Awhile back Dr. Hawking said humans had 1000 years to find another planet. NOW he says humans have 100 years to populate a new planet if we are to survive. When do you plan to move and where oh where will you be "out there"?
Yes, he DOES know what he is talking about!
Firstly, he changed his mind! Yes, of course. Professional scientists are usually prepared to change their minds in the light of new knowledge or better techniques, though they are only human so are not always keen to abandon a pet hypothesis. The accusation would be fair if he stuck to a hypothesis later found mistaken by clear evidence.
That is how science works: it observes, postulates, tests, reviews, accepts, adjusts or rejects, and so on. If it had never done so, if scientists had never changed their minds but all been pure dogmatists, we would not be able talk about it like this. We would not even have electricity supplies, let alone computers.
And Dr. Hawking et al are NOT "Idiots", GoofeyHawk. I have not read his paper or book you have used, but since you haven't cited it I can't follow it up. However I would be very surprised if he had suggested planetary colonising as a serious proposition, for the very reasons you give. Instead it is more likely an attention-grabbing metaphor to alert people to what might happen here on Earth, and how long we have to sort things out. If he'd written it in the bland, passive-tense, all-factual and analytical text suited to the learned journal, far fewer non-specialists would read it.
Note that if you had gained your information from a non-scientific source like a popular newspaper, you can't guarantee that report being fair because even if trying to be balanced, most journalists are not scientists, and have a poor understanding of scientific method.
Durdle 