13 March 1930: Discovery of Pluto Confirmed

On 13 March 1930 astronomers at the Lowell Observatory in Flagstaff, Arizona telegraphed the Harvard College Observatory with the news of the discovery of a new planet, the first since Neptune in 1846. The discovery had been made on 18 February by Clyde Tombaugh less than a year after he had been assigned the task of searching for the ninth planet.

In the late 19th century observations of the orbits of Uranus and Neptune suggested that they were being affected by another planet. In 1906 Percival Lowell, founder of the Lowell Observatory, started a search for the elusive “Planet X”.

Lowell’s ten year search came to an end with his death in 1916. A battle with Lowell’s widow, Constance, over the terms of a funding bequest in his will put the project on hold until 1929 when a new astronomical camera was built.

Tombaugh was hired and set to work examining the photographs taken of the night sky by the new camera. He compared pairs of photographs taken on different nights and attempted to determine if any of the objects shown had moved.

On 18 February 1930 Tombaugh examined photographs taken on the nights of 23 and 29 January. He noticed an object in the plates that appeared to have moved. This was his first view of Pluto. Further photographs of the same patch of night sky confirmed the movement and the discovery was announced to the world through the telegraph to Harvard on 13 March.

After collecting over a thousand suggestions for the new planet’s name it was officially designated Pluto on 24 March. The name Pluto, the Roman god of the underworld, had been suggested by Venetia Burney, an eleven-year-old girl from Oxford.

Over the course of the 20th century estimates for the size of Pluto were reduced considerably. With a radius of only 1,172 km many astronomers questioned its planetary status. The discovery of objects in the Kuiper belt of a similar, or possibly larger, size forced a reassessment of Pluto’s classification. In 2006 the International Astronomical Union (IAU) created a new category of objects called dwarf planets and placed Pluto within it, leaving only eight planets in our solar system.

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