Aliens could be watching us. A survey of the star systems within about 325 light years of Earth has found that 1715 of them have been or will be in the right position to spot our planet with the same techniques we use to find exoplanets, and 75 of the closest ones could even detect the radio waves that we constantly send out into the cosmos.
The easiest way to spot a planet outside our solar system is to catch it passing between us and its star, blocking out some of the star’s light. Lisa Kaltenegger at Cornell University in New York and Jackie Faherty at the American Museum of Natural History in New York examined data from the Gaia space telescope on the positions and motions of nearby stars to figure out which of them could find Earth in this way.
They found 1402 stars that are currently in the right position to see Earth pass in front of the sun, plus 313 that were in such a position in the past and 319 that will be someday.
They ran simulations extrapolating the movements of those stars over a period of 10,000 years, and the average time any given member of the sample could see Earth during that span is 6914 years – plenty of time to notice us, if there are inhabitants of those stellar systems with powerful enough telescopes.
Seventy-five of these systems are also close enough to detect the radio waves that we have sent out from Earth in the last 100 years.
The researchers estimate that there could be more than 500 rocky worlds orbiting in the “Goldilocks zone” of those 1715 stars, where life as we know it could be possible. We already know about some of them, a few of which are famous – for example, the TRAPPIST-1 system, which hosts seven Earth-sized planets, will be able to see Earth starting in the year 3663 and ending around the year 6034.
Kaltenegger points out that these exoplanets would be good targets in our search for intelligent life. “These worlds might be worth the trouble of studying further, because we know they can see us,” says Kaltenegger. “Who would have the most incentive to send us a signal? The ones who could have found us.”