Pluto
Pluto is the farthest planetary object from the Sun, in the deep reaches of the Local Sector. Because of this distance from inhabited space Pluto has come to serve a purpose as the home of the Sol’s maximum security prison, where the most dangerous criminals are sent. Travel to Pluto is mostly forbidden unless you’ve been cleared by the ISSP, which rarely happens due to the dangerous nature of the criminals housed in the colony. Overall inhospitable temperatures and lack of terraforming also makes travel to Pluto undesired by those who would consider deep space travel, in any case.