Percival Lowell, the astronomer who predicted the existence of a planet beyond Neptune, was born on this day in 1855. And on his birthday in 1930, astronomer Clyde W. Tombaugh announced the discovery of Pluto.
According to Lowell’s calculations, the only way Uranus and Neptune should be in their current orbits is if another large planet lurked just out of sight in the far reaches of the solar system, subtly shaping the ice giants’ orbits with its gravitational influence. He dubbed the undiscovered world Planet X, and in 1906 he commenced an enthusiastic search for it.
Thirteen years after Lowell’s death, Tombaugh – working at the Flagstaff, Arizona observatory that Lowell had founded in 1894 – announced that there was, in fact, a planet just about where Lowell’s calculations said Planet X should be. He made the announcement on March 30, Lowell’s birthday, and for 48 years it appeared that the late astronomer had been posthumously vindicated, at least when it came to Planet X. Lowell also famously promoted the theory that there were intelligent, canal-building aliens on Mars, an idea that the general public found fascinating and other astronomers found unlikely.
Then, in 1978, astronomer James Christy discovered Pluto’s moon Charon (although today we see Pluto and Charon as more of a binary pair than a planet and its moon), and observing the pair’s orbital dynamics allowed astronomers to calculate Pluto’s mass for the first time. Pluto, it turned out, was far too small to have an effect on the ice giants’ orbits. Data from Voyager 2, which flew by Uranus and Neptune 11 years later, in 1989, showed that Lowell’s calculations were actually a bit off thanks to an imprecise estimate of Neptune’s mass. Planet X had really been Neptune the whole time (and it would have gotten away with it, too, if it weren’t for those meddling space probes).
But the story may not be over yet. Some astronomers say there’s a growing body of evidence that a super-Earth may be waiting to be discovered out in the cold darkness, 20 times further from the Sun than Neptune.