In principle you can do pretty much whatever you want. If you know it makes no sense, it works better
not to put a spotlight on it and try to rationalize it, because like Mario jumping really high, if it isn't rational, explanations will be stupid.
As well as stupid, they're also superfluous. People are happy to accept any bull crap, often even if they knew it's bull crap.
As a counter example I'd point you at the 1993 Mario movie. They couldn't
just have a guy who jumps high and lives with talking shrooms. They had to have a "de-evolution" gun and rocket boots. Neither of those things made any
more sense, of course, since evolution doesn't work that way, rockets don't work that way and we didn't evolve from fungis. They also didn't add anything useful since people
already accept Mario at face value. This is what I mean by stupid + superfluous.
So, big big planet. If we're talking physics for a moment, the gravity is redonkulous. Jupiter has "only" 2.5 or something times Earth's gravity at its "surface" because it's a giant ball of mostly hydrogen. Earth, for contrast, is mainly
iron.
You wouldn't have a terrestrial planet the size of Jupiter because they apparently do not form (though our knowledge is pretty limited), but if you did, it would be astoundingly massive and the gravity would be crushing. It would have to be dramatically
more dense than Earth because of gravity compression.
If you removed that mass, the planet might disperse, though at this point I'm talking out of my black hole.
So literally if anyone asks why the planet is so big, what with the gravity ya know, the correct answer is "artistic license".
Technology varies greatly from place to place[/quote]
It does that IRL.
Many societies cannot manufacture the highest technology we use, even if they import the gizmos (or are handed them). Look at Afghanistan. They make rugs. And they have some upstart "factories" where they assemble components made elsewhere, like Chinese motorcycles. Though we're getting into semantics, they do not "have" the technology to shoot down a Hind, even if they were handed the (somewhat sophisticated) devices.
Modern countries have spacecraft. What kind of technology gap is rugs and spaceships?
But maybe you want these people to be out of contact with each other...
You might experiment with the geography of the situation. Look at Guns, Germs and Steel. Consider bumping your planet's gravity a tad over Earth's and see what that does to powered flight. Also consider that not all developing societies bother to go out and colonize anything. China sent some ships out but quickly changed their minds.
Heck, right now plenty of people don't want us doing anything in space. Valid or not? Doesn't matter. Xenophobia, fear and anti-ambition are in the human race's intellectual toolbox.
You can also introduce some plot device like armored sea monsters that keeps them apart, but your story will probably wind up being "about" that, at least in terms of plot.