At first I thought we are a long ways off from an April fools prank. This appears to be for real. A couple of Caltech researchers Konstantin Batygin and Mike Brown, have formulated a very intriguing argument for another planet on a widely eccentric orbit, far beyond the Kuiper belt.
There are some known Kuiper belt objects that all seem to have a related orbit. After ruling out a couple of scenarios they found one theory that aligns with the evidence in hand. There is another planet out there.
As of now this is all theoretical, no one has observed this new planet. But there is a lot of evidence indicating it is out there. Stuff like this is what gets me up in the morning. I have said for decades that if you do not learn something every single day, you were not paying attention. These guys were paying attention. Even if this all gets washed away with upcoming observations or new evidence to the contrary. They were still paying attention, and using their skills to solve a solar system oddity. Click the link!