How was Pluto Discovered?

While NASA’s New Horizons Mission flies by the frozen dwarf planet, snapping away and gathering data, we ought to learn more about the history of Pluto and how this planet was found at the very edge of our solar system. It was all the way back in 1930, when on the 23rd and 24th of January a young astronomer Clyde W. Tombaugh was scanning the night sky during work in Flagstaff, Arizona. He was taking pictures of star positions, looking for mysteries beyond this planet which might shine more light on our solar system. He took a couple of pictures, set them aside not realizing that he’d found exactly what he was looking for: a ninth planet in the solar system.

The hunt for Pluto grew out of the search for other planets in the solar system. In the 18th Century, as the field of astronomy expanded with better and more powerful telescopes, astronomers began to scan the heavens to look for additional planets. Sir William Herschel had first observed Uranus, which was originally believed to be a star or comet, in March of 1781 as part of a larger movement by astronomers to discover new features in the solar system. After Uranus was formally identified as a planet, astronomers raced to identify others. In 1801, a new planet called Ceres was discovered in the gap between Mars and Jupiter. It was later downgraded to a new class of astronomical objects, an asteroid, but Ceres’ discovery helped prompt the prediction of other planets throughout the Solar System.

As astronomers observed Uranus and its orbit, they noticed small variations: something large, and further out in the solar system, was affecting the gas giant. In 1821, astronomer Alexis Bouvard published a detailed table of Uranus’s orbit. Drawing from his data, a pair of astronomers, John Couch Adams from England and Urbain Le Verrier from France independently predicted the location of a new planet. Using the data provided by Le Verrier, a German astronomer named Johann Gottfried Galle began to search for this unknown body, and on September 23rd, 1846, he observed it near where it was predicted. Neptune was the furthest planet discovered thus far, but, shortly after its discovery, astronomers began to predict that other planets lay beyond in the far reaches of the Solar System.

To many astronomers, the orbits of Uranus and Neptune seemed to indicate the existence of another body—something altering their movements around the sun. And therefore between 1877 and 1930, a number of theories were proposed. They predicted the existence of anywhere from one to five planets beyond Neptune. However, one astronomer, Jean Baptiste Aimable Gaillot, reran calculations on the planetary orbits, and determined that these theories were put forth in error.

The astronomy community largely abandoned the idea of another planet beyond Neptune. However, there were lingering optimists. One such astronomer was American William H. Pickering, who predicted a Planet O far out beyond Neptune, as well as Planets P through U, all of which were debunked. Another American, Percival Powell, theorized the existence of celestial body he called Planet X. This Planet X eventually turned out to be the planet we call Pluto. It was a long journey – but perhaps not as long as the one New Horizons has covered to reach the secluded planet at the edge of our solar system.

However, the name Pluto was chosen on March 24, 1930 after 11-year-old Venetia Burney in Oxford, England suggested the name “Pluto.” The name denotes both the assumed unfavorable surface conditions (as Pluto was the Roman god of the underworld) and also honors Percival Lowell, as Lowell’s initials make up the first two letters of the planet’s name.


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