They have been rendered using several layers.
The first layer is the diffuse earth colour which has the photographic information and some bump map to add volume and shadows.
The second is the cloud layer. This is made out of two spheres with the same texture but different settings. The bottom of these two layers has a blur factor on the alpha mask to create extra volume under the top sharper layer. These layers cast shadows on the bottom diffuse sphere.
The top is the atmosphere layer. It is made out of a flat bright blue colour made visible using a falloff effect. This effect means that the layer is only visible at shallow angles. The result is the blurry atmosphere at the edges of the earth.