By next month our Solar System is expected to officially contain twelve planets, meaning astronomy texts and astrological charts will have to be republished. The new lineup in order from the sun will now be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto/Charon and 2003 UB313 (nicknamed Xena).
The International Astronomical Union has long been accepted globally as the authorized arbitrator of all things planetary since 1919. The IAU’s Planet Definition Committee (PDC) seven-expert committee is made up of various experts ranging from historians to astronomers. They assembled in Paris this June and July to hear and settle on proposals that were then sent to the International Astronomical Union’s general assembly in Prague convening August 14 through August 25.
The meeting was called as a result of a long-standing fight over whether Pluto is actually a planet or not. The Hubble Space Telescope and other ground based observers have found objects in our solar system that are larger than our most distant orbital cousin. The discussion came to a two-year head just recently, with the discovery of UB313 (nicknamed Xena for now) which will be officially named later. The new planet is beyond Pluto and is larger and has a single moon.
The question was whether to declare UB313 the tenth planet in our solar system or downgrade Pluto. Pluto was originally given planet status in 1930 when it was thought to be as large as Earth. With Pluto’s moon Charon being similar in size to its neighbor, Pluto/Charon is now expected to be classified as a “double planet” because they’ve been found to more or less revolve around each other.
IAU Resolution 5 for GA-XXVI: "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."
The IAU also drafted a resolution to define a new subcategory for planets called a "Pluton". Plutons must take more than 200 years to complete an orbit or must reside beyond Neptune. This would include Pluto, Charon and “Xena”. The new system could cause havoc with astronomers as local celestial bodies are being constantly discovered beyond Pluto’s orbit that will meet the criteria.