Just visiting

The recognition of these hazards may help us understand deeper mysteries about Life in the Universe. Many explanations have been offered as to why we find no evidence for the existence of advanced extraterrestrial life in the nearby Universe.  Perhaps we are too uninteresting to be worth contacting; perhaps life requires extremely improbable events to sustain it. More likely, I feel, is that life never survives for long periods. Asteroidal impacts, passing comets, bursts of gamma radiation, all these external hazards are common occurrences. We are shielded from many of them by the planet Jupiter and our large Moon. Without these gravitational shields we would have suffered a string of catastrophic impacts that would have continually reset the evolutionary clock. Couple with the threat to life offered by internal hazards like war, disease and environmental disaster, we begin to see that it is perhaps not entirely surprising that no one is 'out there' in our part of the Universe. The cosmic environment stretches far wider than Darwin ever imagined.

-- from 'Cosmic Environmentalism' in The Artful Universe (Expanded) by John Barrow