The chemical reactions that give rise to life need enough liquid water and not too much ultraviolet light, which seems to be a rare combination in the galaxy Source: https://www.newscientist.com/article/2197406-weve-found-4000-exoplanets-but-almost-zero-are-right-for-life/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home