How Many Aliens Are There In The Galaxy? Astronomers Turn To “Bayesian Statistics”

In recent years, some astronomers have tried to use a complex form of analysis called Bayesian statistics to provide more empirical evidence for the existence of extraterrestrial life and intelligence in the universe.

They focused on two big unknowns: the probability that life on Earth-like planets emerges from an inanimate environment, a process known as the origin of life, and the probability that intelligence emerges from it. Even with such assessments in hand, astronomers are divided over what they mean for life elsewhere in the universe. The lack of consensus is due to the fact that even the best Bayesian analysis can only do so much with so little hard evidence of extraterrestrial life and intelligence.


Answering questions about the origin of life and the probability of intelligence is difficult because scientists have only one message: life on Earth. “We don’t even have a complete data point,” says David Kipping, an astronomer at Columbia University in the US. For example, we don’t know when life appeared on earth. Even that is uncertain.”

Bayesian analysis is based on Bayes’ theorem, named after Thomas Bayes, an 18th-century English statistician. To calculate the probability of an event, such as the origin of life, astronomers first propose a possible probability distribution. For example, we can assume that life originated with the same probability between 100 and 200 million years after the formation of the Earth, 200 to 300 million years after the formation of the Earth, and any other 100 million years in earth’s history. This assumption is known as Bayesian prior information. Statisticians then collect data or evidence. Finally, they combine prior information with evidence to calculate what is known as a posterior probability. In terms of the origin of life, this probability is the probability of life on an Earth-like planet based on our previous assumptions and evidence. The posterior probability is not a single number but a probability distribution that quantifies any uncertainty. For example, it might show that the probability of the origin of life increases or decreases over time, rather than having an equal probability distribution as prior information suggests.

Kipping assessed the probability of the origin of life and the probability of intelligence. For prior information, he chose the Jeffreys Prior designed by Harold Jeffreys, another British statistician and astronomer. It is said that the Jeffreys priori does not provide information to the greatest extent because it does not incorporate a large number of hypotheses and places more emphasis on evidence.


In the calculation of kipping, this prior information focuses on what he calls the “four angles” of the parameter space: life is common, intelligence is common; Life is common, intelligence is rare; Life is rare, intelligence is common; Life is rare, intelligence is rare. Before bayesian analysis began, the probability of all four angles was the same.

Kipping combined the Jeffries priors with anecdotal evidence of the emergence of life and intelligence on Earth to produce a posterior probability distribution that allowed him to calculate new probabilities for the four corners. For example, he found that “life is common and intelligence is rare” is nine times more likely than “both life and intelligence are rare”. The ratio of “life is common and intelligence is common” to “life is rare and intelligence is common” is as low as 9 to 1.

Kipping said the calculations “suggest that extraterrestrial life should exist.” “This at least suggests that life is not a difficult process,” he said.

Not all Bayesian statisticians agree. And not only they, any person interested in the origins of life issues will doubt base flat answers, because any of such analysis is subject to related to life on earth of geology, geophysics, paleontology, archaeology and biological evidence, the evidence is not clearly show the origin of life a time line and smart.

Caleb Schaaf, an astronomer and astrobiologist at Columbia University, said: “We are still trying to define what we mean by living systems. This is problematic when it comes to making assertions about when the origin of life occurred, or even when intelligence evolved.”