Tiamat supposedly is the name of a former planet in our solar system. According to Sitchin's interpretation of the Sumerian creation tale, our solar system originally consisted of the Sun (Apsu) and nine planets. These were: Mercury (Mummu), Venus (Lahamu), Mars (Lahmu), Tiamat, Jupiter (Kishar), Saturn (Anshar), Pluto (Gaga), Uranus (Anu), and Neptune (Ea). Earth did not exist in those days, nor did the moon. Pluto was in between Saturn and Uranus. And there was a planet in between Mars and Jupiter, called Tiamat.
Then, a foreign celestial body, called Marduk, entered the solar system, clockwise, which is opposing the direction of the planets. The gravitational pull of Marduk changed the orbit of Pluto. The satellites of Marduk collided with Tiamat. Tiamat broke into two parts: one part was thrown in an orbit closer to the sun, in between Venus and Mars and became Earth. The other half was destroyed in the collision with other satellites and created the asteroid belt. Parts of the satellites became the meteors, that keep on having a clockwise orbit, unlike the planets and the asteroids, that go counter-clockwise. Through the gravitational pull of Tiamat, and after having been hit by minor satellites too, Kingu, Tiamat's moon, was thrown into another orbit too, and became Earth's moon.
Now Marduk didn't experience any collisions itself, but its orbit was changed. It was thrown into a major orbit around the Sun, clockwise, that takes about 3,600 years. Since then, Marduk got the name of Nibiru. It's (later) inhabitants, are called the Nefilim.
Since the first appearance of Marduk, our solar system consists of twelve bodies: the Sun, Mercury, Venus, Earth, the Moon, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto and Nibiru/Marduk. Hence the name, the twelfth planet for Nibiru/Marduk.
=> Another theory mentions a former planet with the name of Maldek in between Mars and Jupiter. Maldek would have been inhabited by a humanoid civilization with a highly advanced technology. The inhabitants however destroyed themselves and the planet in a nuclear war, thus creating the asteroid belt.
=> just a thought: Earth and the solar system are about 4,5 billion years old. Humanoid civilizations within this solar system are far more recent. (The furthest back some sources go, is 50 million years ago, which still leaves 4 billion years uncovered.) Maybe, Marduk could have split up in Earth and Maldek, and later on, Maldek self-destroyed.
Leviathan ~ Maldek ~ Marduk ~ Nefilim ~ Nibiru ~