This scene isn’t too busy which makes it easy to explain and to duplicate. Feel free to add more to yours such as an asteroid field, more planets, moons, etc.
To begin, decide what resolution you want your wallpaper to be. I made mine 2048×1080 so that it’s plenty big enough for a widescreen monitor and works on smaller monitors as well. To do this I simply began a new image with the following settings:
I’m running CS4 by the way, so if yours looks different, this may be why!
Now we have a nice blank canvas. Pick a color, or a couple colors that you would like to use for your space background. I chose a gradient of dark purple and dark blue. If you do the same you can either do a radial gradient with a lighter color where your planet (or planets) will be, or a linear gradient across the whole thing. I originally used a linear gradient, but this time I think I will use a radial.
Once you have your gradient, or single color up, it’s time to burn and dodge it to make it look a little less two dimensional. This is a trick I found in a tutorial years ago. Dodge will make parts of the image brighter, and Burn will make parts darker. Select the Burn tool and right click on the canvas and select the maple leaf brush:
This is a default brush that comes with Photoshop so you hopefully shouldn’t have any trouble finding it. The default size may work for you, but you’ll probably want it a little bigger. Begin burning the canvas in different areas, then switch to the dodge tool, using the same brush, and dodge the lighter areas:
Continue dodging and burning until you are satisfied with the look of your background.
Now for the stars. This is the most tedious part. Create a new layer and select a small (3px) brush in a light yellow. Start creating little ‘stars’ across the canvas. Next, change colors to light pink or blue. Add more stars. Pick another color, repeat. Once you have a good number of small stars, feel free to up the size of your brush a little and add in some larger ones. Once you’re satisfied, we’re going to add a soft white outer glow in the layer styles.
Now, again in our soft yellows, pinks, blues, etc. click once over select stars, making some of them appear brighter than others. You can do this on the larger stars as well by picking a larger brush (around 35px). Try different sizes on different stars for a more random look.
Now for the planet. New layer. Set your colors to the default black/white. Go to Filters > Render > Clouds. Select the Elliptical Marquee and, while holding the shift key, create a circle somewhere on the canvas (preferably somewhere where there is an interesting pattern in the rendered clouds). Then go to Select > Inverse, and hit the Delete key. Now you have what looks like a flat moon.
Hit Ctrl D to deselect, then Ctrl click on the layer (in the layers tab) to select just the planet. You should see the marquee moving around it if you did it right. Now go to Filter > Distort > Spherize. Normal mode, 100%, hit OK.
Makes it look a little more round and less flat. While the shape is still selected, create another new layer and grab your ellipse tool. Switch your color to black (if it isn’t already) and draw a circle towards the lower right edge of your planet (or wherever the shadow would be on yours).
Now go to Filter > Blur > Gaussian Blur. Set it to about 50 and hit OK. With the marquee around the planet, it keeps the blur of the ‘shadow’ from extending beyond the boundaries of the planet. If you don’t like the position however, you can step backwards, deselect the planet, move the black circle to wherever you like, reapply the Gaussian Blur, Ctrl-click the planet layer (while still on the layer with the black circle), Select > Inverse, and delete. Should keep a nice blurry shadow on your planet, and nowhere else.
Hit Ctrl-E to merge the shadow layer with the planet layer. Now it’s time for more dodge and burn. This time we’ll be using a regular circular brush, burning shadow into the bottom of the planet, and adding light and highlight to the top. I like the look of a large soft brush (100px, 0% hardness). I also added a color overlay layer style to give my planet some color. You can play around with colors and blending modes to find what you like. I chose a dark yellow and set the blending mode to Soft Light.
Now you can play around with a few other layer styles if you like. In the following picture, I actually recreated my planet because I wasn’t satisfied with the amount of texture on it. Changed the color overlay to a dark orange, added an outer glow in a grayish blue hue (50% opacity, 25px size), and an inner glow in white (50% opacity, standard 5px size).
Almost done! Now to add a little more dimension to the rest of the picture. Zoom out so you can see the whole thing. New layer. Render a new set of black and white clouds. Set the blending mode for the layer on Multiply. Now grab a giant, soft eraser (300px or larger, 0% hardness), and selectively erase out the parts you don’t want to show as dark.
Again, another new layer. Grab your brush tool, make sure you have white or another pale color selected. Hit F5 to bring up your brushes window and add the following settings:
Now.. make some pretty random patterns across the sky. This will probably take a few tries to get it how you like it. In my experience, a soft brush (0% hardness) and a lower flow on the brush seemed to work best. I also dropped this layer down below the planet so that the random starfields were behind the planet. You can, of course, also go back with your giant, soft eraser and take out parts you don’t like.
At this point, your scene is pretty much done. You can go back and play around with some of the different techniques and add a few more layers until the image is satisfactory to you. I added a couple more of the multiply blend mode rendered clouds for a darker image. As I said before, you can add more planets and whatever else you like.