1/5 seems to be the accepted number doing the rounds, but there must be other factors to consider too - I'm sure this number is still overoptimistic? For examp

1/5 seems to be the accepted number doing the rounds, but there must be other factors to consider too - I'm sure this number is still overoptimistic? For example..how many of these habitable exoplanets also do the following earth things 1. Rotate around their axis as opposed to having one side facing the star. 2. have a stabilising moon to stop them from wobbling about. 3. Also how many of these exo solar sytems similarly have heavier elements and have evolved from spent star dust - using spent supernova explosions? To have enough water you need H and enough O Some other important considerations that should perhaps be factored in? The Earth went through a couple of "snowball-earth events" which resulted in a healthy % of oxygen being made available in the atmosphere. We also have an outerlying and right-sized Jupiter to sweep away most of the asteroids and space debris. Even with all the luck in the world, intelligent life only involved in the last million years of our 4 billion year plus Earth history... and we needed an asteroid, the right size again, to wipe out the dominant dinosaurs so that the evolutionary stage was set for our homo sapien species. How much of this is repeatable? We may indeed be the only intelligent life in our galaxy.