FLAGSTAFF, Ariz. (AP) — It’s been 75 years since the discovery of the planet Pluto, but it remains a mystery. Perhaps in 10 more years, when a space probe gets close enough for a good look, some of its secrets will be revealed.
Pluto was 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 — planet — 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.
Mr. 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 ninth planet that had been thought to exist since the 19th century, based on observations of the orbit of the eighth planet, Neptune.
The 26-year-old Mr. 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 thought right away that the recurring speck he saw was the elusive “Planet X,” later named Pluto.
Generations of schoolchildren grew up memorizing solar system charts that included Pluto. But shortly after Mr. Tombaugh died in 1997, some astronomers suggested that the International Astronomical Union 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 might be as many as 100,000 of these bodies in what’s called the Kuiper Belt, said Bob Millis, director of the 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 that 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.
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 generally spherical, like other planets, and has an atmosphere and seasons. Asteroids and comets tend to be misshapen, said Mr. Weaver, a senior scientist at Johns Hopkins University’s Applied Physics Laboratory.
Mr. Tyson’s decision at the Hayden Planetarium to remove Pluto from the planets and lump it in with the Kuiper Belt seemed to strike a nerve. He 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,” Mr. Tyson joked.
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