Dark Angels, one of the first founding, were one of the most secretive of all of the chapters within the Imperium. Unlike the inhabitants of Septimus Quadraticus Sistema IV Alpha-VII Principale, they did not file status reports with Terra. They did not even make the other Imperial forces in the region aware of their presence unless it was absolutely necessary. As a result, no one knew that now the Dark Angels were on crusade not far, in galactic terms, from that mining planet. In fact, it was really in their back pocket in case they needed additional fuel.
A half a dozen planets had actually already felt their wrath. This was actually the 10th in that long line. The hundred or so marines that remained were left to mop-up, exterminate the resistance, or at least cripple any hope for revenge.