There is more information available on this subject at Terraforming on the English Wikipedia.
Terraforming is the process of transforming a dead planet into a living planet. Since there are no aliens encountered in the series besides the Necromorphs and the Tau Volantis natives, all of the human colony planets must have been terraformed in some way if they sustain life. There are numerous methods for terraforming a planet which vary by planet, but the only ones mentioned so far are solar and geothermally powered enviro-cleansers which remove harmful atmospheric chemicals and replace them with a standard Earth atmosphere.
Trivia[]
- Aegis VII may have had a terraforming facility, as various characters managed to breathe the planet's air without a helmet or a gas mask.