Are we alone in the universe? This is one of the biggest questions science is trying to answer. Many are inclined to think that there is indeed life beyond Earth. After all, there are billions of galaxies each with billions of stars. Surely, among them, there must be other Earth-like planets and sun-like stars capable of seeding life.

But however groundbreaking finding microbes on another planet would be, that would pale in significance to making contact with another alien civilization.

There’s actually a way to estimate how many alien civilizations may reside in the Milky Way thanks to a statistical model developed in the 1960s known as the Drake equation, named after astronomer Dr. Frank Drake.

What is the Drake Equation

The Drake equation is a probabilistic method for estimating the number of advanced extraterrestrial civilizations (N) that harbor technology capable of communicating their existence.

The Drake equation itself is:

N = R^* x f_p x n_e x f_l x f_i x f_c x L

Where:

• R^* represents the average rate of star formation in our galaxy
• f_p is the fraction of stars that have planets
• n_e is the fraction of planets that orbit their parent stars in the habitable zone, also known as the Goldielocks zone, i.e. can potentially support liquid water at the surface and life
• f_l  is the fraction of planets that could support life and actually do develop life at some point
• f_i is the fraction of planets that harbor life that evolves into an intelligent species capable of founding civilizations
• f_c is the fraction of civilizations that develop technology that emits detectable signals of their existence into space (i.e. artificial radio signals)
• L is the length of time for which extraterrestrial civilizations release detectable signals into space (before they may go extinct, for instance)

The story of how the Drake equation can be traced back to the early 1950s when radio astronomy — the study of celestial objects’ radio frequencies — became more widespread. If they could detect radio signals from pulsars and far-away galaxies, they should also be able to detect artificial extraterrestrial signals, scientists thought at the time.

First, scientists listened to artificial radio signals from Mars. Then, in the late 1950s, physicists Giuseppe Cocconi and Philip Morrison argued in a milestone paper that radio telescopes had become sensitive enough that they could detect radio transmissions from other star systems. The pair of scientists further argued that some of these messages would likely be transmitted at a frequency of 1420.4 Mhz, which corresponds to the wavelength of neutral hydrogen. Since hydrogen is the most abundant element in the universe, it would only be logical for an advanced civilization to broadcast its existence at this frequency to other star systems.

Dr. Frank Drake, a young astronomer at the time, had independently reached the same conclusion as Cocconi and Morrison. In spring 1960, Drake embarked on the first microwave radio search for signals from another solar system, aiming the 85-foot antenna of the National Radio Astronomy Observatory in Green Bank, West Virginia, tuned to 1,420 Mhz, in the direction of two nearby Sun-like stars.

Shortly after, at a meeting at the Green Bank facility in 1961, Drake had a speech in which he revealed for the first time his famous equation as a way to stimulate scientific discussion and interest around the search for intelligent alien life.

“As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it’s going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms,” Drake said.

These pioneering efforts sparked the Search for Extra-Terrestrial Intelligence (SETI) movement spearheaded by Soviet and American scientists. By the late 1980s, large-scale SETI projects were established that examined thousands of sun-like stars at a time, culminating with NASA’s Project Phoenix —  the world’s most sensitive and comprehensive search for extraterrestrial intelligence. Project Phoenix eventually moved to the now-defunct Arecibo observatory, which collapsed last year in Puerto Rico, and scanned nearly 800 stars, all within 200 light-years distance, at frequencies between 1,200 and 3,000 MHz. 

Although SETI scientists came back empty-handed and interest (along with public funding) waned, the search for intelligent life continues. The privately-funded SETI Institute is currently building a dedicated array of telescopes that will equal a 100-meter radio telescope, known as the Allen Telescope Array (ATA). This will be the first radio telescope designed from the ground up for the sole purpose of performing SETI searches. The first 42 elements have already been installed at the Hat Creek Observatory, situated in the Cascade Mountains about 300 miles north of San Francisco.

So how many advanced alien civilizations are out there?

The Drake equation isn’t exactly rooted in hard science and is more of a speculative framework. Over the years, the equation took a lot of flak from the scientific community due to the many assumptions it makes. For instance, the first exoplanet was only discovered in 1992, more than 30 years after Drake proposed his equation.

Although we can estimate some factors of the equation with relatively high confidence — we know for instance that there are about two trillion galaxies in the known universe and that the Milky Way is home to 200 to 400 billion stars — other variables are far more uncertain. In particular, the odds of an exoplanet in the habitable zone actually hosting life is perhaps the most difficult to gauge since we know of only one planet so far capable of doing so, Earth.

Drake’s equation was made famous by the late Carl Sagan, who featured it during an episode of his timeless series Cosmos. But since Sagan first talked about Drake’s equation, much has changed. Thanks to observations by the Kepler telescope, we now have a much firmer grasp of how many Earth-like worlds may be out there.

Crunching the Drake equation with various values, the number N of advanced civilizations in the Milky Way ranges from as low as 0.000000000091 (we are probably very much alone in the Milky Way) to as high as 15,600,000 (the Milky Way is home to millions of distinct intelligent civilizations).

In a 2016 study published in the journal Astrobiology, Adam Frank, professor of physics and astronomy at the University of Rochester, and  Woodruff Sullivan, an astrobiologist at the University of Washington, looked at this question from another angle.

Rather than asking how many civilizations may exist in the Milky Way — the main premise of the Drake equation — the two scientists calculated the odds that humans represent the only technological species that has ever arisen. Flipping the question means that rather than guessing at the odds of advanced life developing, the two calculated the odds against it occurring in order for humanity to be the only advanced civilization in the entire history of the observable universe.

“This shifted focus eliminates the uncertainty of the civilization lifetime question and allows us to address what we call the ‘cosmic archaeological question’—how often in the history of the universe has life evolved to an advanced state?” said Sullivan.

By applying the most recent data on exoplanets at the time, Sullivan and Frank found that the odds that human civilization is unique in the cosmos (2×10²² stars) is about one in 10 billion trillion, or one part in 10²².

“One in 10 billion trillion is incredibly small,” says Frank. “To me, this implies that other intelligent, technology producing species very likely have evolved before us. Think of it this way. Before our result you’d be considered a pessimist if you imagined the probability of evolving a civilization on a habitable planet were, say, one in a trillion. But even that guess, one chance in a trillion, implies that what has happened here on Earth with humanity has in fact happened about a 10 billion other times over cosmic history!”

The Drake equation and Fermi’s Paradox

But if that were true, where are all the aliens? This is the question that physicist Enrico Fermi, the inventor of the world’s first nuclear reactor, asked as well when he posited his famous Fermi Paradox — the notion of how there is a virtually limitless number of stars, but you don’t see much life floating around.

The question is a valid one when considering:

• There’s nothing special about our sun – it’s young, medium-sized and similar to billions of other stars in our galaxy.
• It’s believed there are between 100 and 400 billion planets in the Milky Way. Considering intelligent life appeared on one, it’s reasonable to consider there should be at least some other kind of intelligent life elsewhere in the galaxy.
• Millions of years of technological progress mean that an intelligent species should have the capability to travel to distant stars and even other galaxies. Just look at how our world has changed in the past 100 years alone.
• According to mathematicians Duncan Forgan and Arwen Nicholson from Edinburgh University, self-replicating spacecraft traveling at one-tenth of the speed of light — admittedly a quick speed — could traverse the entire Milky Way in a mere 10 million years. This means that civilization could potentially colonize the whole galaxy in a mere couple of millions of years. Except it didn’t happen.

The most straightforward explanation for this paradox is that the vast majority of these advanced alien civilizations, if not all, went extinct. Perhaps we too will go extinct as fast as we came into this world.

“The universe is more than 13 billion years old,” said Sullivan. “That means that even if there have been a thousand civilizations in our own galaxy, if they live only as long as we have been around—roughly ten thousand years—then all of them are likely already extinct. And others won’t evolve until we are long gone. For us to have much chance of success in finding another “contemporary” active technological civilization, on average they must last much longer than our present lifetime.”

“Given the vast distances between stars and the fixed speed of light we might never really be able to have a conversation with another civilization anyway,” said Frank. “If they were 20,000 light years away then every exchange would take 40,000 years to go back and forth.”

The inevitability of self-annihilation of intelligent life is an opinion shared by scientists at NASA’s Jet Propulsion Laboratory and Caltech who also made their own spin on Drake’s equation in a 2020 study.

The researchers found that life was most likely to emerge around 13,000 light-years from the galactic centers, where there is the greatest density of sun-like stars. The optimal time frame for the development of alien civilizations was estimated at 8 billion years after the formation of the galaxy. For comparison, Earth is about 25,000 light-years from the galactic core and complex intelligent life evolved around 13.5 billion years after the Milky Way formed.

According to the researchers, most civilizations that have appeared before us have likely self-annihilated. Other civilizations that are still active in the galaxy are likely young, due to the propensity of intelligent life to eradicate itself. Over a long enough timeframe, the probability of self-annihilation borders on certainty.

“As we cannot assume a low probability of annihilation, it is possible that intelligent life elsewhere in the Galaxy is still too young to be observed by us. Therefore, our findings can imply that intelligent life may be common in the Galaxy but is still young, supporting the optimistic aspect for the practice of SETI (search for extraterrestrial intelligence),” the authors wrote in their study.

“Our results also suggest that our location on Earth is not within the region where most intelligent life is settled, and SETI practices need to be closer to the inner Galaxy, preferably at the annulus 4 kpc (kiloparsec) from the Galactic Center.”

Other possible explanations for the apparent silence in the universe include the possibility that life is exceedingly rare (let alone the intelligent variety) or that humanity is simply too early to the party. We may be alone but only for the time being.

This article originally appeared in 2021.