February 18 marks the discovery of Pluto by astronomer Clyde W. Tombaugh. The discovery reshaped the understanding of the solar system, as Pluto was long considered the ninth planet until the International Astronomical Union (IAU) reclassified it as a dwarf planet in 2006.
The search for Pluto began decades earlier with astronomer and author Percival Lowell. Lowell was known for his theories on extraterrestrial life, which he explored in books such as “Mars and Its Canals” and “The Evolution of Worlds”. While his books were very popular among science-fiction writers and the public, many astronomers remained skeptical of his ideas. As a result, Lowell sought to strengthen his scientific credibility by searching for a ninth planet.
Because Neptune had been discovered through its gravitational influence on Uranus’s orbit, Lowell and fellow astronomer William Pickering believed irregularities in the planet’s orbit could mean the possibility of another planet. In 1905, Lowell obtained a camera and began a dedicated search, convinced of the planet’s location. He referred to the undiscovered planet as “Planet X”. However, after ten years of looking, Lowell lost hope in his pursuit and died in 1916 without confirming the existence of Planet X.
In 1929, the Lowell Observatory in Arizona revived the hunt using a 13-inch telescope camera. Drawing on Lowell and Pickering’s findings, Clyde W. Tombaugh led the new efforts. On February 18, Tombaugh finally identified the presence of Planet X by comparing photographic plates taken on different days. After verification by other astronomers, the discovery was officially announced on March 13, 1930, which would have been Lowell’s 75th birthday.
Following the announcement, the observatory received numerous letters proposing new names for the planet. One proposal from eleven-year-old Venetia Burney from Oxford, England, who suggested the name Pluto. The name derives from the Roman god of the underworld (known as Hades in Greek mythology. The American Astronomical Society and the Royal Society both formally accepted the name later that year.
Future research revealed Pluto has a surface temperature of -360 °F. The orbit is highly eccentric, taking 248 years to complete, with an orbital distance of 4 billion miles from the Sun. Pluto also has the most elliptical and inclined orbit of any planet, and at its closest approach to the Sun, it passes inside Neptune’s orbit, the eighth planet.
Pluto’s discovery was extremely popular, capturing global attention. The Associated Press named it one of the top news stories of 1930. However, less than a century later, the IAU revised its definition of a planet, requiring that the orbit is around the sun, be rounded by its own gravity, and “clear the neighborhood around its orbit.” Because Pluto’s orbit does not dominate that of other objects, such as those in the Kuiper Belt, the planet was reclassified as a dwarf planet. The decision proved controversial, sparking ongoing debate among scientists and the public over Pluto’s place in the solar system.
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