trancejeremy
Adventurer
Well, a planet would probably need a magnetic field, because otherwise everyone would get irradiated. Well, depends how much cosmic radiation there is. In something like Planescape, in Sigil, probably none. (And I guess if the Spelljammer cosmology still applies, then that's not a problem, since everything is in a crystal sphere. But it probably doesn't anymore, since 3e)
And IIRC, the direction of the field would generally always be North/South roughly, if it's like our universe and was generated by a spinning core. Since that's the axis of the spin (it's not exactly because it's liquid, which sloshes around, and the earth also wobbles).
But of course, in a fantasy world (or even an science fiction one, like an artificial planet), the magnetic field could simply be because someone buried a really really big magnet in the planet. Or monopoles.
And IIRC, the direction of the field would generally always be North/South roughly, if it's like our universe and was generated by a spinning core. Since that's the axis of the spin (it's not exactly because it's liquid, which sloshes around, and the earth also wobbles).
But of course, in a fantasy world (or even an science fiction one, like an artificial planet), the magnetic field could simply be because someone buried a really really big magnet in the planet. Or monopoles.