Craig Krueger, Sojourn Campus Church
"Now it is true that 40 billion planets in the Milky Way have sufficient liquid water to support biological life as we know it. But what of the other necessary zones? Let's take the photosynthetic zone as an example. Without photosynthesis, only microbes can exist; no plants or animals can survive.
For photosynthesis to occur, seven factors must be fine-tuned: light intensity, ambient temperature, carbon dioxide concentration, regularly occurring season variation, mineral availability, liquid water supply and atmospheric humidity (for land based life). These seven factors alone eliminate all known star planet systems for habitability except our sun earth system! And there are six other zones to explore!"