by Avi Loeb
In the quest to find extraterrestrial life, no question is more important than whether life is broadly deterministic and quite probable or a consequence of random and improbable events. In other words, do the same basic conditions always give rise to life? Or was the emergence of life on Earth a freak occurrence that is very unlikely to happen again?
Numerous fields of study are advancing these questions on all fronts. As they do, one simple observation looms large: the single, substantial source of data we have—namely, Earth—is astoundingly fecund. The factors that allowed life to emerge on Earth, crucial among them our planet’s distance from the Sun, didn’t result in merely a few microorganisms huddled around hydrothermal vents on ocean floors. They produced a cornucopia of life of such rich complexity that today’s flora and fauna rest atop an entire era of reptiles that preceded them. For us to believe that teeming life would be restricted to a single blue marble in the entire expanse of the universe seems the very height of hubris.
Nearly all life on Earth is dependent on the Sun. Not for nothing have humans worshipped it from the dawn of our civilization to the last time you spent an hour lounging on a beach towel. We are literally the stuff of stars; the matter we are made of was produced in the hearts of exploding stars, which then formed planets like the Earth, which then became the material of all terrestrial life, including you and me. And without the warmth and light of the Sun, there would be no plants, no abundant oxygen, no life as we know it.
It is no exaggeration to state the majority of complex multicellular life on Earth is directly or indirectly dependent on the Sun’s existence. But what should the search for extraterrestrial life take from this fact? How can we use the known certainty that the Sun supports conscious intellectual life to inform our search for life elsewhere?
Habitable-zone boundaries around stars with different surface temperatures (vertical axis), ranging from the most abundant dwarf stars, like Proxima Centauri, to rare giant stars, like Eta Carinae. The horizontal axis shows the flux of light shining on the planets’ surfaces relative to sunlight on Earth. Various known planets are labeled in the diagram. The nearest habitable planet outside the solar system, Proxima b, appears near the bottom right.
Image by Mapping Specialists, Ltd.
Knowing whether or not our Sun is anomalous would tell us a good bit about how anomalous (or not) the life that it supports is. If the Sun is a typical host in all respects, and the presence of sentient life in its vicinity is exceptionally rare, if not unique, then our existence is most likely the result of random chance and unusual indeed. But if the Sun is atypical in certain respects, perhaps those atypical characteristics are required for life, making our existence less random and less unique. That, in turn, would make our search for extraterrestrial life less random, for we would have reasons to examine stars like our own.
As it happens, the Sun-Earth system is anomalous in two clear respects. First, the Sun’s mass—330,000 times that of Earth—makes it more massive than 95 percent of all known stars. And while this does not rule out our interest in searching for life on planets orbiting more statistically average stars, given that we have limited resources of time and money, it encourages us to look for stars that are especially massive, like the one that sustains us.
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The qualities of the Sun encourage us to direct our search for extraterrestrial life—at least initially—to stars that are similar to our own. The qualities of Earth guide our search as well, particularly when it comes to picking which planets to study first.
The observable data from Earth, the one planet that we know supports a dense and complex biosphere, allows us to compose a short list of features we should look for on other planets. But paramount among all the parameters that are essential for Earth’s habitability is the existence of liquid water.
Liquid water, often called the universal solvent, is ideally suited to transport energy into and waste out of cells, and no terrestrial life has been discovered that is able to exist without it. It is so important for life, in fact, that astronomers use it to define the habitable zone around each star, measured by a planet’s orbital distance from the center of the solar system. Identifying planets at the Goldilocks distance from a star, that zone in which water neither freezes nor evaporates, is the astro-archaeologist’s starting point in the hunt for alien civilizations.
The universe, it turns out, provides us with an embarrassment of places in which to look.
Over the past two decades, we have learned that the universe contains numerous exoplanets (the technical term for any planet residing outside the solar system). This spate of discoveries began in 1995, when astronomers Michel Mayor and Didier Queloz became the first to find definitive observational evidence for an exoplanet—a close-in, Jupiter-like planet, 51 Pegasi b, around a sun-like star—based on the line-of-sight motion of this star as the exoplanet orbited it. Their pioneering work ushered in the new era of hunting exoplanets and earned them a Nobel Prize in 2019.
The basic contours of this research were not, in fact, new; they had been put forth four decades earlier by the astronomer Otto Struve, who proposed that the hunt for alien planets might profitably target gas giants zipping around their parent stars in tight orbits, big worlds going around their stars within a few Earth days. The existence of such planets, Struve argued in a 1952 paper, was suggested by the evidence that some binary stars (a pair of stars bound together by gravity) whip around their common center of mass in a similar manner. And these big exoplanets should be relatively easy to detect via their powerful gravitational tugs on their host star or their blocking of light during transits across the star’s face.
But Struve’s paper was ignored, as was his proposal to search for close-in Jupiters. The scholars that sat on the time-allocation committees for major telescopes asserted that it was commonly understood why Jupiter lies as far from the Sun as it does, and they saw no reason to waste telescope time in searching for exo-Jupiters that were much closer to their host star. Their prejudice slowed down scientific progress by decades.
Once exoplanets were legitimized as part of the mainstream, their discovery accelerated rapidly. Within a decade after the discovery of 51 Pegasi b, hundreds of other exoplanets were identified. And with the 2009 launch of NASA’s Kepler Space Telescope, built for the explicit purpose of identifying exoplanets, that number, at the time of this writing, has jumped to 4,284, and thousands of candidates await confirmation. What is more, we now know that about a quarter of all stars are orbited by planets of Earth’s size and surface temperature, planets that might have liquid water—and the building blocks of the chemistry of life—on their surfaces.
The abundance of exoplanets upon which we could fix our observational equipment reminds me of a common Jewish tradition during Passover Seder: the hiding of a piece of matzoh, called the afikomen. The task for the children of the household is to find it, and whoever is successful receives a reward.
What I learned as a child—and what I am mindful of now as an adult in the nascent field of astro-archaeology—is that the question “Where to look?” trumps the question “What exactly are we looking for?” And my sisters and I also quickly learned that the best places to start looking were the places where the afikomen had been hidden in the past.
Today, this same strategy is guiding the search for extraterrestrial life. Most of our telescopes and observational instruments are seeking evidence of life on rocky planets with features—most crucially liquid water—that are consistent with the one place we know life exists: Earth.
But is this all we can do? Is there anywhere else we might look, even if we restrict ourselves to the orbits of alien stars?
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Exoplanets that seem similar to Earth aren’t the only places where we could seek life. Additional research that I conducted with my postdoc Manasvi Lingam suggests another highly promising place to seek the chemistry of life: in the atmosphere of so-called brown dwarf stars.
Brown dwarfs ar
e small, less than 7 percent of the mass of our Sun. And since they do not have enough mass to sustain the nuclear reactions that cause other stars to burn so bright (and so hot), they can cool to planetary temperatures. This could result in liquid water existing on the surfaces of small, solid particles in the clouds orbiting a brown dwarf.
We needn’t stop with brown dwarfs. We should also consider examining green dwarfs, dwarf stars that show the telltale “red edge” in reflected light that is evidence of photosynthesizing plants. By our calculations, green dwarfs orbiting sun-like stars might be our best bet for locating an astrobiological afikomen.
Green dwarfs, brown dwarfs, and exoplanets in the habitable zone of suns—these options in no way exhaust the possibilities for astro-archaeologists, especially when you posit civilizations far more technologically advanced than our own. But at this stage of the search for extraterrestrial life, when our theorizing, observational tools, and efforts at exploration are in their relative infancy, these are the best targets available to us. Outside of our own solar system, that is.
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Even as we contemplate seeking out life in interstellar space, we must admit that we’ve not exhausted the possibilities within our own solar system; astro-archaeologists should also seek evidence of extraterrestrial life in our planetary backyard.
We could start by looking for technological equipment floating through the solar system. Much as we spotted ‘Oumuamua, we might discover—and obtain conclusive evidence about—other artificial objects that originated from other stars. In the first century of our own technological revolution, we sent Voyager 1 and 2 out of the solar system. Who knows how many more such objects an advanced civilization might have launched?
The simplest way to detect passing alien technology is to search under the nearest, largest, brightest lamppost—the Sun. Just as happened with ‘Oumuamua, sunlight provides us with valuable information about objects’ shape and motion, and it also makes them more visible. We need all the help we can get in this search because, for the moment, our tools for spotting objects like ‘Oumuamua are relatively primitive.
As I explained toward the beginning of this book, the telescopes that discovered ‘Oumuamua did so accidentally; they were all designed, built, and deployed to accomplish other things. The earliest space archaeologists will likewise have to repurpose existing astronomical tools, at least until such time as the world provides them with instruments explicitly built to their purposes.
In the meantime, perhaps the easiest way for us to look for alien technology in our solar system—and certainly the best opportunity we will have to actually lay hands on it—is to devise a method to detect it as it collides with the Earth. This would require us to find a way to use the Earth’s atmosphere to search for artificial meteors. If the object is bigger than a few meters, it could leave behind a remnant meteorite that—if we could detect and track it—might yield the first tangible evidence of extraterrestrial technology.
We can also search the surfaces of the moon and Mars for extraterrestrial technological debris. Whether we liken the moon (which has no atmosphere or geological activity) to a museum, a mailbox, or a dumpster, we can say one thing for sure: it keeps a record of all objects that crashed onto its surface over the past billions of years. Without checking, however, we will never know if it contains the equivalent of a statue, a letter, trash—or nothing.
We needn’t restrict ourselves to planet surfaces. Jupiter, for example, could serve as a gravitational fishing net that traps interstellar objects that pass near it. Given scientists’ current blinders regarding what’s out there, they assume they’ll recover only natural rocks or icy bodies like asteroids and comets. And no doubt that is the majority of what we would encounter. But perhaps it is not all we would encounter.
Given the rich reward of such a find, we should make the effort. Yes, it will be vastly more expensive and vastly less certain than low-key surveys, but such an undertaking would be in the same vein as my family walking along a beach examining shells. Perhaps tomorrow’s space archaeologists will find the equivalent of an extraterrestrial civilization’s plastic bottle.
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The more tools we arm tomorrow’s archaeologists with, the farther they can extend their search. As I postulated in collaboration with Ed Turner, from the outskirts of our solar system, one can look for artificial lights that originate from distant cities (or perhaps giant spacecraft). One can distinguish an artificial source of light from an object reflecting sunlight by the way it dims as it recedes from us. A source that produces its own light, like a light bulb, dims inversely with distance squared, whereas a distant object that reflects sunlight dims inversely with distance to the fourth power.
One such tool that space archaeologists could use to great effect is the advanced instrumentation at the Vera C. Rubin Observatory. This wide-field reflecting telescope is expected to commence its survey of the sky in 2022. Along with mapping the Milky Way and measuring weak gravitational lensing to give possible insights into dark energy and dark matter, it is expected to increase humanity’s catalog of objects in the solar system by a factor of ten to one hundred. The Vera C. Rubin Observatory is far more sensitive than any other survey telescope, including Pan-STARRS—which, of course, discovered ‘Oumuamua.
With our newfound ability to peer farther beyond the solar system than ever before, we could search for artificial light or heat redistribution on the surface of a planet. Breaking out of the constraints of the Drake equation, we could look for technosignatures beyond just communication signals. To see how this might work, consider an exoplanet that is already in our sights.
The tidally-locked planet Proxima b orbits the habitable zone of Proxima Centauri, the star nearest to our Sun. When my colleagues and I worked on the Starshot Initiative, Proxima b was the exoplanet we identified as a possible target for our lightsails. Though Earth-size, the rocky Proxima b always faces its star with the same side. You may recall that my younger daughter pointed out that it would make sense to own two houses on such a planet, one on the permanent-day side where it is always hot and bright and another on the permanent-night side where it is always cold and dark.
But an advanced civilization might find a solution of greater technological sophistication. As I argued in a paper with Manasvi Lingam, the planet’s inhabitants could cover the day-side surface with photovoltaic cells that would generate sufficient electricity to illuminate and warm the night side. Were we to focus our instruments on such the planet, the varying level of light from its surface as it moves around its star could tell us if a global engineering project of this type had taken place, and the solar cells on the day side would also produce distinctive reflectance and color. Studies to seek out either phenomenon can be done just by monitoring the planet’s light and color as it orbits its host star.
This is just one example of the range of telltale signals that space archaeologists could train their instruments to seek. As our own planet suggests, they could look for evidence of industrial pollution in distant atmospheres. (Indeed, several years before ‘Oumuamua appeared in our solar system, I wrote a paper with my undergraduate student Henry Lin and the atmospheric expert Gonzalo Gonzales about searching for industrial pollution in the atmospheres of exoplanets as a signature for advanced civilizations.) And while the atmosphere’s contamination by a blanket of pollutants could signal one civilization’s failure to outrun the filter, it could signal a different civilization’s efforts to intentionally warm up a planet that was otherwise too cold or cool down a planet that was deemed too hot. Astro-archaeological excavations conducted light-years away from their objects of study could also include a search for artificial molecules, such as chlorofluorocarbons (CFCs). Long after a civilization has expired and so ceased to send out deliberate signals, some molecules and surface effects of industrial civilization will still survive.
The sandbox of space archaeology stretches, of course, to the edge of the universe. There is no reason to confin
e our searches to planets. With that understanding, some scientists could dedicate their efforts to searching for flashes of light from beams sweeping across the sky from great distances. Such beams might indicate a civilization’s means of communication or a means of propulsion. When humanity has taken the necessary steps to send lightsail craft out into the universe using the method my team devised for the Starshot Initiative, such technology will result in bright flashes visible to others due to the inevitable leakage of light over the edge of the sails.
In addition, one could search for a swarm of satellites or megastructures that block a significant fraction of the light from distant stars, a hypothetical system known as a Dyson sphere, after Freeman Dyson, the late, great astrophysicist who first envisioned it. Such gigantic megastructures would confront major engineering challenges, and if they exist, they would be rarities. But they also present a possible technological solution to the great filter, and given foresight, means, and opportunity, a civilization facing its own extinction could set out to overcome those challenges. Finding out if such things exist, however, begins with our seeking evidence of them.
The contemplation of such megastructures raises a recurring question that space archaeologists will have to overcome, given that their efforts must presuppose the existence of intelligences greater than their own. That a project like a Dyson sphere strikes humans as overwhelming—indeed, impossible—may simply reflect the fact that we are not yet intelligent enough to undertake it. A civilization far more advanced than our own may well have overcome the hurdles that we, from our more limited understanding, see as insurmountable.
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