I am just old enough to remember a time when most textbooks only spoke of exoplanets (i.e. those outside our solar system) in the hypothetical sense, their existence taken as an assumption rather than demonstrable fact, while the question of whether our 8-planet solar system was typical or an anomaly remained a hotly-debated topic. Despite over a century of attempts and subsequent erroneous claims, it wasn’t until 1992 that the first confirmed exoplanet was discovered, two of them as a matter of fact, orbiting the pulsar PSR B1257+12. As surprising as this was to the scientific community, the high likelihood that the planetary pair had formed as the result of a merger between two white dwarfs rather than a collapsing gas cloud meant the world would have to wait another 3 years for the confirmation of a “true” planet orbiting a main-sequence star like the Sun. The discovery ushered in a new era for astronomy and as of November 1st, 2020 there have been a whopping 4,370 exoplanets confirmed in 3,230 star systems. This menagerie of other worlds, ranging from “hot Jupiters” and lava worlds to gas and ice giants and seemingly everything in between stands in stark contrast to the apparently silent heavens, leaving a profound question unanswered:
“Where is everybody?”
Despite what many a crackpot relative or internet crank will tell you, humanity is, to the best of our knowledge, completely and utterly alone in a universe so vast it defies comprehension, a contradiction commonly referred to today as The Fermi Paradox. The concept of aliens and the foundations of the Fermi Paradox have been around for a surprisingly long time, entering the Western conscience with the speculation of Epicurean philosophers like Democritus of ancient Greece, who today is famous for the first conception of atoms. From the viewpoint that the world was merely a chance jostling of these atoms, they argued it was not only reasonable, but exceedingly likely that there exist other inhabited worlds similar to our own. To paraphrase the argument: “who ever heard of a single ear of wheat growing alone in a vast, fertile field?”
Should you happen to see this mentioned on the next episode of Ancient Aliens, it’s worth noting that Democritus’s ideas about “aliens” (and atoms) are not quite as modern as they sound, instead more akin to our modern idea of beings from “parallel universes”, not habitable planets orbiting other stars since the ancient Greeks had no idea what stars and planets actually are, much less orbits. To them, these other worlds existed but were forever inaccessible to us. What’s important is that the core idea, that it’s improbable that life should arise on Earth and only on Earth begins here. Its dismissal by the Aristotelians, who preferred what can be known to what might be and whose influence on Western culture remains to this day, caused such discussion to take a back seat during the early Middle Ages, but the idea remained nonetheless, never being fully dismissed as “impossible” by theologians who were wary of placing what they saw as limits on the omnipotence of the Christian God. Contrary to the popular misconception that discussion of such matters would get one labeled a heretic and burned at the stake, the idea of aliens inhabiting other worlds within our universe that were, at least in theory, accessible to us in some way first arose in the Western world during the late Middle Ages. In his influential work De Docta Ignorantia (1440), Nicholas of Cusa argues that:
Life, as it exists on Earth in the form of men, animals and plants, is to be found, let us suppose in a high form in the solar and stellar regions. Rather than think that so many stars and parts of the heavens are uninhabited and that this earth of ours alone is peopled – and that with beings perhaps of an inferior type – we will suppose that in every region there are inhabitants, differing in nature by rank and all owing their origin to God, who is the center and circumference of all stellar regions (II, p. 12).
His work sparked intense discussion among Christian theologians over whether or not such aliens had been “redeemed” and even whether “original sin” would apply to them. After all it was probably a safe assumption that they were not descended from the mythical Adam and Eve.
The idea stuck around and was eventually taken up by the scientists of the modern era, including the likes of none other than Harlow Shapley who stated in an interview for The Sydney Morning Herald (1959) that:
The universe has 10 million, million, million suns similar to our own. One in a million has planets around it. Only one in a million million has the right combination of chemicals, temperature, water, days and nights to support planetary life as we know it. This calculation arrives at the estimated figure of 100 million worlds where life has been forged by evolution (p. 5)
Five months later, Frank Drake performed the first systematic search for extraterrestrial intelligence by monitoring two nearby sun-like stars, 6 hours per day for four months, searching for radio signals around the emission frequency of neutral hydrogen, the most common element in the universe. The reason for this frequency choice is simple: if somebody out there wants their message to be heard, they’ll put it where everyone is already looking. A year later, while preparing to host a meeting for SETI to discuss the future potential of such search efforts, he penned the equation which would bear his name.
Though initially conceived as little more than a mathematical conversation piece, The Drake Equation (as it came to be known) at last allowed astronomers to estimate the number of intelligent civilizations within our galaxy, for the first time couching the discussion in numbers rather than idle speculation. The only problem? Nobody had any real idea what any of these numbers actually were. The various numbers proposed at the meeting by Drake and his colleagues as an “educated guess” give a final value somewhere between a minimum of 20 and a maximum of 50,000,000. Such an enormous range perfectly illustrates their aforementioned uncertainty. In the six decades since, the data from countless observations, surveys, and space missions from NASA, ESA, and observatories around the world have finally shed some light on a few of these parameters which, in theory, allow us to narrow down the estimates a bit.
Mean Rate of Star Formation, R*
Our first parameter in the Drake Equation is R*, the average rate of star formation. For our purposes here, we are only looking for life within our own galaxy so we will use the rate of star formation in the Milky Way. Calculations from the available data suggest that somewhere between 0.68 and 1.45 solar masses of galactic material goes into making new stars every year. To find the star formation rate, we need to also account for the average mass of new stars using the available empirical data which indicates a value of around two stars per solar mass worth of material. Multiplying these two numbers together gives us a value between 1.5 and 3 new stars per year formed in our galaxy, a value slightly higher than the original 1961 estimate of 1 per year.
R* = 1.5 – 3
The Fraction of Stars that have Planets, fp
As mentioned previously, the exoplanet surveys thus far found 3,230 star systems that contain planets. However this figure only tells us the number of stars that have been found to possess planets, not the fraction of them that do. The total number of stars surveyed is much larger at over half a million. Dividing the two numbers gives a value for fp of less than 1%. At first glance, this looks pretty dismal, but there are some important caveats that we need to account for. The majority of exoplanet discoveries were made using the transit method, which makes continuous observations of a star’s light to look for periodic dimming caused by a planet passing in front of it directly along our line of sight. Our chances of observing such dimming is directly related to the star’s radius (bigger is better) and inversely related to the planet’s orbital distance from its host star (closer is better). Should our line of sight to a given system happen to be anything other than virtually edge-on, the transit method will not detect any planets there, even if there actually are. For those that we are fortunate enough to be observing nearly edge-on, if a planet’s orbit is too wide then it will not pass in front of the star from our perspective, thus there will be no dimming and the planet will go undetected. Finally, if a planet is too small, then the star’s dimming could be too small for our instruments to detect and the planet will go undetected.
It perhaps seems that we’re out of luck on getting any reasonable estimates, but statistical analysis of the transit data combined with that of surveys using other methods of planet detection such as doppler spectroscopy and gravitational microlensing point towards a conclusion that even the optimists of the 1960’s would consider absolutely astonishing; fp may very well approach 1, far larger than the maximum estimated value of 0.5 made 60 years ago! In other words, it’s possible that nearly every single star in our galaxy has one or more planets!
fp ≈ 1
Thus far things are looking better than ever for the potential development of intelligent, detectable civilizations (IDCs) in our galaxy, only deepening the paradox. However from here on our estimates for successive factors in the Drake Equation become increasingly speculative.
The Fraction of Potential Life-Supporting Planets that Develop Life, fl
This number is even more difficult to estimate than the previous factor. Our current sample size for life-supporting planets that go on to develop life is 1, with a possibility of 5 more if Mars, Venus, Titan, Europa, and/or Enceladus are eventually found to have or have had life. Some scientists argue that life arises as a rule virtually as soon as conditions are right, so this value should be ‘one’. Others argue it may actually be a freak occurrence even in all the right conditions, so this number should be tiny. In the case of our planet, life appears to have developed as soon as the conditions were right, but any attempt to extrapolate this to other planets contains heavy anthropic bias as our model planet (Earth) was not chosen randomly but rather by the organisms already inhabiting it. There currently is no evidence that abiogenesis occurred more than once on Earth, yet if it were a common phenomenon we might reasonably expect it to have occurred several times. It certainly could have, but all known terrestrial life shares a common origin, meaning if it ever did then those lineages are most likely long gone. If abiogenesis does indeed turn out to be an incredibly rare event, there is still a potential loophole. Remember, we only care about the number of habitable planets that go on to actually develop life, not how that life got there in the first place. It is entirely conceivable that planets, including our own, could have been seeded with microbial life that hitched a ride on a rogue piece of post-collision debris ejected from another already inhabited planet, a hypothesis known as panspermia. A technologically advanced civilization could also choose to do this intentionally in order to spread its genetic family throughout the galaxy. It may sound far-fetched, but humanity already has the capability to do this. Scientists and engineers go to painstaking lengths to sterilize spacecraft in order to avoid unintentionally ‘infecting’ other planets or moons with potentially invasive extremophile Earthlings. The various proposed panspermia hypotheses are a topic that honestly deserve an article all their own, so for now we will wrap things up optimistically and use the original estimate made by Drake and his colleagues back in 1961.
fl ≈ 1
You may have noticed we skipped over a factor, the average number of habitable planets per star with planets. Like panspermia, this factor warrants its own discussion and is the subject of part two in this series. Stay tuned!