Download Unreal Engine Asset – Physically Based Earth | Photorealistic, customisable planet with high res 16k textures, physically based shading, and background environment.
Includes 16k Earth and Mars texture maps free of charge, or you can substitute your own maps as long as they conform to the same structure. As the shading is physically based you can change the Rayleigh scattering to simulate different atmospheres. The blueprint pre-calculates a texture lookup so that the atmospheric scattering will correctly influence the color of the surface shading. With the right combination of settings and maps completely different planets can be created, for example Mars.
This is a fairly heavy shader at 608 instructions, the sampling is scalable to trade quality for speed and I have run it on a GTX680, but it will still require a reasonably strong graphics card for decent performance.
Atmosphere ray marching technique partly based on these resources: http://nishitalab.org/user/nis/cdrom/sig93_nis.pdf and https://www.scratchapixel.com/lessons/procedural-generation-virtual-worlds/simulating-sky/simulating-colors-of-the-sky
Earth textures put together from: https://visibleearth.nasa.gov/view_cat.php?categoryID=1484 and https://earthobservatory.nasa.gov/global-maps