Marduk (or Nibiru) is a hypothetical 12th planet from Sumerian astronomy, which by its collision with Tiamet (Tiamat), a planet that was between Mars and Jupiter, formed the planet Earth and the asteroid belt and comets. This is suggested in The 12th Planet by Zecharia Sitchin; this claims to be a serious theory, but is widely considered false and baseless by scientists and historians.
According to Sitchin, Nibiru/Marduk's inhabitants called Anunnaki (Ningischzida) survivied 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.
The Sumerian Nibiru, also known as Marduk (not related to Marduk in Babylonian mythology), is described by some as being the twelfth planet or Planet X, supposely the twelfth member in the solar system family of planets (which includes 10 planets, the sun, and the moon).
Nibiru stands for "Planet of the Crossing". It was represented in antiquity by a cross and a winged disk; being the supreme deity in the Sumerian and Babylonian pantheons.
Nibiru is also the god of 50 names and king of gods. 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.
However, this idea is fatally flawed in that a brown dwarf with a period of 3760 years would be clearly evident through infrared and gravitational observations.