try adding a low intensity ambient light, because now the transition is to harsh between the light and shadow.
As for the image you placed as a background i would only use the mountain then create a sphere asign a shader with a transparencyramp going from black to purple and on the colorchannel slap a granite(tweak the granite texture to a black background with white dots-simulating stars- on it.
You can also create a moon which emanates a slight blueish light(if i'm not mistaken)
Just spice things up and you will have a
great scene