Are we alone in the universe? It’s a question that has rankled philosophers and scientists for ages, but for the last half-century answering this question has been the exclusive purview of the Search for Extraterrestrial Intelligence (SETI), a scientific enterprise dedicated to looking for traces of alien intelligence in the universe. Since the beginning of modern SETI in 1960, things have seemed hopeful. There are something like a billion trillion stars in the universe, and we now know that most of these stars probably host planets. The odds that at least one of these planets was home to an alien civilization seemed good—it was just a matter of finding them.

Yet nearly 60 years after Project Ozma used a radio telescope to search the universe for alien messages for the first time, we have yet to find any evidence that we’re not alone. Some SETI researchers like Seth Shostak are optimistic that first contact will happen any day now, but according to new research from Oxford University’s Future of Humanity Institute, it might never happen because there’s a good chance we’re the only intelligent creatures in the observable universe.

Cosmic Solipsism?

In 1950, the physicist Enrico Fermi was busy developing a hydrogen bomb at Los Alamos National Laboratory, but he still found time to contemplate the universe when he wasn’t working on one of the most destructive weapons ever built. Considering how big the universe is, Fermi assumed that it must be swarming with intelligent life. How odd, then, that not a single extraterrestrial civilization had ever made contact with Earth or left even the faintest trace of its existence.

The improbable silence in the universe has since become known as the Fermi Paradox. Yet Fermi never pursued the notion beyond an initial thought experiment, and to paraphrase Lord Kelvin, it ain’t science until it has some numbers. Fortunately, only a decade later the planetary scientist Frank Drake turned the Fermi Paradox from parable to physics with the now famous Drake equation, which is used by SETI researchers to estimate the probability of life in the Milky Way.

To get an estimate of the number of intelligent civilizations in the universe, the Drake equation combines the rate of star formation per year, the fraction of stars with planets, the number of these planets that are habitable, the fraction of habitable planets with life, the fraction with life that develop intelligence, the fraction of intelligent civilizations that are detectable and the longevity of detectable civilizations.

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When Drake presented this equation to his colleagues at the 1961 Green Bank conference following the end of Project Ozma, they ended up with estimating that the Milky Way was populated with somewhere between 1,000 and 100,000,000 intelligent civilizations. (Drake himself estimated there to be around 10,000 intelligent, communicating civilizations in our galaxy.) This is a big spread, but that’s because most of the values in the equation are best guesses. The only values that scientists have real data on is the rate of star formation and the number of exoplanets.

Outside of that, the results of the Drake equation mostly depend on whether you’re optimistic about life forming and surviving its own intelligence. (At this time many of the most talented physicists in the world were working on nuclear weapons, after all.) As the astronomer Steven J. Dick has pointed out, "perhaps never in the history of science has an equation been devised yielding values differing by eight orders of magnitude. Each scientist seems to bring his own prejudices and assumptions to the problem.” Drake himself admitted the shortcomings of the equation, which was only devised as a way to articulate research directions in SETI rather than provide a concrete number on the prevalence of extraterrestrial life in the universe.

Scientific SETI

In a paper published to arXiv earlier this month, researchers from Oxford University’s Future of Humanity Institute argue that if anything, the fact that estimates from the Drake equation only vary by eight orders of magnitude is surprising. Due to the biases of scientists using the equation, this range should be much larger. In order to understand the Fermi Paradox, the researchers argue that the values in the Drake equation should not be concrete values, but ranges or probability distributions. By looking using distributions of values, they find that the Fermi Paradox isn’t much of a paradox at all because the odds are actually pretty good that we’re all alone in the galaxy, and perhaps even the entire observable universe.

The problem, according to the researchers, is that plugging numbers into the Drake equation as a ‘best guess’ implies certainty and results in misleading estimates. Probability distributions, on the other hand, allows for uncertainty to be captured in the equation.

To estimate the likelihood that we’re alone in the galaxy and/or the observable universe, the researchers used two different methods. One of them surveyed the current state of scientific knowledge about different variables in the Drake equation (for example, the probability of life emerging from a roiling prehistoric soup). The other method sampled published scientific papers that used the Drake equation to quantify the uncertainty about each variable according to current scientific knowledge.

The result of these methods were comparable and both suggested that there is a substantial probability that we’re alone. According to the researchers, there is between a 38 and 85 percent chance we’re alone in the visible universe and between a 53 and 99.6 percent chance we’re alone in our galaxy.

These are sobering results, but the researchers caution against any kneejerk cosmic pessimism. “This conclusion does not mean that we are alone, just that this is very scientifically plausible and should not surprise us,” the researchers wrote. “It is a statement about our state of knowledge, rather than a new measurement.”