February 18, 1930
Editor’s Note: This article was originally written in August 2001, a full five years prior to the August 2006 decision of the International Astronomical Union to demote Pluto to the status of a “dwarf planet”.
Not many undergraduates enter the University of Kansas having already achieved worldwide fame. And, to date, only one has enrolled at KU after discovering a planet. His name was Clyde Tombaugh, a 24-year-old amateur astronomer from Burdett, Kansas, who positively identified Pluto as the ninth planet in our solar system on February 18, 1930. The finding was the culmination of many years of scanning the night skies by numerous astronomers searching for what had been known as “Planet X,” an effort analogous to looking for a golf ball located 33 miles away.
Percival Lowell, scion of a wealthy and distinguished Massachusetts family, had dedicated himself to finding the mysterious Planet X since the 1890s. Based on a series of complicated mathematical formulations, he became convinced of the existence of a trans-Neptunian planet. He was not alone. Other astronomers suspected that a ninth planet existed beyond Neptune and Uranus, but technological setbacks continually dogged their efforts to find it. Lowell financed the construction of the Lowell Observatory near Flagstaff, Arizona, and in 1905, a comprehensive photographic search for the missing planet began in earnest. Between 1914 and 1916, nearly 1,000 images were made over a large area of the night sky with a telescope borrowed from Swarthmore College. The astronomers also made use of a blink comparator, a device that detects minute changes in photographs taken of the same region of the sky.
Lowell died suddenly of a stroke in 1916 at the age of 61 and the search was halted for the next decade or so. Much of his estate was lost in litigation over a contested will, but in 1927 Percival’s nephew, Roger Lowell Putnam, resumed the search for Planet X. A $10,000 grant was obtained from Harvard University, where Percival’s brother, Dr. A. Lawrence Lowell, was president. The funds were used to build a 13-inch telescope. Meanwhile, in 1927, a Kansas high school graduate named Clyde Tombaugh had constructed a nine-inch Newtonian refractor telescope from used tractor and automobile parts and was scanning the night skies of the high plains. He drew sketches of Jupiter and Saturn and sent them off to the Lowell Observatory for critique but instead was offered a job. Tombaugh arrived in Flagstaff in January of 1929 and began work under the supervision of Lowell Director Vesto Melvin Slipher.
Slipher’s efforts to find Planet X reached a new low in May of 1929; he was under pressure to locate the planet, and after comparing thousands of photographs from the expected region without finding it, he felt he had failed. Tombaugh took up the search in earnest. Initially, Tombaugh encountered some problems with the Observatory’s new 13-inch telescope, and he retrofitted corrective measures into the device, prompting him to paraphrase Socrates’ vocative “Know Thyself!” into “Know Your Telescope!” In September 1929, two plates revealed possible candidates for Planet X, but a third plate taken on a third date was needed for corroboration. On the night of January 21, on what Tombaugh called the “worst seeing in my life before or since,” an exposure was made that revealed the image of Pluto, albeit swollen due to high winds that night. Corroborating images were made on January 23 and 29. On February 18 at 4:00 PM, Tombaugh detected the three corroborating images; Planet X finally had been located.
Percival Lowell’s widow wanted the planet to be named first Zeus, or Lowell, or perhaps Constance – this from the woman who had apparently squandered the original Lowell estate away in litigation. She was ignored, and Planet X became known as “Pluto,” after the Roman god of the underworld, the symbol of which is PL, coincidentally the initials of Percival Lowell. A young English girl, Venetia Burney, originally suggested the name.
Since its discovery, some astronomers have questioned whether Pluto really qualifies as a planet, a debate that continues to this date. Indeed, many authorities now take the position that Pluto should be classified as a comet or an asteroid rather than a planet. It is often considered part of the Kuiper Belt, a cluster of asteroids beyond Neptune. But other astronomers continue to classify Pluto as a planet since it possesses a moon, called Charon, discovered in 1978.
Clyde Tombaugh went on to receive a scholarship to KU where he graduated with a Bachelor of Arts degree in 1936 and earned his Master’s in astronomy in 1939. He accepted a teaching position at New Mexico State University in Las Cruces in 1955 and created and developed the astronomy program there, regarded as one of the best in the nation. Tombaugh retired as Professor Emeritus in 1973, but remained actively engaged with the program.
He went on the lecture circuit in 1980 to raise money for an astronomy endowment at NMSU. Clyde Tombaugh Elementary School in Las Cruces, the Clyde Tombaugh Astronomy Center in Dodge City, Kansas, the Tombaugh Planetarium at the New Mexico Space Center in Las Cruces, and university observatories named for Tombaugh at both NMSU and KU testify to the resilience of his reputation. Tombaugh possessed physical hardiness as well. He lived until the age of 90, dying on January 17, 1997.
But Clyde’s exploration of Pluto did not end with his death. On January 19, 2006, Tombaugh’s ashes were aboard the New Horizons spacecraft as it lifted off on a journey to Pluto and beyond. The craft is expected to reach Pluto in July 2015 and continue on its journey past our solar system.
Department of History
University of Kansas
Description of a blink comparator: http://www.jpl.nasa.gov/ice_fire//vol1a.htm;
Clyde Tombaugh’s own account of the discovery: http://www.jpl.nasa.gov/ice_fire//9thplant.htm
See also David H. Levy, Clyde Tombaugh: Discoverer of Planet Pluto, Tucson: University of Arizona Press, 1991.]