Once the Solar System's Ninth Planet, Pluto Was Reclassified 20 Years Ago. Here's Why Planetary Scientists and the Public Alike Love the Small Icy Body

A 24-year-old observatory assistant from the Midwest discovered Pluto. Clyde Tombaugh, born in 1906 in Streator, Illinois, grew up fascinated with the solar system. A self-taught astronomer, he built his own telescopes to observe Mars and Jupiter.

Tombaugh wrote a letter to the Lowell Observatory in Flagstaff, Arizona, asking for a professional opinion on his celestial drawings, and the staff ended up offering him a job as an assistant, searching for a hypothesized, elusive ninth planet. On February 18, 1930, at the observatory, he was studying photographic glass-plate images he had taken with a telescope. Using a blink comparator, he examined two plates captured days apart, and noticed a tiny dot featured on both of them but in different locations.

That dot was the small and icy Pluto. Tombaugh had found what he and observatory leadership were seeking. After more studying, on March 13, 1930, the observatory team publicly announced the discovery of the planet.

That day would have been the 75th birthday of astronomer Percival Lowell, who had set up the eponymous observatory in 1894. Lowell had suspected that a ninth planet, which he referred to as Planet X, was out there, beyond Neptune. In 1905, he began to formally search for it.

But Lowell died in 1916, after spending the last 11 years of his life, off and on, looking for Planet X. Though he never got to witness Pluto’s discovery, the finding at his own observatory was the realization of his dream. Every February, the Lowell Observatory hosts its I Heart Pluto Festival.

Observatory staff members are already gearing up for Pluto’s centennial in 2030. Now, 20 years after the International Astronomical Union (IAU) voted, on August 24, 2006, to remove Pluto from its ninth-planet status and redesignate it as a dwarf planet, the enthusiasm for the icy body endures. In fact, that decision sparked even more fascination with Pluto, says Schindler.