Taking an astronomy class (something very far outside of my major!) has presented some unique challenges, including but not limited to: reaching into my memory for math and physics principles I haven’t used since high school as a college junior, and a workload much heavier than what I was prepared for. But it’s also been an incredibly rewarding experience so far, and as the semester progresses the class is finally starting to dive into some of the deeper questions about space. How can our telescopes see so much? How hot are stars? What are black holes? Which movies accurately depict space and its principles and which don’t? All of these questions are important, but my favorite one that we’ve explored in class so far is the question that we’ve all asked ourselves at some point: Is Earth the only planet in the entire universe that sustains life? Or are we completely alone?
Despite not having the most scientific accuracy because so many of its parts are estimates, the Drake equation can be used to estimate how likely it is that other life exists specifically in our galaxy (mostly because it would be nearly impossible for us to directly contact a civilization outside of our galaxy). The equation was created by a scientist named Frank Drake in the 60s, and it’s now used mostly as a tool for talking about other intelligent life and advanced civilizations that may exist in the Milky Way as opposed to trying to pinpoint the number of them and their whereabouts. The Drake equation is as follows:
I know at first this equation seems really long and complex and scary, but each part represents something very specific that can be easily found out or estimated.
N
The N in the equation, which is what we’re trying to solve for, represents the estimated number of extraterrestrial (or alien) civilizations in the Milky Way that may be able to communicate with us. In order for a civilization to communicate with us, several other things must also be true, which we’ll get to as we work our way through the equation.
The really cool thing about N is that its value depends on your personal estimates for some of the other aspects of the equation. N will probably be different because every person will have different beliefs about life estimates in the galaxy.
R∗
R∗ is the first part of the equation, and its value is always the same. This is because it stands for the average number of stars that are created in the galaxy each year. This estimate has changed slightly over the years since the equation was created, but the current estimate sits around 3. So, R∗ in the Drake equation is always 3.
fp
The next part of the equation is fp, another fixed number. fp represents the fraction of those stars formed in a year that also have planets that form with them. Scientists used to think that a planet forming along with a star was more rare, but now it’s understood that when a star forms, at least one planet almost always forms with it. Since there is a near-100% chance a planet forms with a new star, the fixed number for fp is 1.
ne
This part of the equation is where things become a little less straightforward. We have to estimate this number largely based on our own solar system because it is what we know the most about. ne stands for the number of planets orbiting a sun that have the potential to support life, also known as the number of planets in the average solar system that lie in that star’s zone of habitability. The zone of habitability is an area around a star where the temperature of a planet in that zone can maintain liquid water. We know that life can’t develop without water, so a planet’s ability to have liquid water is crucial.
In our solar system, we have two main planets that lie in the zone of habitability: Mars and, of course, Earth. Venus is somewhat within this zone, and Venus likely did once have liquid water on it before it boiled off, so some estimates do include Venus. Since our solar system has, let’s say, three planets that lie in the zone of habitability and can theoretically have liquid water, then a safe estimate for the rest of the solar system is that other stars also have an average of three planets in their habitable zones. Therefore, ne is 3.
fl
In the equation, fl stands for the fraction of those planets within a star’s zone of habitability that can support life. We can also base this estimate on what we know about our own solar system, and since Earth is the only planet that seems to be able to maintain life, this means that 1 out of 3 planets in the habitable zone can support life, so that fraction for fl is about 0.33.
fi
This is where the real estimation begins. fi represents the number of planets that support life that will go on to eventually support intelligent life. Not all life is intelligent; everyone may have a different definition of what exactly intelligent life is, but life in the form of bacteria, while it is still life, is not an intelligent species. The value for fi can be really anything, as long as you take into account that not all planets containing life end up developing intelligent life. Personally, my estimation for fi is small–I think about 1% of planets that have life go on to develop intelligent life, so my value for fi would be 0.01.
fc
fc takes life estimations a step further. This part of the equation stands for the fraction of planets that have intelligent life that develop and support advanced civilizations from that intelligent life. An advanced civilization in the context of the Drake equation is a civilization that is technologically-advanced enough to send radio signals into space as a means of communication. Humans only gained the ability to use radio signals in the 1930s, so one could estimate the number of technologically advanced civilizations to be extremely small based on this fact. I think that 0.5% of planets with intelligent life develop technologically advanced civilizations, so my fc value would be 0.005.
L
The final value in the Drake equation is L, which represents the length of time in years that a technologically advanced civilization could emit radio signals into space. This number really depends on how optimistic you are; I personally think that since we haven’t even had radio technology for a hundred years that our civilization has maybe another 100 or 200 years until either radio waves are deemed unimportant and their use is discontinued or there is another mass extinction event that makes it impossible for humans to continue to use radio waves. So, my estimate based on Earth for L would be 300.
Putting it all together
Once you’ve compiled all of the numbers and estimates, to solve the Drake equation for N you need to multiply everything together. When multiplied, all of the elements of the equation including my personal estimates (3 x 1 x 3 x 0.33 x 0.01 x 0.005 x 300) equal about 0.045. If N is the number of civilizations in the Milky Way that could contain an extraterrestrial species capable of communicating with us, my estimate for it is essentially 0.
However, that’s just my opinion based on how optimistic/pessimistic my estimates are! I encourage you to come up with your own estimates and try out the Drake equation for yourself. Whether or not we are alone is pretty much an impossible question to answer as of right now, so have fun with this equation and the idea that there might be something else out there… however comforting that may or may not be.
