Cloud cities. You know them, you love them, and I want them in my conworld. The last story I posted here takes place in one.

Economically, here’s how I see this panning out:

  1. A gas giant has economically exploitable gases.
  2. Floating extraction platforms similar to oil rigs are set up to extract those gases.
  3. These platforms develop ancillary economies to support the people mining those gases.
  4. These ancillary economies attract more and more people, diversifying the overall economy to the point that the platforms become floating cities.

In terms of physics and chemistry I’m on much shakier ground. This isn’t a rock-hard sci-fi setting, so I’m willing to fudge things, but I like learning about the real world through my worldbuilding so it’s fun to try and make it work.

The cities are held aloft by Flanar pontoons and stabilized in part by the extraction equipment hanging down from the underside of the city into the layer where the extractible gases can be found.

At first I imagined the cities being sealed from the outside, but that makes them no different than orbital colonies save for the presence of gravity, so I want them open.

Right now I imagine there being a belt of breathable air, encircling the planet, limited to a certain range of heights and possibly combined to certain latitudes, where the cities can be found. They would drift along with the wind currents, so the air speed would be near zero, allowing people to venture outside without being blown away.

One possibility I entertained was that the whole planet is mostly oxygen and argon, but that doesn’t seem likely.

On other places where this question has come up people suggested a Venus-like super earth, so a massive rocky planet with a very thick atmosphere. That would still necessitate sealed cities I think.

  • The Bard in GreenAEnglish
    5·
    2 days ago

    I’ve always thought this was a really cool idea. There are a few examples in Sci-Fi, other than Star Wars (Bespin). The Bobiverse for instance hints at such a thing (Odin) without getting at all into the mechanics of it.

    The biggest problem is relative weights. Oxygen is going to sink into the planet while hydrogen and other light elements float to the top. Gas giants are mostly hydrogen.

    How could a gas giant be made of mostly heavier elements so that there’s a layer of oxygen near the top (with survivable pressure)?

    • Biological processes - this is the most interesting one to think about IMO. Some kind of very interesting ecology has been going on on the planet for billions of years. Perhaps some kind of silicon based biosphere deep inside the planet uses fusion to generate energy and has been fusing hydrogen and helium into heavy elements for so many billions of years it’s changed the atmospheric composition of the planet. Paleontology on such a planet would be extremely difficult, but also fascinating. This kind of idea has been explored in Sci Fi (for example kind of shallowly in Manta by Timothy Zahn).
    • Terraforming - machines of some kind maintain your air band… probably not very well.
    • It’s actually an ancient alien megastructure - Iain M. Banks had one of these (the Airsphere) in one of the Culture books.
    • It formed in some very weird way, like in the aftermath of a supernova, in which case, what is it orbiting and how is it in a Goldilocks zone? While the idea of habitable planets orbiting black holes as been explored (Interstellar) and even studied, the most likely possibility is that the super nova remnant (black hole, neutron star, pulsar) has a stellar companion that it captured and that in turn captured one or more of the planets that formed from the super nova remnant. An extremely rare situation. IMO, this is the second most interesting possibility. If I were you, I might combine the first one and this one.

    In all of these, storms are an interesting (very dangerous) problem. On Jupiter and Saturn, storms transfer material from deeper in the planet’s atmosphere into higher atmospheric levels (leading to color changes like the Great Red Spot). Storms will dredge up unbreathable material which the very least would require the cities to seal up and ride them out. And storms on gas giants can last for decades or centuries (Great Red Spot) so your cities need to be able to navigate away from them. But this circulation of material is likely critically important for any biosphere maintaining the oxygen atmosphere.