[douglas]

There are two possible ways I can think of for you to start with a domed planet. First, you're playing with a pregenerated map with your start location marked. Second, you're playing with a mod that allows moons large enough to be considered for homeworld selection.

To fix the first problem, edit the map and change your start location to an empty square. When the game starts, if there is already a planet at your start location it will put you on that planet with whatever size and atmosphere it happens to have. For start locations without preplaced planets, however, it will generate a customized planet for you. Planet size will be determined by the homeworld quality setting in game setup and the atmosphere will match what your race breathes.

To fix the second problem, you will have to either change the mod to not generate unusually large moons or select a homeworld quality that will force your homeworld to be larger than the largest possible moon. The homeworld selection process might pick a moon if it's large enough, but will then place the homeworld on the planet the moon is orbiting. I am not aware of any published mods with moons larger than Medium, so unless you've added such things yourself selecting Good quality homeworlds (which forces all homeworlds to be Large) should solve the problem.