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Gas Giants

This article was originally posted to the pre-magazine Freelance Traveller website in 2003, and reprinted in the November/December 2015 issue.

Gas Giant Interior Details by Bob Atmur
Original “Platform 44” scenario 1982 Dave Bryant
Links are to Wayback Machine captures of the Extrasolar Visions website  by John Whatmough (and will open in a new tab/window).

“The Solar System consists of Jupiter and some insignificant debris.” - Unknown

“No, it consists of four planets and some dust specks.” - Bob Atmur

Gas Giant Planets. Jovians. “Gargantuan Farts Floating in Space.” “Big Gs” on the one-line UPP. Free fuel for the skimming, if you’re chintzy and/or like to live dangerously.

But there’s more…


A Gas Giant, as the name implies, is a giant planet primarily made of gases, with little or no solid planet beneath all that atmosphere.

Astronomically, planets are sized by their mass, not their diameter (as in Traveller size). Units of measure are T-mass (ME, one Earth mass) and J-mass (MJ, one Jupiter mass, 318 ME). Since only the smallest of Gas Giants have anything approaching a solid surface, diameter is measured from the 1000-millibar point (pressure equivalent to Earth sea level) in the atmosphere.

Small Gas Giants are less than 50 ME (about 1/6 MJ); as their size increases, the density (and “surface” gravity) decreases—hydrogen is very light. They are usually measured in T-masses.

Large Gas Giants are over 50 ME; between 40 and 50 ME, the increased mass starts to compress the planet, and the density then increases with size. As the size increases, so does the internal heat, radiation emissions, and magnetosphere. They are usually measured in J-masses.

Brown Dwarfs or “substars” are over 13 MJ (about 4000 ME), and form the transition between planet and star. At this size, the core temperature and pressure are high enough to start deuterium and tritium fusion but not the simple hydrogen fusion needed for a true star. These miniature “failed stars” glow infrared- to red-hot from internal heat, and have the radiation output to match. The majority of Brown Dwarfs are found as solo mini-stars, close binary systems with another Brown Dwarf, or as distant binary companions to M-class, V- or VI-size red dwarf stars. However, they do crop up occasionally in other, more “normal” planetary systems.


Like all planets, Gas Giants form from a protostar’s protoplanetary disk. Unlike smaller rockballs, they form from the lightest gases in the disk, not the heavier dust and particles. Because of this, Gas Giants must form quickly, before the protostar ignites and its stellar wind blows the remaining light gases out of the system.

This happens in one of two ways:

Reality Intrudes on Scouts

Book 6: Scouts is the detailed star system generator for Classic Traveller; later revisions of the game simply revised the system generator (and returned to it in Traveller20). In stock Scouts, Gas Giants are rolled for and placed first, asteroid belts second, and other “rockball” planets last. The rules on this state:

Gas giants must be placed in available orbits in the habitable zone and in the outer system. While gas giants can be in inner orbits, they should not be placed starward of the habitable zone unless there are no other orbits available.

Place planetoid belts in available orbits. If possible, planetoid belts should be placed in the next orbit inward from gas giants.

So far, so good. Based on Sol system, with its four outer-system Gas Giants separated by an asteroid belt from the four inner rockballs. (Very symmetrical, if you count Pluto-Charon as part of the Kuiper Belt and not a true planet…)

However, at early Tech Level 9, astronomers were able to discover planets of other suns by Doppler Spectroscopy, measuring their gravitational influence on the star’s spectrum as the planet orbits. And what they found at first was clearly impossible under Scouts: “Epistellar Jovians” or “Red-hot Jupiters”, Large Gas Giants of up to many J-masses in “torch orbits” incredibly close to their suns. Since then, Large Gas Giants (the only planets detectable from interstellar distances) have been found in every orbit imaginable.

The following rule enhancements make Scouts compatible with these later discoveries:

Placement: Gas Giants may be in any orbit whatsoever, including “torch orbits” and under some special cases, “Unavailable orbits”.

Size: Scouts divides Gas Giants into Large and Small Gas Giants, but does not break them down any further. However, size does matter in appearance, number of moons, and other game effects described later.

Satellites: Scouts describes Small Gas Giants as having 2D-4 satellites (moons) and Large Gas Giants as having 2D moons.


The typical image of a Gas Giant is Jupiter in Sol System, with pastel cloud bands broken by one or more “Great Red Spot” cyclonic superstorms. In reality, Gas Giants’ appearance varies greatly depending on the distance from their sun, and secondarily from their mass.

(Due to a physics phenomenon called “Rayleigh Scattering”, almost all atmospheric gases are a very faint blue color. This is the reason the sky is blue on a world with an atmosphere—dark indigo with a Class M sun, sea blue for a Class K, and “sky blue” for Class G and above. The sky is darkest at the zenith because the line-of-sight passes through less atmosphere than near the horizon.)

Orbit/Temperature Effects on Appearance
Size Effects on Appearance

There is a more detailed essay on Gas Giants’ appearance under varying conditions of temperature and size at http://tinyurl.com/xsvisions-specs.


Epistellars and Flare Stars

A Large Gas Giant in a torch orbit can have devastating effects on the entire system. Gas Giants have magnetic fields to match their size, and a torch orbit is close enough so this magnetic field interferes with that of the sun. As the planet orbits low and fast, its magnetic field sets up eddy currents in the star, drawing plasma from the star’s interior to its surface.

When this effect has built up enough—usually around once a century—the star mega-flares, blasting off plasma down the magnetic lines of force. For a short time, the star becomes a miniature nova, increasing its brightness many-fold; the surge of radiation and heat can boil off planetary oceans and melt planetary surfaces smooth. (Larry Niven’s short story “Inconstant Moon” describes the effects of such a mega-flare.)

Naturally, such “flare stars” do not have habitable worlds. The only long-term colonies in such systems would have to be dug-in underground to survive the mega-flares.


Rings are composed of rock and ice particles in close orbit, and vary considerably. Scouts allows for multiple rings in multiple orbital positions, but does not otherwise describe their appearance.

Almost everyone who has never flown a starship thinks of evading pursuit by cutting through a planetary ring. This is possible, but collisions with ring particles will chew up a ship—hopefully not as much as your pursuer. Treat these collisions as equivalent to missile hits; roll 1D for the number of collisions when cutting the ring.

Satellites (Moons)

The moons of a Gas Giant are subject to some unique effects, all having to do with the Gas Giant’s gravity and magnetosphere.

Surface-habitable Moons

The image of a habitable satellite world—with its parent gas giant hanging in the sky—is one of the classic images of “alien world”. As stated in Scouts:

If a main world already exists, it should be placed in the habitable zone. If a gas giant is in that orbit, the main world will be a satellite of the gas giant.

If the “satellite of the gas giant” is to be surface-habitable, this is easier said than done. A habitable satellite world has to be in a certain orbit to be viable; too close and the satellite is bathed in lethal radiation from the Van Allen belts and constantly turning inside-out with tidal-caused volcanism; too far and the orbital period and (tidal-locked) day become too long to support surface life.

A surface-habitable satellite must orbit 10-15 radii from its primary to clear the Van Allen belts, yet have an orbital period less than 80-90 hours (preferably 50 or less).

The optimum size for a Gas Giant with a habitable satellite appears to be between 20 and 200 ME, with 20-40ME and 100-150ME being the most desirable. Larger than this, and the satellite is too close, burning from hard radiation on the outside and volcanism on the inside. At these mass ranges, a satellite should be able to orbit 10-15 radii away without too long a day. (Since Traveller measures orbits in radii of the primary and mass determines the orbital period, the middle range of 50-100ME has the lowest density—and the largest radii—for their mass; at 10-15 radii, the orbital period will be longer than for a denser primary.)

Even so, a satellite world will still be very seismically and volcanically active, and will have a larger amount of background radiation. At 10-15 radii, the primary will appear fixed in the sky, with an apparent diameter of 15-20 times the Moon as seen from Earth, blue banded with white clouds like a mixture of sapphire and quartz in ever-changing detail. By day, the primary is a huge crescent, eclipsing the sun for an hour or two every day. During the eclipse, sunlight leaking around the primary’s atmosphere turns the Gas Giant into a crimson ring flickering with polar aurorae and flashes of mega-lightning, bathing the satellite in a ruddy twilight.

At night, the full or near-full planet shines brightly enough to make the concept of “night” a joke, bathing everything in a blue light second only to daylight.

In extreme cases, tidal effects on long-term orbital dynamics can result in the Type E (Elliptical) atmosphere, where the satellite is elliptical instead of spherical, with nearside and farside tidal bulges extending out of the atmosphere. A more common version of this phenomenon is to have the satellite’s hydrosphere in a “ring ocean” with landmasses/highlands concentrated directly beneath the primary and at the antipodes.

Inside a Gas Giant

Nothing living can survive the temperatures and pressures deep within a Gas Giant; all human experience will be with the “surface” regions and upper atmosphere. The basic rule-of-thumb is the deeper you go, the higher the pressure and temperature and the more extreme the conditions.

A useful analogy is that of a waterworld’s planetary ocean, except the “ocean” is a hydrogen atmosphere that becomes red- and white-hot in its depths. For Traveller purposes, the pressure/temperature is rated in “Crush Depth”, where the pressure and temperature destroy a vessel with all hands—first crushed like a sinking submarine, then melted by the ambient heat and dispersed in the winds. Armored ships (like System Defense Boats) have a greater crush depth; for simplicity, rate crush depth in whatever Armor Factor is in general use for ships—High Guard, Striker, MegaTraveller, whatever. A ship can survive inside a Gas Giant so long as it is above its crush depth.

Complicating this simple image is weather—supersonic winds, updrafts, and downdrafts; gigantic electrostatic effects; and tornado funnels the size of planets. Gas Giant atmospheres are a cosmic exercise in fluid dynamics, and tend to sort out into two main conditions:

These are called “layers” because they tend to form in horizontal layers within the atmosphere, storm layers forming at the interfaces between calm layers of different wind conditions. These layers are not continuous like the skins of an onion; storm and calm layers break the “surface” at different latitudes, forming the cloud bands. Cyclonic storms (“spots” and “speckles” on the “surface”) are spinning vertical columns of concentrated storm layer—tornadoes the size of planets drawing hot impurities from deep within the planet.

For Traveller purposes, treat the vertical cross-section of a Gas Giant as alternating calm and storm layers, with crush depths for increasing Armor Factors at increasing depths. Note that Inner Zone Gas Giants have no clouds to indicate the presence of their (larger and more energetic) storm layers. Small Outer Zone Gas Giants will be mostly calm layers, with storm layers only in the depths.

Beneath crush depth, temperatures and pressures continue to rise with depth—red-hot, yellow-hot, white-hot. Finally the pressure is enough to liquefy the hydrogen; after thousands and thousands of km of red-hot hydrogen fog comes the “true surface”, a planetary ocean of white-hot liquid hydrogen. If any carbon exists at this state, the temperature and pressure compress it into macrocrystalline form—a rain of diamonds down to the molten rock core.

Fuel Skimming (and Complications of Same…)

Travellers will most likely encounter a Gas Giant when skimming fuel. This looks straightforward; just skim the Gas Giant’s hydrogen atmosphere, scoop a load of free fuel, and take the risk of Unrefined Fuel, right?


The optimum Gas Giant for fuel skimming is between 30 and 50ME; this is the point of minimum density and minimum surface gravity, where even a 1-G ship has enough acceleration to climb out in a worst-case situation—all the fuel in the universe won’t help you if you can’t make escape velocity after the skim. (Generally, if the Surface Gravity from the tables is less than the ship’s Maneuver Drive G-rating, the ship can skim safely.) Outer Zone orbits are preferred, as less heat increases the chance of “calm air”. (Cloud banding is the most obvious clue; if the Gas Giant is small and cold enough to not have cloud bands, chances are the calm air extends over most or all of the “surface”.)

There is Unrefined Fuel, and there is Really Bad Fuel—contaminated with heavier elements. If the Gas Giant is less than 20 ME (low-end Table 1) or greater than 1-2 MJ (Table 2C and up), the raw fuel will be not just unrefined, but Contaminated. Less than 20 ME, and the mini-Gas Giant has not accreted enough hydrogen to dilute the heavier gases; more than 1 MJ, and the atmosphere is too active from internal heat, upwelling heavier elements from the depths in planet-sized convection cells. Inner-zone Gas Giants also suffer from this problem—doubly treacherous, as their atmosphere is too hot for any telltale clouds to form.

Contaminated Fuel has the following in-game effects:

At the other extreme, some Gas Giants have little or no heavy gas contaminants, and are natural Refined Fuel. These Gas Giants are always found orbiting the oldest stars in the Galaxy (Class M, Size V “red dwarfs” and Class K or M, Size VI “subdwarfs”) with no other bodies in the system—no moons, no asteroids, no planets, nothing. When these systems formed, the only thing existing in the proto-Galaxy was hydrogen. (The formal astronomical name for this is “low-metallicity”.) Yes, they are natural Refined Fuel, but they’re in another system where all support to exploit this has to be brought in by Jump.

Fuel Skimming is more complex than its general image of “dive in, hit the atmosphere, skim the fuel, and boost out”. The basic idea is to hit a calm layer and run with the wind, avoiding radiation and turbulence.

Class A to C Starports with Gas Giants in-system make use of special Fuel Lighters to skim and process their fuel. A fuel lighter is a streamlined non-starship, usually around 800 tons or so, with powerful Maneuver Drives to pull itself out of a Gas Giant’s gravity well and all possible interior space devoted to fuel scoops, fuel tankage, and fuel processing plant. They boost to the Gas Giant, skim themselves full of fuel, and refine it on the return trip, unloading Refined Fuel at the starport or directly to ships. Tankers are starships similarly equipped; these accompany naval task forces or make runs to nearby Gas Giant-bearing systems (especially the aforementioned “low metallicity” systems) for those systems without a source of fuel.

Some high-tech worlds mine their Gas Giants for exotic chemicals when no other source is available. On small scales, this is done with specially modified fuel lighters whose “fuel refineries” effectively work in reverse, discarding the hydrogen and storing the specific chemical “impurities”. Since these “impurities” are normally found in storm layers, these chemical lighters seek out the edges of storm layers for their skimming. Because of the danger, these craft are often unmanned, with autonomous AI robotic brains controlling them in-atmosphere. Sometimes a live crew flies them to the Gas Giant, “abandoning ship” in a carried small craft and re-boarding upon emergence; others are remote-controlled from a home base on one of the Gas Giant’s moons.

(Legally, this is complicated by the ancient maritime law that a completely-abandoned ship is a “derelict” open to Right of Salvage by anybody; the equally-ancient safeguard to this is to have an animal aboard who is technically on the crew roster as “ship’s mascot” who is left aboard when the crew evacuates. As long as one of the “crew” is aboard, the ship is not legally “abandoned”.)

Large-scale “atmosphere mining” is done by large, heavily armored, purpose-built robotic refining platforms. Essentially giant unmanned chemical lighters, these platforms dive deep into the storm layers to skim and collect their chemicals, surfacing into the upper atmosphere to transfer their refined chemical loads to manned fuel lighters acting as chemical tankers.

Life in a Gas Giant

Some Gas Giants are life-bearing—all the precursor organic chemicals and molecules are present at some point, and the solar and interior heat (and lightning) provide the energy. The best candidates for life-bearing are similar to the best candidates for skimming—small, temperate-to-cool Gas Giants with relatively calm atmospheres.

Gas Giant life forms will be floaters or flyers, analogous to ocean life. Most of the time they will be floating bacteria, microscopic “aero-plankton” visible only as strangely colored clouds or haze. This aero-plankton tends to congregate along the borders of calm and storm layers, where the storm layers bring nutrient chemicals up from the depths like “black smokers” along deep-ocean vents. Alternately, photosynthetic aero-plankton could ride winds in the upper atmosphere and sunlight.

Multicellular life could be simple floating “sheets” of “aero-algae” colonies or more organized forms including the “balloon-beasts” of common imagery (like the Jgd-Il-Jadg); actually, the majority of worlds—Gas Giant or rockball—rarely get beyond bacteria.

Because of the turbulent environment on the edge of storm layers—life-forms could easily be drawn into a storm layer and destroyed by downdrafts into the depths—Gas Giant life would probably reproduce quickly and prolifically, spawning offspring faster than the turbulence and downdrafts could destroy them. Colony creatures have an advantage in this regard; if they are torn apart by winds and turbulence, each surviving fragment can regrow into a new colony. Though there is theoretically no limit to the size of Gas Giant “balloon-beasts”, the winds and turbulence normally limit size to smaller than commonly believed. (Such as 3m gasbag diameter in the case of the Jdg-Il-Jadg.)

Life can complicate fuel-skimming; bacteria and aero-plankton would just be another “impurity” to refine out, but multicellular/colonial life can clog fuel scoops or in extreme cases cause collision damage (as Missiles).

Scenario Nuggets

Inconstant Sun

Belters are operating in a flare-star system with an epistellar Gas Giant. The star is showing massive sunspot activity, prominences flaring visible to the naked eye; a mega-flare is imminent. For various reasons, the belters are staying and extracting until the last minute, betting on outrunning the flare. Do you feel lucky today?

Platform 44 (1982 Dave Bryant)

A chemical lighter (of hundreds to thousands of Traveller tons) starts its climb out of the Gas Giant; above, its crew in their Cutter descend to rendezvous with the unmanned lighter and fly it back to the main world. A freak mega-bolt blanks out the Cutter’s sensors for over a minute, blowing every circuit breaker in the avionics; when they recover, the lighter—now silent, wreathed in St Elmo’s Fire and lightning arcs like a Tesla Coil—is dropping in a glide path back into the Gas Giant. Can the player-character crew board, repair, and salvage it before it hits crush depth?

As the out-of-control lighter drops, it will fall through several calm and storm layers before reaching crush depth. Each storm layer can damage the lighter further, as it falls out of control and is knocked about by the turbulence; this allows the referee to orchestrate the scenario, as updrafts or downdrafts could speed or temporarily reverse the descent.

If the damage (from the initial accident or subsequent storm-layer tumbles) has weakened the hull, it could crush before its official crush depth, killing all aboard—remember, the pressure is always increasing to and past ocean-bottom levels, and any breach or damage weakness only has to let the pressure in. As the pressure increases, the hull will creak and groan ominously. (Don't overlook the “horror value” of the gory details of a crush-depth collapse and the fact that they’re falling without ever hitting bottom because a Gas Giant has no surface to hit.) If the interior atmosphere is breathable, there will also be a fire/explosion hazard long before crush depth as the increasing pressure literally squeezes the outside hydrogen through the hull itself (remember, hydrogen atmospheres are classed as Type C—Insidious—as hydrogen molecules are so small they can interpenetrate almost anything solid, literally squeezing through the spaces between the hull molecules). The lighter having an armored hull and a greater crush depth than the crew’s escape cutter complicates salvage—if the crew stays with the lighter past the cutter’s crush depth, they will be unable to escape. Again, do you feel lucky today?

This scenario was developed by Dave Bryant and run in several incarnations at several conventions in the early 1980s; in the original version, “Platform 44 Linda” was a large dedicated refinery platform; the corporate tanker docking with it was also damaged, but managed to boost out; the player-characters were the crew of a Scout/Courier on detached duty who were skimming fuel near the accident and volunteered to salvage the platform as it plunged through the atmosphere.

From experience in the original scenario, I recommend the referee measure the fall by calm and storm layer, with crush depth for cutter and platform measured in so many layers below the “surface”, not by a hard-and-fast time-and-distance track. This allows the referee to orchestrate the danger for maximum drama without getting bogged down in detail. One crowd-pleasing effect in the upper atmosphere (first calm layer?) was an EVA along the exterior of the platform’s hull to reach a damaged area, vacc-suited PCs dragging themselves from handhold to handhold in a blast of ammonia sleet, like mountain climbers in a blizzard. Later on, when the platform’s emergency “hot-air balloon” gasbags had been deployed to halt the descent, they ran afoul of local balloon-beasts in rut, attracted to the gasbags.

Illustration Links

Illustrations are by John Whatmough, captured by the Wayback Machine from his website “Extrasolar Visions”, formerly at http://www.extrasolar.net/. TinyURL links in this article are to captures at various dates at the approximate time this article was originally added to the Freelance Traveller website.
A full list of images is available from the Wayback Machine captures at http://web.archive.org/web/*/http://www.extrasolar.net/usage.asp.


Table 1: Small Gas Giants
1D Mass Diameter Trav Size Surface Gravity Remarks
ME MJ Terra Jupiter Gs
1 5 0.015 1.7 0.16 14 1.73  
2 10 0.030 2.5 0.23 20 1.65  
3 20 0.070 4.0 0.36 32 1.25 (Uranus/Neptune)
4 30 0.100 5.7 0.52 45 0.92  
5 40 0.130 7.4 0.67 60 0.73 (Point of lowest density)
6 50 0.170 8.0 0.72 64 0.78  


Table 2A: Large Gas Giants
1D Mass Diameter Trav Size Surface Gravity Remarks
ME MJ Terra Jupiter Gs
1 100 0.32 9.0 0.82 72 1.25 (Saturn)
2 150 0.48 9.5 0.86 76 1.66  
3 200 0.64 10.0 0.91 80 2.00  
4 250 0.90 10.5 0.96 84 2.25  
5 300 0.95 11.0 1.00 88 2.50 (Jupiter)
6 Roll again on Table 2B


Table 2B: Large Gas Giants
1D Mass Diameter Trav Size Surface Gravity
ME MJ Terra Jupiter Gs
1 350 1.1 11.2 1.02 90 2.85
2 400 1.3 11.4 1.04 91 3.10
3 450 1.5 11.6 1.05 93 3.30
4 500 1.6 11.8 1.07 95 3.60
5 550 1.8 12.0 1.09 96 3.80
6 Roll again on Table 2C


Table 2C: Very Large Gas Giants
1D Mass Diameter Trav Size Surface Gravity Remarks
ME MJ Terra Jupiter Gs
1 600 1.9 12.2 1.11 98 4.2 (Double Jupiter)
2 700 2.3 12.4 1.13 99 4.6  
3 800 2.6 12.6 1.15 101 5.0  
4 900 2.9 12.8 1.16 103 5.4 (Triple Jupiter)
5 1000 3.2 13.0 1.20 104 5.8  
6 Roll again on Table 2D


Table 2D: Very Large Gas Giants
1D Mass Diameter Trav Size Surface Gravity Remarks
ME MJ Terra Jupiter Gs
1 1500 5.0 13.5 1.25 108 8.2 (Five Jupiters)
2 2000 6.7 14.0 1.27 112 10.2  
3 2500 8.3 14.5 1.34 116 11.8  
4 3000 10.0 15.0 1.36 120 13.3 (Ten Jupiters)
5 3500 11.6 15.5 1.41 124 14.6  
6 Roll again on Table 3  


Table 3: Brown Dwarfs
1D Mass Diameter Trav Size Surface Gravity Remarks
ME MJ Terra Jupiter Gs
1 4000 13 16 1.45 128 16.6  
2 5000 17 17 1.55 136 17.2  
3 6000 20 18 1.64 144 18.5 (Twenty Jupiters)
4 7000 23 19 1.73 152 19.4  
5 8000 27 20 1.82 160 19.8  
6 ≥9000 ≥30 ≥21 ≥1.90 ≥168 ≥20.0 (Thirty Jupiters)