On June 25 the Pentagon and the Office of the Director of National Intelligence released their much hyped report on unidentified aerial phenomena, or UAP. Space alien enthusiasts and skeptics alike awaited it with bated breath. And while the report did not rule out an extraterrestrial origin for much of the documented UAP, it was short on details or bombshells.
But we already know our world is easily detectable by extrasolar observers. A paper published on June 23 in Nature shows that in the past 5,000 years, 1,715 stars have been in the right celestial position to view a populated Earth transiting the sun—with 319 more entering this sweet spot in the next 5,000 years. And seven of these far-off stars are known to have their own orbiting exoplanets that might support life.
“Instead of constantly saying, ‘What can we detect from other worlds?’ and ‘Where are the other worlds that we can detect?’ think about it the other way,” says Jackie Faherty, an astronomer at the American Museum of Natural History in New York City and a co-author of the new study. “What worlds can find us? How many of them and for how long?”
Lisa Kaltenegger, an astronomer at Cornell University, approached Faherty with the idea to create a map showing which nearby stars could see Earth in the past and future. “I wanted to do a billion years!” Kaltenegger says of the proposed time line. “And I was like, ‘No, there’s a finite clock backtrack you can do,’” Faherty explains.
The data set the two researchers used came from the Gaia mission, a spacecraft launched by the European Space Agency in 2013 to tally and track more than a billion stars throughout the Milky Way. It uses a distance-measuring technique called parallax, which can be understood by simply winking one eye, then the other and noticing how objects in your field of view shift in proportion to their proximity to you. “Your eyes are separated by a small amount of a distance, and that distance between your eyes is what allows you to measure depth,” Faherty explains. That is what Gaia does, too, except its baseline is roughly the span of Earth’s orbit around the sun rather than the space between a person’s eyes. This longer baseline allows the spacecraft to more precisely measure celestial distances and motions. But just as with your eyeballs, there is still some uncertainty in establishing the exact kinetics of these uberdistant objects, Faherty says.
So the pair settled on a 10,000-year window stretching from 5,000 years ago to 5,000 years from now. The time line is conservative, Faherty says, considering Earth is 4.55 billion years old. But the temporal component is still especially significant because everything in space is moving over time, says René Heller, an astrophysicist at the Max Planck Institute for Solar Systems Research in Göttingen, Germany, who was not involved with the study. “What’s happening in space is dynamic—it’s not a static picture!” he says.
From the Gaia data set, Faherty and Kaltenegger picked out the stars within about 300 light-years of our sun—those “in our neighborhood,” Faherty says. Thanks to Gaia and other surveys, the researchers already knew how fast each star is moving, so they pushed the stars’ trajectories backward and forward through time on a big virtual map. This approach allowed them to determine when and where these neighborhood stars entered, or will enter, the so-called Earth transit zone, or what Faherty calls the “bull’s eye in the sky”: the area where a star may be aligned just right to get a glimpse of our world crossing the face of the sun.
That is the same method astronomers here on Earth have used with great success to find and study thousands of worlds around other stars. By monitoring a star continuously, observers can seek out a regular pattern of “dimmings and rebrightenings” produced by shadowy planets parading across the star’s face as seen from our solar system. This remarkable method does not just tell us if there are planets encircling a star—it also allows observers to scry the bulk chemical composition of the planet’s air via starlight shining through its upper atmosphere. “When the planet passes in front of the star, it leaves a spectral fingerprint, as we call it—information about its atmosphere in the starlight,” Heller says.
Kaltenegger and Faherty’s study is not, it turns out, the first to look for other planetary systems that could catch Earth in transit. Heller and one of his colleagues created a similar map in 2016, although that earlier work tallied just 82 stars that would be aligned in the right position—and it did not implement the temporal component that the Gaia data set allowed Kaltenegger and Faherty to include in their new paper. “We thought about whether others might look for transiting planets as we do but from an extrasolar perspective,” Heller says of his previous work. “And some of them might be lucky in seeing us earthlings transiting in front of the sun.”
Looking at Earth and the solar system from this flipped perspective is extremely valuable, Kaltenegger says. “The most impressive image ever, I think, is the pale blue dot picture that Carl Sagan helped to make.” In that famous photograph, captured by the outbound Voyager 1 probe beyond the orbit of Pluto, a minuscule pinprick of light (Earth) hangs in a diagonal sunbeam against the dark void of space, its vaguely cyan color hinting at the presence of watery oceans and clouds. The image is a viscerally visual depiction of William Blake’s oft-quoted musing about glimpsing “a world in a grain of sand,” showing how even a single pixel of planetary light falling on some faraway detector can reveal surprising amounts of astrobiologically relevant information. The view from Voyager 1 is a testament to the chilling, exhilarating fact that, just as we can see ourselves from the interstellar depths, others can, too.
Seven of the stars mapped by Kaltenegger and Faherty are known to host possibly rocky exoplanets thought to be passable candidates for harboring liquid water—and thus life as we know it—on their surface. One of these, the world called Ross 128 b, was in Earth’s transit zone for about 2,000 years. It “saw” our planet between the 10th century B.C. and the 10th century A.D., a time period comprising the reign of Alexander the Great, the fall of Rome and the zenith of the Mayan civilization. But the best known view is yet to come and exists around another star called TRAPPIST-1. This star is encircled by seven approximately Earth-sized planets. Four are at the right distance from TRAPPIST-1 to conceivably support life, Kaltenegger says. The star and its retinue of worlds will enter Earth’s transit zone in about 1,600 years.
These stars and related systems should be priority one for current and future efforts to seek out exoplanets that may bear not only life but perhaps even alien technological civilizations, some astronomers say. “I would put the targets mentioned at the top of the list,” says Jill Tarter, chair emeritus for research on the search for extraterrestrial life (SETI) at the SETI Institute, who was not involved with the new study.
Back on Earth, how might we prepare for when TRAPPIST-1 enters our sight line in 1,600 years—or for when any exoplanets do so, for that matter? Heller says that it is a somewhat futile exercise to make 37th-century plans with 21st-century technology. And humanity might not even be here that far into the future, an option Heller has whimsically considered for imagination’s sake. We could install some sort of co-orbiting giant sign or apparatus that would imprint its shadow on the sun’s outgoing light alongside our planet but would stay locked away if someone pressed a button once per year, he says. The sign would thus inflate once we were gone (or had forgotten about it) to display the message “Nice not knowing you” to any beings that came across it in our absence, he jokes.
Faherty says that beyond orienting searches for extraterrestrial life, she hopes this new star map inspires and opens minds. The project expanded how she thinks about our chances of encountering other worlds, she says. “I got an eerie feeling of ships passing in the night [doing this work],” Faherty adds. Heller says he also has a strange feeling about being contacted. “Think of living in a room, and all the windows are open, and you do all your business, and you don’t know that all the windows are open,” he says. “Would you behave differently if you knew that you are being seen all your life?”