What is Planet X?

In the 1930s, when American mathematician and astronomer Percival Lowell decided to search for a planet beyond Neptune, he called the hypothetical planet “Planet X.” This search led to the discovery of Pluto, but for many years some astronomers believed that another world larger than Pluto must exist undiscovered beyond Neptune. They thought this because Neptune's orbit seemed to be influenced by the gravity of an unseen planet. More recent studies indicate that a large Planet X most likely does not exist, but the term Planet X is still used in pop culture for an undiscovered planet in our solar system.