Terraforming is a colony project in Master of Orion. It requires the Genetic mutations technology.
Terraforming a planet will upgrade a planet's biome by one rank: from barren to either desert or tundra depending on planet orbit (inner planets to desert, outer planets to tundra), from desert to arid, from tundra to swamp, and from arid, swamp, or ocean to terran. A planet can be terraformed repeatedly to end up as a terran-class planet. The ocean-to-terran transformation is the only one where the planet stays in the same rank class.
Terraforming is the slow process of altering the environmental characteristics of a planet. This changes weather patterns, stabilizes extreme temperature fluctuations, and adjusts the abundance of surface water. Terraforming will only work on planets that have hospitable environments already.
You can terraform a planet several times, but each application has an increased production cost. Terraforming cost increases according to planet size.
The transformation process doesn't generate pollution in the colony.
Terraforming allows a planet to support a larger population and produce more food per tile. However, the project cannot be bought out to speed it up, so planets should only be terraformed after they have already built up their production capacity sufficiently.
For example, terraforming a Huge Barren planet repeatedly will cost 1.7×250=425 Production to bring it to Tundra or Desert class, then 425×1.5=637.5 to bring it to Swamp or Arid class, and finally 637.5×1.5=956.25 to bring it to Terran class.
Colony projects are not the only ways in which a planet's biome can be improved. A toxic or radiated world can be transformed into a barren world by colony structures, respectively a toxic processor and a planetary radiation shield (or upgraded version thereof). These transformations are instantaneous once the structure is built, but unstable: if the structure is destroyed in any way (scrapped for cash, dismantled by a quake, or razed by enemy bombardment), the planet reverts instantly to its previous biome.
However, terraforming projects are stable transformations: if, for example, a toxic planet has a toxic processor built, and it is then terraformed into a tundra planet, the transformation to tundra is stable. Losing the toxic processor at this point would not cause the biome to revert back to toxic or barren. Only pollution can degrade a planet after a stable transformation.