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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.
Ok, after trying many different things, the problem is that my clouds and atmosphere were too far above the surface of the planet. The light would pass through one side of the icosphere, above the surface of the planet, and come out through another side of the icosphere. The weird caustic effect is caused by the normals of the icosphere, since it doesn't have thickness. This means that the normals were correct for the outer face and incorrect for the inner face, which meant the light came out weird.
The fix for this is can be found in this article: https://www.katsbits.com/codex/backface-culling/#rendered-cycles. The geometry node has a "backfacing" output, which means that putting that into the factor of a mix shader connected to the main texture and a transparent BSDF will make the light hitting the interior of the icosphere transparent, fixing the banding problem.
