By Dr. Gary Deel, Ph.D., J.D.
Faculty Director, School of Business, American Military University
This is the first article in a four-part series discussing the probabilities of extraterrestrial life elsewhere in the Milky Way galaxy.
Perhaps the most existential question humans have ever asked still remains unanswered: Are we alone in the universe?
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We have pondered this question for centuries, beginning with the earliest nomadic tribes gazing up at the stars and culminating in sophisticated scientific missions currently being undertaken by the global astrophysics community.
Although we’ve not yet found any positive evidence of life beyond Earth, there are still plenty of reasons to maintain hope that extraterrestrial life exists elsewhere in our galaxy. The universe is a vast place, and yet, even our cosmic ‘backyard’ gives us reasons for hope.
Drake Equation Created to Estimate the Number of Planets That Could Support Life
No discussion on the possibilities of extraterrestrial life in the universe would be complete without mention of the Drake Equation. The Drake Equation is a speculative concept developed by Frank Drake of Cornell University in 1961.
Drake used a number of variables — the realities of some we are much more certain about than others — to estimate the number of planets in the Milky Way galaxy which might be home to intelligent life. The Drake Equation has been written in different ways over the decades since its inception, but here is a simplified version that serves the purposes of our discussion in this article:
N = R* × fe × ne × fl × fi × fc × L
Breaking Down the Drake Equation
The Drake Equation can be subdivided into several parts.
R* = Rate of Star Formation. The first variable is star formation, or the rate at which stars which might be suitable homes for intelligent life are born. This is a number which we can estimate within an order of magnitude, through observation of our galaxy and the ages of different stars in it.
For example, let’s say we estimated that there are approximately 100 billion stars in the Milky Way Galaxy. Not all of these stars would be viable candidates for life.
The main category of exclusion would be stars that are so big that they burn through their entire supply of hydrogen in just a few hundred million years. These stars would die in supernova explosions long before life on surrounding planets would ever have sufficient time to arise, develop, and evolve to a level of intelligence comparable to that of the human race.
fe = Fraction of Stars with Planets. The next variable in the Drake Equation is the fraction of all stars which might in fact have surrounding and inhabitable planets. To our knowledge, even extremophilic life would be incapable of living on an active star, so any star supporting life would do so via orbiting planets.
Drake could only guess about this variable in 1961. Today, we know from missions such as NASA’s Kepler Space Telescope that many — perhaps even more than half — of the stars in the Milky Way galaxy host planets. Humankind has catalogued thousands of exoplanets in just the short few years since we’ve had the ability to detect them, so preliminary data suggests that planets are a fairly common occurrence.
ne = Number of Planets per Star. The next variable is the average number of planets per star in the Milky Way galaxy. Missions such as Kepler have established that exoplanets exist. We now have a healthy catalog of such planets, and many stars have more than one planet in orbit.
But how many planets per star are there on average? This is a difficult question to answer with precision at this point in our understanding.
For instance, some systems have two planets. Others have three or four. Some systems are binary, meaning that they contain two stars with planets in stable orbits around the mutual center of gravity between them.
Our own solar system has a whopping eight planets — and a few dwarf planets as well. But it’s too early to tell yet whether that is uncommon. So the best we can do here is another educated guess.
fl = Fraction of Suitable Planets. The next variable in the Drake Equation is the fraction of planets that are actually suitable for the evolution of intelligent life. This variable complicates the situation quite a bit more.
One of the fundamental underpinning questions for which we do not have a certain answer is: What circumstances are necessary for life to arise and evolve to intelligence?
We’re fairly certain that life requires planets, but what kinds of planets? We know that life on Earth requires a certain temperature range, where water can exist in a liquid form — that is to say, between freezing and boiling temperatures.
Some extremophiles can persist outside of this range, but the vast majority of life is constrained within these parameters. Intelligent life may actually require water in liquid form as a necessary ingredient to complex growth.
We humans are, after all, predominantly liquid water by weight. But there are other factors that are still more uncertain. Must life be carbon-based? Does it require an atmosphere? What about a magnetic field to protect from harmful radiation?
Again, there are many unknowns. Another educated guess is our only option.
fi = Fraction of Planets on which Life Actually Arises. Once we approximate how many planets are capable of supporting life, we must then ask ourselves what proportion of such planets will actually host life. Just because something is possible does not make it inevitable.
There are other factors to account for, such as planetary collisions, meteor impacts and system collapses which might prevent an otherwise habitable planet from ever becoming a home to anything at all. This variable requires another educated guess.
fc = Time to Developing Technology that Allows Detection by Others | L – Length of Time Detectable by Others. The last two components in the Drake Equation relate to two variables. One is the amount of time required for a species to evolve from its humblest beginnings to the point at which they are capable — presumably through radio astronomy — of making their presence known in the galaxy. The second variable is the average length of time such civilizations would be detectable before they either destroy themselves or are snuffed out by natural events.
We have our own civilization as a kind of baseline for the first variable, though it is, of course, a sample of n = 1. And given that our future is uncertain, we have no frame of reference at all for the second variable, so more guesses must be made here. In any event, these last two factors are obviously more concerned with our ability to find extraterrestrials than whether they exist at all.
Now, you might be disoriented at this point by all of the guesswork and unknowns involved in the Drake Equation. However, the exciting takeaway from this thought experiment is that, even when you use the most conservative estimates for all of the variables in the Drake Equation, the number that this equation yields as an approximate estimate of planets with intelligent life is still in the tens of thousands, hundreds of thousands, or even millions! So although this kind of exercise is not an exact science yet, it does at least serve to give us hope that our efforts are not in vain.
Given the implications of the Drake Equation, we might be tempted to think that life should be abundant in the galaxy. However, an observation from physicist Enrico Fermi challenges this assumption. In the second part of this article series, we’ll look at the infamous Fermi Paradox and how it challenges the idea that the galaxy is replete with intelligent life.
About the Author
Dr. Gary Deel is a Faculty Director with the School of Business at American Military University. He holds a J.D. in Law and a Ph.D. in Hospitality/Business Management. Gary teaches human resources and employment law classes for American Military University, the University of Central Florida, Colorado State University and others.