Pluto Learns Eight Is Enough for Planets

Like the Edsel, the Flying Wing, and the Austro-Hungarian Empire, the ninth planet became a relic of history Thursday when international astronomers meeting in the Czech Republic decided Pluto was too small to remain a full member of the planetary club.

Members of the International Astronomical Union overwhelmingly voted to demote Pluto to a “dwarf planet.” Though still retaining the term planet, it was clear that Pluto had been exiled.

“Pluto’s out,” said Michael E. Brown, the Caltech astronomer whose discovery last year of a planet-like object called UB313 reignited the long-running debate over whether Pluto should be considered a planet. “People are going to be unhappy, but it’s the right thing to do. This is a great moment in science.”

“I’m just glad they decided something,” said Marc Buie, an astronomer at the Lowell Observatory in Flagstaff, Ariz., where Pluto was discovered in 1930.

The vote reverses a recommendation made last week by a committee of the IAU, the sole authority for classifying and naming astronomical bodies.

The earlier proposal said that to be a planet, a body need only be round and orbit the sun.

Under that definition, there would have been dozens of new planets added to the solar system, something the astronomers gathered in Prague refused to accept.

Instead, the astronomers adopted a definition that also requires a planet to “clear its area,” meaning that it dominates in its neighborhood of the solar system and prevents any other similar-sized objects from forming. That eliminated Pluto, one of thousands of objects in the Kuiper Belt, 3.6 billion miles from the sun.

“Poor little Pluto,” said Patricia Tombaugh, the 93-year-old widow of the man who discovered Pluto, Clyde Tombaugh. “Kids are going to be upset.”

Tombaugh, who lives in Las Cruces, N.M., said children love Pluto because “it’s little like they are.”

Children interviewed at the California Science Center in Exposition Park were disappointed. “It’s an awesome planet,” said Jaykb Olivas, 6. “Since Pluto’s the smallest planet, we could visit it and be like a giant.”

Some parents who had grown up memorizing the names of the planets, using mnemonics like “My Very Excellent Mother Just Served Us Nine Pizzas,” seemed more distressed.

“I feel like something’s missing,” said Micaela Chambers, 39, of Granada Hills as she played with her son outside the science center.

Tombaugh said she too was disappointed. “But they had to get that mess straightened out. There’s too many planets.”

Though Pluto’s taxonomy had always been a point of contention among scientists, it was Brown’s discovery last year of UB313, which he nicknamed Xena, that forced the IAU’s hand. That’s because Xena was at least as large as Pluto, and possibly larger. If Pluto was a planet, how could Xena be denied?

“It’s all my fault,” Brown said at a news conference at Caltech on Thursday. “I may go down in history as the guy who killed Pluto.”

As unpopular as the vote may be, it was the only reasonable scientific choice, he said. “There are eight really large objects in the solar system,” Brown said. “They are special.”

The eight planets that meet the new definition are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Under the rejected definition, Brown estimated the solar system could end up with 53 planets or more, many of them smaller than Earth’s moon.

The elasticity of that definition caused a revolt among some of the 3,000 members of the IAU.

Some of the astronomers tried to soften the blow to Pluto fans, urging that the new category of dwarf planets have a sub-category called Tombaugh Objects, or Plutonian Objects in the Kuiper Belt. The astronomical body didn’t approve that recommendation.

In the end, only about 10% of the membership voted on the new definition.

While Brown and the IAU voting members in Prague were convinced that cashiering Pluto was the right choice, the feeling was not unanimous among planetary scientists in the U.S.

“I think the IAU vote is a muddled compromise that will not settle the question of, what is a planet?” said Andrew Chang, a researcher at Johns Hopkins University’s Applied Physics Laboratory.

This is not the first time a planet has been demoted. When Ceres, the largest asteroid, was discovered in 1801, it was listed as a planet. It was banished when scientists began finding other asteroid belt rocks. With this new definition, Ceres has been rejected twice.

Clyde Tombaugh was an amateur astronomer hired on at the Lowell Observatory as a lowly assistant when he spotted a new object on photographic plates in 1930.

The observatory has dedicated an exhibit to Tombaugh that is visited by 70,000 tourists every year.

Buie said the debate over Pluto’s classification is less important to scientists than to the public.

Planet or dwarf planet, “They’re just descriptions,” Buie said. Lowell officials declined to comment on whether they will alter the Tombaugh-Pluto exhibit based on this new definition.

Textbook and encyclopedia publishers must now remap the solar system.

Paul Kobasa, editor-in-chief of World Book encyclopedia in Chicago, said the press run for the 2007 edition was shuffled because of the pending IAU decision.

Kobasa said this shouldn’t crush schoolchildren who have developed an affection for the solar system outcast. “The thing to remember is there is still an object out there in space that still has the name Pluto,” he said.

Expecting that something like this might happen to Pluto, officials at Griffith Observatory have titled their new planetary exhibit “Pluto and beyond,” said director Edwin C. Krupp.

“Pluto, whether it’s a planet or not, is always going to be welcome at Griffith Observatory,” he said.

In fact, Pluto has proved over the seven decades since its discovery to be one of the most reliably popular objects in the solar system among the public.

Much of the story of Pluto has become folklore, including the unusual way it got its name. Venetia Burney, an 11-year-old from England, suggested the name of the Roman god of the underworld because she was interested in mythology.

It was a perfect fit for a planet shrouded in darkness.

Now in her late 80s, Venetia Phair said when reached by phone at her home in England that she had been getting a number of calls about the IAU decision. After more than seven decades of debate over the planet, she said she couldn’t comment on it. “I’m too old,” she said.