Animation by Nicolas Galvani
This shot looks pretty simple!! But I can assure you it's not. To get the such shadows onto the round planet, I used the lightShadowFraction attribute to avoid any fading of the shadows (which occurs because of the curve of the planet) The shadowColor is 'normal' but I have exagerated it with the lightShadowFraction by mapping the ambient attribute of the snow material with 2 different colors.
click here
to see the animation... (divX)
(sorry for the bad anti-aliasing, but grabbed from a playblast)