Provisional Title: Return to the Station
There are not many planets in the Goldilocks zone. Orbital habitats are cheaper and faster than trying to terraform. As time goes on, the majority of our civilization will be living in space instead of on planets.
This also could change how we expand. With slow migration of O'Neill cylinders, you could have an entire civilization pack up and move. Your destination wouldn't even need inhabitable planets.
Though if they do head towards another occupied system, arrangements would need to be made. I imagine it would be quite inconvenient to have a trillion people show up at your door unannounced.