(A rendering thread that's gone unanswered for 2 hours?! This isn't the GameDev that I know and love)
The theory sunlight hitting a planet is the same theory for any light hitting any object. The thing I think you're looking for, though, is Lambert's Cosine Law, which is one of the fundamental concepts in any physically-inspired rendering. Specifically, for any point on your planet, the amount that that place is illuminated is: (brightness of the light) * (cosine of angle between the surface normal and the light source) / (distance from the surface and light source)^2.
Hope that helps!