I just don't get this train of thought. If the plan is to move our population to another planet where we, A) can't breathe the air, B) have to work to find water to drink, C) need to bring all tools, equipment for farming, life support, etc with us, and D) need to use massive amounts of the fuel (ironically obtained from the very plant we are leaving) just to get there, it seems like a good way to die as a species. I've never been to any other planetary body, but I believe the best place we know of in the universe to "start over", IE, make a sustainable habitat from scratch, is right here beneath our feet.
It's an air gap. Literally and metaphorically. Anything that could take out multiple planets is statistically far less plausible. The lack of dinosaurs running around should be enough to convince one that eventually, something changes the world dramatically enough to kill off the majority of lifeforms.
A, B, C are not entirely correct. There's air, water and metal on other orbital tracks. It just takes some work to make it useful for us. Sending probes to pre-position supplies would be the smart thing. If one was sufficiently ambitious... For the cost of the second Iraq war, we could have sent a good number of robots and some nuclear reactors to Mars to get things kicking. Both poles are primarily water ice, with CO2 coatings. Nitrates have been found in the soil. Have robots build shelter, stockpile water/O2, prep soil for crops, form metals of various kinds, build tools and machines for a decade or two before we send people.
Hard and expensive, definitely. Again, the alternative is eventual species death. AJ mentioned only a handful of the ways humanity could die. Mars is the best choice with near term technology. Mid term, Galilean moons. Longer term tech is Venus.