https://www.gamespot.com/articles/destiny-2s-first-dlc-leaks-on-xbox-store/1100-6453216/

Following a report suggesting that Destiny 2's first DLC takes players to Mercury, those details have now been corroborated by a listing on the Xbox Store. The first Destiny 2 DLC pack, called Curse of Osiris, will indeed take players to that planet for an Osiris-centric story.