Volumetric Light

1. Create a new document, or open a starfield with a planet you have made.  Use a black background, and create an object somewhere in the scene:

As you can see, I plan to have my lighting come from the top left of the document

2. Create a New layer, and fill it with black.  Set this layers blending mode to Screen:

3. Create a lens flare (Filter > Render > Lens Flare).  The screen will block out all of the black and keep the white:

4. If it seems a little sharp, just apply a gaussian blur to it (Filter > Blur > Gaussian blur):

5. Your document may look something like this:

6. Now, grab your eraser tool from the tool bar.  Make a brush about the size of your object, with 0% hardness, and erase the light on the opposite side for your object from the light source:

Click to enlarge

Volumetric light means that your lighting is taking on the physical properties that allow it to move around objects instead of just going straight through them.  With a little practice, you can use this technique in space scenes to make your work look like this: