The Drake Equation May Be More Important Now Than Ever

How many intelligent civilizations should there be in our galaxy right now? In 1961, the American astrophysicist Frank Drake, who died on September 2 at the age of 92, devised an equation to estimate it. Dating from a stage in his career when he was “too naive to be nervous” (as he later put it), Drake’s equation has become famous and is named after him.

This puts Drake in the company of top physicists with equations named after him, such as James Clerk Maxwell and Erwin Schrödinger. Unlike them, the Drake equation does not contain a law of nature. Instead, it combines some little-known probabilities into an informed estimate.

Whatever reasonable values ​​are plugged into the equation (see image below) it is hard to avoid the conclusion that we must not be alone in the galaxy. Drake remained an advocate and supporter of the search for extraterrestrial life throughout his days, but has his equation really taught us anything?

The Drake equation may seem complicated, but its principles are actually quite simple. He claims that, in a galaxy as old as ours, the number of civilizations detectable by virtue of their spreading presence must be equal to the rate at which they emerge multiplied by their half-life.

Putting a value on the spawn rate of civilizations might seem like a guess, but Drake realized that it can be broken down into more manageable components.

He claimed that the total rate is equal to the rate of suitable star formation multiplied by the fraction of those stars that have planets. This is multiplied by the number of planets capable of supporting life per system, multiplied by the fraction of those planets on which life starts, multiplied by the fraction of those planets on which life becomes intelligent, multiplied by the fraction of those planets that transmit their presence.

complicated values
When Drake first formulated his equation, the only term that was known with any certainty was the rate of star formation: about 30 a 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 guess. However, observational discoveries of exoplanets (planets orbiting other stars) that began in the 1990s and have flourished in this century, make us confident that most stars have planets.

Common sense suggests that most multi-planet systems would include one at the right distance from its star to be capable of supporting life. Earth is that planet in our solar system. Furthermore, Mars may have been suitable for abundant life in the past – and could continue to cling to it.

Today we also realize that planets do not need to be hot enough for liquid water to exist on the surface to support life. This can be produced in the internal ocean of an ice-covered body, supported by heat generated by radioactivity or tides rather than sunlight.

There are several likely candidates between the moons of Jupiter and Saturn, for example. In fact, if we add the moons as capable of supporting life, the average number of habitable bodies per planetary system could easily exceed one.

However, the values ​​of the terms to the right of the equation remain more contentious. Some hold that, given a few million years to play, life will start wherever is suitable.

That would mean that the fraction of suitable bodies in which life actually gets going is roughly equal to one. Others claim that we still have no proof that life began anywhere other than Earth, and that the origin of life could actually be an extremely rare event.

Will life, once started, end up developing intelligence? It probably has to get past the microbial stage and become multicellular first.

There is evidence that multicellular life began 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, appeared only once and might be rare on a galactic scale.

Intelligence can confer a competitive advantage over other species, meaning its evolution could be quite likely. But we don’t know for sure.

And will intelligent life develop technology to the point of (accidentally or deliberately) transmitting its existence through space? Perhaps for surface dwellers like us, but it might be rare for inner ocean dwellers of icy worlds with no atmosphere.

How long do civilizations last?
What is the average duration of a detectable civilization, L? Our television broadcasts began to make the Earth detectable from afar in the 1950s, which gives a minimum value for L of about 70 years in our case.

However, in general, L can be limited by the collapse of civilization (what are the chances that ours will last 100 more years?) or by the almost 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, then N (the number of detectable civilizations) is likely to be greater than a hundred. In a taped interview in 2010, Drake said his best estimate of N was about 10,000.

Every year we learn more about exoplanets and we are entering an era where measuring their atmospheric composition to reveal evidence of life is becoming more feasible. In the next decade or two, we can expect a much stronger estimate of the fraction of Earth-like planets on which life begins.

This won’t tell us anything about life in the inner oceans, but we can hope that missions to the icy moons of Jupiter, Saturn and Uranus will tell us about it. And, of course, we could detect real signs of extraterrestrial intelligence.

In any case, Frank Drake’s equation, which has stimulated so many lines of inquiry, will continue to give us a thought-provoking sense of perspective. For that we should be thankful.

David Rothery, Professor of Planetary Geosciences, The Open University

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