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Pluto's Out of the Planet Club.
Last year we had 9 planets. Recently we were informed it would grow to 12. Now we've only got 8. The International Astronomical Union, currently meeting in Prague, voted on August 24, 2006 to demote Pluto down from planethood status. Now Pluto, Charon, Ceres and the newly discovered 2003 UB313 (aka Xena) will merely be known as “dwarf planets”. Under the new definition, planets must orbit a star, be spherical in shape, and clear out their neighbourhood of orbital debris. Pluto has failed to fulfill the third requirement, so it's out of the planet club.
It is official: The 26th General Assembly for the International Astronomical Union was an astounding success! More than 2500 Astronomers participated in six Symposia, 17 Joint Discussions, seven Special Sessions and four Special Sessions. New science results were vigorously discussed, new international collaborations were initiated, plans for future facilities put forward and much more.
In addition to all the exciting astronomy discussed at the General Assembly, six IAU Resolutions were also passed at the Closing Ceremony of the General Assembly:
1. Resolution 1 for GA-XXVI : “Precession theory and Definition of the Ecliptic”
The IAU members gathered at the 2006 General Assembly agreed that a “planet” is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
This means that the solar system consists of eight “planets” Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called “dwarf planets” was also decided. It was agreed that “planets” and “dwarf planets” are two distinct classes of objects. The first members of the “dwarf planet” category are Ceres, Pluto and 2003 UB313 (temporary name). More “dwarf planets” are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate “dwarf planets” are listed on IAU's “dwarf planet” watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.
The “dwarf planet” Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects.
Below are the planet definition Resolutions that were passed.
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