Pluto may not be a planet, but it definitely exists. Not every case is so clear. The history of near-Earth astronomy contains many cases of bodies that may or may not have ever existed in the solar system we call home. In this episode of Stuff to Blow Your Mind, Robert Lamb and Joe McCormick continue their journey through space to survey once-hypothetical bodies that took work to prove, once-imagined bodies that are now confirmed phantoms, and the unsolved mysteries where questions remain. This time, the destination is a hypothetical planet beyond the orbit of Pluto.