We still have no idea how exactly Mars and Titan returned in the first place. The implication is that Savathûn was involved in returning Mars for her own ends, potentially the same could be true of Titan what with Ghosts of the Deep and the Lucent Brood mission there. Io is still stuck in the Ascendant Plane, hence why we’re able to visit it through the Court of Blades. Oryx has essentially facilitated that connection for use as an arena to test the contenders to his throne.