Clyde Tombaugh Discovers Pluto (February 18, 1930)
On February 18, 1930, a 24-year-old self-taught astronomer named Clyde Tombaugh discovered the ninth planet while painstakingly comparing photographic plates at Lowell Observatory in Flagstaff, Arizona. The images that revealed the tiny, shifting point of light had been taken weeks earlier, on January 23 and January 29, 1930. The find capped a quarter-century search for a hypothesized "Planet X" and introduced the world to Pluto.
The Search for Planet X
The hunt that led to Pluto's discovery began decades earlier with astronomer Percival Lowell, founder of Lowell Observatory. Lowell believed that unexplained irregularities in the orbits of Uranus and Neptune pointed to the gravitational pull of an undiscovered ninth planet, which he called "Planet X." He spent years from 1905 until his death in 1916 searching for it without success. The observatory shelved the project for over a decade before reviving it in 1929, when director Vesto Slipher hired young Clyde Tombaugh, a Kansas farm boy with no formal astronomy degree but a talent for building telescopes and a fierce work ethic. Tombaugh was assigned the tedious task of photographing sections of the night sky in pairs, then scanning them for any object that moved against the fixed background of stars, a telltale sign of something within our solar system rather than a distant, stationary star.
Did You Know?
A small portion of Clyde Tombaugh's ashes were carried aboard NASA's New Horizons spacecraft, launched in 2006, which flew past Pluto on July 14, 2015—meaning the man who discovered Pluto in 1930 ultimately traveled to it himself, 85 years later.
January's Photographs and the Discovery
Tombaugh's method relied on a device called a blink comparator, which rapidly alternated between two photographic plates of the same star field taken several nights apart, making any moving object appear to "blink" back and forth. On January 21, 23, and 29, 1930, Tombaugh photographed a region of the constellation Gemini using the observatory's 13-inch astrograph telescope. Weeks later, on February 18, 1930, while blinking the January 23 and January 29 plates, he noticed a faint fifteenth-magnitude object shifting position by about 3.5 millimeters, consistent with the parallax expected of a distant planet. Tombaugh spent the next several days confirming the find with additional plates before observatory staff verified the discovery. The object's position was remarkably close to where Lowell's calculations had predicted Planet X might lie, lending the moment an air of destiny even though later analysis showed the alignment was largely coincidental, since Pluto's mass was far too small to have caused the orbital anomalies Lowell had originally observed.
Naming Pluto and Its Legacy
Lowell Observatory announced the discovery to the world on March 13, 1930, deliberately chosen because it was both Percival Lowell's birthday and the anniversary of William Herschel's 1781 discovery of Uranus. The observatory received over a thousand suggestions for naming the new planet. The winning name, Pluto, was proposed by an 11-year-old English schoolgirl named Venetia Burney, who suggested the Roman god of the underworld to her grandfather, a librarian at Oxford, because the planet's distant, dark realm seemed fitting; the name also cleverly began with Percival Lowell's initials, P and L. Pluto was formally adopted on May 1, 1930, and Tombaugh, who continued working at Lowell Observatory for years afterward, became instantly famous. Pluto held the title of ninth planet for 76 years until the International Astronomical Union reclassified it as a "dwarf planet" in 2006 following the discovery of similarly sized objects in the Kuiper Belt, a decision that remains debated among astronomers and the public to this day.