This is something that I created by just playing around. First adjust colors, tones and shadows in camera raw before taking it into Photoshop. Then change it to lab color and apply image, experimenting with the blending modes. Don't forget to return the color back to rgb before you continue on.

Make a copy of this layer before starting to set up your scene, we will use this later on for the extra planets. At this point, draw out a rectangle selection of the lower half of the picture stopping at the base of the mountains. Press command J to bring a copy of this selection to it's own layer. Convert this layer to a 3d postcard and begin playing with the 3D tools to position this so that it gives you a convincing ground layer. Select out the top of the picture from the base of the mountains, command J to raise it to a layer of it's own. You will now use content aware scaling to fill out the canvas so that you will feel like you could step right into the scene.

add a new blank layer, fill with deep navy blue, then add layer mask filled with black. I do this by pressing option down while clicking on layer mask at bottom of layers pallet. Using white at about 30 percent paint onto the mask area to darken the sky only. Try to remain only in the sky area. This will give you a darkened sky so that you will have a base to show off the rest of your handy work. Using a multiply blending mode on this layer brings it all together.

Now is time to create the other planets. You will be using the copy that you made at the beginning. I applied another round of lab color like we did before but make sure it looks really saturated because some of the brightness will disappear as the planets take shape. Now that you have converted it back to rgb again, go into filters, artistic, plastic wrap and play with the way it looks, I wanted mine to have more dimension and shine so I went a little crazy. Next, go up to the 3d menu and select
create shape out of layer, then choose sphere. Make adjustments in the lighting on the sphere by double clicking on it's layer thumbnail and it will open up the different areas you can edit in it.

Once you have your sphere the way you want it, add some inner shadows, inner glows. Duplicate this layer and rearrange the size and rotation, then add a gradient overlay, your choice of gradient, opacity and blending mode. Duplicate once again and create a different look once again and your planets are ready to arrange in the sky. It is at this stage that I add some lightning to the sky and a bit of a glow behind the planets. Did some burning and dodging where I thought it needed highlight and shadows.

The finishing touches that I put on this was to add another layer and use a plugin to add the look of water and masked it off so that it affected only the little lake area. On that layer I lowered the opacity a bit so the water looked fairly real. Combine merge the two layers duplicate that one and set the mode to color and lower the opacity to about 78%. Once it is all merged together, I performed an unsharp mask then fade unsharp mask. And just look at the before and after.