Pluto Gets New Name, as Does "Xena"

Kimberly Johnson
for National Geographic News
September 15, 2006
Pluto is now just a number, officially speaking.

The former planet has been dubbed asteroid number 134340 to reflect its new status as a "dwarf planet."

Pluto was ousted from the official pantheon of planets late last month when the International Astronomical Union, the governing authority over celestial bodies, finalized the first official definition of "planet" at a meeting in Prague in the Czech Republic.

According to the IAU, a planet is an object that orbits the sun, forms itself into a sphere, and has enough gravitational pull to clear its path of space debris.

The move downgraded Pluto and finalized a long-running debate about Pluto's planetary status. Unlike the eight planets once considered its peers, Pluto does not sweep its neighborhood clear and has an orbit that is much more eccentric.

Following the revised conventions, the IAU's Minor Planet Center, which tracks asteroids and comets, gave Pluto the new designation on Wednesday to add it to the standard catalog of numbered objects with well-determined orbits.

But that doesn't mean people will start referring to Pluto differently.

"There is no intention of changing the name," IAU President Catherine Cesarsky said. "The number assigned is just for tables."

Also on Wednesday, the IAU gave an official name to 2003 UB313, a distant icy rock larger than Pluto.

In a tongue-in-cheek choice, 2003 UB313 will now be called Eris, after the Greek goddess of discord and strife. Eris' discovery last year reignited the debate over the definition of "planet" that eventually led to Pluto's ouster.

2003 UB313 had previously been nicknamed Xena by its discoverer—astronomer Michael Brown of the California Institute of Technology in Pasadena—after the title character in the television show Xena: Warrior Princess.

"Now Pluto is not alone anymore. It has Eris," Cesarsky said.

Changing Times

"Astronomers have known for a very long time that Pluto was different," Cesarky added.

So why did Pluto gain planetary status in the first place?

It was discovered in the 1930s "when telescopes weren't very good," explained Don Shapero, director of the Board on Physics and Astronomy at the National Academy of Sciences in Washington, D.C.

Since then, however, telescope quality has developed at a startling pace.

"In this case what ultimately precipitated the change was a better understanding," he said. "As we learn more and understand more, we change how we categorize things, how we name things."

Scientists had been chipping away at Pluto's planetary status for a long time.

In 1978 U.S. Naval Observatory astronomers discovered Pluto's large moon Charon while studying photographic plates of Pluto's position in the sky. The pair form an unusual "double system," since Charon is so large that Pluto and Charon orbit each other, instead of Charon orbiting Pluto.

The scientists also determined that Pluto and Charon's masses were much smaller than previously thought.

In the 1990s astronomers armed with powerful telescopes began finding other objects in the solar system with similar orbits to that of Pluto. This led them to give the area beyond Neptune's orbit its own name: the Kuiper belt, after 20th-century astronomer Gerard Kuiper.

Astronomers say nearly a thousand Kuiper belt objects have been found, several larger than Pluto.

"It appears that Pluto is more a prototype of the Kuiper belt objects," said Geoff Chester, an astronomer and spokesman for the Washington D.C.-based Naval Observatory. (See an interactive model of the solar system.)

Science in Action

The change in Pluto's status also presents a snapshot of the scientific process in action.

"Science should be correctable," said Cornell University astronomer Joe Burns, an expert on small planetary bodies and irregular satellites who attended the IAU vote in Prague.

"If put in the proper light, this really is a shining hour for science."

With so many objects in the solar system with an established orbit around the sun, a clear definition was needed, Chester says.

Otherwise, "How do you distinguish planets from stuff?" he said.

Pluto's new classification could be jarring for schoolkids who have learned otherwise, Shapero, of the National Academy of Sciences, says.

"It's a shocking introduction to the fact that science evolves."

But Pluto's reclassification has also generated interest in astronomy and "catches kids' imagination," added Cecelia Blalock, vice president of the Washington, D.C.-based Young Astronaut Council, a group that promotes science and math education in schools.

"The whole concept of space is changing, and there's a greater appreciation of how much more vast it is than just our solar system," she said.

"These are the concepts that really need to be instilled in children."

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