An as yet undiscovered planet might be orbiting at the dark fringes of the solar system, according to new research.
Or might not.
Either way, is there anything more SF-y than an entire unknown planet? What is it like? How had no-one noticed before? Is it a new arrival, a captured rogue? Or was someone hiding it... on purpose?
Or maybe someone stole it. Or just put it there for us to find...