The appearance of myth about “Nibiru planet” which is connected with the End of the World can be associated with the revival of the old myth about “Nemesis star” with some modifications. You can compare both of them and you'll see that the myth about Nibiru tells about the planet which has a complete revolution in 3.6 thousand years and the Nemesis star – 32 millions years. In both cases authors mention the End of the World and catastrophic changes for Earth.

So, what reasons do we have in a favor of the argument that these “Space objects” are myths?

The time of the existence of our Solar system is estimated in 5 billion years, our Earth is 4.5 billion years. Simply by dividing to the period of rotation of mythical planets it is possible to find the number of revolutions which have made fictitious bodies: for Nibiru - 5 billion years / 3.6 thousand years = 1.4 million times; for Nemesis - 5 billion years/ 32 million years = 156 times.

Please note that we are not talking about one or two revolutions. We are talking about hundreds and millions. If fictional planets must gravitationally affect to all Solar system bodies (and not selectively, exclusively to Earth!), we would have already found traces of such influences in the form of disturbances (changes) of the orbits. Even one pass of a massive body around the main asteroid belt located between Mars and Jupiter is enough to significantly change the orbits of some of these bodies.

Considering the periodicity of such impacts and the limited number of massive bodies, the American astronomer J. Kirkwood, back in 1866, came to the conclusion that the resonances in the motion of bodies in Solar system exist, in particular, his prediction about the resonances in the motion of asteroids (zone of avoidance) has been called "gaps Kirkwood". The existence of orbital resonances guarantees the absence of gravitational effects of the unknown mythical planets on short time scales, because even with a single pass of unknown massive body some of them would be lost.

Authors of myths believe that planet Nibiru and Nemesis are natural celestial bodies, therefore, their movement must be very well predicted by the laws of celestial mechanics. If you expect the passage of such bodies near Earth's orbit in 2012 it must be assumed that the remaining 2 years they will fly over a distance approximately equal to the distance from Earth to Jupiter. Even our little Moon referred to the distance of Jupiter's orbit will have stellar brightness of 7th magnitude (note that reflective power of Moon is 0.136, which describes its surface as a dark).

Thus, naturally, the massive mythical body, intersecting at the present time the orbit of Jupiter, is quite bright and easily detectable from Earth. From the study of statistic of asteroid discoveries it can be concluded that nearly all the asteroids with absolute magnitude 16 and less are open (absolute magnitude - is the measure of a celestial object's intrinsic brightness). This limit corresponds to 22.7 magnitude and less for an object Jupiter's orbit. Thereby, if the mythical planet Nibiru existed it would have been observed much further than the Jupiter's orbit.