why was the probability of one in 10 billion per planet considered highly pessimistic by a recent study on aliens? A recent article about the existence of extraterrestrial life claimed that "unless the probability for evolving a civilization on a habitable-zone planet is less than one in 10 billion trillion, then we are not the first."
It continues to say that "a probability for civilizations to form of one in 10 billion per planet was considered highly pessimistic."
Here's a link to the actual study. The author of the article is a coauthor of the study.
Could someone go through the study and explain the source of the probabilities and their assessments (optimistic vs pessimistic.)
The main reason for this is the new data we aquired with the Kepler telescope and our findings about the goldilocks zone (where temperature is right for water to be liquid on the planet).

In the last decade a lot of discoveries, like the highly likely liquid water on the moon europe (which is way outside of the goldilock zone), the findings of amino acids in stelar clouds, etc, lead to the understanding that the chemical ingrediants and the chemical processes for life are possible in way more places than we thought.

Adding to this, the data from the Kepler telescope suggests that around 25% of all solar systems have a potentially habitable planet in some way (which makes that around 60-100 billion potential habitable planets in the milky way alone)