The Discovery of Pluto – 95 Years Later
On February 18, 1930, astronomer Clyde Tombaugh made a monumental discovery at the Lowell Observatory in Flagstaff, Arizona: the planet Pluto. At the time, the tiny celestial body would be considered the ninth planet of our solar system.
Of course, Pluto was later reclassified as a dwarf planet, but its discovery remains a significant milestone in astronomy.
The discovery of Pluto was the result of a long-standing search for a hypothetical ninth planet, known in the early 20th century as “Planet X.” Percival Lowell, an American astronomer and founder of the Lowell Observatory, had spent years searching for this unseen world. Based on irregularities in the orbits of Uranus and Neptune, Lowell and other astronomers theorized that a massive, undiscovered planet was exerting gravitational influence.
After Lowell’s death in 1916, the search continued under the direction of his observatory. Using a tool called a “blink comparator," Tombaugh painstakingly examined images taken on different nights, searching for a moving object against the background of fixed stars.
On February 18, 1930, Tombaugh spotted a faint dot shifting between images. Further observations confirmed that this was a new celestial body beyond Neptune. The discovery was announced on March 13, 1930 – coincidentally both Lowell’s birthday and the 149th anniversary of William Herschel’s discovery of Uranus.
The newly discovered planet needed a name, and suggestions poured in from around the world. Eventually, an 11-year-old English schoolgirl named Venetia Burney suggested “Pluto,” after the Roman god of the underworld.
Before Pluto’s discovery, Neptune was considered the outermost planet. We knew very little about the boundaries of our solar system. Pluto fueled interest in finding other trans-Neptunian objects, though no more were discovered for many decades.
For most of the 20th century, Pluto was accepted as the ninth planet. However, astronomers soon realized it was much smaller than we initially believed. It had a highly elliptical orbit, sometimes bringing it closer to the Sun than Neptune.
Pluto’s unusual characteristics led astronomers to suspect it wasn’t alone. In 1992, scientists confirmed the existence of the Kuiper Belt, a vast region of icy bodies orbiting beyond Neptune.
Pluto was not a lone oddity anymore. There were many objects like it, and that raised a debate that’s still controversial, even today. If Pluto is a planet in the same way as the other big eight, are all those other objects (many of which are very similar to Pluto) planets as well?
Eris’ discovery in 2005 reignited the debate over what defines a planet. In 2006, the International Astronomical Union (IAU) had to establish a formal definition: a planet must orbit the Sun, be spherical, and clear its orbit of debris. Because Pluto shares its orbit with many Kuiper Belt objects, it was reclassified as a “dwarf planet.”
No surprise, many astronomers and amateur enthusiasts weren’t happy. Pluto, after all, had decorated scientific posters in classrooms around the world as the ninth planet for nearly a century and had been an accepted part of the cosmological pantheon, if you will.
Many scientists (and the public resisted) the idea of Pluto losing its planetary status. The debate continues, and there are still groups that advocate for a broader definition that would reinstate Pluto as a planet.
Even though Pluto is no longer officially a planet, it still has plenty of value as an object of study. In 2015, NASA’s New Horizons spacecraft provided the first close-up images of Pluto, revealing mountains of water ice, nitrogen glaciers, and a possible subsurface ocean.
Whether it’s a planet or not, Pluto is forever enshrined in the mythos of astronomy’s history.
Check out these exclusive Pluto-themed gifts at The Space Store:
Pluto Globe
Thanks to the aforementioned New Horizons mission, we know a lot about Pluto’s geography, and this detailed globe features the most notable named features on the [dwarf] planet.
This bracelet still honors Pluto’s status as an honorary planet, no matter its classification. All beads are made of unique stones, with Pluto represented by Jasper.
For plenty of other popular gifts on sale, check out our daily deals section today while sales are hot.
- Written by Matt Herr