I think it’s best to start with some resources:
- LOD implementation: acko.net: making worlds introduction
-
- Simple description: A cube where each face is a Quad-Tree
- Generated Surface Terrain: The usual suspect here is Simplex(/Perlin) Noise however that’s tricky to get right.
-
- I’m also super-impressed by the Map Generator 2.0 from the forums, I think it could make an amazing basis for this stuff!
- Generated Materials: this one is going to be quite UE4 specific in some regards, in the past I haven’t generated the materials procedurally only textures and other components that are used by a predefined material.
- Atmosphere: You probably want to Google for “Atmospheric Scattering o’neil”.
-
- There’s a bunch of good examples and articles out there like GPU Gems 2, chapter 16.
I’m not aware of anyone trying this with UE4 yet though I have been tempted myself.
A note of caution however is that whilst you should be able to generate some big objects in UE4 you will run into precision issues as you try to get upto planetary scales. Even Earth-like scales will exceed the limits of floating point precision quite quickly.
These are not insurmountable however and there are various techniques to get around them.
These are some link to think about the topic:
unity-notes-on-rendering-the-big-and-the-small
custom-world-coordinate-system-changing-space-world
genesis engine coordinate systems
confused-very-large-environments
A Real-Time Procedural Universe, Part Three: Matters of Scale
However, that’s a lot to think about all in one go 
My advice would be to start with the quadtree-cube, just get something rendering in UE4, nothing fancy just an auto-LOD cube. You can worry about the size of it later!
Good luck.