They won't proc-gen terrain, see
my earlier post.
So you want to say they hand crafted heightmaps for the hundreds of celestial bodies instead of using a noise function? I seriously doubt that (although it is 25 years in the making so who knows, maybe that's what they were doing all the time
), unless each planet has a total surface of 1 square kilometer or something before it starts wrapping around. Furthermore at "kind of" planetary scale that's a fucking lot of data even if you store just the vertex coordinates plus the usual UV, normals and whatever else they need to render the terrain (but still the installation size is fucking huge). Finally, do you truly believe bethesda would pick the hard route requiring a lot of effort rather than take shortcuts?
If the game truly lets you land anywhere and keeps reasonable (but not necessarily 100% realistic) scale for the surface then there is no other option than to use a noise function at least as a basis, with some "manual" modification layer for unique planetary features like say "cool crater visible from space so that we can put it in a trailer" (this is what both Kerbal Space Program games do for example, and those have just 16 celestial bodies at the moment). This is what everyone does with planetary scale terrain because it's borderline impossible to create handcrafted terrain at planetary scale that wouldn't on average look worse than a half-decent perlin or simplex noise-based output.
My wild guesses on the planetary "see that moon, you can land there" exploration based on Toddster's obvious lies are:
If you could drive around the fucking planet or fly around it then the Toddster would hype it, so obviously their implementation can't do that.
So I guess if you pick to land at a set of coordinate on a given celestial body the game runs the noise function for something for a preset size of a square chunk of the surface and generates the terrain. Then it rolls some points of interest from prefabs, places them "randomly" using another noise function. You get a visible or invisible barrier if you try to walk outside of the generated chunk of the planet, because of course Bethesda wouldn't be able to do a proper chunking system that would allow for driving a rover around a whole moon or flying above the planet No Man's Sky style without crashing your PC due to bad optimization or at least enormous stutter. Also this hides the monotony/repetitiveness of the terrain generation algorithm and any other flaws (such as seams) that larger maps might have shown easily.