An easy way to get started is this:
Create a new atmosphere node and connect it to the planet node.
Set the haze density to 4
Increase haze exponential height from 3000 to something like 6000-8000.
Play with the glow settings to regulate the light falloff by the sun.
Beware that if you apply this to your existing scene that you will have to adjust the lighting settings for your clouds, as the atmosphere is key in how shading on surfaces and clouds look.
Cheers,
Martin
