It actually had an endgame! I'm impressed, it took me a minute to figure out that I needed to actually be on the planet in order to see where the others were
even though you said it in you op.
Once I figured that part out though it was pretty fun. I somewhat agree with Podunkian, however my first question before sticking by his thinking would be to figure out what you had planned it to be. As a simple exploration, find all the planets game, this was pretty stable.
One thing I would suggest in the wake of Pokunkian's thinking however would be to possibly have two or three planets orbiting a star of some sort. I suppose then you could incorporate Inanimate's suggestion of having some sort of inhibitor (ie. stars kill joo).
Overall though I thoroughly enjoyed the experience!
e: grammar