Not necessarily, they could just be there for the lore, we know that Mercury is mentioned in the core game, the Vex turned it into a machine or something. Though it wouldn't surprise me if Mercury was released as a DLC around the end of 2015/start of 2016. I can't see Saturn being an unlockable location. I mean, I know that Bungie have done a terrible job of making the planetary bodies realistic (The moon having an atmosphere and Venus' atmosphere isn't crushingly dense) but surely they remember that saturn is not a terrestrial planet, right? ... Right? EDIT: Ok, after reading the Saturn Grimoire card, it looks like the location may actually be its moon, Titan. "Out beyond the Reef its rings shine. Whatever was known of it in the Golden Age is long forgotten, but more ancient truth remains: Saturn has dozens of moons, including the mighty Titan, a world larger than Mercury. A Guardian's ship might, in theory, reach Saturn, especially if supported by a larger vessel and good navigational data. But the hazards of the Reef and the Deep Black beyond threaten to devour anyone who makes the attempt."