According to Geoffrey Mandel's Star Trek: Star Charts, gas giants have an age ranging from two to ten billion years and a diameter between 50,000 and 140,000 kilometers. They are located within the "cold zone" of a star system. A class J planet is characterized by a tenuous surface, comprised of gaseoushydrogen and some hydrogen compounds, and an atmosphere comprised of zones that vary in temperature and composition. The only known lifeforms to be found on J-class planets are hydrocarbon-based. Examples of class J planets include Jupiter and Saturn.