Planet X is a large hypothetical planet with an orbit beyond that of Neptune. The scientific basis of the Planet X hypothesis was broadly discounted in the early 1990s.

The "X" mentioned in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for ten. Although Pluto was discovered as a result of the search for Planet X, it is not considered Planet X. Neither is Eris, even though it was at one point considered for reclassification as a planet under a proposal outlined by the International Astronomical Union.

In popular culture, "Planet X" has become a generic stand-in term for an undiscovered planet in the solar system. Its use by scientists, however, is exclusively in reference to the particular hypothesis.

A lot of scientist and claim it does and that we will come to know it in our life time! These scientists a not mad either they are well respected in their field. One of whom is Zecharia Sitchin. Either way you decide whether or not it’s true. Here is a great site with lots of info on the subject. Post your thoughts about this here!