Space, July 14: 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 aliens world where the residents—past, present, or future—would have a chance of detecting Earth as a transiting exoplanet.

It 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 the month of 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, but it also gives a unique—and arguably, unsettling—viewpoint of our visibility to the rest of the universe.

As the first study of its kind to take into account the changing vantage points of stars as they have shifted over time, builds upon previous research that used only their current positions in the cosmos.


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The net 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 aliens being living on planets circling these stars would, with mature enough technology, be able to detect Earth’s presence for at least a thousand years. 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 catalogue 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.

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.)


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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. 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 travelling further through space, making us even more susceptible to being found. Aliens seeker on Earth has 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 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.

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.


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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.

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.

In the meantime, Kaltenegger and Faherty continue to chart out which of our galactic neighbours 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.