In my opinion, the light source and the actual shadow arent correct that way.
The light is coming from the top left foreground but the shadow is showed at the bottom right
foreground. The shadow has to be at the bottom right background... I hope you know what i mean
Maybe this explains it a bit more: The white point is ment to be the light source.
(this image only explains the light and shadows and is not ment to be rendered perfectly
)
Be sure to place the shadow to the opposit site of the light source (very commonly spoken).
In your case of the chosen light source direction it shouldve been more like this:
But as Coffee said, try a bigger shadow and with this you need to arrange the lights position.
Coffee's posted picture explains it very well. The light comes from the backside of the planent and
so the shadow is more present on the front.
Btw, you did it almost right with the darker areas of the planet (not the black holes) but they need to be more darker.
So long,
Lorath