Alrighty, here are 2 versions of it at higher detail. I tried increasing AA but it didn't have a significant effect. Increasing detail was the only thing that really affected the jagged planet edge.
Here's detail 1 with GI 2/2. Render time was 1 hour and 20 minutes:
[attachthumb=1]
And here's detail 2 with GI 1/1. Render time was 4 hours and 27 minutes:
[attachthumb=2]
Interestingly, although the edge of the planet is smoother here, there is also more banding in the sky and some odd blotchiness in the lower-right. It also loses some of the nice soft lighting of those mountains in that area. A combination of all 3 might be best, in fact, because I also liked the sort of "rainbow sparkles" along the edge of the planet, despite the jaggedness. If you could combine the slight color variations of detail 1 with the smoothness of detail 2 and the lower-right from the original, I think it'd be the best of the lot.

- Oshyan