These kinds of cloud shadows isn´t possible with pure geometry surfacing clouds, then again..these are a bit exxagerated in scale, usually clouds from space are quite flat ..thin in the looks, but it depends on how close you get to the planet, a delicate balance, so these belows are far from perfect either, so I am working on that from time to time when I feel like for it.
Atmosphere needs atmosphere, meaning thickness and anisotrophy/asymmetry values, so you could hint the ground but still feel that there is atmosphere density, this must be balanced with the right settings for density, scattering, asymmetry.
Fog etc below, and clouds casting dark shadows in the atmosphere itself is also something nice to see when there is other cloud layers or dense fog in the atmosphere..
So, one geometry mesh, surface layered, this case pure fractals (fractals are hard to work with and get right, takes longer to render, so finding other painted, created maps at very very high resolution may be the best)
One volume item (has to be none pyroclastic) for the atmosphere
One Volume item with proper fractals for the clouds, weather, turbulence..pick whatever you find interesting.
The hardest part is to get the scaling right on both the distrubition, fine detail scale, thickness scale only at certain areas ( which I need to adress)
Also need to adress thinner edges of the atmosphere a bit more, ogo taiki samples were nicely setup with that, so more work on that.
That said, only use this when you want more close up shots, with controllable light design of the shadows, colors etc, very very render costly, may work for you with stills.
For animations you would need to boost up the CPU as Well as getting render farm access I would guess.
If using distant planets, go with pure surface methods as shown with Octane.