What would be most interesting, after proving that this object is a planet in our solar system, would be to calculate its orbit. Once that is done, you naturally know the period of its orbit, and how close it gets to Earth. Then you can correlate its closest approaches to Earth in the past with large events - if there are any, and there may not be any. It COULD explain a lot of the events of the past.
It may have a long period, and each time it swoops in, it also brings with it a retinue of various objects like comets, moons, rocks and debris rings to wreak havoc in the inner Solar System..................