I can't leave a problem lying around, so I got to experiment a bit today. Used a very simple 24poly imported disc, single side, forced displacement, and added a black default shader masked by a fake stone distribution in the opacity slot (made very white from the stone colors by color adjust shader), fed output into empty surface shader with the same stones set as child.
Soft shadows, detail 0.8 no defer atmo, AA8, rendertime: 6mins! Planet disabled. Two lightsources for the brightest stars, the others are tiny dots, very luminous on a simple (imported) no shadow single sided plane behind the 'planet', opacity-masked the same way.
The planet, btw, is a 3m diameter sphere, with some pf's and a tiny cloud on a 3m diameter invisible planet (which I could have used for the planet anyway, but I added the clouds later).
One thing I have to find out; why the clouds are greyish.