Complex sentence: In the case of Pluto, only the inner 3% of the region where prograde orbits would be stable is occupied by satellites, and the region from Styx to Hydra is packed so tightly that there is little room for further moons with stable orbits.
Split into simpler sentences: In the case of Pluto, only the inner 3% of the region where prograde orbits would be stable is occupied by satellites. In contrast, the region from Styx to Hydra is packed so tightly that there is little room for further moons with stable orbits.