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 Web site:
http://www.badastronomy.com/bad/misc/planetx/science.html

Astronomers are debating what to call objects in the Kuiper Belt, a disk of icy bodies beyond Neptune. Pluto belongs to the Kuiper Belt, yet we call it a planet. But aside from Kuiper Belt objects, the prospects for new planet discoveries in our solar system are small.

However, there is growing evidence for planets orbiting other stars.

Find out more from other sources:

• For links to additional information (mostly technical) about planets outside our solar system, see The Extrasolar Planets Encyclopaedia
  http://vo.obspm.fr/exoplanetes/encyclo/encycl.html