Rendering the planets themselves was simple enough once I figured out how raymarching worked, but once I started adding volumetric atmospheres with scattering (needed to make clouds and their shadows work), it got complex. Coming up with the optimal arrangement of atmospheric samples and how many subrays to cast got very complex very quickly and while I could make it work, it was never very satisfactory and Povray, with its years of optimisation, was vastly faster.

...although I see now that Mitsuba has had a new release. Hmm.