Terminology[edit | edit source]
The term "gas giant" was coined by the science fiction author James Blish. It refers to a planet which is composed mainly of hidrogen 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[edit | edit source]
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 sorround a rocky or nickel/iron core.
Sudarsky's classification[edit | edit source]
Gas giants are split into five classes (numbered using Roman numerals) according to their modeled physical atmospheric properties.
Upper mass limit[edit | edit source]
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[edit | edit source]
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 likelyhood 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 favour 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:
Of course it also possible that organisms alternate between these "roles" during their lifecycle.