Perhaps the planet wasn't always this way. Perhaps the days are only 9 years long now but in the past they were much much shorter. A 9 year long day would require that the planet is almost tidally locked with it'sits star (but not quite) as it would need to slowly rotate in opposition to its orbit.
This would be a peculiar orbit so it's interesting to think about how that might have happened. One possibility is that a large rougerogue gas giant planet passed through the solar system and disrupted the orbit and rotation. Another idea is that some disruption, such as the rogue gas giant, moved a small rocky planetoid into an orbit around the planet such that the it slowed the planet down.
Another idea is that perhaps the planet was originally in a pole oriented orbit with one pole tidally locked. The civilization could have thrived in the thin strip between the dark and light side. Then at some point a cataclysmic shift occurred (again because of external gravitational forces or even an impact) that shifted the entire orientation of the planet.
