After like 4-5 years of banging my head on the challenge of from-scratch procedurally generated continent-scale terrain... I finally figured out a workflow.
It involves rendering like 25 or so 8k terrain tiles(including diffuse, NRM, mask maps etc) in QuadSpinner's Gaea suite and stitching them together into several truly insane files in photoshop. Still not perfect, but its workable. And improvable! Ideally you'd want 16k, but my current workstation isn't PC master race enough.
The other thing was creating an accurate and photorealistic Rayleigh scattering material for the atmosphere that creates a proper fall-off at the transition between day and night. ie why everything goes orange at sunset.
Did that. Chucked in a couple of cloud layers and modelled the ring superstructure kinda lazily... And bang, one hopefully-photorealistic weapon of galactic destruction.