Making a Planet

Make a new document, 1000x1000 with black background.

Let's add some space stars. Go to Filters>Noise>add Noise. In the pop-up menu put a reasonable amount, like 10-15%. Be sure to check Gaussian and Monochromatic. Hit OK.

To make the stars, go to Image>Adjustments>Brightness/Contrast. Move the sliders like in the example. Play with values until you reach your preferred amount of stars. Now you have a nice star space.

Create a new layer. Then use the elliptical marquee tool, and while holding down shift, create a circle selection. Make it big. Fill the selection with black.

Now go to Filters> Render>Difference Clouds. If not satisfied with the result, hit CTRL+F (Command +F on macintosh) again and again until you reach the correct image.

Then go to Image->Adjustments->Levels, or hit CTRL+L (Cmd+L on Mac). Move the center and right slider towards the left, like in my example:

Now go to Filter->Sharpen-> Unsharp Mask and use the values: Amount: 500%, Radius: 2.5 and Threshold: 10. I remember, you can play with these values as you want.

Now go to Filters>Distort>Spherize and put the amount 100%.

Your image should look like this:

Then go to Image->Adjustments->Color Balance Apply the following color levels:

Shadows: +100 / 0/ -100
Midtones: +100 / 0/ -100
Highlights: +70 / 0/ -15

Go again to Filters>Sharpen>Unsharp Mask and use these values: Amount: 275%, Radius: 4.5 and Threshold 40 levels. Finally double click on your Sun layer to bring up the Layer Style dialogue. Check the Inner Shadow and Outer glow (at outer glow you can put the values from my example or you simply can play).

Here is the final image: