[Frederick_d_Ohlmann]

Example : they are CO2 breather and you are oxygéne (O2) breather (it's marked on the race view and you chose it in game création).
Your CO2 planet are domed and have less room than the O2 one with same size.. The O2 one have 5 time more facilities because yours people can breath without problem
But, if there is only aliens CO2 breather the Co2 planet will be undomed and you will gain more room - the same room as if it were an O2 planet with your people on it.
In the other case, if you have CO2 breather AND 02 breather in the same planet, it will be domed anyway - and that's less room for yours facilities.