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No. 2140:
Poor Old Pluto

Today, Pluto Lives. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Two things happened in 1930: Disney gave Mickey Mouse a big floppy, friendly puppy; and a young astronomer, Clyde Tombaugh, identified a new planet. Neither had a name at first. 

A few months after Tombaugh found his planet, it was named after Pluto, god of the underworld. A British schoolgirl had suggested the name since Pluto could render himself invisible. The planet had likewise been very hard to find. A year later, Disney gave that same name, Pluto, to Mickey's puppy. During the 1930s, we school-kids, watching Saturday afternoon cartoons, were all aware that Pluto could be either a planet or a puppy.

Just the other day, another kind of naming plunged Pluto into a maelstrom of controversy. The International Astronomical Union removed Pluto, and other objects of comparable size, from the list of planets. Another was Xena, a bit larger than Pluto and further away. These lie on the inner fringe of the Kuiper belt, a large group of small objects on the outer edge of our solar system. 

The asteroids, by the way, move in a much closer belt, just beyond Mars. The largest asteroid, Ceres, was discovered two centuries ago and it too was called a planet. Fifty years later, astronomers realized that it was just one of a vast number of such objects. So it was likewise downgraded from planet to asteroid.

The controversy over what to call Pluto is tied up in the mathematics of mass and motion. Calculating the motion of more than two objects in a mutual gravitational field is fiendishly difficult. The objects in our solar system all affect one another; the Sun is just a major player. We expect certain regularities in planetary motions and Pluto fails to satisfy those regularities. 

By the way, we now know that our unplanet Pluto has three moons. Her moon Charon is relatively large -- almost enough to make Pluto a double nonplanet. Pluto's two smaller moons, Hydra and Nix, were only found in 2005. 

A recent Science magazine article argues that two objects once collided to form Pluto, Charon and a debris field. Gradually, the interaction of gravity fields changed Charon's orbit from wildly elliptical to circular, and merged the debris field into those little moons. That was learned through computer modeling that would've been impossible when we had only paper and pencils. 

Pluto's orbit around the sun is very different from those of the remaining eight planets in two respects: it lies in a different plane, and it's far more eccentric. Xena is also off in the Kuiper Belt with her own eccentric orbit. One moon has been found orbiting Xena, and it was (predictably) named after Xena's sidekick Gabrielle, from the old TV series. 

So much stuff out there! And, as for poor old Pluto -- well call it what you will. It's still there -- a floppy puppy of a planet, or a rose by some other name. Call it a planet, or a Kuiper object, or a Romulan spy vessel. I will simply call it -- Pluto.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

J. J. Lissauer, Growing Apart in Lock Step. Science, Vol. 313, 15 August, 2006.

For more on the discovery of Pluto, see:
For more on Pluto's reclassification, see:
For a fine perspective on relative sizes of planets and stars, see:

I'm grateful to local astronomers Drel Setzler, John Gordeuk, and John Hunt, for their counsel. John Gordeuk points out that a dynamicist and a geologist will claim different grounds for classifying planets. He puckishly points out that, since Earth's moon (unlike, say, those of Jupiter) is more affected by the Sun's gravitational pull than Earth's, perhaps it should be classified as a planet. 


Pluto and Xena