People have been asking me why I put the planet earth in the background. And the reason behind it was because according to the plot, humans left "Earth" and went to another planet. I assumed in order for the new planet to be inhabitable, there would have to be water on the new planet.
The planet in the image compels the viewer to wonder what exactly is going on and want to see the movie. A viewer may look at the poster, not knowing the storyline/plot, and see the damage buildings and see the planet in the sky and wonder what happened or whats going on.