Step 1 Find an image of some dry mud (Odd I know, but it works).

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Step 2 Now invert the color of the image. (Ctrl+Shift+I)

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Step 3 Select the Ellipse tool and select an area of mud which can be as big or as small as you like just as long as it is somewhat centred.

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Step 4 Invert the selection (Ctrl+I) and then delete it.

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Step 5 Now fill in the blank section with black.

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Step 6 Next step is to bulge the image (Effects -> Distort -> Bulge) This makes it look round, therefore planet-like. For this image I used a bulge of 31%. Do what looks best for the size of image.

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Step 7 Now duplicate the image and on the new layer, adjust the blending to Colour Burn in the Layer Properties.

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Step 8 Duplicate the image you just did the Colour Burn on and adjust the blending to Additive. (You might be able to skip this step if your mud is fairly dark).

Now Merge the Layer down so that you only have one layer to work with.

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Step 9 Time for some glow (Effect -> Glow). You'll have to adjust the glow depending on the effect you want and the image. You'll need more or less depending on how many cracks your "planet" has.

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Step 10 Now duplicate the image. After you've done that, on the new layer do a Zoom Blur. (Effects -> Blurs -> Zoom Blur) I used 48% use it to your liking.

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Step 11 On the image we just did the blur on, adjust the blending to Lighten.

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Step 12 Turn the saturation to zero (Adjustments -> Hue/Saturation) now merge down and you're done. (Again, depending taste and the type of mud you use, you might want to skip the saturation step).

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That's that. Here is an image done with some different mud. The hue was changed before edited.