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Pluto the Accidental Planet
The Controversy Over Clyde Tombaugh's Planetary Discovery

By: John Shepler

On February 18, 1930 a 24 year old self-taught astronomer sat concentrating through the eyepiece of the Blink-Comparator at the Lowell Observatory. Clyde Tombaugh was searching for something special, a dim little needle-in-the-haystack that had eluded the keen eye of the very founder of the observatory, the esteemed Percival Lowell. To find it would make history. It would also prove Lowell's calculations that predicted a ninth planet perturbing the orbits of Uranus and Neptune. Clyde Tombaugh was pursuing the elusive Planet X.

Heart of Pluto mousepad. Get this and other pluto themed producgts now.Back and forth he flipped the shutter of the Blink-Comparator, a machine that presented his eye with two photographs of the identical field of stars taken several weeks apart. Clyde had been at this nearly a year, seeing nothing moving in the eyepiece. Suddenly, there it was, just as Lowell had predicted and searched for in vain for eight years himself. A faint speck jumped between the stars. Only a moving planet would do this, and this one was right where it should be...or so it seemed.

On March 13, 1930, the birthday of Percival Lowell, the Lowell Observatory announced the discovery of Planet X. Some, including his widow, thought the proof of the decades old prediction justified naming the planet after Lowell. But planets were traditionally named for characters in Greek and Roman mythology. So Planet X became Pluto, the name suggested by Venetia Burney, an 11 year old school girl in Oxford, England. Pluto, god of the underworld, ruling in the bleak darkness of an orbit that takes 248 years to complete. It was a name that everyone could agree on, especially since the initials for Pluto are PL, the same as Percival Lowell.

This would be the happy end of the story, except for an increasing number of oddities that have been accumulating over the last sixty plus years. For one thing, we now know there never was a Planet X. The calculations were wrong.

How's that? The slight shift in the orbit of Uranus that astronomers couldn't fully explain, even with the discovery of Neptune in 1846, disappeared when careful measurements were made by the interplanetary spacecraft sent to the outer planets. Almost as strange, astronomers first assessed Pluto to be much larger and massive than later observations justified. It turns out that those observations proved that Pluto is smaller than our own moon and has a mass just two tenths of one percent of the Earth. That's just a fraction of what would be needed to account for the presumed motion of Uranus that started astronomers looking for a ninth planet in the first place.

What's more, Pluto's orbit around the Sun is tilted compared to the other planets. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune line up within a degree or two of a flat plane through the sun. Their orbits are nearly circular. Pluto's path is tilted 17 degrees to that plane and is so elliptical that for the 20 years from 1979 to 1999 Pluto has actually been the 8th planet of the Solar System, closer to the Sun than Neptune.

Some astronomers are starting to take a skeptical view of little Pluto, wondering if what Clyde Tombaugh really discovered was a large comet instead of a small planet. The composition of Pluto is a core of rock and ice with a surface of methane ice and a slight atmosphere of nitrogen, methane and carbon monoxide. If Pluto's orbit was even more elliptical, it would enter the inner solar system and shoot out a comet-like tail as the water and gasses boil off from the heat of the sun.

Then there is the matter of Pluto's moon, Charon. Charon is about half the size of Pluto itself and named, appropriately, after the mythological character who ferried the dead across the River Styx and into the underworld of Pluto. Pluto and Charon are almost a double planet system. They dance around each other, all the while showing exactly the same faces. Charon hangs motionless in the Pluto sky, and vice-versa.

There is a move afoot in astronomy to "demote" Pluto to a "minor planet" and consider it something of a large asteroid that was misclassified in the excitement of being found just where a planet was mistakenly expected. But...not so fast. The whole definition of what is a planet is somewhat murky to begin with. What scientists agree on is that a planet must orbit the Sun and be large enough that its own gravity pulls it into a spherical shape. Pluto easily meets that criteria. Besides, there is no definition as to composition. Jupiter, Saturn, Neptune and Uranus are gas giants. Mercury, Venus, Earth and Mars are rocky. Why not an icy-rocky planet?

New Horizons launchPerhaps the best idea is to get out there and see what Pluto and Charon really look like and what is going on at the edge of the Solar System. After all, Pluto is the only planet missed on the Voyager grand tour of the Solar System. NASA has long had plans for a mission to Pluto. The original concept was the Pluto-Kuiper Express, which was later changed to New Horizons. That mission successfully launched on January 19, 2006 and encountered the Pluto system on July 14, 2015. The Pluto mission hopes to map not just the planet and its moons, but also other objects in the Kuiper Belt, a region of minor icy planets that orbit the sun in almost complete darkness. Planet 9 or Minor Planet 10,000? That is the question.

My vote is to stick with the lineup that includes Pluto and leave the door open for yet a planet 10, 11, or ... who knows? We're just starting to really explore space, and who can say what fortuituous discoveries await us just beyond our present field of view. Considering the happy accident of discovering a planet exactly where one miscalculated it to be suggests there is so much more that we just haven't thought to look for yet. When we do, we might just find it!

Note: Sadly, on August 24, 2006, the International Astronomical Union decided to recategorize Pluto as a "dwarf planet." Only a minority of the members participated in this vote. Perhaps after the stunning success of the New Horizon's mission, they might have a change of heart.


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Books of Interest:

Pluto and Charon by Alan Alan Stern (Editor), David J. Tholen (Editor), S. Alan Stern (Editor). This 756 page book has an extensive collection of scientific information from a description of the discoveries of Pluto and Charon to composition, internal structure and thermal evolution to chemical models of Pluto's atmosphere and interaction with the solar wind. Read the extensive table of contents for more details.

A Double Planet?: Pluto & Charon (Isaac Asimov's New Library of the Universe)by Isaac Asimov, Isaac Asimov, Greg Walz-Chojnacki, Frank Reddy. Discusses the smallest, most distant, and most mysterious planet in our solar system, its discovery, its peculiar orbit, and its recently discovered satellite.

Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System by Alan Alan Stern, Jacqueline Mitton. Now one of the world's leading experts on Pluto shares the most current information on the last exciting frontier for astronomers in one lively, illustration-packed volume. Based on the latest information gleaned from the new technology of groundbased astronomical instrumentation and spacecraft explorations after the 1981 Voyager expedition, this exciting book looks at how Pluto was discovered and explored, and how the pursuit of knowledge about this distant planet has revolutionized the entire field.

Clyde Tombaugh & the Search for Planet X by Margaret K. Wetterer,  Laurie A. Caple(Illustrator). This book for children shares the adventures of Clyde Tombaugh from his early years making telescopes on the family farm to his work in discovering the mysterious Planet X in 1930, which was soon named Pluto.

Also visit Books-A-Million for an excellent selection of new books, magazines, e-books, audio books and more at low, low prices. Who is Books-A-Million?


Also visit these related sites:

Clyde Tombaugh - Articles on the man who discovered Pluto, from New Mexico State University.

New Horizons Mission - Find details on the current mission to Pluto and see the stunning photos of Pluto and Charon. Thanks to NASA JPL for the photos of the New Horizons launch and Pluto shown in this article.

International Astronomical Union - Founded in 1919, this respected body has over 8,700 individual members in 66 adhering countries. It is the internationally recognized authority for assigning designations to celestial bodies and any surface features on them.

American Museum of Natural History - In a earlier decision, the Rose Center for Earth and Space which opened in 2000 at this museum in New York, has issued an opinion that Pluto is simply a lump of ice in the Kuiper Belt. They have removed Pluto from their display of planets. - astronomy and space links, shortcuts, favorites and bookmarks.

Pluto Themed Gifts - Find T-shirts, mousepads, and many other items with an image of the planet Pluto.

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Copyright 1999 - 2016 by John E. Shepler. Contact me at: John (at)

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First Published: January 24, 1999 as part of A Positive Light


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