New planet definition demotes Pluto to dwarf

This isn't your daddy's solar system anymore, says astronomy professor Constantine Deliyannis.\nAs of Thursday, the International Astronomical Union, the group responsible for naming celestial bodies, redefined what makes a planet, kicking Pluto out of the exclusive club.\nUnder the new definition, a planet is any body orbiting the sun that is spherical or nearly spherical because of its own gravitational forces and has "cleared the neighborhood around its orbit."\nPluto is a spherical body in orbit around the sun but fails to meet the last part of that definition.\n"It's a very sensible decision," Deliyannis said in an e-mail. "It's certainly true that given our ever-expanding knowledge of the solar system and its structure, and particularly the large number and variety of round objects that (we) now know it contains, it was time to revisit what we mean by 'planet' and decide upon a more precise definition."\nPluto was declared a planet soon after its discovery in 1930 by astronomer Clyde Tambaugh. \nAstronomers began to doubt its status years later as more and more Pluto-sized objects were discovered in an area beyond Pluto's orbit known as The Kuiper Belt and theorized that Pluto was not a planet but rather the largest of those objects.\nSeveral other definitions of a planet were presented at last week's IAU conference in Prague, Czech Republic.\nOne plan widely reported in the media would have let Pluto keep its planet status and immediately add three more planets to the solar system. Those new planets would have been Pluto's moon Charon; 2003 UB313 (nicknamed Xena by its discoverer), an object in orbit around the sun beyond Pluto; and the asteroid Ceres, located in orbit between Mars and Jupiter.\nMuch like Pluto, Ceres was considered a planet throughout the 1800s but was eventually demoted.\nIf the IAU had gone with that plan, another 12 objects would have been candidates for planethood. Some astronomers estimated that definition could have brought the total number of planets in the solar system up to 200.\n"There must have been substantial sentiment at the IAU that planets must somehow be special -- that we cannot allow every little round object ... to be promoted to the status of 'planet,'" Deliyannis said.\nThe new definition classifies Pluto, Ceres and "Xena" as "dwarf planets." Charon is now recognized as a "small solar system body," a classification that serves as a catchall category for numerous asteroids, comets and satellites, according to The Associated Press.\nCutting the solar system down to eight planets probably won't impact many people's daily lives, but it does mean that textbooks and courses from elementary school to college will have to be revised.\n"When I was in elementary school, I learned a mnemonic for the order of the planets at the McDonnel Planetarium in St. Louis," said professor of astronomy Phyllis Lugger in an e-mail. "It was: 'My Very Educated Mother Just Served Us Nine Pizzas.' Keeping with the theme of food that children enjoy, I suggest the following for the new mnemonic: 'My Very Educated Mother Just Served Us Nachos."\nLugger said she sent her idea to the chair of the planet definition committee, who thought it would work well for young astronomy students.\nDeliyannis also sees some exciting possibilities in classes at IU.\n"The specific new IAU definitions of planet and dwarf planet can lead to whole new sets of discussions and debates, involving students," he said. "For example, panels or teams could be set up within a class to debate the pros and cons of these new definitions."\nThe IAU's new definition isn't the final word on what makes a planet a planet, either. In the past 20 years, dozens of extra-solar planets have been discovered orbiting nearby stars. Most of these planets appear to be extremely large gas giants like Jupiter, but astronomers don't yet have the technology to tell if they are spherical or have cleared the neighborhood of their orbit.\nThese planets also appear to have highly eccentric orbits more similar to Pluto's than Earth's.\n"Today we know very little about extra-solar planets," Deliyannis said. "I am sure that eventually we will have definitions of objects orbiting other stars, and these definitions might perhaps be guided, at least in part, by the definitions of 'planet' and 'dwarf planet' adopted for our own solar system at the IAU meeting in Prague"