Class G - Geocrystalline
Age: 3-4 billion years
Diameter: 10,000 - 15,000 km
Surface: Still crystallizing
Atmosphere: Carbon dioxide, some toxic gases
Evolution: Cools to become Class-K, L, M, N, O, or P
Life-Forms: Primitive single-celled organisms
Class F planets can generally be found in a star's habitable zone, or Ecosphere. Like Earth, they range in size from 10 to 15 thousand kilometers in diameter. The surface of a Class F planet is still crystallizing, and the atmosphere still contains some toxic gases, making it unsuitable for most life forms. As it continues to cool a Class F planet will change to Class K, L, M, N, O, or P.
Delta-Vega, the desolate home to an automated lithium mining station in "Where No Man Has Gone Before" (TOS), is Class G..