I have searched planet city lights tutorial but they are mostly for photoshop. I assume that you know Gimp layers etc.
So you can make quite fast those city light maps for Blender and Gimp. So here we go:
1.First open your planet texture to gimp, save it png format.
2.Next erase sea area (if there is sea in planet texture). With color eraser tool.
3.Go to colors and desaturate colors.
4.Go to colors again and select levels,and move sliders until land area is totally black.
5.Next duplicate layer and name it :lights and hide first layer.
6.Go to filters select hsv noise filter and use these values:saturation 110,value 180.
Or use your own values.
And if you want more brightness go colors and adjust color levels.
7. Unhide your planet texture.
8.Next step is that you lower your lights layer. Under your planet layer.
9.Then start erase coast lines etc “creating cities” and select some fuzzy brush or similar to get
more randomness for cities.
Actually more you test different options more get realistic results.
And blender users: you can use Emitting node (in node editor) to chose city maps brightness and
and city map color.
Have fun. Best regards spacehead2_1
ps i put here my example of planet city light map and zoom in to see those cities
and first image from Blender applied to sphere and rendered with Cycles

That is very interesting spacehead2_1. Thanks for the tut.

thanks, this is my first tutorial here