The planet he is on isnt a planet hes back on the ark it repaired itself the same way it makes halo rings and I believe locke was intended by the UNSC to be chiefs replacement which is why he has to be more than his actions and such
Hes on the ark to get to mendicant bias likely due to the return of the flood or because mendicant bias turned against the forerunners in the past and may know how to defeat them