The planets you can colonize have different atmospheres. If you colonize a planet that has the atmosphere your race can breathe, then you can have maximum number of facilities and population on that planet allowed for that size of the planet. If the planet has atmosphere you can't breathe, then your colony will be domed and will not be able to hold as much facilities and population. The difference in facilities and population between planets with breathable and non-breathable atnospheres is about 5:1. For example; if you colonize a huge planet with breathable atmosphere, you will be able to have 25 facilities on it. However if that planet has non-breathable atmosphere, you will be able to have only 5 facilities on that planet.
When you look at the map, you will notice that all planets that you can colonize have + sign on top of them. The planets with green + have breathable atmosphere, and planets with red + have non-breathable one.
Hope this will help you get started.
