How can we approach numbers on an incomprehensible scale like the number of civilizations in our galaxy? The Drake Equation, devised by the late astronomer Frank Drake, provides a clue.
The equation above might look like gibberish at first glance, but each component has its own function. The variables(letters in the equation) are like puzzle pieces, standing for a certain value. Plug in all of the values and you get N, the number of extraterrestrial civilizations in the Milky Way galaxy. Theoretically.
The Drake Equation is the brainchild of Dr. Frank Drake. He recently passed away at the age of 92 after a lifelong career of searching for intelligent species in our galaxy. In honor of his legacy and contributions to SETI, we'd like to highlight his most famous creation that has shaped the philosophy of SETI.
A Closer Look
Let's take a dive into the parts of the Drake Equation. Here it is again so that you can refer back to it with each step.
What is the definition of each constant?
N is the number of detectable civilizations in the Milky Way galaxy.
R* is the rate of formation of stars in the galaxy in number per year.
fp is the fraction of the stars with a planetary system.
ne is the number of planets per system suitable for life.
fl is the fraction of suitable planets where life appears.
fi is the fraction of planets with life that produce intelligent civilizations.
fc is the fraction of civilizations that send out signals.
L is how many years civilizations send out signals.
If you set this equation up in a fraction format, you can see that every unit can be cross-canceled out in the numerator and denominator. In other words, if you multiply all the values and cancel out the units, you're left with:
Number of detectable civilizations(N) = ?????
Pretty exciting!
Concessions
There are, of course, practical limitations to using this equation.
First of all, we may never be able to find all the variables within the lifetime of human civilization. The galaxy is mind-numbingly expansive, and the variables get increasingly more difficult to find as we move to the right of the equation. Even the leftmost value(rate of star formation) is not yet exactly known, as the galaxy has fluctuating activity levels in different regions and over time(Plaxco).
There's also a more exotic question: what is an environment 'suitable for life'? The habitable zone is defined as the distance from a star where water can remain liquid on a planet's surface(Brennan). But with the hundreds of billions of stars in the galaxy, it might be conceited to assume that intelligent life has the same biological needs terrestrial life does.
Closing Thoughts
The Drake Equation is, essentially, a Fermi Problem: a method to estimate an extreme number's order of magnitude.
It teaches us how to approach the kind of topics that are hard to wrap your head around. Instead of shying away from the big numbers, we can logically estimate very large(or very small) numbers by using what we know and a few educated guesses. Science is all about making hypotheses and revising them as we go along the scientific process.
We can honor Frank Drake's memory and his fight for SETI by staying curious about our place in the universe and all of its possibilities. Are there intelligent civilizations like us looking up at the night sky? If so, how many are transmitting, hoping to reach another lonely island of life in the galaxy? These are the kinds of questions we can work towards answering by applying this mindset.
That's all for today. I hope you enjoyed today's article, and works related to SETI are listed below as always. I'll see you in the next one!
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Bibliography
Brennan, Pat. “The Habitable Zone.” NASA, NASA, 2 Apr. 2021, https://exoplanets.nasa.gov/search-for-life/habitable-zone/#:~:text=The%20definition%20of%20%E2%80%9Chabitable%20zone,nor%20too%20cold%20%E2%80%93%20for%20life.
Plaxco, Jim. “Drake Equation Tutorial.” Astronomical Adventures, Astrodigital, https://www.astrodigital.org/astronomy/drake_equation.html.
Shostak, Seth. “Drake Equation.” SETI Institute, SETI Institute, July 2021, https://www.seti.org/drake-equation-index.
Image by Mohammad Alrohmany, distributed under a CC BY-SA license.
Image by Raphael Perrino, distributed under a CC BY 2.0 license.
Other works about extraterrestrial civilizations:
The Three-Body Problem trilogy by Cixin Liu
Extraterrestrial by Avi Loeb
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