When the main map is sufficiently zoomed out and 3D models are replaced by symbols, Planet Class is indicated by a hollow white circle. The more hollow the representation, the lower the class. High quality planets will appear as virtually solid white circles.
Planet Class generally ranges from 0 to 26, though higher values are possible. Class 0 planets are uninhabitable, while planets of Class 1 or higher are habitable and can eventually be colonized.
Each race has a specific initial Planet Class for its homeworld (from 10 to 15, usually 10) and for the other habitable planet in its home star system (from 3 to 8, usually 4, except for the Thalan Empire which doesn't have one). Earth is a Class 10 planet by default, while Mars is a Class 4. See each race's page for details.
Planet Class may be increased through "terraforming", via three quality enhancement techs: Soil Enhancement, Habitat Improvement, and Terraforming. These tiles are visible in advance as yellow, orange, and red, respectively.
Typically planets of Class 4 or less have 9 additional squares that can be unlocked after fully terraforming them (+3 tiles per tech), while planets of Class 5 or above get only +1 tile per tech -- meaning lower planets, once upgraded, can beat lower-middle class planets. However this 'terraforming potential' can vary between planets of the same Class, or even for a race's secondary planet.
Planet Class may also be improved by the Planet Quality Bonus ability, selectable during race setup.
A planet with a sufficiently high Class gets a bonus to its approval rating (in TA the bonus is a flat +10% added at the end if Class 11 or higher).
Higher quality planets are very advantageous due to their greater capacity for planetary improvements. Invading minor races, which tend to start with higher Class planets (15+), can be an easy way to gain planets and space.
- In one game, I had the Dread Lords appear on a Class 39 planet, so the game definitely does this from time to time.
- It should be noted that the game itself caps Class 30+ planets at 36, before Soil Enhancement and similar improvements. This places the absolute cap, without any mods or hacking (but including cheats), at a Class 39 planet.
- It seems as if the Planet Quality Bonus affects a planet's 'terraforming potential' such that with +20% a Class 6 planet (Class 7 after the bonus) can get +2 tiles per tech, ending up as a Class 13.
- An excellent post about DA from 2007 is consistent with the experimental results from TA I'd added to each race's page about starting planets. It also makes a very important observation I hadn't caught onto about bonus tiles only occurring on secondary planets above Class 4. (Plus it revealed instances where I'd not done enough trials or where the High-Res Mod for TA was making undocumented changes -- at a minimum it tweaks Mars to behave like a typical Class 4 secondary with +3 or +4 tiles per tech, rather than the +2 it gets in vanilla DA/TA.)