Nevertheless, here are the rules as far as I can tell, based on an ongoing game that I'm using to figure stuff out (Easy difficulty, 99 stars, basically won already) and from the Orders\terraforming.lua file and whichever other files seem relevant based on a Shift-Ctrl-F search (but I am wholly unfamiliar with the Lua syntax, since all previous programming experience being in C-family languages).
- Terraforming options appear if they'd increase a planet's max pop figure with either the current population mix or your empire's main race.
- Atmosphere Generation lets you terraform barren planets into the Arid and Glacier types.
- Terraforming lets you terraform planets to Garden and Island.
- Hive Worlds lets you terraform into Hive planets; but this technology is racially locked to be only available to Ashdar (both types), Gremak, and Yoral.
- Coral Bloom lets you terraform into Coral planets, but this technology is locked to Phidi.
- Atmospheric Condensation combined with Energy Refocusing let you terraform into Iceball planets, but these technologies are locked to Orthin. Atmospheric Condensation is also required to terraform Inferno planets, so only the Orthins can do something with them: other races have to hope they can get some Tarib colonists instead.
- Sentient Ecosystems lets you terraform into Paradise planets, however I have no idea how this is unlocked.
- Inferno planets can only be terraformed by the Orthin (see above).
- Cold planets can only be terraformed into Iceballs (so, by the Orthin) or if you have the "Artificial Suns" technology, which again I don't know how you get.
- Warm and Hot planets can be terraformed normally, but if you want to terraform them into Iceball planets you need the Orbital Mirror technology. Since Iceball is the only planet with a Max Temp, it's the only "additional terraforming option on hot worlds" that this tech gives you, making it utterly useless to every race besides the Orthin. So it would probably make more sense to have Orbital Mirrors be locked to Orthins, but Atmospheric Condensation (a tech useful to any non-Tarib race that encounters an Inferno planet) be available to everyone.
- Glacier planets are sometimes Warm, and therefore terraformable. Steppe and Iceball planets are always Cold, however.
- Ocean planets can be Small, but Island and Garden planets cannot. So you can easily remove all of its water to turn it into a small Arid planet, but it's absolutely impossible to only lower the oceans enough to make island chains emerge.
- There's no technology terraform planets into the Ocean and Steppe types.
- Unique planets are not protected against terraforming, I can turn Gremal into a Garden planet. (However, Verrold is Cold, so I can't touch it).
I gotta play as the science bugs now.