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After the discovery of Neptune in 1846, eager astronomers combed the Solar System for signs of another planet that might lie beyond this gas giant. They had good reason to believe one might exist: Neptune’s discovery had directly resulted from French mathematician Urbain Le Verrier’s hypothesis that perturbations in the orbit of Uranus were caused by the gravitational pull of a yet-undiscovered planet. Le Verrier predicted the position of this new planet and sent his calculations to German astronomer Johann Gottfried Galle. With these coordinates, Galle discovered Neptune the very next day—exactly where Le Verrier had predicted. But there remained some discrepancies in the gas giants’ orbits even with the discovery of the blue planet, so the hunt for yet another distant planet in the solar system forged onward.

Percival Lowell, an early twentieth century astronomer, exhibited a particular obsession with this search for a ninth planet. Lowell christened this hypothetical new planet “Planet X,” and, in his 1915 Memoir on a Trans-Neptunian Planet, concluded that Planet X must have a mass roughly seven times that of Earth—about half that of Neptune—and a mean distance from the Sun of 43 AU.* When Lowell died suddenly in 1916, his observatory, led by the efforts of his widow, persisted in the search for the elusive Planet X.

Fourteen years later, an ambitious budding astronomer from a Kansas farming family made the first major discovery in the search for Planet X. In 1929, twenty-three-year-old Clyde Tombaugh arrived at the Lowell Observatory and was given the task of systematically imaging the night sky. Tombaugh captured sections of the sky in pairs of images taken two weeks apart. He then placed the paired images into a machine called a blink comparator, a microscope that superimposes two photographic plates, “blinking” rapidly between them and creating a time-lapse illusion of the movement of any planetary body. In February 1930, after searching for almost a year and examining nearly two million stars, Tombaugh discovered a moving object on photographic plates taken in the previous month. The object lay just six degrees from one of the two locations for Planet X that Lowell had suggested, and it seemed as though, at long last, Lowell had been vindicated.

This supposed new planet was named Pluto, in part to honor Percival Lowell, as his initials make up the first two letters of the word. However, it did not take long before astronomers began debating Pluto’s status as a planet. Observations showed that Pluto was six times dimmer than Lowell had predicted. It also had a far more elliptical orbit than any other planet in the solar system. In 1978, Pluto was found to be too small for its gravity to affect the gas giants, a discovery that resulted in a search for a tenth planet, which was eventually abandoned in the early 1990s when the Voyager 2 spacecraft found that irregularities in Uranus’s orbit were actually due to a slight overestimation of Neptune’s mass.

In 1992, the discovery of numerous small, icy objects similar in size and orbit to Pluto led to a more vocal debate over whether it should remain a planet or whether Pluto and its asteroid neighbors should be given their own separate classification. In 2006, the International Astronomical Union ultimately reclassified Pluto and its friends as “dwarf planets,” leaving the Solar System, once again, with only eight planets. It is not likely that Pluto will ever be called an official planet again, either. Mike Brown, who discovered Eris, a dwarf planet larger than Pluto, in 2005, speculates that there are likely thousands of other “rocks” like Pluto orbiting in the Kuiper belt outside of Neptune.

Today, the astronomical community is largely in agreement that Planet X as it was originally conceived does not exist, but Planet X as a concept has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System. In popular culture, Planet X has become a stand-in term for any undiscovered planet in the Solar System, regardless of whether or not it fits into Lowell’s original theory. Thus, real or not, Planet X remains a fixture in the astronomical universe.

*An AU (astronomical unit) is roughly the distance from the Earth to the Sun.

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