Planet Atmosphere Shader
The planet atmosphere shader can be found by selecting the Planet Atmosphere object and by heading over to the Node Editor.
There, you will see this node group:
Volumetric output for the shader.
Main color of the atmosphere.
Creates the sunset reddening effect of the atmosphere due to refraction. This is essentially a volume absorption node mixed in with the scatter. The color of this effect can be adjusted with this setting.
Amount of absorption effect.
Density of the atmosphere. Controls how thick the atmosphere is.
Controls the amount of light emitted by the atmosphere. Not physically realistic, but helps get more light in shadowed areas and, at larger values, can generate a stylistic effect.