Simon (one of the aliens) starts to explain it to Sean (Jason's character): "Sean, what you need to know is, our people were here before. We were here first. This place was our home before it was yours. [...] my people believe that if we stay here, something will happen. To all of us. We call it The Event. It's a rebirth. The next step for our people. We'll change, evolve... into something else. Something greater. But your people won't survive it."

In the end, Sophia ends up bringing their planet into Earth's orbit. And she explains to President Martinez that humankind will basically be slaughtered by her people to make room for the 2 billion aliens that need Earth as their new home. Awesome.