PRAGUE, Czech Republic – The universe really is expanding: Astronomers are proposing to rewrite the textbooks to say that our solar system has 12 planets rather than the nine memorized by generations of schoolchildren.
Much-maligned Pluto would remain a planet — and its largest moon, plus two other heavenly bodies would join Earth's neighborhood — under a draft resolution to be formally presented Wednesday to the International Astronomical Union, the arbiter of what is and isn't a planet.
"Yes, Virginia, Pluto is a planet," quipped Richard Binzel, a professor of planetary science at the Massachusetts Institute of Technology.
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The proposal could change, however: 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 shouldn't be a planet at all — the new lineup 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 got demoted.
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."
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.
The latter's discoverer, Michael Brown of the California Institute of Technology, nicknamed it Xena after the warrior princess of TV fame — and its moon Gabrielle after Xena's TV sidekick — but it likely would be rechristened something else later, the panel said.
The galactic shift would force publishers to update encyclopedias 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.
Even if the list of planets is officially lengthened when astronomers vote on Aug. 24, it's not likely to stay that way for long: The IAU has a "watchlist" 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 United States and host of PBS'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), which is about 70 miles wider.
But the IAU said Pluto meets its proposed new definition of a planet: any round object larger than 800 kilometers (nearly 500 miles) in diameter that orbits the sun and has a mass roughly one-12,000th that of Earth. Moons and asteroids will make the grade if they meet those basic tests.
Roundness is key, experts said, because it indicates an object has enough self-gravity to pull itself into a spherical shape. Yet Earth's moon wouldn't qualify because the two bodies' common center of gravity lies below the surface of the Earth.
[Pluto and its largest moon Charon, on the other hand, both orbit their common center of gravity, which lies between them, making them effectively co-planets.]
"People were probably wondering: If they take away Pluto, is Rhode Island next?" Binzel quipped. "There are as many opinions about Pluto as there are astronomers. But Pluto has gravity on its side. By the physics of our proposed definition, Pluto makes it by a long shot."
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 quarreling.
"We don't want an American version, a European version and a Japanese version" of what constitutes a planet, he said.
Neil deGrasse Tyson, director of the Hayden Planetarium at New York's American Museum of Natural History — miscast as a "Pluto-hater," he contends, merely because Pluto was excluded from a solar system exhibit — said the new guidelines would clear up the fuzzier aspects of the Milky Way.
"For the first time since ancient Greece, we have an unambiguous definition," he said. "Now, when an object is debated as a possible planet, the answer can be swift and clear."