What is Planet X? Is it true it has been observed/detected?

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There is no astronomical body currently designated as 'Planet X'. A “planet X” has been variously postulated in the past to explain discrepancies in the orbit of Uranus and Neptune, which have since been disproven (for further info, see http://en.wikipedia.org/wiki/Trans-Neptunian_planets).

Currently, many objects past the orbit of Neptune have been discovered, but all of them are minor bodies belonging to the Kuiper Belt.

I hope this answers your question! Let me know if you would like further info.