# why was the probability of one in 10 billion per planet considered highly pessimistic by a recent study on aliens? [closed]

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 numbers in the article that attempt to summarise the paper are wrong and don't appear in the original paper. The one in ten billion being pessimistic is a matter of opinion. – Rob Jeffries Jun 16 '16 at 11:57
• The first quoted statement is a misapplication of probability and expected values. Statistically we would expect there to be/have been more than one planet with a civilization on it, but that says next to nothing about how many there are/have been in reality. The odds could say that 99% of planets develop an intelligent civilization, and we could still be the first. It'd be (extremely) unlikely, but not impossible. We can't prove we aren't the first until we find a planet with a civilization that conclusively predates our own. – zibadawa timmy Jun 16 '16 at 12:39
• @Rob Jeffries Why do you say they are wrong? Doesn't the author seem to try to prove his entire point with those numbers? – user8669 Jun 16 '16 at 12:47
• The great thing about the Drake equation is that you can plug in your own numbers and get different answers. While I wouldn't want to guess those numbers myself, I think it's probably a fair assumption that life on planets might be common, but technological life, quite a bit less so. There's still a great deal we don't know about this though. I think their numbers are probably fair, but very different numbers and conclusions might be fair too. My layman's answer, their estimate is conservative but reasonable. – userLTK Jun 16 '16 at 12:53
• Thank you @userLTK. I would prefer, however, a reasonable explanation of how they got to those figures. – user8669 Jun 16 '16 at 13:00

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.

• Thank you for that insight. However flawed the Drake equation may be (with regards to prediction,) I am still interested in knowing how the authors of the study determined which probabilities are pessimistic and certain ones are not. – user8669 Jun 17 '16 at 15:23

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)

• Thanks for the context on recent findings. The trouble is, that doesn't make the probability figures cited by the article any more or less true. – user8669 Jun 16 '16 at 12:56
• @user8669 Well any assumption on inteligent life up to this date is just an assumption, we can't know since we don't have a big enough data set (we have none) of intelligent alien species. This is all highly speculative. What I tried to say is that recent findings during the last decade or so showed that the possibility might be a lot higher than previously thought. – RononDex Jun 16 '16 at 13:03
• @user8669 also the calculations and data they used are right in that publication you linked – RononDex Jun 16 '16 at 13:07
• @RonenDex do you understand their calculations? I agree that everything is highly speculative, which is why I am all the more curious how they got those numbers. – user8669 Jun 16 '16 at 15:13
• I'd also wonder why the news article's title sounds so assured (science should not be about sensationalism.) – user8669 Jun 16 '16 at 15:17