From Sonic Team:
The Pioneer Project. A plan born of desperation, conceived in response to the imminent destruction of their home worlds. As plans for the evacuation began, unmanned probes were sent into deep space to find a habitable planet. When a potential site was discovered, the first interstellar transport ship, Pioneer 1 was sent to establish a colony. Pioneer 1 confirmed that the Planet Ragol was a suitable location, and the initial colonists started preparing the planet for the main wave of refugees, beginning with the construction of the Central Dome.
7 years later... Pioneer 2 completed its long voyage with the main wave of refugees aboard. But as Pioneer 2 entered orbit and opened a communications link with the Central Dome, an enormous explosion shook the entire planet. All contact with the thousands of men and women of Pioneer 1 was lost.