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Of the characteristics most commonly cited for speculative fiction world-building believability listed below, to which do you assign a greater significance and why? This may be taken as a subjective opinion question but I'm also hoping someone will mention an "unforgivable if ignored" point or two.

  • Gravity
  • Planetary Mass
  • Planet Density
  • Planet Rotation
  • Axial Tilt
  • Presence of a Moon or Moons
  • Magnetic Field
  • Plate Tectonics
  • Wind
  • Ocean Currents
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  • $\begingroup$ I'd add atmospheric pressure in there (heavily related to gravity and wind I guess) $\endgroup$
    – Twelfth
    Nov 13, 2014 at 0:16
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    $\begingroup$ This is far too broad: it depends on your plot and tone. What's central to your plot needs to rest on firm ground if you're doing hard SF. Peripheral concerns can be more easily disbelieved. $\endgroup$ Nov 13, 2014 at 1:48

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This is an incredibly subjective question, but I can provide an interesting answer.

Gravity

If you want realism, you really want to think about gravity (as opposed to the other things you listed). Why? It governs so many aspects of the universe. Gravity impacted spacetime at the Big Bang. It influenced the formation of galaxies, galaxy clusters, and larger structures. It helps stars evolve, planets coalesce during formation and celestial bodies orbit each other. It also impacts most of the other things you mentioned:

  • Planetary mass: Planetary formation could not have happened without gravity. It also influenced just how much material came together to form the planet, or a star.
  • Planetary density: See Planetary mass.
  • Planetary rotation: See Planetary mass.
  • Axial tilt: See Planetary mass, but also consider that if a planet's axis is titled because of an impact from another object (e.g. possibly in the case of Uranus), gravity was responsible.
  • Presence of a moon or moons: See Planetary mass. Don't forget that gravity impacts everything in a stellar system.
  • Magnetic field: The magnetic field of a planet comes from its core, which is under extreme pressure . . . partly because of gravity.
  • Plate tectonics: Gravity probably influences how magma moves in the mantle, thus influencing plate tectonics. At any rate, it influenced how the plates originally formed, which is something.
  • Wind: Okay, gravity doesn't really affect wind.
  • Ocean currents: Gravity probably effects these, just as it does magma. But just like in the case of magma, it's a loose connection.

I'd pick gravity because it influences pretty much all of the other effects you mentioned.

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  • $\begingroup$ The classic book "Mission of Gravity" by Hal Clement involves a world where surface gravity goes from 3.0g to 667.0g at the poles. A probe has failed to take off from the poles and the humans contract a shipload of critters from the planet to go and recover the data. And there is at least one time when gravity did effect even the wind! $\endgroup$
    – Oldcat
    Nov 12, 2014 at 23:55
  • $\begingroup$ @Oldcat Oh, cool! I'll have to read that. $\endgroup$
    – HDE 226868
    Nov 12, 2014 at 23:56
  • $\begingroup$ Hal Clement is the king of the World Builders. He makes everything as accurate as he can, scientifically. $\endgroup$
    – Oldcat
    Nov 12, 2014 at 23:57
  • $\begingroup$ My kind of guy. $\endgroup$
    – HDE 226868
    Nov 12, 2014 at 23:57

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