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August 14, 2006

Dead planet walking

photo of pluto

Tomorrow is the opening session for the General Assembly of the International Astronomical Union. This meeting is the event for the astronomical community, but this time there's a great deal of lay interest in the Assembly. That's because the IAU is also in charge of the official classification of celestial objects, and this time they're going to decide whether or not Pluto gets to stay a planet.

There are strong arguments for stripping Pluto of its planet status... it's tiny (our moon is bigger than Pluto, so are six moons orbiting other planets) and its orbit isn't very planet-like ( the orbit is severely tilted, more closely matching the orbits of some other Kuiper belt objects than those of the planets). A further blow to Pluto's status happened last year when CalTech's Michael Brown discovered another Kupier Belt object even bigger than Pluto. That object is still awaiting official name designation from the IAU, when Brown first discovered it, he called it "Xena". If Pluto is a planet, shouldn't Xena be one too?

On the other hand, there's a lot to be said for tradition and history, and we've had nine planets since Pluto was discovered back in 1930. And lots of school kids are taking the issue seriously, with letter writing campaigns to local science museums. Finally, who says we have to have consistent designations? Lots of scientific classification schemes are messy and inconsistent (You ever try to learn geologic periods? Or cloud types?).

A third possibility is to toss out the whole urinary "planet" thing, and replace it with a three-tier classification...gas giant planets like Saturn and Jupiter, rocky planets like Earth and Venus, and "mini-planets" like Pluto and Xena. That could raise the number of planets in our solar system from about two dozen to more than 50, depending on the minimum size cutoff.

Whatever happens, there's one thing for sure... the solar system is a much more interesting and varied place than you learned back in grade school.

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Posted by Chris Spurgeon at August 14, 2006 04:19 PM

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