Consider implementing shadows on planets based on normal maps.

[smcameron]

Shadows as opposed to just shading. Technique is summarized here: http://enbdev.com/doc_normalmappingshadows.htm
PDF is here: http://enbdev.com/NormalMappingShadows.pdf (PDF just seems to be slides with approximately the same info as the web page, maybe slightly more, but it's not a paper.)

This could theoretically render the long shadows you should see on a cratered planet around the terminator. There would likely be some head-spinning complexity around the cube map seams though.

I think a once sentence summary (found on reddit) is like so: "For each fragment you effectively reconstruct a 1d slice of the height field by integrating along a line on the normal map in the direction of the light source".

or to quote the link:

How it works

The light ray is an axis for computing slope of normal map. Trace from hit point to light direction and compute sum of dot products between normal map and light direction. If slope is bigger than 0, pixel is shadowed. If slope is also bigger than previous maximal value, increase hardness of shadow.