The Mass Terraforming Program was a series of programs designed and set up by Darth Abeonis some time after the Sith Crusade. Its main purpose was to make many of the worlds in the galaxy considered uninhabitable, habitable, and to repair the damage inflicted during the war.
So successful was the program that by the time of Abeonis's death, Korriban, planetary capital of the Decreton Empire, had been transformed from the dead rock it had been before, to a green and lush world (around the equator at least).
During the reign of Darth Krayt, many other worlds would follow suite. Hoth's surface (although much colder than most other planets), warmed up enough to allow a permanent garrison, to guard the planet's trading facilities, and planet's such as Nerezza, became that little bit more hospitable.