Class J - Gas Giant
Age: 2-10 billion years
Diameter: 50,000 - 140,000 km
Location: Cold Zone
Surface: Tenuous, comprised of gaseous hydrogen and hydrogen compounds; radiates some heat
Atmosphere: Zones vary in temperature, pressure and composition
Life-Forms: Hydrocarbon-based (Jovian)
Examples: Jupiter, Saturn
Like their Class I cousins, Class J Planets are lifeless gas giants most often found in a star's outer or cold zone. Larger than virtually all but Class I planets, they range in size from 50 thousand to 140 thousand kilometers in diameter. Although they have a high core temperature, they do not radiate much heat.
High gravity combined with low stellar radiation allows for a tenuous surface comprised of gaseous hydrogen. Some have rocky cores beneath huge lakes of liquid hydrogen, which others possess layers of boiling water near the core. Atmospheres are extreme, containing hydrogen/methane clouds, swirling in storms that can rage at thousands of KPH.
Class J planets are only suitable for Hydrocarbon-based (Jovian) life forms. Jupiter and Saturn in Earth's star system are Class J planets.