A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Whatever happened to "Planet X"?

July 26, 1996

Dear Cecil:

When I was in sixth grade in 2972 I remember reports of the discovery of a tenth planet located beyond Pluto. This planet was referred to as "Planet X." I have heard nothing further about it. Are there ten of us in the solar system, or was there a dust mote on the telescope?

Cecil replies:

Um, 2972? You are ahead of your time. I'll assume it's a typo, although with AOL users one never knows.

The story of Planet X starts in 1846 with the discovery of Neptune. Neptune was the first planet whose discovery had been predicted based on irregularities in the motion of nearby bodies, in this case Uranus (which in this squeamish age we've agreed to pronounce YOOR-uh-nuss).

Scientists guessed these "perturbations" were due to the gravitation of an unknown planet and calculated where said planet could be found. Sure enough, when astronomers looked in the indicated direction, there was Neptune.

Naturally all the other astronomers wanted to duplicate this extremely cool feat. As it happened, Neptune's orbit wasn't precisely as predicted. Within days of its discovery one astronomer was speculating about the existence of yet another planet. Many others chimed in with their own predictions in the following decades.

Some of the most famous predictions came from astronomer Percival Lowell, best known for his belief in the canals of Mars. Lowell dubbed the mystery body Planet X. He never found it, but after his death Clyde Tombaugh, an astronomer at Lowell's observatory, did. Or so he thought.

The new planet, dubbed Pluto, jibed pretty well with Lowell's predictions for Planet X. Just one problem. It was way too small to cause the observed perturbations in the orbits of Uranus and Neptune. (We now know Pluto's mass is only 1/500th that of earth.) Tombaugh's find was a result of luck and his own doggedness. Back to the telescopes.

More Planet X predictions surfaced periodically. The one you remember was made in 1972 by an astronomer who shall remain nameless, who predicted a Saturn-size (i.e., huge) planet that took 500 earth years to revolve around the sun and whose orbit was tilted at a cockeyed angle to the earth's. Not surprisingly, within a year other scientists had determined no such planet could exist.

Finally in 1993 someone recomputed the orbits of Uranus and Neptune using more accurate data gathered by space probes. Guess what? Once you got the slop out of the numbers there weren't any perturbations — never had been. So no Planet X. All that time and brainpower spent on the chase, and at the end there was squat to show for it. I can relate.

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