Step 1
Create a new document of any size, I"m using 500x500 for this example.
Step 2
Flood fill the background with black (#000000).
Then make a new layer and use the brush tool to make a white circle. I?m going to use a 300 pixel brush.
Using the elliptical marquee tool, hold down the SHIFT key on your keyboard and make a selection in the center of the canvas. Holding SHIFT will ensure that the circle is perfectly round. Fill this selection with white (#FFFFFF):
Step 3
Press 'D' on your keyboard to reset your foreground and background colors to black white respectively.
With the selection still active, go to "Filter->Render->Clouds".
Press Ctrl + F on your keyboard three times to repeat the Clouds filter:
Step 4
Press Ctrl + I on your keyboard to invert the colors.
Press Ctrl + L on your keyboard to bring up the Levels Adjustment Tool and apply the following settings:
Result:
Step 5
Press Ctrl + D on your keyboard to deselect the selection.
Press Ctrl + U on your keyboard to bring up the Hue/Saturation tool, and apply the following settings:
Result:
Step 6
Press Ctrl + J on your keyboard to duplicate this layer.
Go to "Filter->Blur->Gaussian Blur" and apply the following settings:
Double click this layer and apply the following blending options:
Step 7
Click this layer, and drag it below our previous layer. This should give our planet a little depth. Try creating several of these with different color settings, to create a great looking space scene.
Try playing around with some of the settings see what you come up with, here is a piece I created using the techniques taught in this tutorial.
More Tutorials: 
