Start by opening a new document in Photoshop. Make the background black and the dimenision 1500x1000. Create a new layer in the layer palette and name it Planet. Select the rounded marquee tool from the tool bar and hold down shift and drag a circle in middle of the canvas. See the final for a better idea of the size of the planet relative to the scene.
Select the gradient tool and click the radial gradient option in the top tool bar.
Set the foreground color to a deep blue, I used #1c62ca . Set the background color to black and drag the gradient tool inside the selection from the top left to the bottom right.
Now let's apply and outer glow to give the planet and atmosphere. bring up the layer styles by clicking the icon at the bottom of the layer palette and apply these settings to the outer glow.
Keeping the layer style window open, apply an inner glow using these settings.
For the planet surface, any texture can be used to make it look like ground but if your following along with me you can down load and use the texture I'm using here: Plant Skin
Drag/drop the texture into the planet scene and name it texture. It's rather large so scale it down so that it just covers the top and bottom of the planet.
After you've scaled the texture to match the picture above, apply the spherize filter to the texture by going to filter>distort>spherize and use these settings.
It enlarges the texture so you'll have to resize it back down so it just covers the top and bottom of the planet again.
In the layers palette. Make sure the texture layer is selected and ctrl +click the planet layer to create a circular selection of the planet and then apply a layer mask to the texture layer by clicking the "add layer mask icon at the bottom of the layers palette. At the top of the layers palette, set the blend mode to Linear dodge and lower the opacity to about 80%. Your planet should look similar to this:
Looking at the above image we need to make it a little darker on the side opposite the light source. Click the layer mask icon on the texture layer then select the paintbrush. Make sure your foreground color is set to black. In top tool bar, set the opacity for the brush to 50% and 300px soft brush.
Then paint the bottom right side of the planet to make it darker by masking out some of the texture.
Let's add a little more color to the planet. In the layers palette, click the "Layer thumbnail" on the texture layer to select it. Set the foreground color to #ff7e7e. With the paintbrush still on the same settings as above, paint on the top and top left edges to add just a tint of color.
To finish the planet create a new layer above the texture layer and name it "cloud". With the cloud layer selected, ctrl +click on the planet layer to create a selection of the planet. Hit D then X to set the foreground to white and background to black. Then go to filter>render>clouds to fill the selection with clouds. Next, with the cloud layer selected and the selection still active apply the spherize filter once again with the same settings as above. Set the blend mode of the cloud layer to soft light and lower the fill to about 60%.
Note: I won't go into how to make the second smaller planet seen in the final because it was made the exact same way only with a different texture and a redish glow rather than blue.
Nebula and Star Field
To make the Nebula gas clouds create a new layer below the planet layer and name it blue nebula. Set your foreground color to a deep blue, I used #191892 and your background color to black. Apply the clouds filter to the new layer, filter>render>clouds then the Difference Clouds filter twice filter>render>difference clouds. This fills the entire scene with clouds so apply a layer mask to to the blue nebula layer then select the paintbrush. Choose a soft brush at 300px, set the foreground to black and paint out the parts of the clouds you don't want. (click for a larger view)
Do this a couple more times with 2 different colors, I used a dark orange and purple on mine. Set the fill level in the layers palette for all three colors to 80%
To make the star field, we'll use the Paintbrush with the "star" brush head and use a combination of these sizes: 8px , 14px, 24px, 42px . Set the spacing to 350.
Create a new layer above the nebula layers but below the planet layer and name it stars. Start with the star brush set at 42 px and just click one time on several areas around the scene. (don't over do it). next take the 24px brush and do a little more. Keep doing this going down in size each time. The object is, the smaller the brush the more you can go crazy with it, try to cluster a bunch around some of the bright spots in the nebula.
Here's the final.
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