Select the planet, create a new layer, modify the selection that it'll be just a bit bigger than the planet's edge. Fill the circle selection with a colour you'd like the atmosphere to be. You modify the selection again, make sure it's just a bit inside the planet's circumference. Delete the rest. What remains is a nifty circle. Set its layer to Screen and apply a Gaussian Blur, about 5-10 strong, depending on the size of the planet. Delete the part of the blurred circle where the planet is dark. Duplicate the layer and apply Gaussian Blur a few times more for a halo effect. That's it.