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Sky Islands are a popular sci-fi trope. Think the Avatar films, the Legend of Zelda Nintendo switch games and so on.

Is there any configuration of exoplanets or moons that could sustain a large body of land visibly near a planet? Given some exoplanets could have the same density as styrofoam, and others are believed to be in complex gravitational systems with binary stars and other planets, moons etc. Is there any configuration that could work?

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    $\begingroup$ It would seem very unlikely. I guess you don't mean "moons" because The Moon is "a large body of land visibly near" the Earth. I guess you mean sky islands in the atmosphere, or at least within a few thousand metres of the surface, and not moving rapidly relative to the surface. Without unobtainium, or zonai magic, that's not really possible. $\endgroup$
    – James K
    Commented Jul 29, 2023 at 8:39
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    $\begingroup$ It's a nice idea for science fiction, but the laws of physics apply universally & sky islands still need to obey the laws of gravity & thus would not exist. $\endgroup$
    – Fred
    Commented Jul 29, 2023 at 10:03
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    $\begingroup$ @JamesK: You might want to add upsidisium to that list. I can recall watching the Rocky and Bullwinkle cartoon some decades ago. $\endgroup$
    – Fred
    Commented Jul 29, 2023 at 10:12
  • $\begingroup$ Planet made out of polystyrene, high winds, polystyrene fragments elevated as wing shapes in the airstream, something something? $\endgroup$
    – tomh
    Commented Jul 29, 2023 at 11:07
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    $\begingroup$ Of course the XKCD guy reads this and made a comic from this discussion xkcd.com/2809 $\endgroup$
    – James K
    Commented Aug 1, 2023 at 6:44

2 Answers 2

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Earnshaw's Theorem tells us that it is impossible for a stationary mass to float in a stable gravitational equilibrium, so there is no configuration of exoplanets or moons that could levitate land islands above a planet.

Levitating islands have been discussed in several Worldbuilding Stack Exchange questions:

The "official" explanation for the floating islands in Avatar is that are largely composed unobtainium, a fictional room-temperature superconductor which levitates due to flux-pinning in Pandora's magnetic field. This explanation is implausible but not impossible. A well-known video reports a lift-to-weight ratio of 70,000 for a superconducting disk in a strong magnetic field gradient, so only a small fraction of unobtainium in the islands might be enough to levitate them in lesser magnetic fields. There are lots of very challenging issues about how the islands could have be initially formed with the flux pinned, and how stable the pinned flux is over geological time, but the idea is perhaps less ridiculous than I initially thought.

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According to relativity gravity is a change in the geometry of space caused by mass. There are other theories of gravity in which gravity is a force. And in some of those theories, it might be possible to produce negative gravity. So possibly sky islands could be lifted up by anti gravity generators.

Otherwise artificial floating islands would be limited to something like fuller's cloud nine concept. https://science.howstuffworks.com/engineering/architecture/flying-cities-buckminster-fuller.htm

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