I think we must all respect Stephen Hawking. He is a very brilliant scientist. That being said, I don't think it's ever good for any scientist to get into these hyper ling-term predictions. There are so many things that can happen to a planet over that period of time. Heck, civilization has only been around for about 5000 years. There's nothing known of to compare our species to relative to intelligence and means, in terms of the ability to survive on a planet on that time-scale. You would have to be fairly certain that there will be some event that will make life actually impossible on the planet to assert 1000 years as a benchmark for "must leave the planet". Unless you're talking about space colonies, it's not even all that likely we'll have anywhere to feasibly get to in that short a time span in terms of a livable planet to settle on. I agree with the idea in principle Hawking is putting forward, but I guess I take issue with his supporting assertion, or I guess the way he makes his case.