Start a new document 600x600, black background.

Choose the circular marquee tool , and while holding down shift, click and draw to make a selection. Make it fairly big.

Create a new layer

Press "D" to reset your colors to default.

Then go to Filter->Render->Clouds

Press CTRL+L to open the Levels window. Pull the middle slider over towards the right by about half way, so your image has more dark patches.

Now open Image->Adjustments->Color Balance (or CTRL+B).

For each of the Tone Balance (Shadows, Midtones, Highlights). set the Color Levels to:

Now go to Filter->Distort->Spherize.

Amount: 100%

Then do it again, this time:

Amount: 70%

CTRL+D to deselect.

Now duplicate this layer. To do this, drag the layer into the new layer icon at the bottom of the layers pallete.

Now apply the plastic wrap filter to this duplicate layer. Filter->Artistic->Plastic Wrap.

Now set the duplicate layer's blending mode to "Hard Light"

That's it for the planet.

Read on if you want to learn how to add stars in the background.

To add stars to the background, create a new layer underneath the planet layer and fill it with black. Then go to Filter->Noise->Add Noise.

Amount: 30%
Distribution: Gaussian
Monochromatic: Checked

Now set your foreground color to black. (Press D).

then choose the Paint Bucket tool , and click on the Noise layer to paint it with black. The number of stars depends on where you click, so if you dont like it, just undo and click elsewhere until you get a nice even star field. Or you can even click again to get rid of more stars.