Gas giant

Terminology
The term "gas giant" was coined by the science fiction author James Blish. It refers to a planet which is composed mainly of hydrogen and helium and whose mass exceeds that of an average terrestrial planet by several times.
The term is a bit of a misnomer insofar as throughout most of the volume of giant planets the pressure is so high that matter is not in gaseous form. The term has nevertheless caught on because planetary scientists typically use "rock", "gas", and "ice" as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what phase the matter may appear in. In the outer Solar System, hydrogen and helium are referred to as "gases"; water, methane, and ammonia as "ices"; and silicates and metals as "rock".

Interior structure
Unlike rocky planets, which have a clearly defined difference between atmosphere and surface, gas giants do not have a well-defined surface; their atmospheres simply become gradually denser toward the core.
Usually the interior structure of gas giants comprises two main layers that surround the rocky or nickel/iron core.
* The first layer is a mixture of metallic hydrogen fluid and neutral helium atoms, with the latter making up about a quarter of the mass but only 7% of the nuclei by number. This type of degenerate matter is the result of immense pressures that transform hydrogen into a solid lattice of delocalized protons and electrons.
* The second layer consists mainly of molecular hydrogen and its outer portion is filled with many strata of visible water and ammonia clouds. The contrasting patterns in this outer region of the atmosphere are the result of coloration by trace chemicals containing Phosphorus and Sulfur and a complex pattern of circulation. Belts are darker appearing regions where material has cooled and is sinking while zones are comprised of warmer material that is rising on a convective air flow. Turbulence at the boundaries between belts and zones leads to the formation of large, whirlpool-like storms. Storms can persist for years on because there are no continental land masses to disrupt the flow that causes them.

Sudarsky's classification
Gas giants are split into five classes (numbered using Roman numerals) according to their modeled physical atmospheric properties.
* **Class I**: Planets in this class have appearances dominated by ammonia clouds. These planets are found in the outer regions of a planetary system. They exist at temperatures less than about 150 K (−120 °C; −190 °F).
* **Class II**: Planets in class II are too warm to form ammonia clouds: instead their clouds are made up of water vapor. These characteristics are typical for planets with temperatures below 250 K (−20 °C; −4 °F). Even though the clouds on such a planet are similar to those of terra planets, the atmosphere consists mainly of hydrogen and hydrogen-rich molecules such as methane.
* **Class III**: Planets with equilibrium temperatures between 350 K (80 °C; 170 °F) and 800 K (530 °C; 980 °F) do not form global cloud cover, because they lack suitable chemicals in the atmosphere to form clouds. These planets appear as featureless blue globes because of Rayleigh scattering and absorption by methane in their atmospheres.
* **Class IV**: Above 900 K (630 °C; 1160 °F), carbon monoxide - rather than methane - becomes the dominant carbon-carrying molecule in a gas giant's atmosphere. These planets form cloud decks of silicates and iron deep in their atmospheres.
* **Class V**: For the very hottest gas giants, with temperatures above 1400 K (1100 °C; 2100 °F) the silicate and iron cloud decks are predicted to lie high up in the atmosphere.Class I: ammonia cloudsClass II: water cloudsClass III: cloudlessClass IV: alkali metalsClass V: silicate clouds

Upper mass limit
The upper mass limit of a gas giant planet is approximately 1.32 × 1029 kg (70 M♃). Above this point, the intense heat and pressure at the planet's core begins to induce nuclear fusion and the planet ignites to become a red dwarf star. Interestingly there appears to be a mass gap between the heaviest gas giant planets detected (about 10 times the mass of Jupiter) and the lightest red dwarfs.

Habitability
It is not uncommon to find gas giants that host unicellular or even multicellular life. To understand why the birth and subsistence of life is possible in the atmospheres of gas giants it has to be taken into account that there are factors which greatly inhibit the formation of complex organisms, but also factors that increase the likelihood of biogenesis and abiogenesis on these planets. An inhibiting factor is the existence of convective currents that constantly transport material between different layers of the atmosphere. Life adapted to the conditions of one atmospheric layer could be transported into another, where it might decompose due to excessive temperature and pressure. This process is referred to as pyrolysis and is a big obstacle to the formation of organic life. Despite these difficulties the variety of different niches and the sheer size of the available reaction volume paired with vast periods of available time favor the formation of life-forms that are able to adapt to the convective currents. Due to parallel evolution organisms that thrive on different gas giants share some basic similarities. Generally, they can be divided in to four categories:
* **Floaters**: Organisms that actively keep their pressure level and stay in the same atmospheric layer for a relatively long period of time.
* **Sinkers**: Organisms that reproduce before falling into the lower layers and decomposing in a pyrolytic process.
* **Hunters**: Organisms that seek out other biota to either prey on them or mate with them.
* **Scavengers**: Organisms, similar to floaters, that spend most of their existence in the lower layers almost at pyrolytic height and consume the organic compounds generated by the pyrolisis of other organisms.

Of course it also possible that organisms alternate between these "roles" during their lifecycle.

1. ↑ https://en.wikipedia.org/wiki/Gas\_giant
2. ↑ https://www.sciencedaily.com/terms/gas\_giant.htm
3. ↑ http://www.pnas.org/content/105/32/11035.extract
4. ↑ http://ircamera.as.arizona.edu/astr\_250/Lectures/Lecture\_07.htm
5. ↑ https://en.wikipedia.org/wiki/Sudarsky%27s\_gas\_giant\_classification
6. ↑ http://encyclopedia.kids.net.au/page/ga/Gas\_giant
7. ↑ http://adsabs.harvard.edu/abs/1976ApJS...32..737S