Are we alone in the universe? The answer is almost certainly no. Given the vastness of the cosmos and the fact that its physical laws allowed life to emerge at least one place—on Earth—the existence of life elsewhere is effectively guaranteed. But so far, despite generations of looking, we haven’t found it. In that time, however, we’ve arguably learned enough to declare that, while we may not be alone, the interstellar gulf between us and our nearest neighbors effectively puts us in an isolation ward. This doesn’t mean we should stop looking—only that we should manage our expectations and prepare for a long and lonely voyage through space and time before meeting them, either virtually or physically.
The possibility of alien life has been discussed since antiquity. But rigorous searches for it have only been within reach for less than a century, following an approach first proposed in 1959 by physicists Giuseppe Cocconi and Philip Morrison, who showed the feasibility of interstellar communications using radio telescopes. A year later astronomer Frank Drake led the first Search for Extraterrestrial Intelligence (SETI) effort, called Project Ozma, which used facilities at the National Radio Astronomy Observatory in Green Bank, W. Va., to look for such signals from putative cosmic civilizations. As part of the preparation for a follow-up meeting on the project, he developed his now famous “Drake equation,” a probabilistic mathematical expression for estimating the number of communicable civilizations, NC, that may exist in the Milky Way.
We are the products of progressive stages of evolution. With this in mind, the Drake equation can be cast as:
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NC = astrophysical evolution × biological evolution × cultural evolution × technological evolution × lifetime of a technological civilization
In that equation, each successive evolutionary phase arises from its predecessor. For us, this progression has taken nearly the entire lifetime of the Earth, some 4.5 billion years.
Attempts to “solve” the Drake equation have been hampered by insufficient knowledge of the likelihood of each evolutionary phase’s completion. Modern observational advances, however, now allow us to make much more reliable estimates of astrophysical evolution, chiefly via finding and studying what are generally regarded as “Earth-like” exoplanets in our galaxy—that is, other small rocky worlds in temperate orbits around their stars. Astronomers’ catalogs now boast over 5,700 confirmed exoplanets, some of which reside in their star’s habitable zone, where starlight can warm them just enough that liquid water can exist on their surfaces. If we extrapolate these results to the entire Milky Way, the estimated number of Earthlike planets, NE, is on the order of three billion. Knowing this empirical result, it is no longer necessary to include an estimate of a technological civilization’s lifetime in the Drake equation, which can now be simplified to:
Nc ≈ αNE
Here, the variable α (alpha) is the fraction of Earth-like planets on which evolutionary thresholds associated with life, intelligence and technology have been achieved. Until alien life is discovered beyond Earth (or, perhaps, created in a laboratory), the biological prerequisites for α will remain highly speculative, with musings about unearthly cognition and technology being even less constrained. For the sake of argument, though, let us assume there is a one-in-100 chance for each of these evolutionary thresholds to be met. In which case:
α = 0.01 × 0.01 × 0.01 = 0.000001
That is, in this (probably optimistic) scenario, the chance that evolution on an Earth-like planet leads to a civilization capable of interstellar radio communication would be literally “one in a million.” If so, the number of communicative civilizations now extant in the Milky Way would be about 3,000. If their home worlds are evenly scattered amongst our galaxy’s hundreds of billions of stars, the average distance between them, rs, would be on the order of 3,000 light-years. Contact via direct interstellar travel thus appears to be exceedingly unlikely. Even under ideal conditions, two-way communications via radio waves (or any other forms of light) would require, on average, circa 6,000 years—a timescale that in terms of cultural longevity may prove challenging, to say the least.
Therefore, as has been suggested many times in the recent past, interstellar communications between civilizations will by necessity be one-way, like a traditional radio or TV broadcast. Our best homegrown example occurred just over 50 years ago, when Frank Drake led the transmission of a message to the globular cluster M13 some 25,000 light-years away on a powerful beam of radio waves from the 1,000-foot-wide reflector dish of Arecibo Observatory in Puerto Rico. This was essentially a symbolic effort—a tech demo that lasted only three minutes—but nonetheless marked a new evolutionary milestone for humanity.
The possibility of another planet emerging from the ravages of evolution since then is small—which suggests in turn that, of all possible communicative civilizations in our galaxy, we may well be the youngest and least advanced. Since it is technically easier to receive radio waves than to transmit them, it makes more sense for us to be in the role of listeners than broadcasters. Searches for less deliberate and directed “technosignatures” from more advanced civilizations are possible. Yet a preliminary search of more than 100,000 galaxies for signs of waste heat from rampant alien technologies yielded fewer than 100 targets showing any enticing infrared excess (all of which may be explained without resorting to pan-galactic supercivilizations). This somewhat disconcerting conclusion and the ongoing null results from decades-long radio SETI efforts does not rule out the possibility that such endeavors may eventually be successful but does reinforce the notion that we are effectively on our own.
For now, our best chance for finding life elsewhere and constraining the value of α will be in the search for biosignatures in the atmospheres of a statistically significant number of exoplanets and, more directly, by a thorough search for life elsewhere within our own solar system. These investigations don’t require millennia. Indeed, they can be completed within a human lifetime using existing technology. We’re already in the process of launching fleets of robotic explorers to seek out life on Mars, as well as on various ocean-bearing moons of the giant planets. We’re also planning advanced space telescopes, such as NASA’s Habitable Worlds Observatory (which may launch in the late 2030s or 2040s), to spectrally survey dozens of potentially Earth-like worlds for signs of life.
Given such seemingly long odds, one might wonder why we should bother looking. Cocconi and Morrison had an answer for this right from the start, noting in their 1959 paper that “the probability of success is difficult to estimate; but if we never search, the chance of success is zero.” Consider this: If just one of these searches were to yield positive results, that would suggest the universe is truly teeming with life. If all the results were negative, that would be the best empirical evidence yet that we are the winners of a cosmic lottery, inhabiting a profoundly precious planetary oasis in a vast galactic desert. If scientists could show we’d hit such a massive jackpot, might we take more care with our world and one another, lest we squander it? I like to think so.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.