From 1905 until his death in 1916, Percival Lowell searched for a ninth planet, which he called Planet X. Lowell had predicted the planet based on irregularities in the orbit of Uranus.
In 1929, Lowell Observatory Director V.M. Slipher hired self-taught astronomer Clyde Tombaugh to resume Lowell’s search for the planet. Tombaugh captured long-exposure photos of the sky and compared them with a blink comparator, which could switch rapidly between plates to check for differences and changes — for example, a planet moving across the sky. On Feb. 18, 1930, Tombaugh saw just such a movement in images of Gemini from the previous month. He verified the object’s movement on other plates, then told Slipher: “I have found your Planet X.”
After confirming the find, Lowell Observatory announced the ninth planet on March 13, 1930 – Percival Lowell’s birthday. An 11-year-old girl from England proposed the name Pluto, which was officially accepted by the American Astronomical Society later that year.
