If you're in the Alpha, or just watching footage, take a look at the screen displaying The Tower, Earth, and the Crucible after you select, "Set Destination". There are six distinct circular outlines not filled in that have the same type of movement as the available locations when you move the cursor. So we already have confirmed Venus, Mars, Europa, and (sort of) Mercury (also I think Saturn but I'm not entirely sure). So this leaves two/one space(s) open. Not to mention broken down ships and stations floating in space (if concept art is to be believed). What do you think the remaining spaces are?