[Slick]

The dome is because the population on the planet can't breathe the atmosphere. Basically you get 1/5 space for population, cargo, and facilities. Two ways to get rid of them:

1) research the Planetary Engineering tech until you get atmosphere converters, then use them.

2) obtain (capture, trade, etc) for alien population which breathes that atmosphere and replace the population with the right breathing race.

#2 is usually faster, but you have to take some extra measures to keep alien population happy.

See the Newbie FAQ for the answers to these and other questions.

Slick.