re: How unlikely is a doomsday catastrophe?
On average we are likely to occupy an average position in the universe (see http://www.pthbb.org/manual/services/grim/) so it is probable (but not certain) that for a planet on the periphery of a middling average spiral galaxy annihalation is realtively improbable (but not impossible) -since we are still here and atatistaically we should be average.