Gas Giant

A Gas Giant is a very large planet with a solid core and thick atmosphere. It mainly consists of various gases. There's no solid surface like a terrestrial planet. The atmosphere can feature gigantic cyclonic storms the size of Earth.

In the Sol system, Jupiter, Saturn, Uranus, and Neptune are gas giants. They have major gravity wells due to their large mass.

Types

There are 10 known types of gas giants.

Type	Description	Rarity
Class I gas giant	Class I or Jovian gas giants have primarily hydrogen and helium atmospheres. Colouration comes from clouds in the upper atmosphere of ammonia, water vapour, hydrogen sulphide, phosphine and sulphur. The temperature at the top of their upper cloud layers is typically less than 150 K.	Common
Class II gas giant	Class II gas giants have primarily hydrogen and helium atmospheres. Water vapour in the upper cloud layers gives them a much higher albedo. Their surface temperature is typically up to or around 250 K.	Uncommon
Class III gas giant	Class III gas giants have primarily hydrogen and helium atmospheres without distinctive cloud layers. Their surface temperature typically ranges between 350 K and 800 K. They are primarily blue in colour because of optical scattering in the atmosphere - with the chance of wispy cloud layers from sulphides and chlorides.	Common
Class IV gas giant	Class IV gas giants have primarily hydrogen and helium atmospheres with carbon monoxide and upper clouds of alkali metals above lower cloud layers of silicates and iron compounds, hence the brighter colours. The temperature of their upper cloud layers is typically above 900 K.	Rare (Common in systems with stars of class A or higher)
Class V gas giant	Class V gas giants have primarily hydrogen and helium atmospheres, with thick clouds of silicates and iron compounds, even metallic iron.They are the hottest type of gas giants with temperatures at their upper cloud decks above 1400 K, and much hotter in the lower layers, often emitting a dull glow from the internal heat within.	Rare (Common in systems with stars of class A or higher)
Helium-rich	Helium-rich gas giants have a greatly inflated percentage of helium compared to the hydrogen in their atmosphere. Much of their hydrogen has been lost over time because they have insufficient mass to hold on to it. It may also be because temperatures in their past were much higher, driving off the hydrogen at a greater rate.	Very Rare
Helium gas giant	Helium gas giants have primarily a helium atmosphere. They have lost most or all of their hydrogen because they have insufficient mass to hold on to it over time. It may also be because temperatures in their past were much higher, driving off the hydrogen at a greater rate.	Extremely Rare (Report discoveries to explorer groups)
Gas giant with ammonia-based life	Gas giant with ammonia-based life. This is primarily a hydrogen and helium-based atmospheric gas giant, but a little below the surface cloud layers, life exists based in the ammonia-cloud layer. The chemistry of this gaseous region is far from equilibrium, with a surprising excess of oxygen and many carbon-based compounds giving it some vivid colours. As with many such gaseous living systems, it is underpinned by vast quantities of free-floating radioplankton - tiny carbon-based algae, each retaining a small quantity of liquid ammonia, extracting their energy from the intense radiation flux.	Uncommon
Gas giant with water-based life	Gas giant with water-based life. This is primarily a hydrogen and helium based atmospheric gas giant, but not far below the surface exists life based in the water-cloud layer just below the atmospheric surface. The chemistry of this gaseous region is far from equilibrium, with a surprising excess of oxygen and many carbon-based compounds giving it some vivid colours. As with many such gaseous living systems, it is underpinned by vast quantities of free-floating radioplankton - tiny carbon-based algae, each retaining a small quantity of liquid water, extracting their energy from the intense radiation flux.	Uncommon
Water Giant	Water giant. Worlds like this have a large atmosphere made mainly of water vapour. It most likely formed when a large icy body warmed up enough to evaporate a large amount of its surface ice, this would in turn trigger a runaway greenhouse effect leading to a very thick atmosphere made of the evaporated ices.	Rare

Lifeforms

Some lifeforms have been found on gas giants such as radioplankton. They live just below the surface in the water-cloud layer or the ammonia-cloud layer of hydrogen and helium based atmospheric gas giants where they extract energy from intense radiation flux and underpin other water-based or ammonia-based lifeforms.

A Green Gas Giant

Green Gas Giants

In extremely rare cases, the radioplankton floating within the upper cloud decks of gas giants can become bioluminescent, so much so that their aggregate light is visible from space. They have been documented to exhibit many bright colours, most notably intense bright green, for which the gas giants they inhabit are classified. Why or how the radioplankton become bioluminescent is not yet known.

Exploration

Most gas giants are worth comparatively few credits when scanned and probed, including those with ammonia or water-based life. The exceptions to this are Class II gas giants, the data for which will fetch a decent price with Universal Cartographics. However, probing an entire gas giant can be time-consuming, and most explorers choose not to go any further than an initial scan.