About 30 years ago, Carl Sagan pointed two different cameras toward the Earth, to see what we could see.
The first camera was strapped to Voyager 1, the probe that left Earth in 1977 for a tour of the outer solar system (also carrying with it the famed Golden Record that bears recordings of human music and whale songs, among other messages, for potential extraterrestrial listeners). In 1990, 13 years after Voyager’s launch with the probe now out past Neptune, 4 billion miles away, Sagan directed the camera to turn back toward Earth and take a picture. The resulting image, of a small, watery point against a vast black sky, became known as the Pale Blue Dot.
Sagan wrote of it, in the book inspired by the photograph, “Look again at that dot. That’s here. That’s home. That’s us. … Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves.” For all that he imagined interplanetary contact as a future turning point for humanity, he also hoped that awareness of our smallness in space could inspire humanity, beset by the threat of nuclear war, to save ourselves.
Then there’s the second camera. Far less famous was the scientific paper Sagan published in 1993 looking at the results of a 1990 investigation of a planet. On a close flyby, a different probe was able to detect the presence of water as gas, liquid, and ice; enough oxygen in the atmosphere that life was quite plausible; and, the loudest signal of all, modulated radio signals that could only conceivably be “generated by an intelligent form of life.”
That life, of course, was us, as that planet was Earth, observed during a flyby of the Galileo probe on its way to Jupiter. Together, the pale blue dot and the Galileo flyby kicked off the modern strain of a research project that continues to this day and in some ways predates Sagan and the whole modern era: trying to look at Earth as an exoplanet.
The latest contribution to this collection was published in the journal Nature in June, a paper by astronomer Lisa Kaltenegger (fittingly director of the Carl Sagan Institute at Cornell). Kaltenegger and her collaborators used new data from the European Space Agency’s Gaia mission to figure out which stars have, have had, or will in the next 5,000 years have the right view to detect Earth in orbit around the sun—using the same technology that we today use to detect planets around other stars. Kaltenegger told me that part of her inspiration—aside from the new availability of this Gaia data—was to help guide future searches for extraterrestrial intelligence; SETI research looks for signals extraterrestrials might be beaming to Earth. She said, “In the ’60s,” at the dawn of SETI, “they started to think if we wanted to send a signal, we would send it to a place where we know there’s a planet. … And so then you reverse that viewpoint.” Who might be out there who knows the Earth is here?
There are a few ways to detect exoplanets, but the most popular—the one, Kaltenegger estimates, by which 70 percent of known exoplanets have been found—requires having an edge-on view of any particular distant stellar system. That way, the planet comes between us and the star, not quite blocking its view in an eclipse but temporarily dimming the star’s light. Scientists hope to use this approach—known as the transit method—and the next generation of telescopes to begin to be able to learn more about exoplanets than just their size, density, and existence. By spying a star’s light filtered through an exoplanet’s atmosphere, astronomers hope to be able to deduce the atmosphere’s composition, which could mean finding potential biomarkers—signs of life.
Astronomer Enric Pallé, a research scientist at the Canaries Institute for Astrophysics, told me, “What we’re trying to understand from looking at the Earth as a planet is to know which are the observables that we can search for in exoplanets, [observables] we see on the Earth that can give us clues about what these planets are made of, and what are their atmospheres made of, and even try to infer if there is life on the surface.”
This was the gist of Sagan’s 1993 Galileo paper. “Life is the hypothesis of last resort,” he wrote, but with dramatic enough data it can be the only plausible explanation. Subsequent research in this vein has looked at all sorts of questions we’d like to answer about exoplanets and asked, Could someone on a distant planet answer these questions about the Earth? Pallé told me, for example, that while Earth’s CO2 cycle—the seasonal fluctuations of carbon dioxide as plants in the Northern and Southern hemispheres bloom and senesce—is a pretty decisive sign of life, it isn’t a viable way to assess an exoplanet. “No way,” he told me, “you will never see that on an exoplanet.” We measure the carbon cycle in parts per million in the atmosphere. “And on an exoplanet,” he said, “we have measurements at 10 or 20 percent accuracy.” The resolution just isn’t there.
Most studies that imagine Earth as an exoplanet do so by reducing it from the complex, detailed sorts of images we’re used to from satellites and local spacecraft to the single-pixel points that would be our best view of an exoplanet. (Imaging an exoplanet at all is tricky business, as planets are small and dim and very close to large, bright stars.) You can do this by taking your observations from very far away—turning a distant probe toward Earth—by compressing a closer global image taken by a satellite into a point, or by using earthshine reflected off the moon during a lunar eclipse, which makes a good stand-in for starlight filtering through an exoplanet’s atmosphere. Research using these methods has shown that a distant observer could detect the abundance of photosynthesis, measure Earth’s rotation, and even determine the general layout of our continents and oceans.
All of these approaches imagine a distant observer at some unspecified place, far enough away from the Earth to see it as a single point, but no more specific than that. Anthropologist Lisa Messeri, who has studied how astronomers turn data into worlds, says this imagined, distant location is an Archimedean point—that is, a point far enough away to offer a different, perhaps more objective or truer perspective. The philosophical term comes from Archimedes’ statement, “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” It’s a bit of collateral resonance, then, that by imagining viewing the Earth from an Archimedean point, astronomers move the Earth, too, if the Earth is the world from which we view the cosmos. Now, that viewpoint can be anywhere out there.
Kaltenegger’s recent study, though, doesn’t imagine an unknown, unspecified viewpoint. It proposes a real one—2,000 of them, in fact. As Messeri put it, “The viewer is emplaced on a star.” Or, to be plausible, on a planet orbiting a star. Messeri has written that much of astronomy research transforms exoplanets from astronomical objects into places, by describing them as worlds that can be experienced from the surface, not just observed through a telescope. “Scientific practices of place-making turn the infinite geography of the cosmos into a theater dotted with potentially meaningful places that are stages for imaginations and aspirations,” she writes in her book Placing Outer Space. But research that makes Earth an exoplanet does more than let us imagine ourselves someday inhabiting a distant planet—it evokes a distant planet that’s inhabited by someone else. And they’re looking at us.
With the latest Gaia data, Kaltenegger was able to deduce not only which star systems can detect the Earth’s transits right now, but who has been able to see us for the past 5,000 years, and who will for the next 5,000. Ross 128, a red dwarf star about 11 light-years from us known to have a planet just a bit bigger than ours orbiting in its habitable zone, first became able to see the Earth transit 3,000 years ago but lost that view around the year 1100. Kaltenegger said, “I started to wonder, if they’d seen a planet [a thousand years ago] and now all of a sudden, if there’s radio waves from the same part of the sky, would they put it together?” Would a Rossian astronomer think, Hey, I know my ancestors found a planet around that yellow star 10 centuries ago, maybe that’s who’s been watching all those sitcoms?
But this doesn’t mean Kaltenegger thinks we should expect a visit. She explains this with a thought experiment she told me she often offers to her students: “I tell them, I have found two exoplanets. Both of them have signs of life in the atmosphere, one of them is 5,000 years older than us, and one is 5,000 years younger than us. And I have money and resources to go to one of those.” Without fail, her students choose the more advanced planet to visit. But Earth, where we’re maybe 200 years past the Industrial Revolution, less than a century into transmitting radio waves to the cosmos—we’re going to be less advanced than almost any technological civilization that might be out there. “Earth is my favorite planet,” Kaltenegger said, “but maybe we’re not that interesting yet.”
She still feels the imagined kinship of extraterrestrial observers, though. “When I was doing this work, I was thinking that if there’s life out there, I really hope that somebody’s rooting for us.” Maybe, she mused, someone saw the Earth 2 billion years ago, when oxygen started building up in our atmosphere, and they kept an eye on us. More recently they would have seen us begin to destroy the ozone layer, and then fix it, and then begin to wreak havoc with climate change. She imagined them cheering us on, “Come on, come on, fix it! Hopefully there’s some nice thoughts out there.”
Astronomer Adam Frank has taken this vein of imagination in another direction, using a hypothetically inhabited cosmos not as a cheering section but as a community of fellow travelers, all facing the hurdle of what Carl Sagan called our technological adolescence—and sometimes, inspiringly, making it through.
In a 2018 paper, Frank modeled a simple version of the climate crisis: A civilization uses its planet’s resources to grow, and the growing population stresses the environment. Then, he added to the scenarios a turning point, a come-to-Jesus moment the likes of which we hope we may be in on Earth, where a civilization realizes it’s fucking up big time and switches to a lower-impact energy source. Frank found that depending on the timing and significance of the adjustments, and the environment’s sensitivity to stress, some civilizations continued their collapse, some leveled out for a while before failing, and, crucially, some were OK.
He wrote in his book, Light of the Stars, that in order to see the climate crisis as survivable, we need to see it as a feature of technological coming of age: “We are not alone. We are not the first.” By imagining Earth as just another planet, just another civilization hitting an inevitable growing pain, Frank hoped to help us see our current crisis not as a moral failing but as a problem to be solved. And a problem that could be.
Not every study that looks at Earth as an exoplanet so explicitly situates us among a galactic community, but that subtext is there. And that’s part of what makes these studies so compelling—and so powerful. They’re cool science (and scientifically important, too, directing further research), but they also capture our imaginations because they presume an inhabited cosmos, taking for granted that there could be other observers out there, other beings, other people, as curious about us as we are about them.
In my conversations with researchers doing this work, often they’d talk not about seeing Earth as an exoplanet, but as a planet. It could be a syllable-sparing shorthand, or something more meaningful. We don’t see Earth as a planet, after all, in daily life. It took math and insight for ancient thinkers to realize the Earth was a sphere and not a flat surface; it took Copernicus’ intuition and Galileo’s telescopes to realize that the untwinkling wandering stars of the heavens and Earth alike were all planetary spheres, all in their parallel orbits. To see Earth as a planet is to see it as one of many, among comrades in the solar system and kin in the cosmos, just one of what turn out to be bountiful worlds. In the 17th and 18th centuries, these discoveries opened the possibilities of imagining other worlds as inhabited, with humanity’s cousins and fantastical beasts. Today, those imaginings are more muted, sometimes only implied, but the possibilities of the cosmos—of someone out there seeing us, of someone, anyone else—thrill us nonetheless.