Young planets, Class G Geocrystaline worlds range in age from 3 to 4 billion years old. Their diameters range in size from 10,000 to 15,000 km. They are located in the biozone region of a star's solar system. Their surfaces are still crystalizing. Their atmospheres contain carbon dioxide and other toxic gases. Class G planets cool over time to evolve into Class N, O or P worlds. Life forms, if any, are primitive single-celled organisms. (Star Trek: Star Charts)