In the end, it's not the aliens or the planet itself (which we're also destroying) that will take out humanity. We're doing a pretty good job of attacking ourselves, thanks to weight-related illness and complications.
Hawking's message comes in an ad posted by the Swedish health organization GEN-PEP. In it, the legendary scientist warns: "At the moment, humanity faces a major challenge, and millions of lives are in danger. As a cosmologist, I see the world as a whole, and I am here to address one of the most serious public health problems of the 21st century. Today too many people die from complications related to overweight and obesity. We eat too much and move too little."
Not only is it a major problem, but it baffles him. "For what it's worth, how being sedentary has become a major health problem is beyond my understanding," he adds. Our laziness and the subsequent self-induced epidemic has even stumped Stephen Hawking, who, again, has forgotten more about the physical structure of the universe than you'll ever know.