My Very Excellent Mail Carrier Just Served Us Nine Pina Coladas. Ugh!
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Posted to SciTech on Fri Aug 18, 2006 at 03:53:26 PM EST. RSS.
Two years ago, the International Astronomical Union met to debate whether "small" objects in the solar system such as Pluto should be counted as planets. On August 16, they released a draft definition of a "planet" that not only counts Pluto, but raises the number of planets in our Solar System to at least twelve.
The definition, hammered out over two years of arguments and compromises, is
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.
So that rules out small asteroids, comets, Nemesis, the Halo, and all the big round objects orbiting the gas giants. But it admits three new objects as planets:
- The 922km-diameter object Ceres, the largest object in the asteroid belt. This would be a promotion of Ceres back to the status of planet, which it lost in the mid-19th century, a few decades after its discovery.
- Charon, currently considered a moon of Pluto. Because the objects rotate around a common point in space that does not reside beneath the surface of Pluto, it is not considered a moon.
- Kuiper belt object 2003 UB313, nicknamed "Xena".
In addition to defining the word "planet", the IAU also came up with the word "pluton" to describe any object that took more than 200 years to complete an orbit around the sun (i.e. it resides beyond the orbit of Neptune most of the time).
The response from the general astronomy community has been mixed, with many astronomers set against the idea. None of the plutons have an orbit in the same general plane as the Big Eight. In fact, 2003 UB313's orbit takes it between 38AU and 97AU of the sun, and is skewed 44 degrees from the general plane. Also, there is little reason to stop at 12 planets when there are dozens more objects waiting in the wings to join the growing family. Mike Brown, who was part of the team who discovered 2003 UB313, has counted 53 objects that may fit the proposed definition.
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