This project has evolved into a procedural real-time-capable modular planet shader, which you can buy here.

This is a demo scene to showcase the real-time capabilities of Eevee when it comes to planets. The planet only consists of a flat procedurally textured surface, no volumes involved. The texture is animated with a curl field displacement node group, and a phong shader node group creates faked rayleigh scattering along the shadow terminator. The project file is meant to be poked around in, not to be used as a production-ready asset. Although the curl field node group is certainly a decent asset for your library. Some of the key concepts are explained inside the node groups.

This scene is not optimized for Cycles, it only works in Eevee. You can copy or recreate the base texture for Cycles, but the rayleigh scattering part will likely break apart in Cycles; for rayleigh scattering in Cycles you should rather use a volume absorption node with a volumetric sphere as atmosphere.

The material shader allows you to change the texture colors as well as the atmosphere color and the strength, position and falloff of the rayleigh scattering.

The curl field node has outputs for 4, 8, 16 and 32 iterations, which will proportionally increase your calculation and render times. I could not find an analytical way to implement curl fields in Blender, so this is a numerical approximation with 4-32 iterations.