The discovery of Pluto – on February 18, 1930 – is a fascinating story,” said astronomer and author Ken Croswell. He told the story to EarthSky:
“Actually, the search for Pluto began a year before Clyde Tombaugh, the discoverer, was even born. It began with the work of Percival Lowell, who during his lifetime, was best known for another planet, not Pluto, but the planet Mars.
“Lowell thought that Mars had intelligent life. He thought that there were canals on Mars that the Martians had built. And he, as a result, suffered a lot of ridicule from other astronomers. (I should say that Lowell in the 1890s had actually founded an entire observatory, which still exists to this day in Arizona, to study Mars and the other planets.)
“In order to get even with these other astronomers who were ridiculing him for his Mars work, he wanted to discover a new planet in the solar system. He thought that would be the perfect revenge. He thought that if he could predict the location of another planet, and then find that planet, he would attain the prestige that had eluded him in all his work on Mars.
“And so he began his calculations, using observations of the motion of planet Uranus. He was hoping to see deviations in that planet’s motion around the sun that would signal the gravitational effect of a more distant planet beyond the planet Neptune. Unfortunately, Lowell died in 1916, before Pluto was ever found.
“But then in 1929, Lowell Observatory hired a young man from Kansas, by the name of Clyde Tombaugh. And he implemented a search for Pluto. Tombaugh was using a telescope that took photographic plates of the sky. During the day, he would examine those photographic plates for a moving object. The word ‘planet’ actually means ‘wanderer,’ something that moves night-to-night in the way that stars don’t.
“On February 18, 1930, a little after 4 p.m. Mountain Time. He saw Pluto for the very first time. And his first words to himself were, ‘That’s it!’
“The discovery of Pluto was inspired by Percival Lowell, and is a testimony to the hard work that Clyde Tombaugh put into searching millions of stars to find this tiny, faint object in space.”