February 18, 1930. On this date 90 years ago, a young astronomer was working at Lowell Observatory in Flagstaff, Arizona. Clyde Tombaugh had just turned 25 years old. He’d been hired to continue a search for a ninth planet, begun by Percival Lowell. He’d been at it for about one year. On February 18, 1930, he compared photos of a single star field – taken six days apart a few weeks earlier – and noticed an object that was moving against the backdrop of distant stars. It was a small, dim, remote body in our own solar system. Today, we know this little world as Pluto.
On the occasion of the 90th anniversary of Pluto’s discovery, Thomas Zurbuchen – associate administrator for NASA’s Science Mission Directorate – commented:
What Tombaugh didn’t know then was that Planet X would launch the era of exploration in the 3rd zone of the solar system. Science builds on science, and this discovery helped pave the way for [the spacecraft] New Horizons’ exploration of this uncharted region.
NASA also said that, although he died in 1997, Tombaugh’s ashes were aboard the New Horizons spacecraft when it launched from Cape Canaveral, Florida, in January 2006. Those ashes, carried in a small canister on the spacecraft, traveled with New Horizons on a 9-year, 3-billion-mile journey to Pluto, which it swept past in 2015, revealing a complex world with mountains and weather, and with a large, young, heart-shaped region of ice on its surface.
The mystery of Pluto began long before Clyde Tombaugh’s momentous discovery. Astronomers in the 19th century knew the seventh planet Uranus as outermost planet in our solar system. But they believed something was gravitationally disturbing Uranus’ orbit, and they concluded another planet must exist beyond Uranus. The location of an eighth planet was mathematically predicted. Not long afterwards, in 1846, astronomers searching with telescopes found Neptune – the 8th planet – based on those predictions.
Yet the mystery wasn’t solved. More observations indicated that Uranus’ orbit was perhaps being influenced by yet another planet beyond Neptune. Astronomers referred to it as Planet X.
Enter Percival Lowell. He was a wealthy American businessman with a passion for astronomy. Lowell had gained fame for his notion of canals on the planet Mars. Then he got interested in Planet X. He established Lowell Observatory in Flagstaff and began to search. The search for Planet X remained a priority even after Lowell’s death in 1916.
Clyde Tombaugh was hired by Lowell Observatory in 1929 to continue the search Percival Lowell had begun. Tombaugh, born in 1906, grew up on a farm in Streator, Illinois. He’d dreamed of becoming an astronomer, but gave up the possibility of attending college after his family’s crops were destroyed by a hailstorm. However, he had taught himself mathematical skills required for astronomy, including geometry and trigonometry.
And he observed the skies with his homemade telescope.
When Tombaugh sent drawings he’d made of the planets Mars and Jupiter to Lowell Observatory in Flagstaff, Arizona – hoping to get back some advice – he instead received a job offer. Ultimately, he worked as an observer for Lowell Observatory from 1929 until 1945. After his discovery of Pluto, Tombaugh was awarded a scholarship and began studying astronomy at the University of Kansas, completing his formal education in 1939.
Hear Clyde Tombaugh’s voice on this 1949 radio show We the People. Shout-out to EarthSky community member Dan Schwarz for pointing it out!
All that time, the mystery of Pluto continued.
Very soon after Pluto’s discovery, astronomers realized it was too tiny to cause the supposed irregularities in Uranus’ orbit. Was there yet another planet lurking out there? This mystery would be solved about 50 years later. New calculations using an accurate mass determination for Neptune, obtained during Voyager 2’s 1982 encounter with the planet, eliminated the need for a Planet X to explain Uranus’ orbit.
Besides Pluto, there are now numerous bodies in the outer solar system, a realm now called the Kuiper Belt. Several of these worlds also carry the dwarf planet label, such as Haumea, Makemake, and Eris.
Astronomers believe we will eventually find many more small, spherical worlds in the outer solar system.
And what of Planet X? Interestingly, in 2016, astronomers from CalTech announced new theoretical evidence that a large planet – a Planet X – exists beyond the orbit of Pluto. The video below has more about their work. These astronomers hope this theoretical work will inspire other astronomers to search for Planet X.
And so astronomers are still speculating, and searching.
Bottom line: Clyde Tombaugh discovered Pluto on February 18, 1930, at the Lowell Observatory near Flagstaff, Arizona.
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