yeh i just was going through it.... its the population of planets causing the problem.
Suggestion: dont use populations of big planets

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heres a 2 minute render (i dont have your image maps so its not exactly same) with your same planet done as not a population...
basically, create a new single planet, then copy the current settings on that one of yours (all the center, radius, and angular position (heading, elevation, distance), and set the shaders back up with just a single planet and then you can up the render detail to .5 again, wallah.