In an historic vote, the International Astronomical Union has finally agreed upon the definition of a “Planet”.
The International Astronomical Union (IAU), the internationally recognized organisation for naming and defining all things celestial, has closed their triennial General Assembly conference in Prague by deciding that there are only eight planets currently present in our solar system. Pluto has been demoted to “dwarf planet” status – albeit a new family of objects within its own right. The eight planets are (in order from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The first “dwarf planets” to make the grade, consist of Ceres, Pluto, and 2003 UB313 (commonly referred to as “Xena”), with more to be added in the near future. The concluding vote took place on Thursday, August 24th, 2006.
It’s not all bad news for Pluto though, as it becomes the prototype for a yet-to-be named new class of objects that exist in the trans-Neptunian region. The IAU intends to set up a dedicated process for naming these bodies in the near future, which typically consists of large bodies within the Kuiper-belt region. Having visited all eight planets in the solar system, NASA's "New Horizons" spacecraft is currently enroute to Pluto and is expected to reach the dwarf planet in July of 2015. In June of 2007, NASA plans to launch off on the "Dawn" mission to fellow dwarf planet, Ceres. Though the chance to send your name to Pluto has passed, the opportunity is now available to be part of the Dawn mission by doing the same.
The passed IAU resolution reads:
RESOLUTION 5A
The IAU therefore resolves that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A "planet"1 is 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.
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Article comments
1 - Victor Plenty
Are you really saying the New Horizons spacecraft has itself visited all 8 classical planets, or is that just an accident of sentence structure?
Leaving that minor quibble aside, thanks for the excellent explanation of the issues contributing to the IAU's decision.
Now it'll be interesting to see whether the new definition will stick, both among the astronomers and in the larger cultural context.
2 - PoizonMyst
Oops .... accident of sentence structure ... sorry about that! The craft will encounter Jupiter for gravity assist in 2007 and will pass closer to the red giant than the Cassini mission, but I don't believe it is scheduled to meet with any other planets before it's arrival at Pluto.
3 - RECEYA
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