In this cool tutorial :-) I will show you how easy it is to create planet from the map of your favorite city. To start, we need space with stars. For that, I used my favorite ContextFree program. I wrote about it detaily in one of my earlier tutorials, so I will keep it short. Download this file - Linkstars.cfdg - renders it several times at large size, and save as PNG images. One example is this image - lots of black dots on white background.
 
Planets - making of - Step 1

Load image created by ContextFree in Photoshop, invert it (Ctrl-I).
 
   
Planets - making of - Step 2

Then create new layer, fill it with blue color, and set this layer blend mode to Multiply. it will make stars blue, not white.
 
Planets - making of - Step 3

I also wanted map of USA to shine on space as constellations. For that, I found outline map of USA states, and made all lines white.
 
Planets - making of - Step 4

Copy and paste white lines into our artwork in new layer...
 
Planets - making of - Step 5