Pluto is a planet after all

CHICAGO – Astronomers, hold on to your telescopes.

The solar system has 12 planets, not nine.

That’s the earthshaking conclusion of an influential international committee, which on Wednesday will recommend a new definition of what qualifies as a planet.

The move is necessary, experts say, because of discoveries in the past decade that have revealed a glut of Pluto-sized bodies beyond the orbit of Pluto – until now considered the farthest planet from the sun.

Those findings sparked an intense debate among planet-watchers: Should the new worlds be welcomed as planets, or was it a mistake to call tiny Pluto a planet in the first place?

Now there’s an answer that just might satisfy Pluto-boosters and Pluto-phobes alike.

A seven-member panel of astronomers, historians and one science writer gathered in Paris last month under the auspices of the International Astronomical Union to settle the question. After a sleepless night, they agreed on a simple yet revolutionary approach to the problem.

A planet, they decreed, is any star-orbiting object so large that its own gravity pulls in its rough edges, producing a near-perfect sphere.

That definition excludes some 200,000 small, odd-shaped rocks, comets and asteroids that wander around the sun.

It also means Pluto gets to remain a planet.

But the new definition also includes three other big space rocks, including one currently considered an asteroid and another long described as a moon of Pluto.

Also to be included is an icy body beyond Pluto, which would belong to a class of planets to be known as “plutons.”

“In a day and a half of hammering it out, we came up with this unanimous recommendation,” said Owen Gingerich, chairman of the IAU’s “planet definition committee” and an emeritus historian of astronomy at Harvard University.

Because planet-seekers are finding new worlds beyond Pluto at a steady clip, the list of newly defined planets could grow well beyond 12 – perhaps dozens more worlds await.

Astronomers from around the world are scheduled to vote on the new definition on Aug. 24 at the IAU’s meeting in Prague. It would constitute the first official recognition of new planets since Pluto’s discovery in 1930.

Gingerich said Tuesday he has already received backing from 10 of the group’s division chairmen. Although there’s nothing binding about the upcoming vote, the IAU is considered the world’s authoritative source on the naming of heavenly bodies.

The proposed planet definition got an endorsement Tuesday from an unlikely source – Neil deGrasse Tyson, director of the Hayden Planetarium in New York, who has argued that Pluto is not in the same class as the other eight “classical” planets. He said although there were other good definitions that would have left Pluto out, he supports the new proposal because it offers the first clear standard of planethood.

“What a planet is has never been defined, not since 2,500 years ago in ancient Greece,” Tyson said. “Provided this definition is unambiguous, I’ll take it.”

Tyson said the requirement of roundness gives a tidy standard. Objects typically do not have enough mass for gravity to pull them into spheres unless they are at least 500 miles across.

“By and large, these things are either round or not round – they’re not sort of round,” Tyson said.

Some experts floated the idea that nothing smaller than Mercury should be called a planet. But that seemed arbitrary, and no better than keeping Pluto’s status solely out of sentiment.

Members of the IAU committee kept their deliberations and conclusion under close wraps over the past month. Richard Binzel, a committee member and asteroid specialist at the Massachusetts Institute of Technology, compared the decision to white smoke emerging from the Vatican upon selection of a new pope.

Binzel said that after a discouraging first day of deliberations, the members were surprised to find a definition that appealed to all of them.

“We want this definition to apply not only in our solar system, but in other solar systems as well,” Binzel said.

The three new planets encompassed by the group’s definition would be the asteroid Ceres, Pluto’s moon Charon and an object beyond Pluto called 2003 UB313, unofficially known as Xena.

For Ceres, it would be a belated promotion. By far the largest asteroid at 580 miles across, Ceres actually was called a planet when first discovered in 1801. But further findings of asteroids between the orbits of Mars and Jupiter led astronomers to class it with those smaller objects. A study last year of Hubble Space Telescope images proved that Ceres is round, placing it within the new definition.

Although Charon is called Pluto’s moon, it was included in the new planet definition because many astronomers believe the two worlds comprise a “double planet” system. Pluto and Charon orbit each other, and their common center of gravity lies outside of Pluto, unlike any other planet-moon system.

The existence of Xena was announced last year by Mike Brown, an astronomer at the California Institute of Technology, who also served on the IAU committee. Hubble Space Telescope images have shown the object is at least as big as Pluto. Brown and other experts believe there may be dozens of such worlds in an area called the Kuiper Belt.

That doesn’t mean schoolchildren need to ditch reliable old mnemonics for the planets’ names and order – such as My Very Earnest Mother Just Served Us Nine Pizzas. Gingerich said that although our definition of a planet may expand, the solar system still is dominated by the eight largest planets.

“I’d suggest [students] focus on the classical eight planets, plus this category of plutons,” Gingerich said.

Tyson believes the great planetary debate has distracted from teaching about the origins of the solar system.