This is the same procedural trick all over again... A planet is an enormously complex thing; it is basically impossible to model one by hand so procedural generation is key to making a believable looking planet. This planet is generated using the built in procedural textures of Blender - mostly perlin noise which defines the height of the land, texture of the water and location of the clouds.
You can watch a video of this planet in action on Youtube.
Blender: Modeling, texturing and rendering.
ffmpeg: Video transcoding.