In 2016, astronomers announced there was a new planet in the outer solar system. Planet Nine, supposedly larger than Neptune and located far beyond the orbits of the planets known so far, is a particular mystery since no one has yet observed it. Scientists have merely tracked its supposed orbit by watching the gravitational pull the planet exerts on the asteroids and space debris near it.

But this isn’t the first time scientists have predicted planets before spotting them. It’s not even the first time they’ve done it because of hard-to-explain orbits. And they’ve usually been wrong.

The solar system grows

Way back in 1845, two astronomers named Urbain LeVerrier and John Couch Adams independently looked at the orbit of Uranus as measured by astronomers, and the predictions of its orbit based on Kepler’s Laws and noticed something was amiss. Unaware of each other’s work, they both concluded that another planet must be present to perturb the orbit in such a way, and each calculated its position and mass.

LeVerrier worked with more alacrity, and gave his predictions to astronomers Johann Gottfriend Galle and Heinrich Louise D’Arrest, who found the body the very next night, exactly where LeVerrier had predicted. Neptune joined the pantheon of planets, and Adams wished he had worked faster.

But after watching Uranus and Neptune for a few decades, astronomers decided their orbits still didn’t seem quite right. Reasoning varied: Some didn’t think Neptune’s mass could perturb Uranus into its observed orbit, others thought Neptune’s orbit wasn’t quite correct.

Ultimately, many astronomers convinced themselves another planet must be hiding out there beyond Neptune’s orbit. And the hunt began again. Percival Lowell in particular dedicated himself to hunting for the so-called Planet X, even founding a famous observatory toward that end that still bears his name. He died before the mystery was resolved, though not before feuding with other astronomers about the qualities (and quantity) of the undiscovered planet(s).

In 1930, Clyde Tombaugh, working at Lowell Observatory 14 years after its founder’s death, found a tiny speck moving slowly along the outer solar system. Pluto was celebrated as the ninth planet. But there were problems almost immediately. The long-sought planet wasn’t big enough to have disturbed the gas giants’ orbits in the way scientists were observing. It was proved, fairly quickly, that Pluto could not the be planet scientists had been looking for.

In 1992, updated Voyager data revealed a slightly embarrassing mistake: Neptune and Uranus’ orbits are fine, and always were (once Neptune entered the picture). Neptune simply was less massive than originally thought. But we got a planet, Pluto, out of the mistake — at least for 76 years.