As the COVID19 pandemic surges into the summer months with no end in sight, it’s sometimes nice to forget about the virus completely and ponder other subjects, like the possible existence of intelligent alien life forms in our galaxy. As both a scientist and a long-time science fiction fan, I’ve often wondered about sentient alien beings and whether or not they are out there in some distant corner of space. Given the vastness of the universe, the gigantic number of stars, and the growing scientific evidence that many stars have associated planets, it’s always seemed reasonable to believe that life could have arisen on other planets. While the conditions on Earth that made it a suitable host for life to evolve may be quite rare, it seems unlikely that there is no other planet where similar conditions occurred. However, turning supposition into more quantitative scientific estimates is difficult as there are many parameters to consider and often scant data to set limits on the parameters. One of the earliest attempts to quantify estimates was the Drake Equation that predicted intelligent civilizations in our galaxy (the Milky Way). The equation states that the number of planets (N) with intelligent civilizations is the product of seven parameters:
R∗ = the average rate of star formation in our galaxy;
fp = the fraction of those stars that have planets;
ne = the average number of planets that can potentially support life per star that has planets;
fl = the fraction of planets that could support life that actually develops life at some point;
fi = the fraction of planets with life that actually go on to develop intelligent life;
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space; and
L = the length of time for which such civilizations release detectable signals into space
However, when Drake proposed this equation in 1961 there was little data available for these parameters. Consequently, the range of uncertainties was so huge that the equation predicted between 1000 and 100,000,000 civilizations depending on what values were assigned to the seven parameters. Subsequent models have been developed, though most still have estimates ranging from hundreds to tens of thousands of planets with intelligent life.
A new study in The Astrophysical Journal adds to that literature by calculating the number of Communicating Extra-Terrestrial Intelligent (CETI) civilizations that might currently exist in the Milky Way using a very thoughtful revision of the Drake equation and the latest astrophysical data. For example, there is now adequate data that addresses the first three parameters (R∗, fp, and ne) so reasonable values can be assigned to the parameters. Second, the authors simplify some of the Drake parameters that are unknowable, e.g., fl, the fraction of planets that could support life that actually did develop life at some point. They argue that there is nothing special about the Earth, so every planet that could develop life should develop it and this fraction becomes 1 and drops out of the equation. They also add in some interesting new parameters such as how long an intelligent civilization might exist before an extinction event, either self-imposed or cosmological (e.g. an asteroid strike), and the metallicity distribution function (MDF). MDF is a measure of the relative iron to hydrogen ratio in stars which is important to identify stars with the potential to form Earth-like planets. Using this new equation and their most stringent estimates of the parameters the authors calculate that there is a minimum of 36 CETIs existing today in the Milky Way, a number that rises to around 3000 CETIs with less stringent parameters. Unfortunately, they estimate that our nearest intelligent neighbor is thousands of light years away, so communication is not likely in our lifetimes. Still, it’s appealing to have a scientific estimate that we are not alone in the galaxy and that someday alien encounters may actually happen.