Very unlucky, we've finally unlocked the sector 8, while our airship was rank 26 and equipped with invincible parts for many visits around the sector 7 (and 6 and 5).
Tired of waiting, we've sent our airship to the sector 7 alone, and that's when we've finally been blessed by the new discovery.

Also, the voyage log shows that we've unlocked the sector 10, while the chat log says "sector 8 has been discovered" (and it indeed has been as we've sent the airship there).