What's that? A planet pack from the Kopernicus Dev himself?
Yes, and it's presently what I am playing. Though to be frank, this is probably because I suck at KSP.
Best thought of as a form of a "realistic physics Kerbolar system," this mod does the following:
1.) Resizes Kerbol/Suns mass to be more like a red dwarf star, this means much more mass and gravity with the same radius. Light output is also slightly tinted, but not too much, as my understanding is it would mostly diffuse out to a whiteish hue from aparent intensity (fun fact, Kerbin falls into the habital zone on red dwarf modeling).
2.) Scales the gravity and mass of all planets down by 0.1x. Yes, this means 10% gravity and 10% mass on all bodies.
3.) To prevent you from killing yourself with overpowered rocket engines in the new low gravity environment, every engines max thrust max is quartered. Kerbals themselves have also had their jump reduced from 10 to 2 to prevent them from seeming superhuman and to represent growing up in low gravity.
4.) Fuel density has been kept the same. There is absolutely no physical reason the kerbals couldn't still push the same fuel to orbit, and this mod is about realism in a red dwarf setting.
5.) For Principia support (below) Minmus and Bop have had their orbits slightly tweaked, and been made retrograde. The Mun and gilly have also been moved slightly, but the order of the orbits is still the same.
Given all that, out of the box, it plays sort of like a "KSP Easy mode." A stock flea engine and pod for example will get you to space, but not orbit. There are still challenges, but they are fewer and farther between. Support is offered for the following mods to spice up the challenge a bit (I play with these):
1.) Principia (highly recommended, after much work and a lot of sim time I can safely say the system is stable!)
2.) Kerbalism (ignition count and runtimes are buffed to represent the lower power engines being more reliable).
In addition, if you want to "Play like the chief Kopernicus Dev, I use/recommend the following mods:
1.) Parallax Stock Textures
2.) Scatterer + EVE
3.) Spectra
4.) RealPlume-Stock, Waterfall
5.) Restock