Are astronomers still looking for "Planet X," a 10th planet in our solar system?

For most of the 20th century, some astronomers were convinced that a 10th planet — Planet X — lay somewhere beyond Neptune. They inferred the existence of such a planet by its apparent effects on the positions of Uranus and Neptune. Both planets seemed to be tugged along slightly faster than expected, as if in response to the gravitational pull of another body. In 1993, astronomer Myles Standish of the Jet Propulsion Laboratory in Pasadena, California, performed a thorough reanalysis...