Kryptonians and Red Sun Planets
Kryptonians hail from a planet that orbits a red dwarf sun- specifically LHS 2520, which is a real-life star. That’s more important than it seems- all red dwarf planets are tidally locked, with a freezing cold nightside(facing away from the sun), a searingly hot dayside(facing towards the sun), a terminator zone around the “equator”(which is a habitable temperature), and the majority of the water on one side of the planet due to tidal forces.
There’s also the threat of solar flares/coronal mass ejections, which are the nasty combination of more intense due to the small size of the star(bringing convection currents closer to the surface) and closer to the planet because the habitable zone of a red dwarf is very small- if Mercury orbited a red dwarf at the same distance that it orbits the Sun, it would be outside the habitable zone.
On a planet with water on the dayside, the terminator zone will either be too high elevation to have liquid water, or be flooded by the storm-tossed sea on the dayside of the planet. (And it will be constantly hurricaning with 50-foot waves on that sea, because of tidal forces and heat from the star. Life could evolve there, but it probably wouldn’t be humanoid or able to survive on Earth without some shenanigans.)
On a planet with water on the nightside, most of it will be frozen on the “back” of the planet(along with much of the atmosphere), but some of it will melt and flow as rivers into the terminator zone around the equator. The dayside will probably be a rocky hot desert, with basically no water to speak of.
Kryptonians specifically evolved in the terminator zone of a planet with its water on the nightside, since that is the red dwarf planet environment that could produce Kryptonians and Kryptonian society as we see them.
Krypton’s nightside is largely icy wasteland, and Kryptonians don't venture that close to either the night or day poles- the poles don’t even have atmosphere(it boils/freezes due to the extreme temperature).
However, both sides of the planet are relatively heavily colonized for deserts(do you see humans making cities in the Sahara or Antartica? No. Kryptonians, on the other hand, have major cities on the border regions of the terminator zone.)
Pretty much all Kryptonian cities are built as domes and giant multi-level structures connected by covered passageways with occasional towers instead of having open-air paths, urban sprawl and independent buildings- this is because it’s possible to insulate an entire city built that way from radiation and temperature extremes, and that way you don’t encounter the structural and space problems that come from building a single large dome over a bunch of smaller buildings.
Kryptonians are slightly stronger and faster and moderately more resistant to damage than humans even outside a yellow sun’s orbit, and their X-ray vision is natural, though less powerful(they can’t change the spectrum or generate very large amounts of X-rays, and thus don’t have heat vision). All other powers are the result of being infused with yellow sun radiation.
Their X-ray vision is an adaptation to the low luminosity of their sun, as well as an outgrowth of their resistance to X-ray radiation. It’s simply turning a passive sensor(hearing/vision) into an active sensor(sonar/lidar)- instead of just letting passive stellar X-rays reflect into their eyes, they generate short bursts of X-rays and measure the resulting distortions and frequency shifts as well as the time it takes to get a return signal.
I will also note another thing- the rotational periods and solar orbits of a tidally-locked planet are by definition identical- one solar year is one solar day, and there is no distinction between the units.
This is how Kryptonian time units measure in approximate Earth time units:
18 Kryptonian Amzel = 25 Earth Years.
Amzel - K-year(438 Zetyar, or 507.3 E-Days) with 6 Loraxo in it.
Lorax - A K-month of 73 Zetyar(84.55 E-Days)(which is not divisible by 6, so I can only assume that each lorax is a different length)
Fanff - K-week of 6 Zetyar (6.95 E-days)
Zetyar - K-day of 10 Woluo(27.8 E-hours)
Woluo - K-hour equal to 100 Dendaro(2.78 E-hours).
Dendar - K-minute equal to 100 Thribo(1.67 E-minutes)
Thrib - K-second, nearly the same length as an E-second(presumably because they’re both based on atomic principles).
In this case, we can see that several things have been lost in translation. All units larger than the fanff are purely calendar convention, and all units smaller than the fanff are based on metric SI units instead of easy time blocks based on sunset/sunrise. The fanff is presumably not actually a week, but something closer to one full solar year, since it’s the smallest unit that breaks the even time convention(like how 60 seconds is 1 minute, 60 minutes is 1 hour and then days are 24 hours long due to being based on the referent of Earth’s rotational period instead of a flat increase in orders of magnitude).
This is because there is no such thing as sunset or sunrise on a red dwarf planet, only how far away you are from the day pole, since the sun is always at the same angle relative to a given point on the planet’s surface. If you’re standing at the terminator, it’s always “dawn”. As you walk towards the day pole, the sun slowly “rises”(and seems to get bigger) as you get closer. The sun is directly overhead at the day pole itself, and probably fills all or most of the sky.
Thus, the fanff is actually a day/year. All of the smaller units(including the zetyar/K-day) are just convenient timekeeping benchmarks with no grounding in the actual astronomy of the planet(much like hours and minutes). Likewise, the larger units are calendar benchmarks with little grounding in the actual astronomy of the planet(much like weeks and months). Therefore, our chart actually looks like this:
Amzel - K-century(438 Zetyar, or 507.3 E-Days) with 6 Loraxo in it.
Lorax - K-decade(73 Zetyar or 84.55 E-Days)
Fanff - K-day/year of 6 Zetyar (6.95 E-days)
Zetyar - K-hour of 10 Woluo(27.8 E-hours)
Woluo - K-minute equal to 100 Dendaro(2.78 E-hours).
Dendar - K-second equal to 100 Thribo(1.67 E-minutes)
Thrib - K-millisecond, nearly the same length as an E-second