Spaceborns
Before men came to planets, they built spaceships. And once they reached a place, they often constructed a station. A small one… just something to sustain and study this planet while waiting for others to come. And sometimes, and even, often, it will never go further than that.
This led to many generations being born and actually living their entire life in small ships in space.
You already know about the Nomad Fleet - one of the most important factions of spaceborn people, with some gigantic transports.
… but many of the spaceborn are just isolated families.
Spaceborn know their stuff about spaceships, making them quite able in Mechanics and Spaceships. But, they are at a complete loss once on a planet, making them low in Planetary skill.