What is the importance of Ukbar in the novel, The Land Without Stars (Valerian)?

Ukbar is a small solar system that consists of four planets as the novel opens. Valerian and Laureline visit each of the four. At the first three, they congratulate them on their recent successful colonization. While they are at the final of the four planets, they learn about Zahir, which is the planet that is about to strike Ukbar. As the novel comes to an end, that planet falls into orbit, making the Ukbar system a system of five planets.