Try this: (of the top of my head)

FIrst image,: 2-color image, first color represents unlit
areas. 2nd color is lit areas.
Second image: existing planet mapping image

Use 1st image as a material map.  Map the 2nd image
onto color one of the material map.  Onto the second
color, map a glow/light texture, something like:

  #declare city_glow = texture {
    pigment { rgb 1 }
    finish { ambient 1 diffuse 0 }
  }

Pete

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