At least 1,715 nearby star systems sat at a perfect angle to view Earth during the past 5,000 years, meaning aliens living in those systems, if they exist and have the right technology, could have watched our planet from afar as early human civilizations first emerged.

Of those star systems, 313 exited the special viewing zone, known as the Earth transit zone (ETZ), sometime in the past few thousand years, leaving 1,402 star systems capable of providing a glimpse of our planet today, according to a new study, published Wednesday (June 23) in the journal Nature. And over the next 5,000 years, 319 additional stars will enter the ETZ for the first time.

All of the identified stars lie within 326 light-years of the sun, and 75 of those stars are less than 100 light-years away. The team chose to search within this 326 light-year radius since that area is the focus of the European Space Agency's Gaia mission, which aims to create a 3D map of our galaxy, and new Gaia data recently became available.

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Given that humans began transmitting radio signals about 100 years ago, the 75 closest stars are near enough that "our radio waves would have washed over them already," first author Lisa Kaltenegger, an associate professor of astronomy and director of the Carl Sagan Institute at Cornell University, told Live Science.

Kaltenegger said she thinks this is the "most interesting subset" of stars for the search for extraterrestrial intelligence (SETI). But even the far-off stars in the viewing zone may have orbiting planets with the right conditions to support life, she said. The question is, is that life looking back at us?

Assuming that aliens exist and that some possess astronomical instruments similar to ours, extraterrestrial life-forms in these star systems could, theoretically, spot Earth and even detect signatures of life on the planet, in the form of atmospheric oxygen and methane, for example, Kaltenegger said. Because oxygen and methane react to form carbon dioxide and water, the two gases would need to be produced in large quantities to show up in a planet's atmosphere, she explained. On planets of a similar temperature to Earth, the only explanation for atmospheric oxygen and methane is the presence of life, she said.

Astronomers hunt for these signatures of life on faraway exoplanets by monitoring the stars those planets orbit, Kaltenegger added. From Earth, a star's light dims when a planet passes between the star and our telescopes; scientists analyze exactly how the light changes to determine the chemical composition of the passing planet's atmosphere.

This method of analysis works only for planets whose orbital paths happen to cross between their host stars and Earth, Live Science previously reported. That got Kaltenegger and her colleagues thinking about how many planets might "see" Earth in the same way, as a wandering rock that occasionally crosses in front of the sun. (Of course, this is assuming that these hypothetical aliens possess the same technology as us; it doesn't tackle the question of whether some alien civilizations have more advanced means of spotting us, Kaltenegger noted.)

The team tackled this question in 2020, using data from NASA's Transiting Exoplanet Survey Satellite and the Gaia mission. This data indicated that 1,004 star systems within 326 light-years of Earth can "see" our planet right now, the team wrote in a report published in October 2020 in the journal Monthly Notices of the Royal Astronomical Society.

Although that finding gave the team an idea of how many stars (and hypothetical aliens) could be watching us, it didn't answer one big question: How long do these stars actually remain in the ETZ? "The cosmos is dynamic, so the vantage point is not forever; it is gained and lost," Kaltenegger said.

In December 2020, the Gaia mission released more data, including a detailed census of stars located within 326 light-years of the sun. This so-called Catalogue of Nearby Stars was more complete than previous surveys and captured the movements of stars through time.

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"What Gaia gives you is the movement of the star over a couple of years," Kaltenegger said. Within a constrained time window, stars can be expected to move at about the same pace and in the same direction, unless they encounter a gravitational anomaly like a black hole, she said. So with the new Gaia data, Kaltenegger and her team could rewind the movement of nearby stars to essentially peer back in time. This allowed them to check where the stars were located 5,000 years ago and whether they provided a view of Earth at that time. They used the same method to look 5,000 years into the future.

"For the first time … we could take the movement of everything around us into account." she said.

Among the 2,034 stars the team identified, seven are known exoplanet hosts, they reported. This group includes the star Ross 128, which could "see" Earth in the past, as well as Teegarden's Star and TRAPPIST-1, which will be able to spot our planet in 29 and 1,642 years, respectively. The TRAPPIST-1 star hosts seven Earth-size exoplanets, four of which sit in the so-called Goldilocks' zone, where conditions are "just right" for liquid water to form.

Among the 75 closest stars the researchers identified, which may have received Earthlings' radio waves, they estimated that these stars may play host to 29 potentially habitable worlds. This estimate is based on the "pessimistic" assumption that only 25% of the stars have rocky, Earth-like exoplanets orbiting them, although the exact occurrence rate of rocky planets in the galaxy is unclear, the team noted. A recent analysis, based on data from the Kepler space telescope, suggested that about half the stars similar in temperature to our sun may host a rocky planet in the Goldilocks' zone, according to a 2021 report published in the The Astronomical Journal.

Now that the team has identified these star systems of interest, they can begin to narrow down which nearby exoplanets might be most interesting to probe for extraterrestrial intelligence. Scientists will be able to get a closer look at these exoplanets after the launch of the James Webb Space Telescope, a large infrared telescope whose cameras and spectrometers can pick up very faint signals, according to NASA. Thanks to its sensitivity, the telescope, which is expected to launch later this year, should provide detailed data about the atmospheres of far-off worlds, allowing astronomers to detect signs of life.

"I think SETI is the search for a very specific kind of life — one that would like to communicate with us," Kaltenegger said. "But life that wanted to communicate with us might be just a very small part of life-forms that hopefully exist in abundance in the cosmos," she said.

So far, humans have only ventured no farther than our own moon. To an alien watching from afar, "maybe we are just not that interesting — yet," Kaltenegger said.

Originally published on Live Science.