According to Sitchin's interpretation of Mesopotamian iconography and symbolism, outlined in his 1976 book The 12th Planet and its sequels, there is an undiscovered planet beyond Neptune that follows a long, elliptical orbit, reaching the inner solar system roughly every 3,600 years. This planet is called Nibiru (although Jupiter was the planet associated with the god Marduk in Babylonian cosmology). According to Sitchin, Nibiru (whose name was replaced with MARDUK in original legends by the Babylonian ruler of the same name in an attempt to co-opt the creation for himself, leading to some confusion among readers) collided catastrophically with Tiamat (a goddess in the Babylonian creation myth the Enûma Eliš), which he considers to be another planet once located between Mars and Jupiter. This collision supposedly formed the planet Earth, the asteroid belt, and the comets. Sitchin states that when struck by one of planet Nibiru's moons, Tiamat split in two, and then on a second pass Nibiru itself struck the broken fragments and one half of Tiamat became the asteroid belt. The second half, struck again by one of Nibiru's moons, was pushed into a new orbit and became today's planet Earth. Sitchin also speculated that Pluto was originally a satellite of Saturn but Nibiru's gravity perturbed it, sending it to the outer Solar System and giving the body its peculiar orbital path, intersecting the orbit of Neptune.