Planets may need their own currencies, but if instant communications are possible then I don't see why they would, they'd be better connected than nodes are right now.

Actually the idea of a separate chain on other planets seems shaky to me. If miners on some planet are a lot faster or more efficient which seems likely then they can double spend or otherwise hassle the lesser for only a the one time fixed time cost of transporting the work. Maybe the lesser ought just accept a delay and/or use processors who settle on the greater planets chain. That should be workable, but I bet something smarter than that arrangement gets invented.