Step outside on a clear night and gaze up at the night sky. Depending on the sky conditions where you live, you may see as many as 2,000 stars.
That’s just a tiny fraction of the Milky Way galaxy, which may have between 100 billion to 1 trillion stars. Most of these stars host exoplanets, and we’ve already zeroed in on a few that may be Earth-like.
So where are all the aliens? This is a key question in the field of SETI, the Search for Extraterrestrial Intelligence. While SETI includes everything from listening for radio signals to examining odd fluctuations in starlight, theoretical work in the field has been dominated by two key concepts: the Fermi paradox and the Drake equation.
The Fermi paradox ponders why Earth has not been visited by aliens, while the Drake equation tries to estimate the number of intelligent civilizations in our galaxy. Both concepts involve a lot of uncertainty, because when it comes to extraterrestrial life, there’s a lot we don’t know.
“These two tools are foundational in the sense that they were made towards the beginning of the field and they are rather profound, but not foundational in the way, say, Newton's Laws are,” said Jason Wright, a SETI researcher at Penn State University.
What is the Fermi paradox?
The Fermi paradox is named after Enrico Fermi, a scientist best known for overseeing construction of the world’s first nuclear reactor and using it to conduct the first controlled nuclear reaction in 1942.
During a 1950 visit to the Los Alamos National Laboratory in New Mexico, Fermi and some colleagues were discussing extraterrestrials and interstellar travel over lunch, in conjunction with a cartoon from “The New Yorker” showing aliens stealing New York City trash cans.
As the story goes, Fermi famously asked, “where is everybody?” Three people who were part of the discussion later reported that Fermi was specifically talking about interstellar travel: If aliens exist and are capable of flying between the stars, then they should have visited us already.
Fermi never published any work on his off-the-cuff remark, and died just four years later. However, his question lived on and became known as the Fermi paradox.
What is the Drake equation?
The Drake equation is named after Frank Drake, an astronomer who in 1960 led the first official search for extraterrestrial radio signals at the National Radio Astronomy Observatory in Green Bank, West Virginia.
During a followup meeting with an eclectic group of thinkers at Green Bank in 1961 that included Planetary Society co-founder Carl Sagan, Drake introduced a formula that could conceivably calculate the number of civilizations currently transmitting signals out into the Milky Way:
N = R* × fp × ne × fl × fi × fc × L
N, the number of civilizations currently transmitting signals, depends on seven factors:
R* is the yearly formation rate of stars hospitable to planets where life could develop
fp is the fraction of those stars with planets
ne is the number of planets per solar system with conditions suitable for life
fl is the fraction of planets suitable for life on which life actually appears
fi is the fraction of planets with life on which intelligent life emerges
fc is the fraction of planets with intelligent life that develops technologies such as radio transmissions that we could detect
L is the average length of time in years that civilizations produce such signs
“The Drake Equation goes in order from easiest to hardest,” said Kaitlin Rasmussen, an astrophysicist at the University of Michigan. While variables like L remain purely speculative, scientists can now answer with some certainty things like average star formation rates in the Milky Way, and the fraction of stars with planets.
Although some researchers have tried to estimate the number of habitable planets using statistics, Rasmussen is hopeful that upcoming generations of large telescopes will allow us to peer into the atmospheres of Earth-sized planets, giving us better estimates on the number of planets per solar system with conditions suitable for life.
Are the Fermi paradox and Drake equation still relevant?
More than half a century has passed since Drake proposed his famous equation, while it’s been over 70 years since Fermi asked where all the aliens are. Are these questions still the best way to think about intelligent life beyond Earth?
There are a myriad of possible solutions to Fermi’s paradox that have been proposed over the years. Perhaps aliens already visited Earth in the past. Interstellar travel may be either impossible or impractical. Or we may be all alone.
Another possibility is the so-called Great Filter: Intelligent species may run up against barriers like climate change or nuclear war that keeps their lifespans short and prevents them from spreading throughout the galaxy. Whether or not Earthlings have evolved beyond such a filter is anyone’s guess.
In a paper outlining the current state of SETI research, Wright says that while he believes theoretical work on the Fermi paradox is still important, “I fear we have approached a point of strongly diminishing returns that will persist until a detection is made.” He advises any would-be Fermi theorists to “stay close to the data.”
As for the Drake equation, any attempt to solve it requires guesswork for many of the variables, leading to wildly different results.
“I would say that its main drawback is that it cannot account for things which we have not even thought to consider,” said Rasmussen. For instance, a carbon dioxide-choked world like Venus could host life that’s completely different from Earth-based organisms, or what looks like a cluster of habitable exoplanets could be affecting each other’s orbits in unpredictable ways.
While some scientists have proposed tweaks to the Drake equation, the concept of using a mathematical formula to calculate the odds of life on other worlds still has merit. As the legendary SETI scientist Jill Tarter once put it, the equation is “a wonderful way to organize our ignorance.”