Depends on what you need from the texture. To backtrace actions you would do with Gimp, and replicating with Pascal is not that hard. But does it need to support close-up generation, like zooming in tiny section of the planet and generate a new high quality texture to that? Then you need a real formula, and can't rely on random, blurring or anything like that. I could suggest sine-waves in multiple layers and in different frequencies. You can have 1 set of waves with X-coordinate and other for Y, then sum them up for RGB or heightmap representation.

Say, sin(X) does a very static wave that you can see repeating. But (sin(X)+sin(0.9+0.31235*X)*0.7124) will not make any repetitions. Although, for it to wrap around seamlessly you might want it to have some repeat, scale it with proper PI-multiplications etc...