Rendering Emission Shaders only in Shadows

Hello There!
I want to render a Procedural Planet, but my problem is that the citys, that would appear in the real world only in the shadow of the earth are visible on the bright side aswell. I don"t want to render the image twice, with and without the emission and the mix it in the compositor with the Shadow pass as a factor, because it would take too long to render an animation with it.
Thanks ahead!