Solar system count now set to swell to 12 planets

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THE CELESTIAL ARRAY: An illustration of the largest known Kuiper Belt objects, image provided by NASA's Hubble Space Telescope. Photo: AP

They will be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and, provisionally, `2003 UB313'

PRAGUE: Our solar system will have 12 planets instead of nine, under a proposed "Big Bang" expansion by leading astronomers. This will change what billions of schoolchildren are taught about their corner of the cosmos.

Much-maligned Pluto will remain a planet and its largest moon plus two other heavenly bodies would join the earth's neighbourhood under a draft resolution to be formally presented to the International Astronomical Union, the arbiter of what is and is not a planet.

"Yes, Pluto is a planet," quipped Richard Binzel, a Professor of planetary science at the Massachusetts Institute of Technology.

Subject to vote

The proposal could change, however: Prof. Binzel and the other nearly 2,500 astronomers from 75 nations meeting in Prague to hammer out a universal definition of a planet will hold two brainstorming sessions before they vote on the resolution next week. But the draft comes from the IAU's executive committee, which only submits recommendations likely to get two-thirds approval from the group.

Besides reaffirming the status of puny Pluto whose detractors insist it should not be a planet at all the new line-up would include 2003 UB313, the farthest-known object in the solar system and nicknamed Xena; Pluto's largest moon, Charon; and the asteroid Ceres, which was a planet in the 1800s before it was demoted.

New category

The panel also proposed a new category of planets called "plutons," referring to Pluto-like objects that reside in the Kuiper Belt, a mysterious, disc-shaped zone beyond Neptune containing thousands of comets and planetary objects. Pluto itself and two of the potential newcomers Charon and 2003 UB313 would be plutons.

Astronomers also were being asked to get rid of the term "minor planets," which long has been used to collectively describe asteroids, comets and other non-planetary objects. Instead, those would become collectively known as "small solar system bodies."

New order

If the resolution is approved, the 12 planets in our solar system listed in order of their proximity to the sun would be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, and the provisionally named 2003 UB313. Its discoverer, Michael Brown of the California Institute of Technology, nicknamed it Xena after the warrior princess of TV fame, but it likely would be rechristened something else later, the panel said.

The galactic shift would force publishers to update encyclopaedias and school textbooks, and elementary school teachers to rejigger the planet mobiles hanging from classroom ceilings. Far outside the realm of science, astrologers accustomed to making predictions based on the classic nine might have to tweak their formulas.

Watch list

Even if the list s is officially lengthened when astronomers vote on Aug. 24, it is not likely to stay that way for long: The IAU has a "watch list" of at least a dozen other potential candidates that could become planets once more is known about their sizes and orbits.

"The solar system is a middle-aged star, and like all middle-aged things, its waistline is expanding," said Jack Horkheimer, director of the Miami Space Transit Planetarium in the U.S. and host of Public Broadcasting's Stargazer television show.

Opponents of Pluto, which was named a planet in 1930, still might spoil for a fight. Earth's moon is larger; so is 2003 UB313 (Xena), 113 km wider.

But the IAU said Pluto meets its proposed new definition of a planet: any round object larger than 800 km in diameter that orbits the sun and has a mass roughly one-12,000th that of the earth. Moons and asteroids will make the grade if they meet those basic tests.

Shape is key

Roundness is key, experts said, because it indicates an object has enough self-gravity to pull itself into a spherical shape. Yet the earth's moon would not qualify because the two bodies' common centre of gravity lies below the surface of the earth.

"There are as many opinions about Pluto as there are astronomers," Prof. Binzel said. "But Pluto has gravity on its side. By the physics of our proposed definition, Pluto makes it by a long shot."

A cosmic consensus

IAU president Ronald D. Ekers said the draft definition, two years in the making, was an attempt to reach a cosmic consensus and end decades of quarrelling. "We don't want an American version, a European version and a Japanese version" of what constitutes a planet, he said. AP



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