How many intelligent civilizations should be in our galaxy right now? In 1961, the American astrophysicist Frank Drake, who died on September 2 at the age of 92, proposed an equation to estimate it. Drake’s equation, which dates from a stage in his career when he was “too naive to be nervous” (as he later put it), became famous and named after him.
This puts Drake in the company of prominent physicists whose equations bear his name, including James Clerk Maxwell and Erwin Schrödinger. Unlike the latter, the Drake equation does not summarize a law of nature. Rather, it combines a few poorly known probabilities into an educated guess.
No matter what reasonable values you introduce into the equation (see image below), it’s hard to avoid the conclusion that we shouldn’t be alone in the galaxy. Throughout his life, Drake remained a proponent and advocate of the search for extraterrestrial life, but has his equation really taught us anything?
The Drake equation may seem complicated, but its principles are actually quite simple. It stipulates that, in a galaxy as old as ours, the number of civilizations detectable because of their presence must be equal to the rate of appearance of civilizations, multiplied by their average lifespan.
Assigning a value to the spawn rate of civilizations might seem like guesswork, but Drake realized it could be broken down into more manageable pieces.
He said the total rate equals the rate of proper star formation multiplied by the fraction of those stars that have planets. This figure is then multiplied by the number of planets capable of supporting life per system, multiplied by the fraction of these planets where life begins, multiplied by the fraction of those where life becomes intelligent, multiplied by the fraction of those which signal their presence.
When Drake formulated his equation, the only term known with certainty was the rate of star formation – about 30 per year.
As for the next term, in the 1960s we had no evidence that any other stars had planets, and one in ten might seem like an optimistic estimate. However, observational discoveries of exoplanets (planets orbiting other stars), which began in the 1990s and have multiplied during this century, allow us today to believe that most stars have planets.
Common sense suggests that most systems composed of several planets would include one at the right distance from its star to be able to support life. Earth is that planet in our solar system. Also, Mars may have been home to abundant life in the past – and it could still cling to it.
Today we also realize that planets do not need to be hot enough for liquid water to exist on their surface in order to support life. This can occur in the inner ocean of an ice-covered body, supported by heat generated by radioactivity or tides rather than sunlight.
There are several likely candidates among the moons of Jupiter and Saturn, for example. In fact, if we add the moons as being capable of supporting life, the average number of habitable bodies per planetary system could easily exceed one.
The values of the terms to the right of the equation, however, remain more questionable. Some maintain that, given a few million years, life can develop wherever it is possible.
This would mean that the fraction of suitable bodies where life actually starts is roughly equal to one. Others argue that we still have no evidence for the beginning of life anywhere other than Earth, and that the origin of life may in fact be an extremely rare event.
Will life, once started, eventually become intelligent? It must probably first go beyond the microbial stage and become multicellular.
There is evidence that multicellular life started more than once on Earth, so becoming multicellular may not be a barrier. Others, however, point out that on Earth, the “right kind” of multicellular life, which continued to evolve, only appeared once and may be galacticly rare.
Intelligence can confer a competitive advantage over other species, which means that its evolution could be rather probable. But we are not sure.
And will intelligent life develop technology to the point of (accidentally or deliberately) broadcasting its existence into space? Possibly for surface dwellers like us, but it could be rare for inner ocean dwellers of frozen worlds with no atmosphere.
How long do civilizations last?
What about the average lifespan of a detectable civilization, L? Our television transmissions began to make Earth detectable from afar in the 1950s, giving a minimum value for L of about 70 years in our case.
In general, L can be limited by the collapse of a civilization (what are the chances that ours will last another 100 years?) or by the near total disappearance of broadcasting in favor of the Internet, or by a deliberate choice to “shut up” for fear of hostile galactic inhabitants.
Play with the numbers yourself – it’s fun! You will find that if L is greater than 1,000 years, N (the number of detectable civilizations) is likely to be greater than a hundred. In a 2010 taped interview, Drake said his best estimate of N was around 10,000.
Every year, we learn more about exoplanets and we are entering an era where it is increasingly possible to measure their atmospheric composition to detect traces of life. Within the next ten or twenty years, we can hope to get a much more robust estimate of the fraction of Earth-like planets where life began.
It won’t tell us about life in the inner oceans, but we can hope to get a glimpse of it thanks to missions to the icy moons of Jupiter, Saturn and Uranus. And we could, of course, detect actual signals from extraterrestrial intelligence.
Either way, Frank Drake’s equation, which has spurred so many avenues of research, will continue to give us a sobering sense of perspective. We should be grateful to him.
David Rothery, Professor of Planetary Geosciences, The Open University.