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Circular Reasoning

February 11, 1999
Sometimes timing is everything. Normally, I would consider the following error by CNN's website to be more than a nitpick but less than a serious error (from CNN's website):
Two new planets found outside solar system

January 9, 1999
Web posted at: 11:52 p.m. EST (0452 GMT)

AUSTIN, Texas (AP) -- Two additional planets orbiting
distant stars have been discovered by astronomers, bringing
to 17 the number of worlds known to exist beyond Earth's
solar system.


Geoffrey Marcy, leader of the San Francisco team, said nine
of the new planets have egg-shaped, eccentric orbits. This
finding means it is unlikely that their parent stars could also
host smaller planets.

In the solar system, the Earth and its sister planets are in
stable, circular orbits, which means they are not as
susceptible to gravitational gyrations that could be caused
by oval orbits.

The problem is that the orbits of all the planets in our solar system are ellipses, and not circles. The reason I said before that this isn't a big error is that for most of the planets, the orbits are very close to circular. Mars' orbit is actually quite elliptical. Pluto, though, is the king of ellipses: it's orbit is so elliptical that it is 60% farther from the Sun at maximum distance than at its nearest!

The reason the timing for this is so funny is that this article was posted to CNN's website just one month before Pluto once again passes Neptune as the farthest planet from the Sun. For 220 or so years out of its orbit, Pluto is the farthest planet from the Sun. But because of its elliptical orbit, for 20 years Pluto is actually closer to the Sun than Neptune. So not only are orbits not circular, but some are so far off that it can change the order of the planets from the Sun.

My thanks to Robert Delaney for bringing this one to my attention!< /small>

©2008 Phil Plait. All Rights Reserved.

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