A new study focuses on the possibility that extraterrestrial intelligence might be looking for us.
Astronomers reversed a technique used to seek life on other planets so that instead of looking for what's out there, they sought to find what planets might see us.
Apparently, there's a lot.
"When I look up at the sky, it looks a little bit friendlier because it's like, maybe somebody is waving," the study's lead author Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University, said.
The new study, conducted by researchers Lisa Kaltenegger and Jacqueline Faherty, used data from the European Space Agency's Gaia mission, which was launched into space to create a three-dimensional map of the Milky Way.
The Milky Way, however, is not static; its stars rotate in unison, but they also move relative to one another. Gaia's repeated measurements of many stars allowed them to estimate their motion relative to the Sun. This enabled Kaltenegger and Faherty to expand their investigation forward and backward in time, resulting in a 10,000-year-wide window centered on the present.
Kaltenegger and Faherty limited their analysis to the nearest 325 light-years. The researchers also examined within a 100-light-year radius because that is roughly how far our radio waves will have traveled.
As it turns out, Earth might have been seen by a multitude of stars. The two authors count over 2,000 of them, with the vast majority of them currently capable of detecting Earth's transits. Earth's transits would last at least 10 hours for around two-thirds of these stars, making them relatively easy to spot; 868 of them will spend over 10,000 years in places where the transits would be visible.
Overall, there are a lot of opportunities to see the Earth among this population.
"If we're looking for intelligent life in the universe, that could find us and might want to get in touch," Kaltenegger said, "we've just created the star map of where we should look first."
Kaltenegger and Faherty's work was funded by the Carl Sagan Institute and the Breakthrough Initiative.
