In a new study published this week in the Proceeding of the National Academy of Sciences, Dr. David Kipping of Columbia University and Flatiron Institute used a statistical technique called Bayesian inference to estimate the odds of complex life and intelligence emerging beyond Earth.
A fundamental question to modern science concerns the prevalence of life, and intelligence, within the Universe.
Searches within the Solar System have not yielded any direct evidence for extraterrestrial life, and the remote detection of chemical biomarkers on exoplanets remains years ahead of present observational capabilities.
The search for intelligence, through the signatures of their technology, may be detectable under certain assumptions and limited observational campaigns have been attempted. However, the underlying assumptions make it challenging to use these null results to directly constrain the prevalence of life or intelligence at this time.
“The rapid emergence of life and the late evolution of humanity, in the context of the timeline of evolution, are certainly suggestive. But in this study it’s possible to actually quantify what the facts tell us,” Dr. Kipping said.
To conduct his analysis, Dr. Kipping used the chronology of the earliest evidence for life and the evolution of humanity.
The astronomer asked how often we would expect life and intelligence to re-emerge if Earth’s history were to repeat, re-running the clock over and over again.
He framed the problem in terms of four possible answers: (i) life is common and often develops intelligence; (ii) life is rare but often develops intelligence; (iii) life is common and rarely develops intelligence; and (iv) life is rare and rarely develops intelligence.
This method of Bayesian statistical inference — used to update the probability for a hypothesis as evidence or information becomes available — states prior beliefs about the system being modeled, which are then combined with data to cast probabilities of outcomes.
“The technique is akin to betting odds. It encourages the repeated testing of new evidence against your position, in essence a positive feedback loop of refining your estimates of likelihood of an event,” Dr. Kipping said.
From these four hypotheses, the scientist used Bayesian mathematical formulas to weigh the models against one another.
“In Bayesian inference, prior probability distributions always need to be selected,” he said.
“But a key result here is that when one compares the rare-life versus common-life scenarios, the common-life scenario is always at least nine times more likely than the rare one.”
His analysis is based on evidence that life emerged within 300 million years of the formation of the Earth’s oceans as found in carbon-13-depleted zircon deposits, a very fast start in the context of Earth’s lifetime.
“The ratio is at least 9:1 or higher, depending on the true value of how often intelligence develops,” he said.
“If planets with similar conditions and evolutionary time lines to Earth are common, then the analysis suggests that life should have little problem spontaneously emerging on other planets.”
“And what are the odds that these extraterrestrial lives could be complex, differentiated and intelligent? Here, my inquiry is less assured, finding just 3:2 odds in favor of intelligent life.”
This result stems from humanity’s relatively late appearance in Earth’s habitable window, suggesting that its development was neither an easy nor ensured process.
“If we played Earth’s history again, the emergence of intelligence is actually somewhat unlikely,” Dr. Kipping said.
“The odds in the study aren’t overwhelming, being quite close to 50:50, and the findings should be treated as no more than a gentle nudge toward a hypothesis.”
“The analysis can’t provide certainties or guarantees, only statistical probabilities based on what happened here on Earth.”
“Overall, our work supports an optimistic outlook for future searches for biosignatures,” he said.
“The slight preference for a rare intelligence scenario is consistent with a straightforward resolution to the Fermi paradox. However, our work says nothing about the lifetime of civilizations, and indeed the weight of evidence in favor of this scenario is sufficiently weak that searches for technosignatures should certainly be a component in observational campaigns seeking to resolve this grand mystery.”