You’re looking at few years of R&D for solution that will require a small computer cluster to run it, and you’ll need custom engine for that.
AFAIK, there are NO games on the market right now that offer anything similar in terms of scale/size.
One way to go about it is some custom based voxel tech that is geared towards planets (higher level detail near surface, no detail or very low level of detail below crust, but memory requirements will be insane.
It is definitely possible to easily explain this step by step, but there’s no point, since the dude has no resources to actually make this thing.
Apply gravity force (directed towards center of the planet) for every object. See Newton’s laws.
Write custom voxel-based solution.
Same as in usual game. Close to player spawn physic-driven objects, far away from player replace them with placeholders. See TwoWorlds II for example.
Same as in usual game with throwable object.
Being wet is matter of changing material parameters.
For water currents, google “GDGPU fluid sim” which will lead you towards papers and experimental tecihniques used for water simulation.
Dig up bunch of meteorology papers, build up a 2d-layer model which simulates humidity/temperature/wind changes over surface, based on the data spawn particle systems an dlighting effects in regions where predicted weather pattern takes place.
Day and night is matter of being in sun’s shadow + light scattering added to it. Research lighting models and light scattering. There’s bound to be some SIGGRAPH papers on the subject.
Same as in usual game.
Generate the planet by deterministic algorithm based on random seed. To undo damage, re-generate specified region only using the same seed.