I am shading a planet, made up of three ico spheres, one for the planet itself, one for the clouds, and one for the atmosphere. However, the cloud layer is introducing this weird band/artifact on the planet that runs behind the beginning of the shadow. Up close, it almost looks like water caustics.
Here is the node group for the cloud layer.
The atmosphere also produces a similar effect, but it is harder to see. It is clearest on the edges of the planet.