(Off Topic) Does "Planet X" exist, and could it come close to Earth?
Is there really a 10th planet?
Many people are fascinated by the idea of finding another planet in our Solar System, but as yet there is no good evidence that there is another large body beyond Pluto.
There are several "Planet X" stories that circulate from time to time in the popular press.
The more scientific Planet X story concerns several astronomers who, after analyzing the orbits of a large number of comets, suggest that there may be a large body in the Oort cloud which is disrupting the orbits of some of the long-period comets that we see, so that they swing into the inner Solar System. Other astronomers have done a similar analysis, though, and they do not see any evidence for a planet. Most astronomers would need better evidence to be convinced of the existence of another planet.
A less scientific Planet X story concerns Zechariah Sitchin, who published a book called The Twelfth Planet in 1976. (Ancient peoples considered the Sun and the Moon to be planets because the move with respect to the stars, so those plus the usual nine give a total of eleven planets that we can all agree on.) Through a combination of some suspect Sumerian archaeology and a complete ignorance of the laws of physics, Sitchin cooked up a hypothesis about a giant planet named Marduk or Nibiru. According to Sitchin, Nibiru swings through the inner Solar System every 3600 years, and the planet and its angel-like alien inhabitants are responsible for everything from the tilt of Uranus to the origin of the human race to the parting of the Red Sea.
I hear that some follower of Sitchin recently predicted the return of Nibiru in the spring or early summer of 2003. Needless to say, Nibiru has yet to be detected by astronomers. [
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Source: Curious About Astronomy? Ask an Astronomer
See also: 10 FAQs about Planet-X (The Astronomy Cafe)