(CNN) – It's a question science fiction writers asked long before man could fly: Are we alone? And if we're not, are we in trouble?

The cosmologist Stephen Hawking is issuing a warning in a new documentary, saying that we are not alone, that there may be hostile life out there, they may be hungry, and we should be careful about going out looking for them.

NASA just announced it's going ahead with a new plan to search for extraterrestrial life. It includes sending a lander to Mercury and exploring the moons of Jupiter and Saturn.

James Green is the director of NASA's planetary science division and he joined us on Thursday's American Morning to discuss the possibilities of life beyond Earth.