest international astronomical organization that brings together distinguished astronomers from all nations of the world. IAU’s stated mission is “to promote and safeguard the science of astronomy in all its aspects through international cooperation.” The IAU General Assembly is held every three years and is one of the largest and most diverse meetings in the astronomical community's calendar.

The IAU’s Planet Definition Committee (chaired by Owen Gingerich, a Harvard astronomer) will be announcing at the end of this year’s conference a new definition for “planet.” (Want to read the draft definition?)

The astronomers will vote at the conference on the definition on Aug. 24. The result could increase the number of planets in the solar system to 12 (or more) from 9. Or it could reduce the number to 8, much to Pluto’s (and Charon’s) chagrin.

Mike Sitko, professor of physics at the University of Cincinnati, says that size is a problem.

“Pluto is smaller in diameter and mass than our own Moon and the moons of many of the giant planets,” he says. “So if Pluto is a planet, why not Ganymede, Titan, etc.? A number of astronomers believe that if Pluto had been discovered in 1995, it would NEVER have been classified as a planet. It certainly is no Earth or even a Mercury! But since it was called one, and it has been called one (perhaps incorrectly) for decades, and even schoolchildren ‘know’ it is a planet, what to do now?”

Sitko says that the problem is what to do about all those objects being discovered in the Kuiper Belt, beyond the orbit of Neptune.

“The first one found was Pluto, because of its combination of size and ‘relative’ closeness, in 1930. The next Kuiper Belt Object (KBO, but sometimes called Trans-Neptunian Objects or TNOs) had to wait until 1995. Now there are hundreds known.”

In the last decade of the 20th century, in the IAU’s 80th year, people began to question Pluto’s credentials.

“In 1999, it was formally suggested to ‘demote’ Pluto from the planetary list,” says Sitko, “and even before that there were many arguments as to what a planet was, and whether Pluto qualified as one. These included size, location and roundness."

Then there’s the name. Traditionally, planets were named after the Roman names of the Olympian gods. When the asteroids (also called “minor planets”) were discovered, the naming convention continued.

“However, astronomers soon ran out of gods — there are over 10,000 asteroids known today,” says Sitko, “and so they began naming them after spouses, children, boy/girlfriends, pets, computers, and even TV characters. There is an asteroid ‘Mr. Spock,’ for example.”

We can handle “Spock,” but “Quaoar?”

“In ‘fairness’ to other cultures, large KBOs were named for other deities, such as Inuit and others. Hence we have Varuna and the almost unpronounceable Quaoar,” Sitko explains. “Now we come to the KBO 2003 UB 313. It is larger than Pluto. So if Pluto is a planet, so is 2003 UB 313. So the leader of the team that discovered it (Mike Brown) needed an unofficial ‘code’ name that began with an X, for Xth planet. Hence ‘Xena.’ So now we have a situation where one of the traditionally Olympian-named planets might (if the IAU goes along with it), get the name of a modern fictional character!”

IAU President Ronald D. Ekers was recently quoted as saying, “People have to be able to agree on a terminology that’s used to describe things in the universe. We don't want an American version, a European version and a Japanese version.” During the conference, scientists will be trying to agree on an internationally accepted definition of what constitutes a planet, and to decide whether Pluto should keep its status. Scientists were also expected to consider whether to bring a new planet, nicknamed “Xena,” into the fold, possibly along with dozens more.

Besides naming other heavenly bodies as planets, another option is to demote Pluto and put it in a new classification, along with other similarly sized objects. Sitko says that there are a few proposals floating around.

“Well, one can use another name for these small objects,” he says. “‘Plutons’ is one. ‘Ice Dwarf Planets’ (one I have used) is another. But that would probably remove Pluto from the list of ‘major planets’ in school books. The alternative is to allow Pluto, Xena and a bunch of other objects in the list of ‘planets.’”

Sitko notes that that human definitions are often arbitrary.

“When does a knife become long enough to be called a sword? Or a rock large enough to be a boulder? And who decides?”

So what’s all the fuss about? When Gustav Holst published his stellar suite, “The Planets,” in 1915, he created themes for the seven planets that were known at the time, besides this one. With the passage of time comes new discovery. The world's not flat. Chemists discover new elements. Astronomers learn new planet names. And so the world turns.

...Or does it?