I wanna build a habitable desertworld which is a gas giant moon. The desired and already semi-fixed parameters are:

  • mass between 0.4 and 0.25 Earth-masses

  • only 10 to 30 percent global ocean cover, water is mostly near the poles

  • orbits a 13,1 jupiter-mass warm superjovian

  • extensive vulcanism and some plate tectonics

  • global average temperature between 40 C and 45 C, just shy of a runaway greenhouse effect

  • only one atmospheric circulation cell due to slow rotation (the moon is tidally locked)

  • sun is an old F-type with only a few hundred million years left on the main sequence

For story purposes I want the poles to be separated by a huge, scorching, equatorial desert. I don't want any rainforest or even savanna near the equator. Hot-semi arid (Koepen climate class BSh) bands on the equator would be the most I'd be willing to accept, oasis of it would even be welcome.

Messing strongly with axial tilt is not an option, sice I don't want a pole-equator climate switch or exteme seasons.

Will my setup already deliver what I want? If not, what could I tweak?

  • 1
    $\begingroup$ Simply curious. A gas giant that massive shouldn't turn into a star? $\endgroup$
    – Lupus
    Jul 16, 2019 at 10:09
  • $\begingroup$ @Lupus You are not far off. The still debated upper mass limit for gas-giants is 13.8 Jovian masss as far as I know. Above that Deuterium fusion would start, turning the thing into a brown dwarf. At 65 Jovian masses Lithium would also undergo fusion. Only once the thing has 0.08 solar masses (84 Jovian masses) hydrogen fusion will start, turning the object into a proper star. $\endgroup$ Jul 16, 2019 at 10:17
  • $\begingroup$ Cha 110913-773444 is an 8 Jovian mass brown dwarf so at 13.1 Jovian masses composition will play an important role in your planet's classification. $\endgroup$
    – Ash
    Jul 16, 2019 at 11:39
  • $\begingroup$ @Ash HR2562 b is considered a planet at 30 Jupiter masses. The cutoff between gas giant and brown dwarf is poorly defined and debated. How would I have to manipulate composition to get a gas giant instead of a brown dwarf? $\endgroup$ Jul 16, 2019 at 11:47
  • 1
    $\begingroup$ Brown dwarfs rely on deuterium, rather than hydrogen, fusion so if deuterium is very rare in the planet as a whole then fusion cannot occur, this is how much more massive objects stay planets instead of becoming stars. Given the age of the star system in question it's possible that it was a brown dwarf but it has burned out most/all of its deuterium supply, brown dwarfs don't stay hot for nearly as long as "proper" stars. $\endgroup$
    – Ash
    Jul 16, 2019 at 12:00

3 Answers 3


If you don't want rainforest around the equator, you have to get rid of the rain. How?

Easy! Take away the ocean!

If the belt comprised between the tropics and the equator is above the sea level, there will be no ocean from which water can evaporate and then condense into rain. Rain will only happen around those areas where the ocean is present and atmospheric circulation can bring the humidity generated above them.

Removing rain from the equator, combined with the strong illumination, will necessarily turn that region into a scorching desert, as you desire.

  • $\begingroup$ Thanks, that pretty much what I thought. Out of curiosity, during phases in the moons history where there where drainage/ocean basin at the equator there would have been rainforest there, right? Additionally a few hypersaline lakes which evaporate as quickly as they stirred up the salt pen they flooded won't give the equator enough water fir rainforests, or will they? $\endgroup$ Jul 16, 2019 at 10:23
  • $\begingroup$ @TheDyingOfLight, that's plausible: the less water is present, the less rain it can form. $\endgroup$
    – L.Dutch
    Jul 16, 2019 at 10:27

The gravel sea, it will actually be largely composed of much larger rocks but, if the equatorial/tropical zone is largely composed of plains of boulders and gravel interspersed with remnant hard rock peaks it can rain as much as it likes at any latitude but the water table will be inaccessibly low for vegetation to grow. This is a self reinforcing cycle as lack of vegetation cover increases the erodibility of finer rock particles. This does require relatively consistent wind conditions to move fine dust continually away from the equatorial latitudes but that makes sense in a tidal locked situation. Water will occasionally be available in places:

  • where larger than usual rocks create openings that extend down to the water table, this water will probably be pretty brackish or possibly even actively saline.

  • where pockets of finer sediment support capillary migration of water from depth to the surface, such water may be quite fresh, resembling a mineral spring or quite nasty depending on the sediments in question.

  • where a layer of fine sediment at depth terminates and gravitational head creates springs, this water will probably be quite fresh.

But these will support only isolated pockets of life in an otherwise inhospitable, and quite impassable terrain, the lighter the colour of the rocks the more of a baking desert the area becomes.


Salt pan of death, there once was an equatorial ocean on your moon but after the gas giant captured another large moon in a resonant orbit the habitable moon got a lot warmer. There are several other mechanisms that could also raise the temperature but the new resonant orbit is needed because crustal flexing makes the moon more oblate, the once reasonably deep ocean is now very shallow, the whole region resembles the Salar de Uyuni salt flats in Bolivia as seen below. The area has plenty of water at or just below the surface of an unstable shifting salt crust some of that water is extremely, lethally, deep. There are mechanisms to form occasional fresh water springs in isolated parts of the pan mainly due to gravel deposits near the edges. The former islands of the equatorial sea are now mountain peaks that may be vegetated, the reflected light from the pan will keep effective temperatures higher than most plants deal with well but desert species should thrive.

Bolivia's Salar de Uyuni, 12,000 square kilometres of shifting salt and hyper saline lake water.

  • $\begingroup$ Not really what I was looking for, but an interesting biome fir my planet for sure, thanks. $\endgroup$ Jul 17, 2019 at 19:10
  • $\begingroup$ @TheDyingOfLight No I didn't really think it would be a best fit for this scenario but it's an alternative harsh, hard to cross, environment that may be worth considering for part of your planetary divide, or for a different project, not even necessarily yours, with similar requirements. $\endgroup$
    – Ash
    Jul 17, 2019 at 19:13

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