For decades, astronomers have probed the universe for signs of life by watching distant planets glide past their host stars, studying the tiny dips of starlight that give away clues about an exoplanet’s atmosphere. Hundreds of those worlds could bear life. And if there’s life, what’s to say it isn’t looking back at us?
New research published Wednesday entertains that question, finding nearly 2,000 star systems that have had a front-row seat to observe Earth in recent millennia. The study suggests that hypothetical extraterrestrial civilizations in those star systems could’ve been in the right position to watch Earth transit our Sun and look at us the same way we attempt to look at other planets.
A total of 1,715 stars within a range of 326 light-years have been in the right place to spot life on Earth, either currently or sometime around the wee ages of human civilization roughly 5,000 years ago, astronomers from Cornell University and the American Museum of Natural History wrote in a paper published in Nature Astronomy on Wednesday. The paper, relying on a recently released trove of data on distant stars detected by the European Space Agency’s Gaia spacecraft, indicates 1,402 of those systems can currently see Earth from a finite perspective called the Earth Transit Zone (ETZ).
“I started to wonder what stars could see us if we change the vantage point... The cosmos is dynamic, so the vantage point is not forever — it is gained and lost,” Lisa Kaltenegger, an associate professor of astronomy at Cornell and a co-author of the paper, tells The Verge. “And I was wondering how long that front row seat to find Earth through the dip in brightness of the star lasts. No one knew.”
Witnessing distant planets move around their star is among the primary techniques astronomers use to observe exoplanets. They watch for a “transit” — when an exoplanet’s orbit takes it between its star and Earth — dimming the star’s light ever so slightly. The idea behind the Earth Transit Zone retools that technique to imagine Earth itself as the exoplanet. As star systems swirl around the center of our galaxy, many fall into the position from which Earth’s silhouette can be seen crossing in front of our Sun, just like astronomers see exoplanets “transit” their own stars.
In recent decades, scientists have put themselves in the shoes of imaginary alien astronomers to get a sense of how detectable we are in the universe, a unique “reciprocal” perspective in the Search for Extraterrestrial Intelligence (SETI). Some of those studies looked at star systems — many confirmed as exoplanet hosts — that are presently positioned to spot Earth. “If any of these planets host intelligent observers, they could have identified Earth as a habitable, or even as a living, world long ago, and we could be receiving their broadcasts today,” researchers wrote in a separate study published in the journal Astrobiology in 2015.
But Kaltenegger and Jackie Faherty, an astrophysicist and senior scientist at the American Museum of Natural History, looked backward and forward in time. They found that 313 star systems were previously in the right spot to see Earth photobomb the Sun in the last 5,000 years. Ross 128 b, an Earth-sized exoplanet orbiting one of the closest stars to our Solar System, was in the right spot to see Earth for over two millennia ending at least 900 years ago. Exoplanets in the Trappist-1 solar system, some 39 light-years away, will be able to see our planet 1,642 years from now. And some exoplanets farther away have been in the ETZ for thousands of years and will remain for thousands more.
If anyone was living on these exoplanets and if that life had mustered the same technology humans currently have, they could have already found Earth. It’s a lot of “ifs” and assumptions, but given the course of human development, we know that it’s at least possible. “Although we cannot infer the time needed for life to start on any exoplanet from Earth’s history, the early signs for life on Earth are encouraging,” Kaltenegger and Faherty’s report says. The star systems closest to Earth generally spend over 1,000 years in the ETZ, the paper says, leaving open “a long timeline for nominal civilizations to identify Earth as an interesting planet.”
Human-made radio waves that started pinging from Earth roughly 100 years ago have washed over about 75 different star systems, the paper says. Forty-six of those systems are also currently in the ETZ, meaning any exoplanets within those star systems probably offer the best seat in the cosmic theater for detecting life on Earth.
Meanwhile, humanity is continuing its own exploration of our neighborhood. Projects using the James Webb Space Telescope, a multibillion-dollar observatory now slated to launch to space by the end of the year, will be able to get a closer look at these nearby celestial bodies.
“We don’t know yet how likely it is to have life on planets or how likely it is that curious technological civilizations evolve from there,” Kaltenegger says. “We don’t know at all if there is life out there in the cosmos, but we are on the verge of finding out.”