This is what you are going to make:
Find a starry background from the internet and paste it into photoshop. It should now be your background layer. Or, create your own background by starting out with a black background and using a small white brush to create stars. I chose this picture I found on google:
Next, find some thin and wispy clouds like cirrus or cirrostratus clouds. Copy the picture into photoshop. Desaturate the image all the way until it is black and white (Image>Adjustments>Hue/Saturation). Then go to Image>Adjustments>Levels, and Move the black marker to the right until the background is completely black. Change the mode to "Screen" and change the opacity so it then looks something like this:
Then, find a planet texture you like. Paste it into a new document and use the circular marquee (Hold shift) to select a circular part of the texture. Then go to Select>Inverse, then press delete. Paste it into your original document. It should look something like this:
Selecting your planet, go to Filter>Distort>Spherize, and make sure it is on 100%. The distortion of the shape creates a three dimensional appearance to give the basic form of a planet. Do this once or twice. Then, go to Layer>Layer Style>Outer Glow and change the blending mode to Linear Light, and create a light blue outer glow. Click OK. Go to Layer>Layer Style>Inner Glow. Use a slightly darker blue.
Next, we are going to create a shadow by using the Ellipse Tool. Create a new layer and make a circle about the size of your planet. Use a Gaussian blur to smooth it out. Adjust to your liking.
Create a new layer and dab white spots over the edge of your planet using a soft brush.
Then change the layer to overlay. It will make the edges of your planet look brighter.
Create a moon and use the same steps as you did when making the planet. Add more planets if you like! But make sure you remember where the light source is, and be consistent with it in your planets!
Start brightening the background! Create a layer in front of your clouds, but behind your planets. Using your brush, start dabbing some light pinks and greens (or whatever color you like) all over, especially where your clouds are.
Then, change the layers style to "color dodge" and lower the opacity if it is too strong. I lowered mine to about 50%.
Jazz up your background by adding real nebulas, more clouds (cirrus clouds would work and could look like meteorites), more colors, more stars, anything you like. I just added some more clouds.
Find a bright lens flare. Paste it into your document and cut it out.
Darken the sky behind the flare to black, then change the blending mode to Screen to render the black area transparent. Put the flare on the brightest part of your planet, and so it's just creeping over the edge of the planet.
Duplicate your flare layer, to make the flare look bigger and more intense.
Now create a new layer and dab a little spot of white, using your brush. CTRL-T and stretch the white spot vertically, so that it looks like this.
Now move it over to your flare so that it looks like this! Adjust the opacity to about 80%. You now have a very nice looking glare on your planet!
Create one more layer, on top of the others. Use a black brush and go around the edges with about a 30-50% opacity and make it a little darker. Change the blending mode to Overlay.
You now have a beautiful space scene!
Tutorials I used when learning how to do this: