Planetary Classification

Planetary classification systems were used by many races to categorize planets. Various factors were taken into consideration, including atmospheric composition, surface temperature, vegetation, and size. The classification system used by the Alliance used single-letter designations such as class M to describe a planet able to support humanoid life for long periods, while the Vulcans used the term "Minshara class" to describe a similar planet.

Planetary Classes
http://sttff.net/planetaryclass.html