In the opening video, the narrator makes mention of five sleeper ships: Liberty, Bretonia, Rheinland, Kusari, and Hispania. Going into the Outcast-controlled system Omicron Alpha, you can find the wreckage of the fifth sleeper ship.
The original version of the opening movie depicted what happened to the solar system after the sleeper ships had left. During the final battle, an alien ship appeared. "It was in this moment", the narration said, "that we were unified in our awe." The alien ship then launched a projectile into the sun, destroying it and everyone else in the system. Only one survivor remained in the whole system, a soldier lucky enough to be on the far side of Pluto. He took off in a ship to warn the sleeper ships of a new threat, the Nomads.