Planet X is a hypothetical planet purported to be on a collision course with the Earth, though this hypothesis has been refuted by most of the mainstream astronomical community. However, there is strong evidence for the presence of a number of large objects in the Oort cloud outside Pluto's orbit.
Originally, Planet X referred to a planet predicted by French mathematician Urbain Le Verrier in 1940 to have an orbit outside that of Uranus, a hypothesis that later turned out to be true with the discovery of the planet Neptune. Since then, astronomers and mathematicians have followed small variations in planetary and asteroid orbits to identify several similar anomalies. Astronomers have used these anomalies to locate Pluto and a number of other dwarf planets, but there's never been any evidence that these planets were headed toward the inner solar system. They are, however, often referred to as Planet X.