In both cases, there was an anomaly, an oddity in the motion of known objects—the planet Uranus for Neptune; six distant bodies in the Kuiper Belt for Planet Nine. The Kuiper Belt is a swarm of dwarf planets and even smaller objects beyond Neptune of which Pluto is the most famous (and contentious) member. Le Verrier had shown that Neptune could settle Uranus’ books, while Batyagin and Brown account for the peculiar behavior of the relevant Kuiper Belt objects (KBOs) by invoking a planet roughly ten times the mass of Earth.

In 1846, the discovery of Neptune turned Le Verrier into a celebrity; for a time, he was the most famous man of science in the world. He went on an international tour and seized the moment to rise to the top of power in the highly contentious and hierarchical world of French astronomy. Batygin and Brown are taking a much more measured tack with Planet Nine—and for good reason. “We felt quite cautious about making the statement we made,” Batygin says. Why such concern? Because, he says, “immediately after the detection of Neptune spurious claims of planets in outer solar system began to surface. We didn’t want to be another red herring.”

There aren’t any obvious errors in Batygin and Brown’s gravitational argument, but nature has plenty of ways to fool astronomers into seeing planets where there are none. Any mass exerts (as Newton saw it) a pull on everything else, and Newton’s universal law of gravitation describes how strong that tug will be, and what motion would result. In the case of Neptune and, presumptively, Planet Nine, undiscovered objects reveal themselves in the unexplained residues of motion of what’s already been observed, once all the known gravitational influences have been tallied up.

That may sound simple, but gravitational glitches can be deceiving. Le Verrier himself was one of the first to be fooled by the impeccable logic of Newton’s theory. After Neptune, he turned his attention to the inner solar system, and he showed (correctly) that Mercury’s orbit wobbles in a way that a Newtonian accounting couldn’t explain. With his own recent triumph so fresh in his—and everyone else’s—mind, the explanation was obvious: There must be a planet hidden in the ferocious glare of the sun, one soon dubbed Vulcan. Within months of making that prediction, Le Verrier trumpeted as a settled discovery an amateur astronomer’s claim to have seen Vulcan crossing the face of the sun. Over the next two decades, at least a dozen other reputable observers reported similar sightings.

And yet, Vulcan doesn’t exist. Le Verrier’s argument for why it should have been real is perfectly consistent, and it matched the state of knowledge at the time—but it turns out that Mercury travels close enough to the great mass of the sun to be subject to the effects Albert Einstein would describe in 1915, with his General Theory of Relativity. The moral of this story isn’t that scientists make mistakes that get resolved through further work. Rather, it lies with the fact that so many observers persuaded themselves—and Le Verrier—that Vulcan was so clearly necessary that it had to be real. That same treacherous combination of hope and expectation has bedeviled planet hunters searching the space beyond Neptune.