spacetoday.net: IAU proposal would increase planet count to 12

Posted: Wed, Aug 16, 2006, 7:46 AM ET (1146 GMT)
12 planet solar system illustration (IAU)

A proposal announced Wednesday by the International Astronomical Union (IAU) regarding the definition of the term "planet" would retain Pluto as a planet while including three other worlds. The IAU proposal, to be voted upon by astronomers at an IAU meeting this month in Prague, defines planet as an object massive enough to be roughly spherical, and also not be a star or in orbit around another planet. The IAU proposal also draws a distinction between "classical" planets and "plutons", planets with orbital periods in excess of 200 years and typically with high orbital eccentricities and/or inclinations. Under the IAU proposal there would be three plutons: Pluto, its moon Charon (massive enough to make Pluto-Charon a double-planet system), and 2003 UB313, informally known as "Xena". In addition, Ceres, the largest asteroid, would also be classified as a planet. A number of other large objects recently discovered in the outer solar system may also qualify as planets once more information about their sizes is known.