A year ago, astronomers announced evidence for the existence of an undiscovered planet hiding out in the boonies of our solar system. Nobody's found the mystery world yet, but that hasn't stopped scientists from speculating about how such a planet could have gotten there.

New simulations, presented this week at the American Astronomical Science meeting, suggest that it may have once been a rogue planet, stolen by our sun.

If it's out there, Planet Nine is expected to be 10 times the size of Earth and 1,000 times farther from the sun. By comparison, Pluto is about 40 times farther than Earth, so Planet Nine is a pretty big outlier. How did it get so far away?