Gas Giant

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

• Starships fuel themselves by diving into this atmosphere and skimming hydrogen from within it.
• The planets Jupiter, Saturn, Uranus, and Neptune, in the Terra system, are examples of gas giants.

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 and hotter toward the rocky core, with hydrogen in solid, liquid, or liquid-like states beneath the mantle layers.

• The 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 1 atm. This corresponds to the mean surface pressure of Terra's atmosphere, which is used as a benchmark.

This definition of the surface is very arbitrary: a gas giant's atmospheric pressure varies greatly from region to region, and even the smallest gas giant will have many hundreds of kilometers of complex, stratified "upper" atmosphere above the measuring point.

• 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 outer regions of a star system, or more rarely within the habitable zone. They are usually found in those regions of the system.

• Occasionally, celestial mechanics and gravitational forces may cause a gas giant world to migrate within the star system, rearranging the orbits of other worlds and even causing them to be ejected from the system, becoming starless Rogue Worlds. Some migrate inward and a few, rather than rapidly burning up in the star's photosphere or being flung out of the system, instead settle into a stable orbital position very close to the star. These worlds become Hot Gas Giants, often referred to as Hot Jupiters or HGGs.

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 infrared. 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.

• Different chemicals and compounds may precipitate out of the atmosphere at different altitudes, temperatures and pressures. They manifest as fogs, rain, sleet, snow, or as heavier solids (typically ices, salts, or sands).
• 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.
• Individual weather events, such as large storms, can cover millions of km² and last for decades or centuries.

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.

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
World Type	Abbrev	Zone	Remarks
Large Gas Giant	LGG	Habitable Zone Outer System	Gas giant planets composed mainly of hydrogen and helium, with diameters ranging from 60,000 km to around 200,000 km. They formed within the outer system, around stony or metal-rich cores which accumulated gases and volatiles from the outer part of the protoplanetary disk. Large gas giants experience extremely high internal pressures. Their interiors are intensely hot, with a dense mantle around the core composed of metallic hydrogen (a degenerate material with similar properties to superdense armor) and liquid hydrogen, a turbulent layered deep atmosphere filled with clouds composed of salts and silicates, and a relatively shallow, largely transparent outer atmosphere where pressures drop below 1 standard atmosphere and eventually give way to the vacuum of space. There are no distinct layers within the world: the mantle transitions gradually into the deep atmosphere, which in turn gradually transitions into the outer atmosphere. Large gas giants have powerful weather systems and violent winds. They may have one or more gigantic world-sized storms that last for centuries. Fuel skimming is possible but requires more than 2-G of acceleration to regain orbit. (This is possible by overloading 2-G drives, but not recommended.) The "standard" large gas giant is Jupiter, in the Terra system.
Small Gas Giant	SGG	Habitable Zone Outer System	A type of gas giant, with diameters ranging from 20,000 km to 60,000 km. Small gas giants generally have similar sized cores to large gas giants and formed within the outer system, gathering material from the outer part of the protoplanetary disk. They are composed mainly of hydrogen and helium: during their formation they were unable to gather the vast amounts of materials necessary to enable them to develop into large gas giants or the relatively large amounts of volatiles characteristic of ice giants. Small gas giants and ice giants are similar in size and appearance. Small gas giants experience lower internal pressures than large gas giants, though those pressures are still very extreme. Their interiors are intensely hot, with a dense but relatively shallow mantle composed of liquid hydrogen, a turbulent deep atmosphere filled with cloud layers composed of silicates and salts, and a transparent outer atmosphere that is generally far taller than those found on large gas giants. Fuel skimming is possible but requires at least 2-G of acceleration to regain orbit. (This is possible by overloading 1-G drives but not recommended).
Ice Giant	SGG IG	Habitable Zone Outer System	A type of small gas giant planet with diameters typically ranging from 20,000 km to 60,000 km. Ice giants generally have similar sized cores to other type of gas giants and formed within the outer system, gathering material from the outer part of the protoplanetary disk. They are primarily composed of hydrogen and helium, but contain significantly higher amounts of elements such as oxygen, carbon, nitrogen, and sulfur than other types of gas giant. Under the extreme pressures and temperatures found deep within the world these elements, and the complex compounds they form, exist as fluids and hot crystalline structures. These are referred to as exotic ices and give the world type its name. Fuel skimming is possible but requires at least 2-G of acceleration to regain orbit. (This is possible by overloading 1-G drives but not recommended). The "standard" ice giant is Neptune, in the Terra system.
Hot Gas Giant Hot Jupiter	HGG	Inner System	Hot close gas giants orbit very close to their star and are relatively young in terms of stellar evolution: the energy streaming off of the star relentlessly strips away their atmospheres, 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 and into interstellar space on the stellar wind. The world forms, like other gas giants, within gas-rich regions of the outer part of the protoplanetary disk, but at some point the gas giant changes its orbital distance around the star. Most probably this migration is caused by the interactions of multiple large planetary bodies during the evolution of the system, but in some cases it may be due to disruption generated by the passing of a wandering star or very large rogue world. Some gas giant worlds migrate inward through their star system as a result of this cosmic reshuffling, eventually moving extremely close to the star. Some, rather than burning up in the star's photosphere or being slung out of the system, settle into a stable orbit. During their movement into the inner system, migrating 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 and these in turn may have settled into stable orbits around the star and become independent planets in their own right, or the gas giant may have "towed" former outer system worlds inward behind it. The extreme temperatures that Hot Gas Giants experience causes them to swell and become puffy: younger examples may be up to 400,000 km in diameter and may stream trails of hot gas from their boiling atmospheres. Their diameter steadily reduces as the stellar wind erodes their atmosphere away. Fuel skimming is possible but extremely hazardous in the very high temperatures and extreme weather conditions found on the world. Drive requirements depend on the remaining mass of the world: at least 2-G of acceleration is recommended to regain orbit.
Brown Dwarf	BD	Inner System Habitable Zone Outer System	Brown dwarfs are massive gas worlds that lie between large planets and small stars. They are classified into three broad types: Y Type dwarfs, with masses between 13 and 30 Jupiter masses. These objects are known as ultra-cool dwarfs and resemble large gas giants. T Type dwarfs, with masses between 30 and 55 Jupiter masses. These objects are known as methane dwarfs because of methane emission lines within their spectra. L Type dwarfs, with masses between 55 and 80 Jupiter masses. These are substellar objects, not quite massive enough to ignite hydrogen fusion. Brown dwarfs are so massive that they were able to ignite limited lithium fusion within their cores during their formation: they do not display hydrogen fusion, which marks true stars. Depending on their mass they may superficially resemble Hot Gas Giants or the smallest type M stars. Despite of their very high mass, brown dwarfs have smaller diameters than gas giants: their incredibly high gravity physically contracts the world, causing electron degeneracy within the material that they are composed of (electrons fill the lowest quantum states, forming electron degenerate matter, similar to superdense armor). Generally speaking the higher the mass and gravity, the smaller the brown dwarf's diameter. Fuel skimming is possible but extremely hazardous in the extreme conditions found on such worlds. Drive requirements depend on the mass of the world: at least 6-G of acceleration is required to regain orbit around Y Type dwarfs: being able to regain orbit around T Type and L Type dwarfs is beyond the capability of conventional drives.

See also

Universal world profile

§ == ( Please refer to the following AAB Library Data for more information: ) == §

• Main world
• Hex Number
• Universal World Profile
  • Starport (Sp)
  • Planetary Size (S)
  • Atmosphere (A)
  • Hydrosphere (H)
  • Population (P)
  • Government (G)
  • Law Level (L)
  • Tech Level (TL)
• Trade classification & Sophont Codes
• Importance Extension (Ix)
• Economic Extension (Ex)
• Cultural Extension (Cx)
• Nobility
• Bases
• Travel Zone
• PBG - Population, Belts, Giants
  • P: Population Multiplier
  • B: Belts
  • G: Gas Giants
• Worlds
• Allegiance Code
• Stellar Data
• COG Sequence
  • C: Climate
  • O: Orbit
  • G: Gravity

References & Contributors (Sources)

This page uses content from Wikipedia. The original article was at Gas_Giant. The list of authors can be seen in the page history. The text of Wikipedia is available under the Commons Attribution-ShareAlike 3.0 Unported License.

This page uses content from Wikipedia. The original article was at Sudarsky's_gas_giant_classification. The list of authors can be seen in the page history. The text of Wikipedia is available under the Commons Attribution-ShareAlike 3.0 Unported License.

This list of sources was used by the Traveller Wiki Editorial Team and individual contributors to compose this article. Copyrighted material is used under license from Far Future Enterprises or by permission of the author. The page history lists all of the contributions.

• Marc Miller. Twilight's Peak (Game Designers Workshop, 1980), .
• John Harshman, Marc Miller, Loren Wiseman. Library Data (A-M) (Game Designers Workshop, 1981), .
• Marc Miller. Imperial Encyclopedia (Game Designers Workshop, 1987), 24,88,93.
• Matthew Sprange. "Gas Giants, Planetoid, and other bodies." Journal of the Travellers' Aid Society volume 2 (2019): 80-85.
• Martin Dougherty. Referee's Handbook (Mongoose Publishing, 2021), 91.
• Ken Pick, Gas Giants at Freelance Traveller