And they succeed, on paper:
The researchers worked with a few different prior distributions for this probability. They also assumed that intelligence took some fixed amount of time to appear after abiogenesis.
Given such assumptions, the geophysical and paleontological evidence of life’s genesis on Earth and what evolutionary theory says about the emergence of intelligent life, Turner and Spiegel were able to calculate different posterior probability distributions for abiogenesis. Although the evidence that life appeared early on Earth may indeed suggest abiogenesis is fairly easy, the posteriors did not place any lower bound on the probability. The calculation “doesn’t rule out very low probabilities, which is really sort of common sense with statistics of one,” Turner says. Despite life’s rapid emergence on Earth, abiogenesis could nonetheless be an extremely rare process.
Turner and Spiegel’s effort was the “first really serious Bayesian attack on this problem,” Kipping says. “I think what was appealing is that they broke this default, naive interpretation of the early emergence of life.”
Even so, Kipping thought the researchers’ work was not without its weaknesses, and he has now sought to correct it with a more elaborate Bayesian analysis of his own. For instance, Kipping questions the assumption that intelligence emerged at some fixed time after abiogenesis. This prior, he says, could be another instance of selection bias—a notion influenced by the evolutionary pathway by which our own intelligence emerged.
Anil Ananthaswamy, “How Many Aliens Are in the Milky Way? Astronomers Turn to Statistics for Answers” at Scientific American
We are left with a calculation that is “ “a positive sign that life should be out there,” he says. “It is, at least, a suggestive hint that life is not a difficult process.” As if they didn’t all believe at least that already.
Paper. (open access)
In the real world, we are hoping to find fossil bacteria on Mars. There may or may not be fossil bacteria on Mars but the search feels more like science.
As for the aliens, of course, They’re Out There. They’ll always be Out There. As long as there’s an Out There out there, there they’ll be. And there will always be an explanation why they never write, never phone… Can’t fight it. Why bother?
See also: Tales of an invented god