In the search for extraterrestrial life, we’re usually the ones doing the snooping. But Lisa Kaltenegger, an astronomer at Cornell University, wanted to know who out there might be watching us. “For whom would we be the aliens?” she asks.
So Kaltenegger enlisted the help of Jackie Faherty, an astrophysicist who works at Hayden Planetarium, part of the American Museum of Natural History, in New York City. Together, they took on the task of identifying stars that might host alien worlds where the residents—past, present, or future—would have a chance of detecting Earth as a transiting exoplanet. That means their planet would have just the right vantage point to observe a slight dip in the brightness of our sun as Earth crosses, or transits, in front of it. This is the most successful method we Earthlings use to find planets beyond our solar system as they orbit around their own host stars, creating tiny blips in the light we can see with astronomical instruments.
In June, Kaltenegger and Faherty announced their results in Nature with an extensive inventory of stars that have either had, or will later have, the proper orientation to discover our planet. They identified over 2,000 stars, using a time range from 5,000 years ago, when civilizations on Earth first began to bloom, to 5,000 years into the future. Not only does the study provide a resource to exoplanet hunters by pinpointing which stars they should pay attention to, it also gives a unique—and arguably, unsettling—viewpoint of our visibility to the rest of the universe. “I felt spied on a little bit,” Faherty says, remembering the uncanny sensation of being overexposed. “Do I want to be on a planet that can be found?”
“It’s a lovely piece of scientific poetry, to think about the way all of these objects are moving through space in this elaborate ballet,” says Bruce Macintosh, an astronomer at Stanford University who was not involved in the work. As the first study of its kind to take into account the changing vantage points of stars as they have shifted over time, it builds upon previous research that used only their current positions in the cosmos. “We can now construct movies of how the universe will look 5,000 years from now in the future, imagining all of the stars winking out as planets get in their way,” he says.
The new result was made possible thanks to the latest data release from the European Space Agency’s Gaia mission, an orbiting observatory with the ambitious goal of creating a three-dimensional map of the positions and velocities of a billion stars. Combined with the planetarium software Faherty uses to visualize stellar motions, she and Kaltenegger found exactly 2,034 stars within Earth’s transit zone. For nearly all of them, any alien beings living on planets circling these stars would, with mature enough technology news, be able to detect Earth’s presence for at least a thousand years. “In the cosmic time scale, that’s a blip on the radar,” says Kaltenegger.
But for human lifetimes, she says, it gives astronomers ample time to develop the tools necessary to peer into other worlds. Kaltenegger and Faherty hope astronomers will use the catalog to find new planets, especially around stars that aren’t very well known or well studied. From there, large-scale missions like NASA’s future James Webb Space Telescope, set to launch by the end of the year, can be used to study planetary atmospheres and look for signs of life. “This is a treasure trove of planets just waiting to be discovered,” Kaltenegger says. “I’m looking forward to what people find.”
The scientists identified 75 stars that were, or still are, close enough for any nearby planetary residents to detect the signals we have been unintentionally sending into space for the past 100 years via television and radio broadcasting. Another 42 stars will enter this zone in the future, with one reaching this vantage point in the next three decades. Of these stars, the researchers conservatively estimate that 29 have rocky planets like our own existing within the star’s “habitable zone” that is temperate enough for liquid water to exist. (Four of these stars have planets that have already been discovered.)
That begs the question: Should we be actively trying to make contact, or hide? John Asher Johnson, an astrophysicist at Harvard University, says that hiding is not an option—if intelligent life exists, they could find us. “We are a civilization that relies very heavily on the radio transmission of information across the globe,” he says. These signals aren’t limited to Earth-bound antennae, but “can be picked up by receivers across the galaxy” up to a hundred light years away. That range will only grow with time as the signals keep traveling further through space, making us even more susceptible to being found. Alien seekers on Earth have been using the same technique for the past 20 years at the SETI Institute, analyzing data from radio telescopes in search of civilizations on other worlds that might be transmitting similar signals into space.
Macintosh agrees that it’s too late to shield proof of our existence, especially across the span of 10,000 years, because any society with technology news comparable to—or better than—ours would have seen Earth’s atmosphere change as we pumped carbon dioxide into the air. (Earlier this year, other researchers published a paper arguing that we could find advanced civilizations by looking for their smog.) But Macintosh also says that it’s a very human-centric approach to assume that aliens would use the same tools we do to explore the universe. “At this instant, transits are the way we’ve discovered most exoplanets,” he says. “But that wasn’t true 20 years ago, and it’s probably not true all the way into the future, either.”
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In his own work, Macintosh uses direct imaging, in which researchers attempt to block out the host star’s light, and then take infrared pictures of the faint dot of a planet next to it. Direct imaging is difficult, and at times impossible, to achieve because stars are so much brighter than the planets around them. But when it can be done, it’s a much more flexible approach, since, unlike detecting transits, it doesn’t require a special orientation between star, planet, and observer. Despite the popularity of the transit method, Faherty says the chances to “hit that bull’s-eye” with just the right vantage point between all three objects is slim.
And while transits are great for detecting planets orbiting close to lower-mass stars, it doesn’t mean those are the only places worth looking. With proposed advancements in telescopes over the next couple of decades, Macintosh thinks direct imaging would be better suited to find Earth-like planets with distant orbits around more massive stars, like ours. “Transits are a bit like the ‘looking for your keys under the streetlight’ joke,” he says, in that they work well for places that are easiest to see.
One underexplored location in the search for habitable worlds is around white dwarfs, the dense, stellar corpses left over when a star explodes. Last year’s discovery of a Jupiter-sized planet circling a white dwarf made scientists reconsider the possibility of finding life in unlikely places. “If life could survive even the death of its star,” Kaltenegger says, “then the future of the universe would be a lot more interesting.” She and Faherty have identified over a hundred white dwarfs in their star catalog for astronomers to study.
Plans are already set for the two researchers to expand upon this work, as they anticipate the next Gaia data release in December 2022, which will fill in missing information about the movement of the stars toward and away from Earth. With this precision, Kaltenegger and Faherty will be able to reach across cosmic time even further, up to a million years in either direction. Someday, Kaltenegger hopes, scientists will be able to cover a 2 billion-year span, stretching all the way back to when life on Earth first started to alter our atmosphere.
Faherty also dreams of eventually sharing this work at the Hayden Planetarium with something like an immersive three-dimensional flight simulation, where visitors can “take off” on a spaceship and experience the motion of the stars that they otherwise could never see. “This is how we can tell the stories of astrophysics, of what we’re doing as researchers, by bringing it to the public and really showing people how we do our science,” she says.
In the meantime, Kaltenegger and Faherty continue to chart out which of our galactic neighbors may also be searching for us, and how their vantage points would shift across time. They liken the nearest stars to ships passing in the night; those with the shortest windows for detecting us might zip right by without a trace. But faraway spectators, ones with a higher chance of catching a transiting Earth, would find a very different world than the one we live on—and given the interstellar distances signals have to travel to reach them, they may not spot us until we are gone.
“This is a reminder that we are all in motion,” Faherty says. Our planet continues to move around the sun, the sun moves around the galaxy, and nothing in the cosmos stays the same. “Perspective,” she says, “is everything.”
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