Sun. Oct 2nd, 2022
The moon can be seen behind an antenna at the Nancay radio telescope site on October 3, 2019, near Vierzon, central France.
enlarge The moon can be seen behind an antenna at the Nancay radio telescope site on October 3, 2019, near Vierzon, central France.

Guillaume Souvant | Getty Images

In the search for extraterrestrial intelligence, there is really only one big question: Where is everyone? This question has haunted alien hunters since Nobel-winning physicist Enrico Fermi put it to some colleagues over lunch 70 years ago. There are billions of sun-like stars in our galaxy, and we now know that most of them contain planets. But after decades of searching, astronomers haven’t found one that appears to harbor life. This is the so-called Fermi paradox: our galaxy seems to be teeming with extraterrestrial civilizations, but we can’t find any.

Researchers working on the search for extraterrestrial intelligence, or SETI, have proposed a number of solutions to the Fermi paradox over the years. But the most compelling answer is also the most obvious: Perhaps intelligent life is just much rarer than we thought.

How rare? Many scientists have tried to answer this notoriously tricky question. Based on their conclusions, there are between zero and 100 million alien civilizations in the galaxy. That’s not a particularly useful set of estimates, so a few physicists in the UK gave it another try recently and came to a remarkably specific conclusion. As detailed in a new article published this week in the Astrophysical Journalthe duo calculated that there should be at least 36 communicating alien civilizations in our galaxy.

That’s…not much, of course, and it has some depressing implications. According to the paper, this would mean that we would probably have to search hundreds of years for an alien civilization before finding one, and it also suggests that our nearest neighbors could be up to 17,000 light-years away. “We’ve gone from quite optimistic about the existence of life in the universe to a little more pessimistic with the passage of time,” said Christopher Conselice, an astrophysicist at the University of Nottingham and one of the authors of the paper. “I think that’s natural, but now we have the kind of information we need to make real estimates based on reasonable assumptions about how life might form on other planets.”

Attempts to estimate the prevalence of intelligent life in the galaxy date back to the very beginnings of modern SETI. In 1961, just a few months after completing the world’s first radio search for ET, planetary astronomer Frank Drake convened a small gathering of leading American scientists to discuss the future of SETI — or whether it should have a future at all. To organize the meeting, Drake made a list of questions he deemed relevant to determine the odds that the search would be successful.

Some of these questions, such as determining the average rate of star formation in the galaxy and how many stars contain planets, scientists were able to answer before first contact. Others — such as which part of the planets spawn intelligent life and how long that life sends messages into space — could only be guessed at. But Drake realized that if you multiply the answers to these questions, they can be used to get a rough estimate of the number of intelligent civilizations in the galaxy. This formula is known as the Drake equation.

Today, astronomers can confidently fill in some of the blanks in the Drake equation, such as how many stars have planets (most) and the average rate of star formation in the galaxy (a handful per year). And when a new generation of exoplanet telescopes like the James Webb Space Telescope comes online, we’ll also have a better idea of ​​how many of these planets are in their star’s habitable zone. This means that liquid water could exist on those planetary surfaces, which as far as we know is a prerequisite for life – intelligent or otherwise.

A bit of a guessing game

But “as far as we know” is exactly the problem with the Drake equation. The number of communicating alien civilizations in our galaxy is a statistical estimate, and like all statistical estimates it can vary widely depending on the assumptions used to make it. In the Drake equation, about half of the unknowns are about alien civilizations. Since we don’t know anything about ET, astronomers have to make some guesses. And in their new paper, Conselice and his colleague, University of Nottingham engineer Tom Westby, make two very big assumptions when reworking the Drake equation.

First, the researchers looked at the only planet we know for sure spawned intelligent life — our own — and used it as a model for any other planet that could harbor extraterrestrial intelligence. Humans showed up and started spewing radio waves into the cosmos about 4.5 billion years after Earth formed, so Conselice and Westby assumed this would be the case on other Earth-like planets as well. But they went further and assumed that all Earth-like planets in their star’s habitable zone inevitably produce intelligent life after about 5 billion years.

“To say that all Earth-like planets will produce intelligent life is a huge assumption and has some serious problems,” said Seth Shostak, senior astronomer at the nonprofit SETI Institute in California. “The habitable zone of our own solar system includes Mars and — depending on who you ask — Venus. But they aren’t populated by intelligent creatures, even if they’ve been sitting around as long as Earth.”

One way statisticians learn about a large, unknown population is by taking a small sample and extrapolating to the larger population. This is essentially what Conselice and Westby did in their paper. The problem is, they’re extrapolated from a sample of one, which is a bit like trying to predict a national election by just examining yourself. Small sample sizes lead to greater variation in results, which is why the Drake equation reliably produces such vastly different estimates of the prevalence of extraterrestrial intelligence. This was even demonstrated by Conselice and Westby in their own paper.

Two hypotheses

The researchers put forward two hypotheses: one strong and one weak. In the strong hypothesis, the researchers assume that an Earth-like planet should produce an intelligent species when it is between 4.5 billion and 5.5 billion years old. That’s how it went on Earth, where people made the technology their own after about 4.5 billion years. The weak hypothesis relaxes the time frame a bit and assumes that an Earth-like planet could be producing life anytime after 5 billion years. Since the average age of stars in the Milky Way is about 10 billion years old, this creates a larger pool of alien societies that could still exist today. (This assumes alien societies won’t last 5 billion years — more on that later.)

The strong hypothesis results in an estimate of at least 36 alien civilizations in the galaxy, but with a very large margin of error. The researchers calculate that the lower bound of the strong hypothesis could be between four and 211 alien civilizations in the Milky Way. Things are more hopeful with the weak hypothesis, which estimates that the lowest number of possible alien societies is somewhere between 100 and 3,000.

That’s a pretty wide spread, but even the most optimistic lower bound of 3,000 societies is still quite small given the size of the Milky Way. If most of the galaxy’s 250 billion stars contain planets, and some of those planets are habitable, you could still estimate that there are millions of civilizations. So why do both the strong and weak hypothesis both produce such small estimates? It all comes down to one key variable in the Drake equation: lor the longevity of a broadcasting civilization.

N = L

“The lifespan of an alien intelligence — how long they stay in the air — is the real crux of the argument,” Shostak says. “All the other terms in the Drake equation tell you how many societies there are, but maybe they won’t be on the air for long because once they invent the radio, they’ll invent the H-bomb and self-destruct as well.”

Indeed, Drake famously reduced his namesake formula to “N=L”, or the number of civilizations in the galaxy is equal to the lifespan of those civilizations. (He even put this equivalence on his license plate.) So the longer you expect an intelligent species to last, the more you should expect to find. Humans have only had radio technology for about 100 years, and in that time we have also created existential threats, such as nuclear war and climate change. How long will human civilization last? It depends on how we deal with the dangers we pose to ourselves.

In their paper, Conselice and Westby take the pessimistic route for their second major assumption. In both their strong and weak hypotheses, they assume that all alien civilizations will only broadcast their existence to the galaxy as long as humans have had radio — about 100 years. This basically assumes we’re on the brink of destruction, but choosing a different estimated lifespan for alien civilizations would also be completely arbitrary. Do intelligent civilizations tend to last 500 or 10,000 years? The only way to know for sure is to find one.

“Humans have had technology for a very, very short time relative to the age of our galaxy,” said Andrew Siemion, the director of the Berkeley SETI Research Center, who was not involved in the study. This means that if – or when – we make contact with an alien society, chances are it is much older than ours. “It would tell us that it’s possible for a technologically capable civilization to last a long time,” Siemion says. “That’s very heartwarming.”


Siemion is the lead scientist on the Breakthrough Listen project, the largest SETI experiment ever undertaken. Breakthrough Listen has been funded at the cost of $100 million by Russian-born billionaire Yuri Milner and in recent years has used some of the most powerful radio telescopes on Earth to scan the cosmos for signs of life. If we’re ever going to find aliens, Siemion and his colleagues are the ones likely to make it happen. While he acknowledges that trying to estimate the prevalence of alien life is an interesting thought experiment, there’s no way to tell if one estimate is better than the other until we connect.

“No pre-argument should serve as a substitute for an experimental program,” says Siemion. “The only way we will ever answer this question is by looking for extraterrestrial intelligence.”

This story originally appeared on

By akfire1

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