Friday, August 25, 2006

Pluto Demoted

Yesterday, Hugh Hewitt and the Great Bleatist were bemoaning the reports that Pluto is no longer considered a planet. What actually happened was demotion of Pluto from full planet status to "dwarf planet." Hey, guys, Pluto is still there, but it really doesn't fit with the other 8 planets. Sorry.

Update: The BBC reports that this vote has stirred up a ferocious backlash. Talk about taking yourself too seriously.

The new definition of "planet" adopted by the International Astronomical Union is based on the discovery that Pluto so small that treating it as a full fledged planet would require that other newly discovered bodies orbiting the Sun be granted planet status. There have been hundreds of thousands of such bodies discovered beyond the orbit of Neptune, creating the prospect of an unending list of planets.
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.
Our moon is far larger than Pluto and would meet that definition of "planet" if it were not orbiting Earth as its primary. There are lots of moons in the Solar System bigger than Pluto.

From now on Pluto will be considered a dwarf planet (or plutonian objects), the first of its kind to be identified, which preserves its claim to fame somewhat. There will be a class called Trans-Neptunian Objects, including Pluto, Sedna, Quaoar and 2003 UB313, the one nicknamed "Xena" by its discoverers and some which belong to the Asteroid Belt, such as Ceres. The rest of the asteroids, comets and meteoroids are called Small Solar System Bodies. These will be found in the Asteroid Belt, the Kuiper Belt and the Oort Cloud.

It doesn't really matter much, since Pluto as the ninth planet has entered popular consciousness and will continue to be thought of affectionatley by most non-scientists. I watched a program on the Science Channel in which the director of The Hayden Planetarium at the American Museum of Natural History, Neil deGrasse Tyson showed all the mail he'd received from school children because he hadn't shown Pluto in the exhibits.


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