The New Satellite

The mysterious disintegration of the planet Mercury presaged mutational changes in the solar system. Next, the Moon followed Mercury into oblivion, bombarding Earth with giant meteors and causing more earthquakes. Then a giant metal globe that had apparently been buried inside the Moon took up an orbit of the Earth. The new satellite is an alien spaceship carrying the survivors of a super scientific race forced to abandon their own world to emigrate to our solar system.
The aliens seeded the then primeval Earth with life cultures in their own image, and then placed themselves in suspended animation whilst evolution took place. They have awakened to assist humanity to avert the mutational danger now facing the remaining planets...