Dawn of Extinction

The plot of this book is about aliens communicating with earth people and trying to make atomic energy to power their spaceships and eventually make an atomic bomb.
"I know it seems weird but that explains why the caretakers, who were really aliens looked like us," Bill answered. "Somehow they have learned how to take a human body and transform their bodies into ours".
Bill discovers a way to exterminate them, and just as he thinks they have been eradicated, he is beamed up by a spaceship.
The alien introduces himself as Bodar and begins by saying, "We are facing a desperate situation because our sun is not putting off enough energy". "Next to the planet Juno on one side is Zenok and on the other side is Alexis." "This planet is closer to the sun." "Since we formed an alliance with the planet Zenok, the two planets have decided to take over the planet Alexis."
With this introduction, Bill finds that they need him to create an atomic bomb. To persuade him they tell him his three children's bodies were taken from the earth to the planet Zenok to be changed into aliens. They will help him get them back if he helps them.
Who is going to be extinct? You find this out at the end of the book.