Now, we all know that Earth (EDZ), Io, Titan, and Nessus are destinations. Well, thankfully those aren't the only ones.
Due to this being a demo version of the game, the empty slots above are... well empty. However, I do believe that each of those will contain other destinations. It is possible that the one in the middle could be an 8th destination.
With this in mind, I can guess that the remaining destinations are as follows:
top middle - Mercury
middle - Venus
left - Mars
far left - Reef
Who knows (besides Bungie) at this point?
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Guess we'll find out in DLC 3