Gas Giant

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A Gas Giant is an extremely large planet, primarily composed of gases, and usually with an extensive atmosphere of hydrogen and hydrogen compounds.

Please see the following AAB Library Data articles for more information:

Description (Specifications)

Gas giants may have a rocky or metallic core—in fact, such a core is thought to be required for a gas giant to form—but the majority of its mass is in the form of the gaseous hydrogen and helium, with traces of water, methane, ammonia, and other hydrogen compounds.

Planetary Characteristics

Gas giants do not have a well-defined surface; their atmospheres gradually become denser toward the core, perhaps with liquid or liquid-like states in-between.

  • These deep interior regions may be composed of metallic hydrogen or metallic helium, kinds of degenerate matter that are still fluidic but behave like electrical conductors.

As a matter of practicality, the diameter of a gas giant is generally measured at the mean altitude where the atmospheric pressure equals 1000 millibars. This corresponds to the mean surface pressure of Terra's atmosphere, which is used as a benchmark. This procedure is very arbitrary: a gas giant's atmospheric pressure varies greatly from region to region, and even the smallest gas giant will have thousands of kilometers of complex, stratified "upper" atmosphere above the measuring point.

  • Thus, terms such as diameter, surface area, volume, surface temperature and surface density effectively refer only to the outermost layer visible from space.
  • The third digit of the "PBG" element of the Universal World Profile indicates how many gas giants are present within the star system.

Gas Giant Life

It is possible for lifeforms (and even NILs) to originate on gas giants

  • Lifeforms that have evolved on gas giants are called jovionoids.

Probable Planetary Orbit & Climate

Due to the mechanics of system formation, related to the distribution of material within the protoplanetary disk, gas giant worlds most commonly form within the habitable zone and the outer regions of a star system. They are usually found in those regions.

  • Occasionally, celestial mechanics and gravitational forces may cause a gas giant world to migrate inward through the star system. Some, rather than rapidly burning up in the star's photosphere or being flung out of the system, instead settle into a stable orbital position close to the star. These worlds become Hot Jupiters.

The color of a gas giant depends largely on its chemical components and the various compounds and substances that form within it; these stain the atmosphere. Colors typically include white, gray, and shades of blue, green, yellow, orange, brown, tan and red. Gas giants look very different if viewed in different spectra such as IR. Gas giants are most commonly monochrome, mottled or banded. A Hot Jupiter may be incandescent.

The upper layers of a gas giant's atmosphere appear similar to the skies of a terrestrial world: the gas mixture is generally transparent, with vast banks of mountainous clouds divided by abyssal chasms and sporadically lit by the flashes of enormous bolts of lightning. The gas mixture becomes denser and more opaque with depth.

  • Radiation belts, shaped by magnetic fields, surround gas giant worlds. Travelling through radiation belts may cause exposure to dangerous levels of radiation.
  • Huge, spectacular auroras may be seen above the polar regions.
  • Many gas giants produce a great deal of electromagnetic noise across a broad spectrum. This may seriously hamper sensor operations and communications.
  • Wind speeds within the upper atmospheric layers of a gas giant can exceed 600 kph: in some atmospheric layers the local windspeed may exceed the speed of sound. Different atmospheric layers have prevailing winds travelling in different directions: the convergence zones between different atmospheric layers may be extremely turbulent.
  • Different chemicals and compounds may precipitate out of the atmosphere at different depths, temperatures and pressures. They manifest as fogs, rain, sleet, snow, or as heavier solids (typically ices and salts).
  • Individual weather events, such as large storms, can cover millions of km² and last for decades or centuries.

History & Background (Dossier)

The Solomani sometimes refer to gas giants as a Jovian planet after the planet Jupiter in the Terra system.

Gas Giant Types

Gas Giant worlds most commonly form within the habitable zone and the outer system and are usually found in those regions.

Gas Giants
Zone Remarks
Large Gas Giant Gas giant planets composed mainly of hydrogen and helium and with diameters ranging from 60,000 km to 400,000 km.
Small Gas Giant Gas giant planets composed mainly of hydrogen and helium and with diameters ranging from 20,000 km to 60,000 km.
Hot Gas Giants Occasionally, celestial mechanics and gravitational forces (most probably due to the interactions of multiple large planetary bodies during the evolution of the system, or perhaps disruption generated by the passing of a wandering star) may cause gas giant worlds to migrate inward through the star system. Some, rather than burning up in the star's photosphere or being slung out of the system, instead settle into a stable orbital position close to the star.
  • Hot close gas giants are relatively young in terms of stellar evolution. The energy streaming off of the star gradually strips away their atmospheres, typically reducing them to a dense near-vacuum core remnant (a "cthonian world") within a billion or so years. The former atmosphere may exist as a sleet of ionised gases and more complex compounds, with a relatively high density compared to the normal interstellar medium, being carried back through the system on the stellar wind.
  • During their movement into the inner system, hot close gas giants may have cleared orbital positions of existing planetary bodies. Their gravitational influence may have shattered worlds and created planetoid belts, they may have shed their moons during their transit which in turn may have settled into stable orbits around the star and become independent planets in their own right, or they may have "towed" former outer worlds behind them.
Hot Jupiter A Hot Gas Giant orbiting close to its star. Hot Jupiters are relatively young in terms of stellar evolution. The energy streaming off of the star gradually strips away their atmospheres, typically reducing them to a dense core remnant within a billion years. The interstellar medium generally has elevated levels of particles spaceward of the Hot Jupiter.
Ice Giant A type of small gas giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. Diameters typically range from 20,000 km to 60,000 km.

References & Contributors (Sources)

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