I find rendering planets in Cinema 4D can be a little tricky when trying to get the atmosphere right. Michael Welter, a talented C4D hobbyist has made an atmosphere shader for C4D. It's free, head over to his site to get it, along with some other really usefull plugns.
I have been using his plugin for a teaser for an upcoming show on FOX8. With a bit of tweaking and diplacment on the cloud layer you can get some really nice shadows along the terminator. I took some screen grabs. It took me about a day to pull togther the rough setup.