What will happen if planet X, Nibiru exist?
They don’t exist, so nothing will happen. Nibiru was predicted to appear in 2003. When it (unsurprisingly) failed to do so, its followers merely decided to latch on to the Mayan Calendar 2012 nonsense and postponed its arrival until then (easy to do when you’re talking about an imaginary object). Back in May 2003, the time of its alleged closest approach to the Earth when it was supposed to cause Earth’s poles to flip over, its followers were still drawing attention to earthquakes and storms (events which take place all the time anyway) desperately trying to convince themselves that these were caused by Nibiru, despite the total absence of any such object in the skies. They will probably do the same in 2012. These people are fools. Don’t take any notice of them.