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 original intent of the Drake equation was to estimate the number of extant radio-emitting intelligent species in the galaxy. If the nearest extant radio-emitting intelligent species is in some other galaxy cluster (i.e., hundreds of millions of light years away), it doesn't really matter as far as the Drake equation is concerned. The intent was to find local intelligences, ones we could hear, and possibly talk to. The number of intelligent species in the observable universe, ever, is pushing the Drake equation a bit too far.
Here's my question: why was the probability of one in 10 billion per planet considered highly pessimistic?
Ward and Brownlee, the authors of the rare Earth hypothesis, would beg to differ. They argued that the Drake equation is fundamentally flawed.
Depending on how one spins the numbers, we might be completely surrounded by alien intelligent species that for some reason have left us alone. Or we might truly be alone in the galaxy. Or we might even alone in our galaxy cluster. This wide variation across many orders of magnitude makes the Drake equation non-scientific. It has zero predictive power.
However, going so far as to say that we are the first intelligent species to have occurred across the observable universe? Even Ward and Brownlee would say "no" to that. Their hypothesis is that intelligent life is rare rather than unique.
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)