Okey, I'll show some pictures then ;

1.) The planets without shadows or light, should be a bit neutral, but you could play with this to get a higher contrast.


2.) With the white sun sprite visible;


3.) Through a shader you multiply the underlying pixels to make them lighter;


It's all 2D, the shader might need pro or sylex to get it to work perfectly though,

The biggest advantage of this is that you can move the sun and it's lighting will simply follow changing on all the planets accordingly practically automatically.. Disadvantage is that it's pretty hard to get it to look really realistic perhaps,

Cheers