Gas Giant

Gas Giants are giant planets composed largely of hydrogen, similar in composition to Jupiter and Saturn. They are composed of roughly 90-95% hydrogen and around 2-10% helium, and are structured in the following layers (from inwards to outwards): Gas Giants can be classified based on their atmospheres. The classes and cloud composition are as follows: Gas Giants tend to have massive magnetic fields (which may even extend beyond some of the planet’s moons and thus shield them from radiation) as a result of the ionization of the giant’s metallic layer.
 * A rocky metal-silicate core similar in composition to silicate terrestrial planets or perhaps even carbon planets.
 * A layer of liquid metallic hydrogen (making up the bulk of the planet).
 * A layer of atmospheric hydrogen, full of many clouds.
 * Class I - Ammonia Clouds. Examples include Jupiter and Saturn. Class I gas giants would orbit either around dim stars or would be found in the outer reaches of their planetary system, and would have temperatures less than about 150 K.
 * Class II - Water Clouds. Class II gas giants would have temperatures of less than 250 K. The clouds would be very similar to those found on Earth.
 * Class III - Cloudless. Class III gas giants have temperatures of between about 350 K and 800 K, and lack elements suitable for clouds. They would appear as featureless blue globes and would likely orbit very close to a star.
 * Class IV - Alkali Metals. Class IV gas giants have temperatures of between 900 K and 1400 K, and would likely be Hot Giants. They would have clouds of carbon monoxide, sodium and potassium.
 * Class V - Silicate Clouds. Class V gas giants have temperatures of greater than 1400 K, and would almost certainly be Hot Giants. They would have clouds of silicates and iron in their upper atmosphere.

Gas giants follow the H/He band on the Mass-Radius graph, and range in radius from about 6 R⊕ to 15 R⊕. Such a high mass range but a low radius range results in a wide density range.

Worldbuilding in Practice
"Hyrja is a class II gas giant of 1.99 MJ and 0.90 RJ orbiting the blue giant Hrimnir at an enormous distance of 1,437 AU. At such a distance, the planet completes an orbit every 6,515 Earth years, but still receives more sunlight from its incredibly luminous star than Earth does from the Sun. Hyrja has over 40 moons, many of which are exploited by ESIC mining teams. The moons alone supply a steady income of mineral wealth and water, supplemented by a vast agricultural habitat on Hyrja's moon Ari, to keep the inhabitants of the Hrimnir system self-sufficient."