Nibiru is a possible planet (or perhaps a very large asteroid) that is theorized to orbit the solar system.
Nibiru is said to be extremely large (4x bigger than Jupiter ). While it was a very popular doomsday theory, it was unlikely that Earth would be hit in December of 2012.
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.