Using just one texture in Photoshop, I'm going to teach you how to make a planet that looks somewhat realistic although, in the end, whether it looks realistic or not is entirely up to you and the way you play with settings.
Texture: I used a texture from a rock. You can try different textures such as leather, and things of the like. If possible, avoid uniform textures, since they will make the planet look too uniform. Open up your texture with photoshop, increase your canvas size to a 1:1:1 ratio (EJ: 3000x3000, 5000x5000, etc).
Because I will be working at high resolutions, I increased the canvas size to 4000x4000.
Note: If you have 512 megs of RAM or less, don't use such high resolutions, because Photoshop will not be able to apply certain filters.
Now you should have the texture in the center, surrounded by empty space. Use the clone stamp tool (in Photoshop toolbar, 1st row, 5th down) to fill up the empty space (hold Alt+click the section that you want to clone, and then start brushing). Make sure that your hardness is 0%, otherwise it's going to look disastrous. Remember that this texture will be twice as big as the planet we will be making. Once you're done with this, go to the Edit menu, and select Define Pattern, then name it whatever you want.
Now we proceed to making a new document. What size? Since my previous one was 4000x4000, the new one will be the same size. Now fill the background with black, pick the Elliptical marquee tool (press M until you see it at the top left of the toolbar), hold down shift and drag the mouse so that you get a perfect circle.
Now that you have a nice circle, select the Paint bucket tool, and change the setting to pattern instead of foreground. Make a new layer (ctrl+shift+N for windows, apple+shift+N for Macs), and fill the circle with the pattern. You should end up with something like this:
Now all we need to do is to spherize it. Make sure that the layer is still selected (dotted line around the circle), if not, ctrl+click (for Windows) or apple+click (for Mac) to select it. Go go filters/distort and select Spherize. Set to 100% and apply. Now we have something that slightly resembles a sphere, but it doesn't look round enough, so we go ahead and apply the filter again under the same settings. The center might look distorted, but it will not be a problem because we will be resizing this anyway and it will be unnoticeable.
Now we have a nice texture that we can use, you may go ahead and save it, but don't close the document yet.