It's likely this is down to procedural generation of the planet textures. The engine is capable of generating LODs for planets on-the-fly. Rather than having fixed assets for planetary bodies, the engine models them based on various processes such as calculating crater distribution and ejecta, landmasses, etc. AFAIK the surface is split into squares of a predetermined size, and each of those squares is calculated per frame sent to the GPU. So while this work is ongoing on your GPU it'll stall the normal render pipeline and leads to stuttering. If you approach a planet slowly in supercruise you should notice that after a while the stutter goes away; this indicates that the LOD for the visible part of the planet has been calculated. If you supercruise around the planet a bit you'll hit textures that haven't been generated and the stuttering will start again.
My feeling is that Frontier aren't putting too much effort into directly fixing this issue as it's something they'll have to tackle with the PG engine for the planetary landing expansion. They'll have to get this aspect right, otherwise approaching planets for landing will make the game unplayable.
If this *is* the problem you're describing there are some settings in the graphics configuration files (can't remember where, sorry!) such as the work done per frame and the size / detail of the different LODs for planet textures.
If it *isn't* the problem, there are also reports of microstuttering in the game that the devs are aware of but haven't yet got to the bottom of. I'd describe the planet texture generation issue as definite stutter though, as frame rates clearly take a large dip.