THREE/5 Gas Giants

STEP ONE: Generate the magnetic field factor on 3.5.1.

STEP TWO: Go to FOUR/1 to decide the base temperature of the gas giants.

Table 3.5.1 Magnetic Field Strength

1d10  Mass
≤50 (small)  51-200 (medium)  201-500 (large)  ≥501 (superjovian)
1  1d10 x 0.10  1d10 x 0.25  1d10 x 0.50  1d10 x 1.50
2-4  1d10 x 0.25  1d10 x 0.50  1d10 x 1.00  1d10 x 2.50
5-7  1d10 x 0.50  1d10 x 0.75  1d10 x 1.50  1d10 x 5.00
8-9  1d10 x 0.75  1d10 x 1.00  1d10 x 2.00  1d10 x 10.00
10  1d10 x 1.00  1d10 x 1.50  1d10 x 3.00  1d10 x 25.00


For ease of reference, gas giants are divided into three size classes - small (like Uranus and Neptune), medium (like Saturn) or large (like Jupiter), and into superjovians.


Gas giants are composed of gas, but for small gas giants the majority of their mass is actually the core of rock and ice. Gas giants have a core of rock and metal, surrounded by an outer core of liquid/solid "ices", compunds made out of carbon, oxygen, nitrogen and hydrogen - such as water, ammonia, carbon dioxide etc. Large and medium gas giants have a layer of metallic hydrogen outside that (a layer that is most of the planet on large gas giants, but not at all as large part on medium gas giants), and all gas giants have a deep atmosphere primarily made up of hydrogen forming the final part of the world. The pressure deep in a gas giant is far greater than anything on normal planetary surfaces. Hot gas giants (those close to the star) are a subject less known, but the basics should be about the same.


Gas giants can form in two ways - they can condense from the protostellar disc or they can be "budded off" by the protostar. The later formation is more likely for close gas giants and superjovians, while more distant gas giants conceivably could form in both ways. In any case, gas giants tend to have a composition very similar to the star in elements, as they are made of the basic building blocks much more than planets and chunks, who are composed mainly by heavier elements.


A gas giant's atmosphere is mostly hydrogen (about 90%). About 10% is helium. On some gas giants, the helium part is smaller - because helium can precipitate in the planetary atmosphere. Cold gas giant atmospheres (those in the Outer Zone) also have parts of ammonia, methane and various carbon compounds - hot gas giants more likely have carbon dioxide and water vapor.


Most gas giants generate interior energy, either by gravitic contraction (common on large gas giants) or helium precipitation (common on cooler gas giants). This interior energy is important mainly to create the complex cloud patterns on many gas giants - giants with little interior energy will have much less in the way of patterns as there is so little internal convection. However, many gas giants also have distinct east-west bands due to their fast rotation.


Gas giants tend to rotate fast, and this gives most of them, especially the medium and large ones, a distinct flattened shape, or oblateness. Most planets are oblate, including terrestrial ones, but generally it is significant only on gas giants.


Gas giants are tinted by their atmosphere, and while this may only be interesting as a note to world generation these are some suggestions:

  • Hot gas giants will tend to be bluish, perhaps dark blue, due to atmospheric scattering of sunlight. Sulfuric compounds may form whitish or yellowish clouds, perhaps tinted by phosphorous compounds.
  • Warm gas giants (Life Zone) will also tend to have a basic bluish color, but water clouds would form notable white systems and they may be tinted by various other chemicals.
  • Cool gas giants (outside life zone, but within the Inner zone) will have pale blue clouds of water and carbon dioxide, mostly covered by yellow-orange-brown clouds based upon ammonium hydrosulfide.
  • Cold gas giants (inner part of the Outer Zone) would have clouds of ammonia covering most of them. Ammonia clouds are white, but they will be stained yellow, red or brown by various other chemicals. Slow rotation or little convection may diminish the bandedness of these worlds.
  • Very cold gas giants (further out) will likely have clouds of methane and a bluish color, with whiter clouds of methane ice.
  • Extremely cold gas giants will be dark blue and virtually without clouds, as methane freezes out.

Still, there are many possibilities. Atmospheric smog, haze, internal convection - all can affect how a gas giant look. Superjovians will also be affected by this, of course, tinted by the clouds which condense in their warmer atmospheres.

Large moons of gas giants in the life zone may well be capable of sustaining life. The potential problems for moons are the rather long day (as the "day" is the orbital period around the planet) which gives the moon itself a weaker magnetic field and distinct diurnal temperature differences, and that some large gas giants and superjovians may have strong magnetic fields which could provide too much dangerous radiation, like the Io's position around Jupiter. Another problem may be that moons of enough size could be rare, though arguably moons of gas giants in the inner zone have the potential to become rather big.