Do You Know the Way to Planet X?

Where is Planet X? And for that matter, what is it?

The search for Planet X began in 1841 as the search for the eighth planet in our solar system and continues today as the search for the
ninth. Planet X was first renamed Neptune (we know where that is), then Vulcan (Urbain Le Verrier's intra-Mercurial planet), then Pluto (a planet no more), then Niburu (Zecharia Sitchin's "12th planet") then Xena (named after the protagonist of the TV series "Xena, Warrior Princess"), and now Tyche (a hypothesized gas giant planet located in the Solar System's Oort cloud). Planet X is not a real planet, but rather a placeholder for planets yet to be found. In a mathematical sense, it is a variable: X = n +1, where n is the number of the last discovered planet.