At least several dozen alien civilisations able to communicate with us may now exist in our galaxy. As scientific claims go, they don’t get much more extraordinary than that.
Yet according to new research, it’s a reasonable best-guess answer to one of the most profound questions in science: are we alone in the universe?
The claim, made by two British academics, appears in the current issue of The Astrophysical Journal, the most prestigious in its field.
Even so, it has already sparked controversy among experts. And small wonder, given the seeming impossibility of producing a remotely credible answer to such a question.
It’s actually a demonstration of the astonishing power of what scientists often jokingly call a “back of the envelope” calculation.
This involves identifying the key factors needed to give a rough answer, combining them in a reasonable formula, then plugging in some plausible figures. The aim isn’t to produce a precise answer, but a ball-park estimate of something seemingly unfathomable.
Admittedly, this often requires considerable expertise – and a large envelope. And in the case of alien civilisations, it’s a calculation that astronomers have been returning to repeatedly for nearly 60 years.
The original attempt was made by Frank Drake, an American astrophysicist and organiser of the first-ever scientific meeting to discuss the search for alien life, held in Green Bank, West Virginia in 1961.
In trying to bring some rational to the debate, Drake wrote down a simple formula combining the various factors he thought were needed to come up with a rough estimate of the number of alien civilisations in the galaxy.
They included the fraction of Sun-like stars that have planets, the number of such planets capable of supporting life and the fraction that actually do so, among others.
What Drake couldn’t do, however, was plug any reliable numbers into his formula, and thus give an estimate of the number of alien civilisations. That, in part, was what the meeting was about.
Ever since, filling in the gaps in what is now called Drake’s Equation has remained one of the biggest challenges in science.
Over the decades, some of the gaps have been filled. Following the discovery of the first planet around another star in 1995, astronomers have found thousands of planets orbiting stars in our cosmic neighbourhood. This suggests the fraction of Sun-like stars with planets is close to 100 per cent.
Yet big unknowns remain – most of them related to the processes that lead to life capable of making contact with us.
Now Professors Tom Westby and Christopher Conselice of the University of Nottingham have suggested a way of cutting through the complexity.
They exploit one incontrovertible fact about the evolution of intelligent life: it happened on our planet around five billion years after the Sun was formed.
To make their estimate, they then assume that if the same conditions exist elsewhere, then after around five billion years the outcome will be the same: the evolution of intelligent, communicating beings.
The researchers show that this allows some of the hard-to-guess factors in Drake’s equation to be replaced by others for which data is actually available.
For example, instead of having to estimate the fraction of planets that develop life, professors Westby and Conselice need to know the fraction of Earth-like planets orbiting at the right distance from suitable stars to allow life to evolve. Estimates for these are now emerging from satellite surveys of planets beyond our solar system.
But one key unknown remains: how long an advanced civilisation survives once it is able to transmit signals. If this is very short – perhaps because such civilisations have a tendency to self-destruct – then the chances of detecting their signals before they fall silent are very small.
Professors Westby and Conselice exploit the fact that we humans still exist around 100 years after we began radio transmissions. Plugging this into their version of the Drake equation gives that headline result: there should be at least a few dozen alien civilisations in our galaxy able to communicate with us.
As with any back of the envelope estimate, the actual number is pretty uncertain. The researchers estimate that under the most pessimistic assumptions, the number could lie somewhere between a handful of alien civilisations to 200-plus.
But their estimate also raises another question: if any such civilisations are out there, why haven’t we heard from them?
Despite decades of effort and several false alarms, no alien communications have ever been detected. One obvious explanation is that the galaxy is a huge place. As such, the nearest alien civilisation is likely to be thousands of light years away – making its signals too faint to detect.
But there are other, more unnerving, possibilities. Perhaps technologically advanced civilisations really do tend to be very short-lived – a possibility which will not surprise any environmentalist.
Or perhaps such civilisations are visited and destroyed - inadvertently or not - by others once they make their presence known.
Whatever the truth, this latest attempt to understand if we are alone in the galaxy carries an intriguing implication. The longer the search for alien signals goes unrewarded, the more likely we humans are to be exceptions in the cosmic game of life.
Robert Matthews is visiting professor of science at Aston University, Birmingham, UK