Im gonna teach you how to make something like this:
Allright. Start off with a 500 x 500 image with a black background.
Create a new blank layer.
Select the round selection tool, hold shift and drag your mouse and release when you see something like this:
Select a foreground and a background, for example:
This will select the colors of the planet you're creating. You can also select something like red, if you want to make a doom planet, or green for a natural feeling. Dont forget blue, the water. You practically get 2 colours in your image.
Then, click Filter -> Render -> Clouds. You'll see something like this:
You might want to press Ctrl+F a couple of times so you get the right colour balance. This is important, becuase it will make the form of your planet!
IMPORTANT: Do not deselect !
Next, you go to Filter -> Sketch -> Notepaper. This will turn the clouds into flat colors. Make sure you set the settings like this:
You should play a bit with the Image Balance. Just make sure the colours are balanced well. If you cant get it right, try making the foreground color more darker than the background color, and also try redoing the clouds.
If you click OK you should see something like this:
Next, do NOT deselect yet, click Filter -> Distort -> Spherize.
Set the settings like this:
If you click OK, you will get an awesome world effect!
Now, lets go for the stars.
Deselect first.
Select the background layer.
Click Filter -> Noise -> Add Noise.
Make the settings like this:
Now click OK.
Finally, to hide some of the stars.. becuase they are too close to each other, duplicate the background layer, put the layer above the original background and below your planet, and put the layer style to Overlay.
If you've all done it right, you'll end up with something like this!
Congratiolations, you just made your own Photoshop planet!
Edit
Playing with another color could end up in funny results just try it yourself!
Doom planet.