Terraform: The majority of planets colonized by the Federation had no native biospheres of note when discovered, including two of the three Core Worlds. While it took over five centuries to terraform SecLand advances in nanotechnology have given shirtsleeve environments to other planets in less than a single century, often while the colonists are still in transit.
There’s some debate on whether Outworlders should be sent to inhabitable or terraformed planets. When the program started the concern was about “wasting” inhabitable planets on exiles, now those who object to dumping them on inhabitable planets are either concerned for the welfare of the involuntary colonists or the environments they’re being inflicted upon. After all, just because one can survive in an environment without an enclosure suit doesn’t mean its’ friendly.