Maldek was a planet of this solar system, located between Mars and Jupiter. It was inhabited by a humanoid species. Just like the Apex-planet, Maldek was destroyed by nuclear war. The Asteroid Belt between Mars and Jupiter are the remains of Maldek.
There is another theory about the asteroid belt, and how it came into existence.
According to Sitchin, Sumerian Mythology mentions the existence of a former planet between Mars and Jupiter that was called Tiamat. When Tiamat collided with the satellites (moons) of Marduk, a foreign celestial body, Tiamat was half destroyed. The surviving half was to become Earth, the destroyed half formed the asteroid belt. Marduk got an orbit around the sun, and would be the "twelfth planet."
=> 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.