I'm glad you like it :)
About the collisions, there is not really a circumference that determine when you can go to a planet to another, it is based on which planet has the stronger gravitational pull on you, that is determinated by the mass of the planet divided by the player distance from it center, so it wold be harder do do a visual representation of that.
And there is a mechanic that is not easy to get, but when you do you wont get "stuck" in the moons, is that when you get horizontal velocity you can actually kind of enter in orbit, so to not get stuck just run and jump ;). This mechanic is not as transparent as I wold have liked to be.