Is there a massive undiscovered planet on the outer reaches of the Solar System? The idea has been around since before the discovery of Pluto in the 1930s. Labelled as planet X, prominent astronomers had put it forward as an explanation for Uranus’s orbit, which drifts from the path of orbital motion that physics would expect it to follow. The gravitational pull of an undiscovered planet, several times larger than Earth, was seen as a possible reason for the discrepancy.
That mystery was ultimately explained by a recalculation of Neptune’s mass in the 1990s, but then a new theory of a potential planet nine was put forward in 2016 by astronomers Konstantin Batygin and Mike Brown at Caltech (the California Institute of Technology).
