Creating Planets - Where to Start?

It’s not to human scale.

If you wanted to do that, you’d need some serious custom heavy lifting under the hood. A common approach is to always have your camera and character at 0,0,0 in world space, and move everything else instead of the character. This get’s around a lot of precision errors - but is also a pain in the **** to manage.

Also, to store that much data - you usually have to use procedural generation. Smooth Voxel methods are pretty common.