GeekOut, episode 3!

Alien sentient life is easily one of the most common recurring tropes in science fiction. The thought of other intelligence somewhere out there in our universe has fascinated our species for a long time, and may even have roots in our ancient ancestors competition with other humanoid species that evolved alongside us. But how likely is it, exactly, that sentient alien life actually exists, especially life that we might have a chance of communicating with?

This question is part of what drove Dr. Frank Drake to design an equation to drive conversation around the topic. The famous Drake Equation takes into account several variables that could have an impact on the chances for life to arise, gain sentience, and begin using communication methods that we could detect. It wasn't designed to produce accurate estimates, as there are simply too many unknowns even with modern technology. Instead, it was meant to highlight our current understanding of the conditions needed for life, the size of our universe, and the amount of time a civilization might spend with communications technology that produces signals we can detect.

Here is the equation itself: N=R*⋅fp⋅ne⋅fl⋅fi⋅fc⋅L Lets take a quick look at the variables.

• R* takes into account the average rate of star formation in our galaxy.

• fp is the fraction of those star systems that contain planets.

• ne is number of planets in a star system that could potentially support life.

• fl represents the fraction of the planets in ne that actually do develop life.

• fi, then, is the fraction of planets with life that develop sentient life.

• fe is the number of civilizations that develop technology that produces detectable signatures.

• L, finally, is the average length of time these civilizations produce those detectable signals.

When this equation was first introduced, much of the data for the variables was based on pure conjecture: what scientists thought the likely values should be based on our current knowledge. Since then, we have developed telescopes that are powerful enough to detect planets around other stars. Our observations there have revealed that nearly all stars have planets in their orbits.

Likewise, we have discovered all sorts of astonishing new life forms here on our own planet since the Drake Equation was introduced, which expands the definition of what a 'habitable' planet might be. Missions to bring back samples from asteroids have shown that the building blocks for life as we know it are abundant in our own solar system, and there is no reason to think our system is special somehow. We have even been able to detect earlier and earlier signs of the first biogenesis here on Earth. The more we learn, the more it sure seems like the ingredients for life are common and that life arrises rapidly when the conditions allow for it.

If that is the case, though, why haven't we found any signs of sentient life out there in our galaxy? Well, for the most part the answer there is that even just our galaxy is mind numbingly large. Our primitive brains that evolved to deal with small, familiar numbers and scales can't easily comprehend just how big the universe is.

Taking our own evolution as an example: Earth has been around for about 4.5 billion years. Life arose rapidly, but it hasn't been until the past million years or so that we started to see the first ancestors of humanity, and modern homo sapiens first appearing 300,000 years ago. For much of that time, we were simple hunter gatherer tribes, living off the land and making simple tools. It wasn't until a mere 139 years ago that we first began experimenting with radio waves. Higher power radio signals that would travel deeper into space have only been getting produced en masse for about a hundred years, then.

Those radio waves extend outward at the speed of light once they leave our atmosphere. So our signals theoretically could have reached 139 light years away from Earth. However, interstellar radiation distorts those signals and caused them to decay. Depending on the amount of radiation at the time, our sginals would likely become unrecognizable from background radiation within 1o - 100 year light years. That means there are anywhere from 8 - 8,000 star systems that would have a chance at detecting our signals.

If we take what we know about our galaxy and plug in the variables for the Drake Equation, one number that we could arrive at is a total of 12,600 civilizations in the galaxy that we might be able to communicate with. I'll spare you the additional math here, but if we take the galaxy's volume into account, assume even distribution of civilizations (highly unlikely), we find that the odds of having another civilization of similar tech levels to our own within a 100 light year radius come out to 0.66%. That is less than one percent.

Again, there are so many more unknowns that go into a proper estimate. We could get a better volume estimate by mapping out the actual density of star systems in our galaxy. Larger telescopes will allow us to gather data on atmospheric compositions of planets around other stars. And if we manage to find signatures of life on other planets in our own solar system, that could increase or decrease the odds of life arising elsewhere in the galaxy. We might even start to find that the ingredients for life tend to occur in clusters, being more rare in other parts of the galaxy. Of course, the ultimate question of how many planets with life lead to sentient life will remain a mystery until we start detecting such planets. Proper answers to these questions likely will remain unknown until we are a galactic species. But that won't stop us from trying to figure it out in the meantime!