As above, 2 spheres - you use an additional mesh for this. Apply the atmospheric effect to the larger one.
Marketplace if you want a paid shortcut:
Or free YT tuts in the previous post. Or so many results on the forums if only you searched:
I’ve been using the above since ever. The thread has numerous links and ideas, worth the read.
It’s a different story if you need to enter the atmosphere:
Scroll down to: Planetary Atmospheres Viewed from Space for a vid demonstrating the effect:
It might be an overkill if you’re only after the effect as seen from outer space.
And finally, if you need to get serious about it, do look into Rayleigh Scattering. Understanding how the phenomenon works makes it easier to work towards a goal.
Hope this is enough.