Planets

First you need to create a background. Create a new document, make the background black, than add some noise by going Filter>Noise>Add Noise with a setting of 90 - 110 with Mono checked. Once you've done that, adjust the levels by hitting Ctrl+L (Mac: Command+L), so that your background looks something like this.

Now create a new layer. Using the Elliptical Marquee create, a perfect circle holding Shift and dragging. Fill the circle with a texture. I simply selected a dark red and a light pink color and went to Filter>Render>Clouds to fill the circle with cloudy texture.

With your selection still active, go to Filters>Distort>Spherize. Use a setting of 100.

Now, create a new layer, grab the Airbrush Tool, set its pressure to 10-15. Then, add a shadow to your planet, try to get it to look something like mine.

Using the Filter>Render>Lighting Effects filter, add a light source to you planet for an added level of realism.

If you want your stars to be stronger duplicate the background and go Filter>Blur>Gaussian Blur to blur it a little bit. Finally, set its Blending Mode to Screen, and duplicate it again if the stars aren't strong enough.