Your renders do look very nice and I agree with Eric's comments.
Coincidentally, I just watched a tutorial last night using Cinema 4D for creating realistic rocks using procedural materials. I would imagine you could translate the tutorial into other software.
Nice work. I agree on the asteroids. Not sure in C4D but in Max, this would be easily fixed by moving the UV mapping modified up the modifier stack so that it applies to the mesh after you've twisted/distorted it. Put another way - apply a new spherical UV map to the finished mesh and you should fix the issue.
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Your asteroid textures look twisted. You might want to remap the textures.
The planet atmosphere is too far away from the planet and has no blur. However, that is my favorite of your three images!
Coincidentally, I just watched a tutorial last night using Cinema 4D for creating realistic rocks using procedural materials. I would imagine you could translate the tutorial into other software.
https://www.youtube.com/watch?v=S-KHaeWfbwc
The presenter also does a great tutorial on creating a realistic Earth.
https://www.youtube.com/watch?v=cCyiHJ-q9NI