Why did astronomers search for Planet X beyond Neptune?

Astronomers searched for a ninth planet, often referred to as "Planet X," beyond Neptune primarily because they observed irregularities in the orbits of known planets, specifically Uranus.12

In the early 20th century, the American astronomer Percival Lowell conducted an elaborate mathematical study of Uranus’s orbit. He concluded that the observed deviations from predicted paths were caused by the gravitational influence of an unseen, massive body located further out in the solar system. This hypothesis instigated a systematic search at the Lowell Observatory, which eventually led Clyde Tombaugh to discover Pluto in 1930.123

It was later determined that Pluto was too small to account for the gravitational perturbations originally attributed to Planet X. Ultimately, more accurate measurements of Neptune’s mass provided by the Voyager 2 space probe revealed that the perceived irregularities in the orbits of the outer planets were not real, effectively eliminating the scientific necessity for a massive Planet X.2