Discovery of Pluto

92 Years Ago, Here’s How A Young American Astronomer Discovered Pluto Within a Single Week.

When Clyde Tombaugh built his first telescope at the age of 20, he didn’t even know that it was setting him forward on a path that would eventually lead him to discover the first-ever known dwarf planet, Pluto. The conclusive evidence regarding an unknown ninth planet beyond Neptune was instrumental in establishing Clyde Tombaugh as the first American to have discovered a planet.

Composite image of Pluto – as seen by NASA’s New Horizons spacecraft when it swept past in 2015. (Image via NASA / JHU APL / S
Composite image of Pluto – as seen by NASA’s New Horizons spacecraft when it swept past in 2015. (Image via NASA / JHU APL / SWRI.)

Although the fact that whether Pluto was a planet or not has remained controversial ever since its discovery, and it was regarded as a dwarf planet after it was found to not abide by the definition of the termplanet,” as provided by the International Astronomical Union (IAU). Still, Pluto’s discovery paved the way towards discovering several astronomical bodies orbiting the Sun in the same region as Pluto, now known as the Kuiper belt region. This makes Pluto’s discovery to be considered of supreme importance in the history of observational astronomy.

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Clyde’s Association With The Lowell Observatory:

As a young researcher, Clyde worked for the Lowell Observatory in Flagstaff, Arizona. His journey in the Lowell Observatory started interestingly and unexpectedly. Unhappy with the store-bought telescopes, Tombaugh used to construct his own telescopes. In 1928, he put together a 23-centimeter reflector out of a 1910 Buick crankshaft along with parts of a cream separator.

Using this telescope, young Clyde made detailed observations of Jupiter and Mars, which he sent to Lowell Observatory to garner some feedback from professional astronomers. However, instead of receiving some constructive criticism, Tombaugh was surprisingly given the job to perform a systematic search for a trans-Neptunian planet (also called Planet X), which Percival Lowell and William Pickering predicted. And this is how the journey towards the discovery of Pluto by Clyde Tombaugh began.

Clyde Tombaugh at his family’s farm with his homemade telescope in 1928, 2 years before his discovery of Pluto. Image via Wikimedia Commons.
Clyde Tombaugh at his family’s farm with his homemade telescope in 1928, 2 years before his discovery of Pluto. Image via Wikimedia Commons.

The Hunt For “Planet X” :

Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus’s orbit was being disturbed by another planet besides Neptune. This unknown planet was known as Planet X. Lowell, and his observatory conducted this search until he died in 1916, but to no avail until Clyde took over the responsibility.

The telescope which Clyde was supposed to operate was equipped with a camera that was used to take two photographs of the sky on different days. A device known as a blink compactor rapidly flipped back and forth between the two photographs. Stars and galaxies essentially remained unmoving in the images, but anything closer could be visually identified by its motion across the sky. Tombaugh spent approximately a week studying each pair of photographs, which contained over 150,000 stars and sometimes nearly a million. 

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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. (Image via 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. (Image via Lowell Observatory)

The discovery of Planet X (later named Pluto) :

On Feb. 18, 1930, Tombaugh noticed the movement of a starry speck that moved in relative position from one plate to the other by the amount he had hoped for. It was planet X. After studying the object to confirm it, Lowell Observatory’s staff officially announced the discovery of a ninth planet on March 13, which was later named Pluto.

Original plates from Clyde Tombaugh's discovery of Pluto in Lowell Observatory Archive. (Image credit: Lowell Observatory)
Original plates from Clyde Tombaugh’s discovery of Pluto in Lowell Observatory Archive. (Image credit: Lowell Observatory)

Pluto was considered a planet at the time of discovery but was later controversially reclassified as a dwarf planet in 2006, though this came out to be heartbreaking for many!

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