Pluto: Planet Or Junk Snowball?

It's been 75 years since the discovery of Pluto, but it remains a mystery. Perhaps in another 10 years some of its secrets will be revealed when a space probe gets close enough for a good look.

Pluto was quickly heralded as the ninth planet in the solar system when it was spotted Feb. 18, 1930 by Clyde Tombaugh, a young amateur astronomer at Lowell Observatory. It still holds that title today, if somewhat tenuously.

"It's a misbehaved planet if you want to think about it as a planet," said Neil deGrasse Tyson, director of New York's Hayden Planetarium at the American Museum of Natural History.

Tyson provocatively removed Pluto from his exhibit of planets five years ago, lumping it instead with a belt of comets at the edge of the solar system.

"I still have folders of hate mail from third-graders," he said.

Pluto was discovered in a search for a theoretical ninth planet. The 26-year-old Tombaugh was given the assignment. Had he not been so attentive, he might have missed Pluto as he stared through an eyepiece while switching back and forth between photographic images of the night sky over northern Arizona. But he believed right away the recurring speck he saw was the elusive Planet X later called Pluto.

Generations of schoolchildren grew up memorizing solar system charts that included Pluto. But shortly after Tombaugh died in 1997, some astronomers suggested that the International Astronomical Union, a professional astronomers group, should demote the tiniest planet.

At the time it was discovered, Pluto was the only known object beyond Neptune in the solar system. When its moon, Charon, was spotted, that seemingly confirmed Pluto's planet status.

But astronomers also have found about 1,000 other small icy objects beyond Neptune rotating around the sun. There may be as many as 100,000 of these bodies in what's called the Kuiper Belt, said Bob Millis, director of Lowell Observatory.

Pluto, with its elongated orbit and odd orbital plane, seems to behave more like other Kuiper Belt objects than other planets, some astronomers say. They also point out Pluto is very small, smaller than Earth's moon.

"You start to see where Pluto fits in better with Kuiper objects," said Hal Weaver, project scientist on the New Horizons mission, which hopes to launch a probe to Pluto next year, possibly reaching it as early as 2015.

Tyson's decision at the Hayden Planetarium to remove Pluto from the planets and lump it with the Kuiper Belt seemed to strike a nerve. Tyson speculates that the name — the same as the Disney character which also debuted in 1930 — and its position as the littlest planet make it a favorite with schoolchildren.

"The Plutocracy, as I like to call it, is greater than we want to admit to ourselves," Tyson joked.

But others have pointed out that Pluto remains unique among known objects.

"If you don't call it (a planet), what else do you call it?" asked Kevin Schindler, senior supervisor of public programs at Lowell.

Pluto is very spherical like other planets. Asteroids and comets tend to be misshapen, said Weaver, a senior scientist at Johns Hopkins University's Applied Physics Laboratory.

Pluto also has an atmosphere and seasons.

Complicating the debate is that there is no official definition for a planet. Setting standards like size limits or orbital patterns potentially invites other objects to take the "planet" label, while throwing Pluto out.

"It's a controversy that flares up and then subsides. I don't know what the outcome will be," said Millis. "It's more interesting to me: What is Pluto really like, than what is Pluto?"

Astronomers hope to get some answers to that question, and the birth of the solar system, with the New Horizons mission. But it will take nearly a decade to reach the icy rock.

In any event, the debate over what to call Pluto is mostly a question of semantics, Weaver said. And even if it somehow loses planet status, he believes it might still be the largest of the Kuiper Belt objects.

"Some people thought (the debate) was a slight of Clyde Tombaugh. You were trying to take away his planet," Weaver said. "I'm sure that it was nothing personal."

By Michelle Roberts