During the outbreak of the Blue Plague, five star systems where put under a permanent quarantine by Council of Worlds. Such worlds where designated by the following Greek letters: Alpha, Beta, Delta, Epsilon and Gamma. Any spaceship seen attempting to leave one of these systems is to be destroyed by Spacefleet before it has a chance to spread the plague any further.
The fate of the five quarantined systems remains unknown...
(Info cited form Zebulon's Guide to the Frontier)
- ↑ Note that this implies some sort of permanent Spacefleet presence or monitoring in-system.