If Aliens Exist in These 1,700 Solar Systems, They Can Probably See Earth

Hypothetical alien astronomers orbiting these stars could have seen the sun’s light flicker as Earth passed in front of it

Earth and sun
An artist's rendering of the Earth and sun seen from space. Astronomers on Earth can detect other worlds when far away stars flicker as an orbiting planet passes in front of them, partly blocking our view. New research asks how many alien worlds might have been able to detect Earth in this way. OpenSpace / American Museum of Natural History

Astronomers on Earth can discover far away planets by watching the light of distant stars and waiting to see if that light ever wavers as an orbiting alien world passes by. But as Nadia Drake reports for National Geographic, a new study turns a hypothetical extraterrestrial telescope back on Earth.

Astronomer Lisa Kaltenegger of Cornell University, lead author of the study published this week in the journal Nature, posed the question: “Which stars could see us as the aliens, as the transiting planet where the Earth blocks out light from the star?”

By analyzing the shifting cosmic lines of sight for more than 300,000 stars within about 300 light years of our sun, Kaltenegger and her co-author identified more than 2,000 stars with the right vantage to have detected Earth sometime in the last 5,000 years—or in the next 5,000 years to come.

For example, a mere 12 lightyears away from us there are two planets roughly the size of Earth winding their way around Teegarden’s star, reports Nell Greenfieldboyce for NPR. By astronomers’ reckoning, these worlds could be hospitable enough to potentially support life.

"If they have the same technique as we do, and if there is a 'they,' then they wouldn't know yet that we exist," Kaltenegger tells NPR. "In 29 years, they would be able to see us."

These two planets in the Teegarden Star solar system are among the 319 stars identified by the study that will come into the right position to see Earth pass in front of the sun sometime in the next 5,000 years. Over the previous 5,000 years, the researchers identified 1,715 star systems with the proper vantage on our solar system. Of those solar systems, 1,402 have the right angle to be looking at us right now.

The study further identified 75 stars that are so close to Earth that radio waves generated by our species could have already reached them, reports Lisa Grossman for Science News. Of those stars, seven of them are paired with potentially habitable planets.

Kaltenegger tells Leah Crane of New Scientist that this subset of so-called exoplanets would be a good place to focus a search for extraterrestrial life.

“These worlds might be worth the trouble of studying further, because we know they can see us,” Kaltenegger tells New Scientist. “Who would have the most incentive to send us a signal? The ones who could have found us.”