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.