IIRC, there was the Planet X search ~100 years ago. Much later it was pointed out either there was a miscalculation of Neptune's mass or a slight issue with Newton's theory. Since then it we know that Planet X does not exist.
Now we have the search for Planet 9, details for both:
Now we have the search for Planet 9, details for both:
https://en.wikipedia.org/wiki/Planets_beyond_Neptune