I think i may be on to something. It doesn’t look great so far and the planet model is not too good (I used one i made a while back) plus i’ve only just started testing this method but i think with some fine tuning it could turn out some nice results. I’ve attached some images of my first test render below, along with the node setup. (A very simple node setup lol) The fade into space is terrible at the moment, it’s too sudden, but i’m sure i can find a way to sort that out 
The colour ramp only has two colours (it could do with more) a darkish grey at position 0.000, and a full black at position 0.020
P.S - I had my multiply node set so high to better highlight the density change from ground upwards
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