Clyde William Tombaugh (; February 4, 1906 – January 17, 1997) was an American astronomer and telescope maker, best known for discovering Pluto in 1930, marking the first detection of what would eventually be recognized as the Kuiper belt. At the time, Pluto was referred to as the ninth planet in the Solar System, a classification that stood for over seven decades.
Born in Illinois, and raised on farms in Kansas, Tombaugh was largely self-taught in astronomy and optical engineering, building his own telescopes from spare parts and grinding lenses by hand. His skill and determination led to a position at Lowell Observatory in Flagstaff, Arizona, where he conducted photographic surveys of the night sky. In addition to Pluto, Tombaugh identified hundreds of asteroids and a variety of star clusters, galaxies, and variable stars, thereby contributing significantly to the cataloguing of the night sky.
As a professor at New Mexico State University, he led the Planetary Patrol project, which helped determine Mercury's rotation period, monitored Jupiter's Great Red Spot, and developed new photographic techniques for satellite searches. After retiring in 1973, Tombaugh continued to construct telescopes, support observational programs, and advocated continued study of unidentified flying objects (UFOs).
Tombaugh was born in Streator, Illinois, son of Muron Dealvo Tombaugh, a farmer of Pennsylvania Dutch descent, and his wife Adella Pearl Chritton on February 4, 1906. He was the first of six children in the family with his sister, Esther being born 2+1⁄2 years after Clyde followed by his brother Roy in 1912, Charles in 1914, Robert in 1923 and Anita Rachel in 1929. Tombaugh's interest in astronomy appears to have begun when he visited the Yerkes Observatory in Lake Geneva, Wisconsin when he was 12 years old in 1918. His uncle Lee also helped spark his interest in astronomy as he was an amateur astronomer who used a 3-inch (76 mm) diameter non-achromatic refractor telescope and gave him several astronomy-related books.
The family's poor finances, due to years of bad weather on the farm and a bad corn harvest in 1921, caused them to move to a new farm in Burdett, Kansas in 1922. As the family had moved to the farm in August 1922 and the crops needed to be prepared urgently, Tombaugh had to drop out of high school for a year to help his father prepare the crops on the family farm. Tombaugh graduated from high school in 1925. Tombaugh's plans for attending college were frustrated when a hailstorm in June 1928 ruined his family's farm crops. Tombaugh read an article in Popular Astronomy in 1924 by an amateur astronomer named Latimer J. Wilson, titled "The Drift of Jupiter's Markings" showing sketches of Jupiter and decided he wanted to make his own telescope so he could see the features shown on Jupiter in sketches.
Beginning in 1926, he built several telescopes with lenses and mirrors by himself. To better test his telescope mirrors, Tombaugh, with just a pick and shovel, dug a pit 24 feet long, 8 feet deep, and 7 feet wide. This provided a constant air temperature, free of air currents, and was also used by the family as a root cellar and emergency shelter. He sent drawings of Jupiter and Mars to the Lowell Observatory, at Flagstaff, Arizona, which offered him a job. Tombaugh worked there from 1929 to 1945.
It was at Lowell in 1930 that Tombaugh discovered Pluto. Following his discovery, Tombaugh earned bachelor's and master's degrees in astronomy from the University of Kansas in 1936 and 1938.
While a young researcher working for the Lowell Observatory in Flagstaff, Arizona, Tombaugh was given the job to perform a systematic search for a trans-Neptunian planet (also called Planet X), which had been predicted by Percival Lowell based on calculations performed by his student mathematician Elizabeth Williams and William Pickering.
Starting April 6, 1929, Tombaugh used the observatory's 13-inch (330 mm) astrograph to take photographs of the same section of sky several nights apart. He then used a blink comparator to compare the different images. When he shifted between the two images, a moving object, such as a planet, would appear to jump from one position to another, while the more distant objects such as stars would appear stationary. Tombaugh noticed such a moving object in his search, near the place predicted by Lowell, and subsequent observations showed it to have an orbit beyond that of Neptune. This ruled out classification as an asteroid, and they decided this was the ninth planet that Lowell had predicted. The discovery was made on Tuesday, February 18, 1930, using images taken the previous month.
Three classical mythological names were proposed: Minerva, Cronus and Pluto. Minerva was eliminated as it was found to already be in use as the name of an asteroid while Cronus was written off as it was feared the astronomer Thomas Jefferson Jackson See who was the primary supporter of it would falsely take credit for it and was widely disliked. In Roman mythology, Pluto can render himself invisible, his name's first two letters are Percival Lowell's initials, and it was proposed by an 11-year-old English girl, Venetia Burney. In order to avoid the name changes suffered by Neptune, the name was proposed to both the American Astronomical Society and the Royal Astronomical Society, both of which approved it unanimously. The name was officially adopted on May 1, 1930.
Following the discovery, it was recognized that Pluto was not massive enough to be the expected ninth planet, and some astronomers began to consider it the first of a new class of object – and indeed Tombaugh searched for additional trans-Neptunian objects for years, though due to the lack of any further discoveries he concluded that Pluto was indeed a planet. The idea that Pluto was not a true planet remained a minority position until the discovery of other Kuiper belt objects in the late 1990s, which showed that it did not orbit alone but was at best the largest of a number of icy bodies in its region of space. After it was shown that at least one such body, dubbed Eris, was more massive than Pluto, the International Astronomical Union (IAU) reclassified Pluto on August 24, 2006, as a dwarf planet, leaving eight planets in the Solar System.
Tombaugh's widow Patricia stated after the IAU's decision that while he might have been disappointed with the change since he had resisted attempts to remove Pluto's planetary status in his lifetime, he would have accepted the decision now if he were alive. She noted that: “He was a scientist. He would understand they had a real problem when they start finding several of these things flying around the place." Hal Levison offered this perspective on Tombaugh's place in history: "Clyde Tombaugh discovered the Kuiper Belt. That's a helluva lot more interesting than the ninth planet."
Tombaugh continued searching for over a decade after the discovery of Pluto, and the lack of further discoveries left him satisfied that no other object of a comparable apparent magnitude existed near the ecliptic. No more trans-Neptunian objects were discovered until 15760 Albion, in 1992.
In 1936, Tombaugh may have discovered the Perseus–Pegasus Filament, a galaxy filament that stretches for roughly a billion light years in diameter, making it one of the largest structures in the known universe. He referred to it as the Great Perseus–Andromeda stratum of Extra-Galactic Nebulae.
Tombaugh is officially credited by the Minor Planet Center with discovering 15 asteroids, and he observed nearly 800 asteroids during his search for Pluto and years of follow-up searches looking for another candidate for the postulated Planet X. Tombaugh is also credited with the discovery of periodic comet 274P/Tombaugh–Tenagra. He also discovered hundreds of variable stars, as well as star clusters, galaxy clusters, and a galaxy supercluster.
The asteroid 1604 Tombaugh, discovered in 1931, is named after him. He discovered hundreds of asteroids, beginning with 2839 Annette in 1929, mostly as a by-product of his search for Pluto and his searches for other celestial objects. Tombaugh named some of them after his wife, children and grandchildren. The Royal Astronomical Society awarded him the Jackson-Gwilt Medal in 1931.