I started out with a square document, created a new layer opened a layer mask on it.
Running
clouds in the layer mask and repeating until I got an interesting distribution, then auto-levelled it to up the contrast.
Then using
Threshold on the clouds and playing with the slider to create a black/white 'continents' map.
Loading the layer mask as a selection and filling the selection with with blue in the layer itself, then inverting the selection and filling with green made the 'map'.
The bottom layer makes the planet shape, and a
Filter > Distort > Spherise at 100% makes the 'map' more 'realistic'
I just dropped a gradient to see if it would look reasonable with an appropriate shadow added.
Obviously, if more time were taken and a bit of care on colour selection a MUCH better result could be achieved ;\
say adding more variation in the land mass for different terrain etc...