After reading The Ellimist Chronicles by K.A. Applegate, I wondered what conditions would have led to the environment found on the planet "Ket" featured in the story. Ket has the following properties:

  • Turbulent, unlivable surface dotted with numerous volcanoes.
  • An atmosphere that extends many hundreds of kilometers above the surface. 480 kilometers is the typical cruising altitude of the flying cities (see below).
  • Natural background radiation that prevents radio traffic between on-world settlements. (Radio transmissions may still be beamed into outer space, as pointless as that is.)
  • A preponderance of floating, crystalline rocks. (Native inhabitants are capable of flight. Cities, hollowed out of flying mountains, require around 90% of their population to carry the weight of the mountain at any time because whatever they fill the mountain with overwhelms whatever force holds it up.)

What sort of planetary conditions would create an environment like this? High gravity? Dense atmosphere? An extremely powerful magnetosphere?

EDIT: After researching the floating mountains from James Cameron's Avatar, I found it is technically possible to have floating mountains composed of room-temperature superconductor propelled by a powerful magnetosphere and would cause massive radio and electromagnetic interference. The downside is that the magnetosphere would be strong to shred vehicles and rip the iron from your blood, but the alien biosphere and civilization presumably adapted to use non-ferrous metals and shielded electronics.

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    $\begingroup$ It's not clear what you're asking for. On the one hand, it seems like you're asking for a plausible scientific explanation for this environment. On the other hand, anti-gravity crystals, cities held aloft by bird-people, and jamming all communication frequencies move this into the realm of magic. What sort of answer are you looking for? $\endgroup$
    – Schwern
    May 27, 2016 at 21:25
  • $\begingroup$ @Schwern: The floating rocks are straight out of James Cameron's Avatar, but I have read that it is hypothetically possible with a strong enough magnetic field due to the Meissner effect. However, the downside is that it would not only interfere with radios, it would rip apart metal-based vehicles and proteins such as hemoglobin (Source). $\endgroup$
    – Anonymous
    Jun 1, 2016 at 12:40
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    $\begingroup$ 'One thing that I have found most annoying about the movie Avatar is the chorus of praise for its "scientific accuracy."' Geez, I agree. Anyhow, you could clarify what you're looking for by choosing a reality-check or science-based tags... or not if you want to choose the Unobanium route. $\endgroup$
    – Schwern
    Jun 1, 2016 at 17:21
  • $\begingroup$ "Beyond a certain mass the rocks require lift from an outside source." - this cannot work that way. Not in scientific means. Ability to float depends on density, not mass. If small rocks float, big rocks float. And that's it. If this requirement is set in stone, I'll make this comment into an answer. If it can be changed, change it to something less impossible. $\endgroup$
    – Mołot
    Jun 2, 2016 at 14:30
  • $\begingroup$ @molot not necessarily so, depends if the mechanism is based on relative density or some other cause. For example, it might relate to area vs mass if the cause is "per unit area", or to amount of some material contained (if due to some kind of repulsive effect between specific materials) $\endgroup$
    – Stilez
    Jun 2, 2016 at 15:30

3 Answers 3


(Really a comment but it won't fit)

Something to be aware of: The density curve of the atmosphere is based only on gravity. For a given planetary gravity the ambient pressure will be pretty much fixed at a given distance from the "top" of the atmosphere, not the planet's surface. This means you can't have a whole bunch of atmosphere on top of your living space without having crushing pressures at your living space.

The deeper you go the higher the pressure until it's not a gas anymore--look at the "gas" giants of the outer solar system. A drawing of Jupiter:

enter image description here

Note how thin the atmospheric layer is (I'm having no luck finding an actual value) compared to the planet despite the layers underneath being things we would expect to be a gas.


I can make a stab at the first few.

  • Volcanism would contribute to a dense atmosphere,so those are compatible and work well.
  • Dense gases in the atmosphere would make it easier for the atmosphere to be much deeper than on earth. There's an issue here since earth has a 100 mile atmosphere but it is fairly tenuous just 10 miles up. The question doesn't say if the 300 mile atmosphere means 300 miles able to be flied in; it could be 300 miles deep but only the bottom 5-20 miles fliable, that would be easy and is allowed by the question wording.
  • If the atmosphere is deep or dense, or the ground has strong sources of heat, then inhabitants could more surely fly (air pressure or strong rising convection currents). There may be a premium on flight if the ground is more dangerous. And even on earth flight is widespread. Not an issue.
  • Background radiation - whether this is electromagnetic radiation or alpha/beta radiation (from radioactivity), this isn't an issue. Volcanism can lead to radio static. Natural radioactivity might not be inconsistent with life that has adapted to it. Flight and shielding may both allow space-bound communication.
  • $\begingroup$ Sorry, I forgot to specify that 300 miles is the typical cruising altitude for the flying cities. I'll add that to the question. (Also, radio transmission is simply never used by the civilization, the transmissions beamed into space were only received by passing aliens.) $\endgroup$
    – Anonymous
    Jun 2, 2016 at 14:14

Okay, I'm going to take a stab at the floating mountains question in a way that doesn't involve magnetism.

Anybody can feel free to correct me if I'm wrong, but I'm purely speculating here. Recent findings have proven the existence of gravity waves. Now, this would be a different kind of wave than EM radiation, which is transverse, as in a sine wave or helical pattern. Gravity waves would be longitudinal, as in areas of compression followed by areas of expansion. For gravity waves, the medium being compressed and expanded is actually the fabric of space itself, or space-time.

The closest analogy would be sound waves.

Now, sound waves can be affected by other sound waves. Sound wave cancellation can occur when two opposing waves meet that are 180 degrees out of phase, or the areas of high pressure of one wave line up with the areas of low pressure of the other wave. It also has to be at the exact matching frequency.

Constructive interference, on the other hand, can occur when the waves are perfectly in sync.

Now, we know of resonating crystals, like quartz, that respond to electricity by vibrating mechanically, and respond to mechanical vibration by producing electricity. These are called piezoelectric crystals and used for electronic timing and oscillators as well as sparking elements in lighters.

Now suppose this 'crystal' you talk about resonates in some way, destructively or constructively with the gravity waves being emitted by the planet. With destructive interference, I would imagine, that the crystals could be relatively 'weightless'.

There may be other ways by applying the analogy of sound waves to gravity waves to explain how the crystals might be able to levitate, but I am reaching the limit of my knowledge.

There is also acoustic levitation, using nodal points, but I feel this answer is getting a bit long, and that method of acoustic levitation would not permit movement or an anti-gravity effect like you are imagining.

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    $\begingroup$ Gravitational waves are transverse, not longitudinal. $\endgroup$ Jun 3, 2016 at 7:05
  • $\begingroup$ "gravity waves being emitted by the planet." - Sorry, but only source of gravity waves that is somehow measurable requires a black hole. Or rather two black holes. Hell of a planet... "Don't dig too deep or we are all doomed!" $\endgroup$
    – Mołot
    Jun 3, 2016 at 9:32
  • $\begingroup$ A less extreme jump which does not require talk of gravity waves is acoustic levitation (youtube.com/watch?v=669AcEBpdsY ). However, long term stability of such a large object could require active feedback. $\endgroup$
    – Cort Ammon
    Jun 3, 2016 at 14:51

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