Nibru:the Villain

Nibiru's orbit around the Sun is highly elliptical, according to Sitchin's books, taking it out beyond the orbit of Pluto at its farthest point and bringing it as close to the Sun as the far side of the asteroid belt (a ring of asteroids that is known to occupy a band of space between the orbits of Mars and Jupiter). It takes Nibiru 3,600 years to complete one orbital journey, and it was last in this vicinity around 160 B.C.E. As you can imagine, the gravitational effects of a sizable planet moving close to the inner solar system, as it is claimed for Nibiru, could wreak havoc on the orbits of other planets, disrupt the asteroid belt and spell big trouble for planet Earth."

June 19, 1982 edition of the New York Times:

Something out there beyond the furthest reaches of the known solar system is tugging at Uranus and Neptune. A gravitational force keeps perturbing the two giant planets, causing irregularities in their orbits. The force suggests a presence far away and unseen, a large object, the long-sought Planet X. Astronomers are so certain of this planet's existence that they have already named it "Planet X - the 10th Planet."

What Will Be the Effects on Earth?
As stated before, the gravitational pull of a planet entering the inner solar system would have profound effects on the other orbiting bodies, including Earth. In fact, the Anunnaki story says that a previous appearance of Nibiru was responsible for the "Great Flood" recorded in Genesis, in which nearly all life on our planet was extinguished (but saved, thanks to Noah). Going even further back, some researchers into this topic suspect that Nibiru once even collided with Earth millions of years ago, creating the asteroid belt and resulting in the enormous gouges in our planet that the oceans now fill.
Mark Hazelwood and others say that Earth is in for some massive and catastrophic changes as Nibiru approaches. Floods, earthquakes, volcanic eruptions, a pole shift and other natural disasters will be so severe, Hazelwood says, that "only a few hundred million people will survive." Another site says the gravitational pull of Nibiru might even stop the Earth's rotation for three days, citing the "three days of darkness" predicted in the Bible.
Some of the Nibiru researchers also cite the prophecies of Edgar Cayce who predicted that we would soon suffer monumental Earth changes and a pole shift, even though he did not attribute them to anything as specific as a visiting planet. And, of course, the much-analyzed Mayan calendar is said to set the "end of world" in December, 2012.
Astronomers and other scientists who would seem to be in a position to know such things have made no announcements about the approach of any planet-sized body. Apparently, they have not detected anything of the kind. Those who believe Nibiru is approaching, however, say that scientists do know all about it and are just covering it up.