Now the mod has Overworld and the Moon. I guess in the future there will be more planets. There can be layers. These are worlds like spacestation worlds, and all of them has a "map". The topest layer (universe) contains galaxys, galaxy layers contains planets, moons, spacestations (or contains solar system layers what contains planet etc.). Galaxy (or solar system) layers are worlds contains little (maybe 60 x 60 x 60 block) spheres looks like planets (sphere blocks depends on the type of planet what it represents) and mini spacestation. These are just models so all spacestation has the same "icon" and all the Earth type planet has the same "icon", all moons has the same "icon" etc. If the player reach an "icon" he will be teleported to it's world. If player reach 1200 Y position he will be teleported to the parent layer. About spaceships: Do you know Haribote aircraft mod? He shared source of the mod, so everyone can use it: http://forum.minecraftuser.jp/viewtopic.php?f=13&t=10418 Maybe you can user it in your code. Whit that mod players can make spaceships what can go from one layer to an other. You current spaceships can use the current planet-switch system. So it can be use to travel in a solar system, but these rockets can't change layer.