...And no, it's not Pluto (RIP). Researchers have found evidence that there may very well be a ninth planet, aptly named "Planet Nine" or "Planet X," in our solar system that's about the size of Neptune.
The catch? No one has been able to find it. Yes, you read that correctly. This hypothetical planet is so far away that it takes between 10,000 and 20,000 earth-years for it to complete a full rotation around the sun.
Sadly, no scientist has been able to concretely prove the existence of such planet; they've simply predicted its existence based off of the similarly peculiar orbits of several objects in the Kuiper belt.
Still, we're going to hold onto hope that Planet Nine exists because that's just straight-up cool as hell.