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 other aliens encountered in the series besides the Necromorphs, all of the human colony planets have been terraformed 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 because Kendra Daniels managed to breathe the planets air without a gas mask.