What is Planet X?

When astronomer Percival Lowell decided to search for a planet beyond Neptune in the 1930s, he called the object of his search "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 Neptune's orbit is not influenced by undiscovered bodies, and that a large "Planet X" most likely does not exist.

Some of the myths and misconceptions about Planet X are debunked on Phil Plait's Bad Astronomy website:

The prospects for new planet discoveries in our solar system are small. However, astronomers are now finding hundreds of planets orbiting other stars.

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