To pass the terraformability test, a planet has to answer the following questions correctly:
- Is your surface gravity between 0.40 and 1.99 Earth-standard gravities?
- Is your semimajor axis of orbit inside the Goldilocks Zone of the star system?
That's it, really. If the planet can say "yes" to both of those questions, then it should be terraformable. Changing the current composition of the atmosphere is considered an engineering problem, not a laws-of-physics problem. Likewise the presence or absence of liquid water.
The "Goldilocks Zone" question doesn't quite work as one might expect. A planet with two widely-spaced stars in theory ought to be less habitable than a single-star system, because the goldilocks Zone would keep moving about and a planet landing in the Zone for all or most of its orbit would be highly improbable. But the Stellar Forge seems to regard multiple stars as a positive, rather than a negative, creating a very wide Goldilocks Zone that can fit multiple planetary orbits inside it.
There's also something odd happening with moons; they tend to be noticeably warmer than they "ought" to be, as if the planet they are orbiting is significantly contributing to the heat levels. It's probably an attempt to model tidal heating, but it seems to be present even on moons where tidal heating ought to be nearly nonexistent (like on Earth's moon). This effect tends to make otherwise Terraformable moons too hot, and moons that "ought to be" too cold can be pushed into terraformability. It's rarely seen, because moons large enough to qualify by gravity are rare to begin with.
The "semimajor axis" question can lead to some rather amusing terraforming candidates, if the planet happens to have a high eccentricity. High eccentricity would regularly push the planet way outside the Goldilocks Zone, theoretically making the planet much less likely to be capable of supporting an Earth-like atmosphere. It ought to be really, really hard to terraform a gigantic comet, but the Stellar Forge doesn't seem to care.
Finally, there are very rare glitches where we simply can't figure out why a certain planet is classed as terraformable, and an apparently otherwise identical planet right next to it is not. There may be some other characteristic in the algorithms we're not aware of. My guess is that, rather than current atmosphere being irrelevant as I stated earlier, that there is a factor that makes planets with extra-thick atmospheres less terraformable. But if this effect is real, it's very hard to pin down.
Feel free to post some pics of some of the planets you're having trouble with, and we'll see if we can explain why they unexpectedly pass or fail the terraformability test.
), but there are binary pairs where one is terraformable and the other is not while both falling inside those parameters you outlined.
