What if I said "Yes!" ? Could you prove that I hadn't?
This is the whole point of the matter. There is absolutely no evidence for Sitchin's Nibiru, save his interpretation of ancient writings and reliefs.
Besides, if I recall correctly, didn't Sitchin claim that Nibiru collided with Earth billions of years ago, thus giving rise to life on our planet? So it can't have been a recent addition to the Solar System.
Correct me if I'm wrong...