On Planets and Plutons

Since the discovery of the solar system body 2003 UB313 - nicknamed "Xena" - a debate has emerged among astronomers as to the definition of a planet. Some have argued that Pluto, much smaller and physically different from both the terrestrial planets (Mercury, Venus, Earth, and Mars) and the gas giants (Jupiter, Saturn, Uranus, and Neptune) should not be categorized as a planet; others have proposed to label both Pluto, 2003 UB313, and other potential large Kuiper Belt objects as planets. To put an end to this debate, astronomers at the the IAU General Assembly in Prague are currently debating the topic, and are hoping to reach a conclusion on August 24th. The current proposal is to have eight "classical" planets and three "plutons," bodies large enough to be spherical that orbit the Sun rather than another planet. The "plutons" - which would include Pluto, Pluto's moon Charon, 2003 UB 313, and the largest asteroid Ceres - would still qualify as planets. The full definition of a planet was four paragraphs long, with four footnotes. Very helpful. (Image credit: IAU)