Nibiru
The Sumerian culture describes, according to some sources, a planet Nibiru (or Marduk) being the twelfth planet. Nibiru is thus a hypothetical 12th planet or Planet X.
12th Planet
Nibiru, named from Sumerian astronomy, is supposely the twelfth member in the solar system family of planets (which includes 10 planets, the sun, and the moon). Its collision with Tiamet (Tiamat), a planet that was between Mars and Jupiter, formed the planet Earth and the asteroid belt and comets. It is reported that Nibiru stands for "Planet of the Crossing". It was represented in antiquity by a cross and a winged disk.
Pantheons
According to some being the supreme deity in the Sumerian and Babylonian pantheons. Though Nibiru is the god of 50 names and king of gods, according to others he is not related to Marduk in Babylonian mythology (Marduk being the a god of Babylon, who was inserted into the old creation epic)
Books
A suggestion in The 12th Planet by Zecharia Sitchin, who considered his proposal a serious theory. Sitchin's claims are generally considered pseudoscience by mainstream scientists and historians aware of the book. According to Zecharia Sitchin, Nibiru has an orbit around the sun of 3,600 Earth years. It is suggested that current astronomy points to the possibility that Nibiru is a brown dwarf or dark star rather than a planet. This has the implication that our solar system, like the majority in the known universe, is a binary star system; in other words, Earth has two suns with Nibiru being the second and less bright.
According to Sitchin, Nibiru/Marduk's inhabitants called Anunnaki (Ningischzida) survived and afterward came to Earth. Sitchin says some sources speak about the same planet, possibly being a brown dwarf star and still orbiting the Sun with a perihelion passage some 3600 years ago and assumed orbital period of about 3600 to 3760 years or 3741 years. Sitchin attributes these figures to astronomers of the Maya civilization, but the supposed sources are unfamiliar to Mayanists.
In a recently published book, titled 2012: Appointment With Marduk, Turkish writer/researcher Burak Eldem presents a new theory, suggesting a 3661 years orbital period for the planet and he claims a "return date" in the year 2012. According to Eldem's theory, 3661 is one-seventh of 25,627, which is the total time span of "5 World Ages" according to Mayan Long Count Calendar system. The last orbital passage of Marduk, he adds, was in 1649 BC and caused great catastrophes on earth, including the Thera Eruption.
Flaws
The brown dwarf idea is fatally flawed in that a brown dwarf with a period of 3760 years would be clearly evident through infrared and gravitational observations.