Good thoughts.We all have thoughts. The NX-01 is where my thoughts took flight. All the saucers through the movie era don't really have a usable deck on the lower deck at the edge. So I thought this was an ideal place for sensors and science equipment. They just made the hull cover them for protection. Franz Joseph noted several places where the hull was designed to allow signals to pass through easily so I just put the two together and figured that ring is either the lower part of something accessed from above or packed with equipment that might occasionally need service but that scans through the hull.
Incidentally I didn't mean to be mysterious, I just didn't have time to complete my post this morning!
Basically, I postulate that the first wave of Starship (AKA Constitution) Class ships had much thinner saucers., about one deck thick. However, as they achieved higher and higher speeds the outer half of the saucers began to fracture. The overall shape was still fine for optimal warp geometry, it just needed to be stronger - so an array of structural integrity boosters was installed around the rim. These were in turn covered up by standard hull plating, which was blended into the existing shape of the hull.
The atmospheric aerodynamics of a separated saucer were just a bonus!

Future builds of this class simply retained what worked.
