By Sophie Mendell and Jacob Ramsey
Science fiction tells tales of incredible worlds, inhabited by countless curious and intelligent alien lifeforms. With over 100 billion planets in our galaxy, Yoda has to be lurking out there somewhere, right? The search for extraterrestrial life has confounded astronomers, space enthusiasts, and curious minds for centuries. The modern hunt for life in our own galaxy began in the mid-twentieth century, with prominent researchers considering the possibility and concluding the likely inevitability of alien life. This is the Fermi Paradox — with so many planets out there, it is highly likely that extraterrestrial life exists, yet we haven’t found any evidence of it. But without star charts and hyperdrives, just exactly how do we find alien life, if it even exists?
To begin this process, researchers needed to identify what they were looking for. Would this be microbes? Certain types of chemicals? Or, perhaps, long-range forms of communication? In 1961, a meeting was held to determine what to look for; this was the start of the Search for Extraterrestrial Intelligence (SETI), where physicist Frank Drake provided a framework for how we can approach the Fermi Paradox: The Drake equation.
Figure 1: The Drake Equation. N: Number of civilizations in the galaxy with which communication could be possible. R*: Rate of star formation in our galaxy. fp: Fraction of stars that have planets. ne: Average number of planets that can support life for every star that has planets. fl: Fraction of planets that could support life that actually do develop life. fi: Fraction of planets that develop intelligent life. fc: Fraction of intelligent civilizations that develop technology to communicate their existence into space. L: Length of time a civilization releases detectable signals into space.
The equation consists of a series of factors which are believed to contribute to the probability of intelligent life existing in our own home, the Milky Way Galaxy, and each comes with unique considerations. However, they all fall into one of three main categories: physical, intellectual, and timing factors. Despite its name, the Drake equation is not something you can actually solve, as some of its variables are difficult to define. Instead, it is more like a thought experiment, allowing us to consider, with some hand-waving, the mathematical probability of finding extraterrestrial life.
The Physical Factors
This group includes concepts such as the rate of star formation in the galaxy, which astrophysicists have determined to be 6 to 7 per year! As new stars form, so do new planetary systems, increasing the places where new life could develop. How many planets does each star have? Do all stars even have planets, and how many of them can actually support life? The answers to these questions are all numbers and probabilities that are relatively easy to estimate with astrophysics.
The Intellectual Factors
The intellectual factors are much more subjective. Just because life can develop doesn’t mean that it will develop. And if it does, will it be as intelligent as human life? More intelligent? Perhaps it will simply consist of small microbes. The lifeform’s intelligence will impact how easy or difficult it is for us to find. An alien species could possibly be missed entirely if we’re not looking in the right places.
The Timing Factors
It is entirely possible that intelligent life has previously existed or will exist in the future in our galaxy. Have or will humans be around to see it, though? The timing factors consider the simple fact that civilizations aren’t around forever. In addition, they may only be intelligent enough to transmit communicable signals for a small fraction of their total existence. Humans have been around for 200,000 years, but we have only been actively sending signals into space since 1962. Even if we continue our space communications for thousands more years, this isn’t very long on a galactic timescale! Considering how little overlap civilizations would have to try to reach each other, perhaps we missed our chance.
One study suggests that there are up to 40 billion earth-like planets in the galaxy, and by Frank Drake’s most recent estimate just 0.000025% of them may have housed an intelligent civilization at some point (or could do so in the future). While this is a very small fraction, it nevertheless indicates that there could be millions of civilizations out there! However, this equation cannot be exact. Consider this: One of the variables is the average length of time that civilizations communicate. Having never encountered another civilization, we cannot make an educated guess about how long we can share information with them. It’s like asking a penguin how long to bake cookies for; the penguin can’t read, work an oven, nor understand the concept of timekeeping.
At this point, it is clear there is no simple solution to the Drake Equation, and this leaves some doubts about its utility. Nevertheless, the thought experiment has succeeded in generating intense discussion on how much life is out there in our own galaxy and what defines “intelligent life.” If the numbers are supposedly this high, why haven’t we made contact with anyone yet? Are we just not able to receive their messages? These questions are at the heart of the Fermi Paradox. Maybe we are alone in the universe; or, maybe, just waiting for the right time to use the Force to say hello Yoda is.
