Well, it's not technically an exploit so much as it is bad pathfinding.
You can get enemy ships stuck on planets if your ship is fast enough to turn around and run behind the planet you are defending. This allows you to conduct long-range attacks past the planet, like say, if you are playing tinkers who can manufacture missiles or othrin who have gunships. The enemy ships can't seem to properly figure out how to get around the planet if it is directly between them and their target. With large fleets, some might manage to pick their way around the planet, but it's definitely funny to have them sit there helplessly sitting there as you pick them off.
An easy fix might just be to either make the planet block attacks, so you can't shoot past the planet, or not block movement, so you can't get your enemies stuck behind the planet.