The 30K Space Marine's Encounter with the Ark

A detachment of around 50,000 Space Marines was trapped in the Warp for one Terran month, for reasons unknown, before the Horus Heresy. When they finally emerged from the Warp, the Astronomican's light was nowhere to be seen. They had arrived in a star system they had never visited before. It contained only one planet inhabited by demi-humans, while the other planets in the system were largely uninhabitable. Where would they go from here?