Since it won't let me edit:
Though you should make sure to preview the map when you are finished.

Haha i swear those last 2 replies went up when i was typing.. oops haha...

If you want to place planets deliberately, controlling the location of every planet and it's type, then you need to get Galaxy Forge, which you can find in the Downloads section. There's a tutorial thread for it in the Modding forum. Also, your friends will need to learn how to download (perhaps via e-mail attachment) custom maps and how to add them to the game (very easy--open main Sins folder...put .galaxy files in Galaxy folder and any .tga files in Textures folder).