Nibiru is a possible planet (or perhaps a very large asteroid) that is thought to orbit the solar system.

Nibiru x ray-1-

Nibiru is said to be EXTREMELY Large (about as big as Jupiter ). While it is a very popular doomsday theory it`s (thankfully) unlikely.

For it to be as big as theorized it would have to be very far away from the solar system, to not adjust another planet`s orbit and to be out of telescopic sight.