Today in science: Clyde Tombaugh discovers Pluto

Daytime photo of a young man standing next to a telescope pointed at the sky.
Clyde Tombaugh at his family’s farm with his homemade telescope in 1928, 2 years before his discovery of Pluto. Image via Wikimedia Commons.

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.

Full disk image of Pluto taken by New Horizons.
Composite image of Pluto – as seen by NASA’s New Horizons spacecraft when it swept past in 2015 – showing what’s informally called Pluto’s heart, and officially called Tombaugh Regio. This heart-shaped region is a vast plain on the planet covered with nitrogen ice. New research shows that Pluto’s renowned nitrogen heart rules its atmospheric circulation. Image via NASA / JHU APL / SWRI.

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.

Man in early 20th century clothing, sitting at the eyepiece of a large telescope.
Wealthy American businessman/ astronomer Percival Lowell popularized the search for a planet beyond Neptune. Here he is in 1914, at the 24-inch telescope at Lowell Observatory in Flagstaff, Arizona. Image via Wikimedia Commons.

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!

Read more about Percival Lowell’s search for Pluto via Lowell Observatory

Clyde Tombaugh looking into eyepiece of a device to 'blink plates' at Lowell Observatory.
Clyde Tombaugh using a device to ‘blink plates,’ that is, to click back and forth between two images of the same patch of sky, taken on two different nights.On those two images, the distant background stars would not appear to move, but closer objects would move from one night to another. Comparing thousands of images, Tombaugh discovered Pluto. Image via Lowell Observatory.

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.

In 2006, the International Astronomical Union changed the status of Pluto from one of nine major planets in our solar system to a dwarf planet.

Spacecraft image of mountains on Pluto.
New Horizons images of a region near Pluto’s equator revealed a range of young mountains rising as high as 11,000 feet (3,500 meters) above the surface of the icy body. Scientists base the youthful age estimate on the lack of craters in the image above. Like the rest of Pluto, this region would presumably have been pummeled by space debris for billions of years and would have once been heavily cratered — unless recent activity had given the region a facelift, erasing those pockmarks. Image via NASA/ JHU APL/ SwRI.
Dark silhouette of backlit Pluto, surrounded by blue haze.
The blue skies of Pluto as seen by New Horizons after closest approach, with Pluto backlit by the sun. It is one of the most iconic images of the mission. Image via NASA/ JHUAPL/ SwRI.

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.

February 18, 2020

Like what you read?
Subscribe and receive daily news delivered to your inbox.

Your email address will only be used for EarthSky content. Privacy Policy
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

More from 


View All