Proposal would increase planets from 9 to 12
'Big Bang' expansion would keep Pluto in the mix
The International Astronomical Union is debating a plan to establish that our solar system has 12 planets. WATCH Browse/Search Will Pluto stay a planet? (5:11) RELATED FACT BOX THE NAME GAME
Many people say the sentence (called a mnemonic) "My Very Educated Mother Just Showed Us Nine Planets" as a way to remember the order of the planets.
Starting from the sun the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
If 2003 UB313 is dubbed a planet, what new sentence will we use to remember it?
Source: NASA QUICKVOTE Should Pluto keep its status as a planet? Yes No or View Results YOUR E-MAIL ALERTS Astronomy Space Programs or or Create Your Own
PRAGUE, Czech Republic (AP) -- Our solar system would have 12 planets instead of nine under a proposed "Big Bang" expansion by leading astronomers, changing what billions of schoolchildren are taught about their corner of the cosmos.
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 is not a planet.
"Yes, Virginia, Pluto is a planet," quipped Richard Binzel, a professor of planetary science at the Massachusetts Institute of Technology. (Watch an expert explain how a planet can just be a big, round thing -- 5:11)
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 should not 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 was 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. 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 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 is 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 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), about 70 miles (113 kilometers) 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 would not qualify because the two bodies' common center of gravity lies below the surface of the Earth.
"There are as many opinions about Pluto as there are astronomers," Binzel said. "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 planetarium 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."
Notre système solaire. Si les astronomes adoptent la nouvelle définition, les planètes de notre système solaire seront Mercure, Venus, la Terre, Mars, Ceres, Jupiter, Saturne, Uranus, Neptune, Pluton, Charon et 2003UB313.

L'union astronomique internationale (UAI) a proposé une nouvelle définition du mot planète qui pourrait voir augmenter le nombre de planètes de notre système solaire de neuf à douze. 

Le nouvelle définition proposée repose sur la gravité et non sur la taille de lobjet. La définition serait la suivante : Une planète est un objet céleste qui a une masse suffisante pour que sa propre gravité l'ait amené à un équilibre hydrostatique (quasi-sphéricité).

Cette nouvelle définition pourrait inclure Cérès, 2003 UB313 et Charon (lune actuelle de Pluton) dans la liste des planètes du système solaire. Une douzaine d'autres objet attendent l'évaluation de l'UAI dont (90377) Sedna, (50000) Quaoar, (2) Pallas, (4) Vesta, (28978) Ixion, (90482) Orcus, (10) Hygie, etc.