Unfortunately, Corona can not deal with this yet, as it does not support heterogenous media. You should cast the vote in most requested features list thread.
Does needing this for one test project really makes it the most wanted feature? Remember, that thread is not "every feature needed ever", but "what hurts you the most generally"
No, but heterogenous volumetric media is needed by a lot of people for many different use cases, and if implemented, it would cover this use case as well.
Actually, rendering planets is not always a test use case. I remember helping a studio render planet atmosphere in Corona couple of years ago, because they could not find a way around it, and ended up suggesting to use the falloff map fake, which worked, but the workflow proved so tedious they reverted to V-Ray for that project. And I myself was doing some planet renders not that long ago, and had to resort to that ridiculous workaround too.
I think when people think of heterogenous media, planet atmosphere is one of the last things they consider, and despite that, it's now #1 request. That speaks for something, especially considering Corona has mainly archviz userbase now.