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7.35 The Discovery of Pluto

Before Pluto's existence was known, the astronomer Percival Lowell predicted that another planet orbited beyond Neptune. He called it Planet X."" Based on his calculations, perturbations in Neptune's orbit could be explained by a ninth planet in the outer Solar System. Lowell started searching for Planet X in 1905 using the observatory he founded in Flagstaff, Arizona. But search after search turned up nothing. In 1916, Lowell died, his dreams of finding Planet X unfulfilled.

A photograph of Clyde Tombaugh. Click here for original source URL.

Lowell's nephew chose to persist with his quest for Planet X when he assumed control of the observatory. He hired a young amateur astronomer, Clyde W. Tombaugh, to perform the arduous search using a new telescope designed specifically for that endeavor. He studied photos of the ecliptic taken one week apart in the hopes that Planet X would be glimpsed in different positions. After more than a year of studying the cumbersome photographic plates, Tombaugh discovered Pluto on February 18, 1930. 


Percival Lowell?observing?Venus in the daytime from the observer's chair of the 24-inch (61 cm)?Alvan Clark & Sonsrefracting telescope, installed in the summer of 1896 at the?Lowell Observatory, which he established in Flagstaff, Arizona (USA). Click here for original source URL.

The ninth planet was not exactly as Lowell had conceived it, however. It was in another area of the sky — only by chance did Tombaugh catch it. They were also looking for something much larger; Pluto is actually much too small to affect Neptune's orbit. Because Pluto is brighter than most bodies in the outer Solar System, the discoverers thought it was more massive than it really is. An accurate mass of Pluto was not determined until after Charon was discovered in 1978. In fact, it turned out that the calculations of Neptune's orbit had been wrong. There were no perturbations, and there is no Planet X. This wasn't known until much later, however, when Voyager 2 returned more accurate data on Neptune's mass and orbit. At the time, they were convinced that Pluto was their elusive Planet X. 

Discovery image of Charon. Click here for original source URL.

Are there any other substantial planetary bodies beyond Pluto? Several astronomers have sought dynamical or photographic evidence of a planet there. Clyde Tombaugh's searches ruled out any planet as large as Neptune near the plane of the Solar System out to a distance of around 100 A.U. However, surveys of the outer Solar System have found many small objects in the region of Pluto. The diameters of these objects range from a few hundred kilometers or about a tenth the size of Pluto, to nearly the size of Pluto. These discoveries convinced many astronomers that the outer fringe of the Solar System is full of icy bodies of which Pluto may be only the largest (or one of the largest). We probably will never find a full-fledged Planet X, Earth-size or larger. However, additional Pluto-size bodies may lurk in the outermost regions, waiting to be discovered as astronomers search for fainter and fainter bodies on the outskirts of the Solar System