Step 1: Firstly, you have to look for a nice background. You can use this background here or use the image below, which is in a lower resolution:

Open it in Photoshop. If you download the original image then you should resize it to 800x533 pixels by going to Image > Image Size and set a width of 800 pixels, the height will be auto resized accordingly.

Step 2: Create a new layer (Ctrl-Shift-N) and use the Elliptical Marquee Tool (M) to select a circular area

Step 3: Press D and X to reset your foreground and background colors. Press Ctrl-Backspace to fill the circular selection with black

Go to Filter > Render > Clouds to fill it with clouds. Go to Filter > Render > Difference Clouds to put your clouds in contrast. Press Ctrl-F to repeat this effect again

Step 4: Press Ctrl-D to deselect it. Press Ctrl-U to open up the Hue/Saturation dialog box. Make sure to tick on the Colorize checkbox and use these settings:

Now our circle turn out into an orange planet but we have to make it more real

Step 5: Double click on Layer 1, which contains our planet and use these effects

    + Drop Shadow:

    + Inner Shadow:

    + Outer Glow:

    + Inner Glow:

Now our image will look like this:

Step 6: Use the Erase Tool (E) to erase the bottom of our image so that our audience believes it lying at the horizon

Step 7: Our planet does not look impressive now, it needs more contrast so go to Image > Adjustments >Levels and use these settings:

Now it looks really impressive, doesn't it?

Step 8: And finally, we need some lighting effects to make our image brighter. Click on  your background in the Layers tab to set it as the current layer. Go to Filter > Render > Lighting Effects and use these settings:

Finished! Here is the final result:

You can add some effects (like Blur, Noise ...) to make it more real, I don't use these effects to keep this tutorial small

---