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. Labeled 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.