What We Know About the Drake Equation

A spray painted exoplanet scene created by my astrophysics students, summer 2022

Written by Lily M.

In 1961 an astronomer named Frank Drake created the Drake Equation. He created it for a way to understand the factors involved in finding life outside our Earth in our galaxy and to roughly calculate the amount of life that can communicate with us. The equation he developed is the following:

N = R* • fp • ne • fl • fi • fc • L

It is a complicated equation and contains quite a few variables.

N means the number of extraterrestrial civilizations we as humans can communicate with.

R* is the rate of star information, which we now know to be about 1.5 stars per year.

fp is the number of stars that have planets. When our Sun was born our solar system organized as a natural consequence with planets forming inside eddies of the solar accretion disk. Astronomers feel that this process would occur around other stars as they formed, and that on average every star should have at least one planet, so this factor approaches 1 or 100%.

A painting of the Epsilon Indi system. An orange dwarf star is orbited by two brown dwarfs which revolve around each other

ne means the number of planets per stars that could possibly support life, or be like Earth. ne is even more difficult to figure out then R* and fp,but was assumed by Drake to be around 3, since there are three planets in our solar system that are or were within the habitable zone.

fl is the fraction of planets that support life and can develop it. With the different types of extremophiles we have found on Earth it looks like life could exists in very hostile environments. This means that fl approaches 1 or 100% with enough time.

fi is the fraction of life-bearing planets that have intelligent life on them, finding out the value of fi is really hard since we only have one example: humans. Some anthropologists still argue on why one of the branches of the ape family evolved into the human species. We don’t really know if this is really unavoidable or just a coincidence. Scientist think that there are other species that could potentially evolve into intelligent beings such as dolphins and chimpanzees. Scientist think that if life could evolve then intelligent life could too, given enough time, since it has happened on Earth and there is no reason to think Earth is a special case.

fc is that fraction of planets with intelligent life where that life is willing and able to communicate with us across interstellar space. Scientist believe that if there is intelligent life out in our galaxy that they will eventually advance to a technology level to send out a signal into space. Knowing the human species, we can only guess that if there is another human-level species they would have either deliberately or accidentally shown that they exist by now. Which could mean that fc could be as high as 1.

A system of exoplanets with two gas giants and a habitable moon

Finally, the last letter L means the time span that the other life can communicate with us. The Earth’s population has grown so much over the past few centuries that the human species has the potential to destroy itself with man-made catastrophes, wildlife disasters, diseases, and nuclear war. If we all start to have a pessimistic view on this then we cannot be sure that human race will live through to the next century, that we will destroy ourselves and our advancement in technology will only have existed for a couple of centuries. It is difficult to say if another species will have the same rate of technological advancement or will have the same tendencies for self-destruction. Scientist think that L is approximately 200 to 10,000 years.

Putting in all the estimates for these variables we come up that N would be about 50, or 50 communicating civilizations per year would arise in our galaxy. The Drake Equation does not have a concrete solution yet because the last four variables are very hard to figure out. You may also have heard of the Drake Equation as the Green Bank equation or Green Bank formula, as the first meeting of astronomers interested in SETI, or the Search for Extra-terrestrial Intelligence, occurred at the Green Bank radio telescope facility in West Virginia. Carl Sagan was one of the astronomers at that meeting, and brought up the Drake Equation in his television series Cosmos. The Drake Equation has been a very popular equation to help us identify the factors we need to solve to look for intelligent life in our galaxy.

An orange dwarf star orbited by a blue gas giant with two red moons

About davidvblack

I teach courses in multimedia, 3D animation, Earth science, physics, biology, 8th grade science, chemistry, astronomy, engineering design, STEAM, and computer science in Utah. I've won numerous awards as an educator and am a frequent presenter at state and national educator conferences. I am part of the Teachers for Global Classrooms program through the U.S. Department of State and traveled to Indonesia in the summer of 2017 as an education ambassador. I learned of the Indonesian education system and taught classes in astronomy and chemistry at a high school near Banjarmasin in southern Borneo. I am passionate about STEAM education (Science, Technology, Engineering, Arts, and Mathematics); science history; photography; graphic design; 3D animation; and video production. This Spaced-Out Classroom blog is for sharing lessons and activities my students have done in astronomy. The Elements Unearthed project (http://elementsunearthed.com) will combine my interests to document the discovery, history, sources, uses, mining, refining, and hazards of the chemical elements.
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