mozz: "Why must they? They could be on planets which have existed long before Earth did. " - no, do you understand the concept of 2nd gen star? Planets need heavy elements, they are created by stars, QED, all the material that is not hydrogen was created in a star, all the elements in us, and our planet were created in a star that lived and died, shedding it''s elements through this part of the galaxy. So stars have had to live and die at least once to create what we have, thus our star is 2nd gen. Any aliens out there and there are lots, would have had the same parameters thrust upon them, they would have had a similar time in galactic terms to evolve. The distances are too vast even for light speed communication, we or they could not build a transmitter/receiver to communicate even with the closest star and that's to your garden gate in galactic terms.