Planet class
From the Galactic Civilizations Wiki
Planet Class (also designated Planet Quality) is a number which represents the number of usable tiles on that planet for planetary improvements. A planet with a sufficiently high Class will benefit from an increase in the approval rating of that planet. It is also the factor that determines the planet's maximum Population.
The planet class ranges from 0 to 26 by default, and higher than 26 for colonized planets which have been improved (eg Soil Enhancement, terraformed). Uninhabitable planets are class 0, while planets above this are habitable. For reference, Mars is a class 4 planet, while Earth is class 10.
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.
Higher class planets are very advantageous, due to the large capacity for planetary improvements. Invading minor races, which tend to start with higher class planets, can be an easy way to gain planets and space.
Apparently, the system can glitch and you will end up with a class 31 or higher planet. Note that a class higher than 72 has no meaning, as there are only 72 total tiles on a planet. No, there is not a super-planet in every galaxy.
