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In the story I'm writing, a gas dwarf (named Eden) is discovered in a triple star system. Its atmosphere is mainly oxygen and water based, and its gravity is slightly higher than Earth's.

Unlike our gas giants, however, Eden has 2 "surfaces". The upper layer is made up of millions of floating islands hovering just above its cloud layer. (A moon or a passing planet probably broke as it passed Eden and was lock in its magnetic field). Known as the Aether, most of the planet's population lives up here. There are even cities built in the sky that simply hover from the planet's magnetic field.

Far below, however, is an unstable icy crust known as the shell, which is constantly changing due to the forces of the water and oxygen ocean layers below it. Shell quakes and rifts occur on a daily basis here, dramatically changing sections of the Shell in an instant.

Could such a world exist, and am I missing any important details?

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  • $\begingroup$ I suggest reviewing the answers to What factors could cause a world to see “northern” lights much further down the globe?. You may find those interesting. $\endgroup$ – a CVn Jun 16 '16 at 20:54
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    $\begingroup$ You have one heck of an imagination, I'll give you that. Even if it's impossible I think you should still write your story in that setting, because it seems pretty epic to me. $\endgroup$ – AndreiROM Jun 16 '16 at 20:57
  • $\begingroup$ A gas planet that has cities on floating islands, built by the aetherians. If you add flying fauna I'll accuse you of writing the prequel to Windforge, which means you will be forced to finish developing that game (BTW have my upvote). $\endgroup$ – Renan Jun 16 '16 at 21:09
  • $\begingroup$ Gas giants do have a rocky core at the center, formed by the immense pressures. I'm unsure of what the geography of the core would look like. $\endgroup$ – Ethan Jun 16 '16 at 21:24
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    $\begingroup$ Astrological term?! $\endgroup$ – JDługosz Jun 17 '16 at 18:53
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Short answer: As described Eden could not exist naturally, you need a consult with Magrathean engineers to see if they could build it for you.

Magnetic levitation of millions of islands cannot occur naturally. The conditions necessary for magnetic levitation are very strict. You need carefully balanced magnetic fields of opposite polarity to support the floating object. A small imbalance and the floating object will tend to flip over and then the magnetic repulsion becomes magnetic attraction and you quickly plummet.

As your moon crumbles, the pieces descending would almost always naturally orient themselves such that they would crash into Eden.

As I type, I see that HDE 226868 has already answered the impossible field strength required aspect. I would also add that unless you manage a monopole somehow, the magnetic field cannot be in the same direction all over the planet, you could in theory have islands hovering over the north pole and others hovering over the south pole, but really nothing in between. Such hovering would not be stable as they would all tend to crash together directly over the magnetic poles.

You could get more stable magnetic support by making the floating islands superconducting magnets due to flux pinning, though unless your planet is extremely cold (where oxygen would be solid) they would not be superconducting naturally.

Lighter than air islands are the only "realistic" way to have floating islands. Perhaps you could have a large scale biological source of aerogels that are somewhat stronger than ones we can currently make and are filled with a lighter gas, perhaps methane (hydrogen and helium are very difficult to contain over time due to diffusion). Aeorgels are not very strong as construction materials though. You definitely need a stronger platform than any aerogel we have. If you add natural clumping of the floaters, you might have respectably large stable floating platforms, though clumping behavior does not result in a strong bond. An aerogel of graphene or similar fullerene structure might be a good place to start. You also have the problem of your islands catching on fire due to lightning strike, etc. but hey stuff happens.

For high buoyancy, a dense atmosphere is desirable, Oxygen toxicity limits the amount of oxygen, Nitrogen narcosis limits the amount of nitrogen and the narcosis problem is more general, essentially all of the heavy inert gases have the same problem, so there are definite limits of habitable atmospheric density.

Overall, it seems to me that Eden is not aptly named.


The tendency to flip over is much more pronounced that you might imagine. You pretty much have to cheat in one way or another to achieve magnetic levitation. According to Earnshaw's Theorem, static levitation is impossible for the most common types of simple magnetic materials. I apologize for being far too generous when I simply described it as a tendency to flip.

Superconducting magnets are not subject to Earnshaw's because of the way the flux lines penetrate the floating magnetic - causing flux locking. Rotating floating objects also bypass the assumptions of Earnshaw's. You can also construct a composite of a number of different magnetic sources on the ground and the floating object that don't match Earnshaw's model.

However, when you are talking about Eden as described, Earnshaw's is a pretty good model of what must occur. Since the model is not 100% accurate in this physical case, I described it in weaker language assuming that an edge case might float. It would almost certainly flip over due to perturbations from storms, etc. even when carefully balanced initially. So, floating for any length of time would be extremely rare at best.

Without going into the physics, you can levitate some things (such as a frog) using very strong fields that is diamagnetic (again, Earnshaw's does not apply), but as you can see it is not stable

Of course, the other problems such as the impossible field strength, etc. would still prevent Eden from existing.

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  • $\begingroup$ Wouldn't they be very resistant to flipping over if they had a very large radius compared to their thickness? $\endgroup$ – Ovi Jun 17 '16 at 14:08
  • $\begingroup$ @Ovi -- the tendency to flip over could be reduced due to geometry, but it would still be a common problem. You have to engineer the polarity and center of gravity correctly to get stable levitation. In natural breakup fragments, it would be unusual for the magnetic polarity to be strongly biased as needed for stable levitation (or even levitation at all) $\endgroup$ – Gary Walker Jun 17 '16 at 17:55
  • $\begingroup$ Hm maybe the OP can reduce the mumber of islands and have a few lucky ones which had just the right properties $\endgroup$ – Ovi Jun 17 '16 at 18:23
  • $\begingroup$ @Ovi Imagine balancing a coin a pin. It certainly is possible with enough effort and engineering, but it's still going to be very prone to falling into a more energetically stable state. $\endgroup$ – CircleSquared Jun 17 '16 at 19:59
  • $\begingroup$ @Ovi - updated my answer with more info re: flipping $\endgroup$ – Gary Walker Jun 19 '16 at 2:26
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In the story, a gas dwarf (named Eden) is discovered in a triple star system. Its atmosphere is mainly oxygen and water based, and its gravity is slightly higher than Earth's.

You don't need to resort to gas dwarfs if you want gravity slightly higher than Earth's. While Jupiter is a heavy beast, Saturn has a gravitational pull only 6.5% greater than Earth's at depths with 1 bar of pressure (slightly less than one atmosphere of pressure at Earth sea level). The level in gas giants where pressure equals 1 bar is often used as the level for surface gravity since gas giants have no surface.

If you want an oxygen and water heavy giant, consider an ice giant. Uranus and Neptune are ice giants. Although, don't be confused by the name. They aren't made of ice or mostly made of water. They simply have a larger ratio of volatile compounds (colloquially known as "ices") than gas giants. For comparison, gas giants are about 90% hydrogen and helium, whereas ice giants are only about 20%.

On the issue of atmospheric composition, don't worry too much about how much molecular oxygen the planet naturally had from its formation. Earth didn't have much O₂ in the air until the Great Oxygenation Event. It was originally closer to that of Titan than the Earth of today. Photosynthetic life nearly suffocating itself on its own waste is what changed that for us. So, there's more than one way to skin this particular cat.

Unlike our gas giants, however, Eden has 2 "surfaces". ... Far below, however, is an unstable icy crust known as the shell, which is constantly changing due to the forces of the water and oxygen ocean layers below it.

This wouldn't be possible in any of the gas/ice giant observed and I can't think of any math to make such a thing work. Physics has limits.

The upper layer is made up of millions of floating islands hovering just above its cloud layer. (A moon or a passing planet probably broke as it passed Eden and was lock in its magnetic field)

This is something else the physics isn't amenable to. If they were magnetically attracted to the planet, they wouldn't be locked in place. They would fall in. Anything else just wouldn't be thermodynamically stable.

Known as the Aether, most of the planet's population lives up here. There are even cities built in the sky that simply hover from the planet's magnetic field.

While I suppose the cities could magnetically hold themselves in place if they have the power to super cool massive supermagnets built into the city foundations/superstructure, that would be obscenely power intensive. A more sane solution might be buoyant cities, similar to what some people have proposed doing on Venus.

In short:

  • Ice giants might be the sort of planet you want.
  • Their formation alone could be enough to give a good atmosphere, or you could use life (or intentional terraforming) to get the rest of the way there.
  • Floating rocks in the upper atmosphere is almost impossible to imagine and an ice shell nested between layers of a gas/ice giant's atmosphere would be an impossible natural phenomenon.
  • Floating cities are conceivable. Magnetism can work, but conventional floating is already considered plausible for even modern technology. (So, you have multiple options there, too.)
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  • $\begingroup$ I know that a gas giant could have similar surface gravity to Earth. But, like I said, Eden has 2 deep liquid oceans. I picked a gas dwarf because most of the planet's mass would be concentrated in these two regions and the planet's core. $\endgroup$ – Mattias Jun 18 '16 at 16:50
  • $\begingroup$ Well, gas/ice giants have liquid oceans too. Things being liquid is a function of temperature and pressure. The problem with gas dwarfs is they're smaller than giants. 1: That means they often have weaker gravity than Earth. 2: Temp and composition being equal, you won't find smaller planets with larger liquid to gas ratios than larger planets. You need a the same amount of air pressure before you find any substance in its liquid form, no matter the planet. The reason Earth has surface water under such a thin atmo is it's mostly rock. I.e. Earth is actually denser than Jupiter (on average). $\endgroup$ – CircleSquared Jun 18 '16 at 18:20
  • $\begingroup$ Basically, if you want a gaseous planet with Earth-like gravity, you're looking for giant planets. Yes, they're massive, but they're more dispersed due to being made of lighter materials. If you want a thin atmosphere, lots of water, and a small planet, you want a rocky planet. That's why I suggested going for an ice giant, with eccentric levels of oxygen/water. They have the right gravity; and with the proper origin, one could even be water-heavy with a mostly oxygen atmosphere. $\endgroup$ – CircleSquared Jun 18 '16 at 18:31
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Here are some jumbled thoughts I had:

There's no way the planet can have a magnetic field strong enough to levitate rocks. In my answer to Can there be planets with extremely strong magnetic fields?, I calculated that for Earth to have a surface magnetic field as strong as a kitchen magnet, it would need to have a magnetic moment stronger than a magnetar, by a factor of about 1.5. You'd need to have an even larger magnetic moment here, because the radius would be bigger because $$B\propto\frac{p}{r^3}$$ where $B$ is the magnitude of the magnetic field, $p$ is the magnetic moment and $r$ is the radius, and also because you would need a stronger magnetic field to levitate the rocks.

This would seem to indicate that the planet cannot be colonized; the cities cannot be built.

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The closest valid thing I can think of would be a nebula in which the gases create a deep enough gravity well for it to be compressed to a life sustaining pressure but there's not enough mass for the gravity well to become any deeper than that.

There may be asteroids in this nebula and as long as they don't have enough mass to disrupt the gravity/pressure balance they wont form a planet, but you wouldn't be able to walk on them, not unless all non-gaseous matter had some sort of alternate gravity.

Inside the nebula would be warm, indeed the denser depths may be quite hot, great columns of hot/moist air rising from the depths, while any free-floating moisture near edges would condense to frost and fall back in, I leave it up to you to figure out where all this heat is coming from.

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  • $\begingroup$ "The closest valid thing I can think of would be a nebula in which the gases create a deep enough gravity well for it to be compressed to a life sustaining pressure but there's not enough mass for the gravity well to become any deeper than that." Unfortunately, mass centralized enough to squeeze inwards would inevitably lead to colapse $\endgroup$ – CircleSquared Jun 17 '16 at 4:19
  • $\begingroup$ The early years of a stellar collapse must surely pass through this phase. Thermal radiation, shedding rotational momentum, and sorting of EM fields all take some time. The question is how hot and how long before conditions become untenable. $\endgroup$ – Jim2B Jun 17 '16 at 18:58
  • $\begingroup$ @Jim2B See the page I linked: "These [gas cloud] fragments then form small, dense cores, which in turn collapse into stars. [...] They possess diameters of 2,000–20,000 astronomical units (0.01–0.1 pc) and a particle number density of roughly 10,000 to 100,000/cm^3 (160,000 to 1,600,000/cu in). Compare it with the particle number density of the air at the sea level—2.8×10^19/cm^3 (4.6×1020/cu in). The initial collapse of a solar-mass protostellar nebula takes around 100,000 years.[...] This core forms the seed of what will become a star." $\endgroup$ – CircleSquared Jun 17 '16 at 20:09
  • $\begingroup$ I realize that stellar nebulae are extremely rarefied. However, consider that at some point a kernel of that nebula begins stellar collapse. At the very beginning it is nearly pure vacuum by terrestrial standards. By then end you have a star. Somewhere in between those periods the gases pass through a density we would find comfortable. The questions are "for how long?" and "how hot is it?". Your answer implies it's somewhat shorter than 100,000 years - which means it could make an interesting story setting, yes? $\endgroup$ – Jim2B Jun 17 '16 at 20:32

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