A new star map reveals more than 2,000 stars, some with their own planets, that have a direct view of our planetary presence
An illustration of the Earth and Sun from thousands of kilometers above our planet. Stars with a past, present, or future view of Earth as a transiting exoplanet appear brightened for emphasis. Credit: OpenSpace and American Museum of Natural History
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.