It's called rim lighting, and as a comic book colorist I used it all the time.(And smokey was a little harsh, but completely 100% right)
My method of achieving it is this:
Select the object you want to put a rim light on. With a poly lasso, pen tool or optimally you would have the object o a separate layer.
Once the object is selected, grab a completely soft round brush(size around 200-250) on around 10% opacity. you need two colors here, one a slight shade of your main object color, the other a very light yellow or what ever color your main lighting on the object is.
In long strokes(a pen tool is invaluable here, but a mouse will work) brush your darker shade around the edge of your object, blending it as you stroke in and making a smooth transition from dark to light. As you can see from the example pics, you don't want to come in too far. If the transition is completely smooth, you can use the blur tool to smooth things out a bit.
Once your shadow is laid down, create another layer over the original, set to hard light blending mode. With your light yellow color picked, and your soft brush on 10% opacity(size around 30-40) set to screen blending mode. Using long strokes, slowly brush in your rim light. You can go over each stroke until the desired brightness is achieved.
I did this exact method with this pool ball, the only difference is the lines are hard and crisp instead of fuzzy, because of the obvious material difference.
I'm not saying you'll get it on the first, second, or even third try. But if you practice it will come to you.
