Aliens are out there. But a new study suggests they're more rare than we think.

The research has raised eyebrows because it suggests humans are extraordinarily special, despite the fact that the galaxy is stuffed with planets.
Image: A glowing white humanoid figure on a black background.
Are our nearest cosmic chums at least several thousand light-years away?Duncan Fotos / Getty Images
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By Seth Shostak, senior astronomer at the SETI Institute

According to a new analysis from scientists at the University of Nottingham in England we don’t have a lot of alien company.

On June 15, two researchers published a paper in the Astrophysical Journal arguing that the Milky Way — which sports an estimated several hundred billion stars — could host as few as 36 alien societies. That’s a surprisingly tiny number, although the authors also make a second, more generous analysis in which they say that the count might be as many as a thousand.

The implication is that our nearest cosmic chums are at least several thousand light-years away.

Either way, their conclusion is that, like stick-shift cars, extraterrestrial civilizations are few and far between. The implication is that our nearest cosmic chums are at least several thousand light-years away.

You may wonder why this story has raised eyebrows. Well, it would make Homo sapiens extraordinarily special, despite the fact that the galaxy is stuffed with planets. It discomfits scientists (including me) because, historically, every time we’ve thought we occupy a privileged position in the universe, we were wrong. Remember that six centuries ago, learned folk would have assured you that Earth was the center of the cosmos.

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How did the British experts arrive at such a stunningly low estimate? After all, there have been enough previous studies on this topic to fill a small horde of hard drives. Astronomer Carl Sagan figured that the Milky Way houses a million societies. A more conservative claim is that the number is closer to 10,000. So why do these Brits disagree?

The scientists arrived at their low-ball tally by using a variant of the Drake equation — everyone’s favorite method to gauge alien head counts. This famous formula, which was introduced by astronomer Frank Drake in 1961, is a string of seven parameters that, when multiplied together, estimate the number of technologically adept societies in the galaxy. The parameters include the abundance of Earth-like planets, the fraction that spawn life, etc.

However, there’s one term in this equation that generates a lot of debate because it’s exclusively sociological. It’s the length of time that a technological civilization maintains its mojo and beams radio or light waves into space. That’s important because if an alien culture goes silent, we may never find it.

In estimating the lifetime of a technological species, the paper’s authors make a big assumption. They note that humans have been beaming signals into the ether for about a century. That’s fair enough. But then they invoke something they’ve coined the Astrobiological Copernican Principle (what others modestly call the principle of mediocrity) and say that the universe is engaged in a massive “Simon Says” game. Whatever we on Earth have done, the rest of the universe has imitated perfectly.

So because we’ve had radio for about a hundred years, the Nottingham duo assume that all technological cultures will use this technology for a century. But then they’ll stop, and radio will go the way of the buggy whip. Radar, broadcast television, Wi-Fi — all these uses for radio will disappear, and the aliens will move on to some other, unspecified technology.

You might not have any problem with that assumption. After all, we don’t know how long radio technology will last here on Earth, so it’s tempting to take our own experience — which extends for just a century — and apply it to everyone. But that’s dicey. Would you say that, because we’ve had airplanes for a hundred years, the aliens will have airplanes for a century, and not longer?

Radio is a very useful technology, based on some fundamental physics. It might be around for as long as the wheel. So it would certainly be reasonable to guess that the technological lifetime of societies is 10,000 years, not 100. Choosing the larger number increases the tally of inhabited worlds by 100 times.

Earth-like planets can spontaneously generate living organisms, and some worlds will eventually spawn an intelligent species. But surely not all such worlds will do so.

In other words, this arbitrary assumption — that the use of radio is short-lived — is largely responsible for the authors’ unusually low estimate for the number of alien societies.

But wait, there’s more.

A second premise in the Nottingham paper is equally astonishing: Namely, that every Earth-size planet with a temperate climate will produce life, and after about 4 or 5 billion years, intelligent life will appear.

Now, of course, most scientists agree with the obvious: that Earth-like planets can spontaneously generate living organisms, and some worlds will eventually spawn an intelligent species. But surely not all such worlds will do so. That’s like saying that every kid who takes piano lessons will inevitably win the Van Cliburn prize.

The Nottingham paper has drawn a lot of attention because it claims that the number of inhabited worlds is likely paltry. But, in fact, by making your own assumptions you can derive just about any estimate for the number of intelligent cosmic species. For myself, I figure that an absolute minimum would be 70, the number that managed to bag speaking roles in "Star Trek." Even that total would beat the lower limit published in this paper.