Same scenario, but with five gears. The actual planetary part was easy…but that center gear was a bear! I finally had to go look up a formula on Wikipedia because everything I tried just wouldn’t get the darn thing to mesh properly. This is just an output window render, didn’t have time for a longer one with materials and lighting. I will do that later. Click thumbnail to go to animation. Blend file here.
It is a bit vain perhaps to point at my own work , but you might want to look at this: http://www.swineworld.org/blender/gears/ . Or if you want superior mechanical accuracy, check Stefano’s original work on http://www.selleri.org/Blender/ (my own work is more aimed at maintainability and low poly animatable meshes).