This is caused by the atmosphere between the object and the camera, receiving light where in real life it would be in shadow. The standard way to fix this is to enable "receive shadows from surfaces" in the Quality tab of the Planet Atmosphere. If you have a scene where there are clouds in front of the object, you need to check it on the cloud layer(s) too.
This setting usually increases render times quite a bit.
If you have this problem in a scene where you really can't afford to increase render times, there is an advanced feature in the atmosphere and cloud nodes that allows you to use function nodes or shaders to mathematically define where the volumetric shadow should be, avoiding the need to enable "receive shadows from surfaces".
A third strategy you can use is a small, localised cloud layer where the object is. You can make it invisible (by turning off "enable primary"), but it will cast shadows into the atmosphere, avoiding the need to enable shadows from surface.
Matt