Usually it’s we who are searching for planets outside our own Solar System. Lisa Kaltenegger from the Carl Sagan Institute at Cornell University and Jackie Faherty from the American Museum of Natural History in New York looked at it the other way around. Assuming aliens have the same level of technology that we do, would they be able to detect us?
To try to answer that question, the authors consulted a huge data set of precise star positions, from the European Space Agency’s Gaia mission. Taking into account the past, present, and future locations of stars and changing viewing geometry, they determined how many star systems would be in position for observers to see Earth transiting in front of the Sun—an important technique for deducing the presence of life. Their analysis identified 1,715 stars within 100 parsecs (about 326 light years) from the Sun that were in the right position to have spotted life over the past 5,000 years, with an additional 319 stars entering this special vantage point in the next 5,000 years.
Most of these stars can be assumed to host planets, many of which could be in the habitable zone, and perhaps could even be Earthlike. The authors estimate that 29 potentially habitable worlds might exist within 100 light years from us. All of them would, in theory, be able to detect radio signals from our planet, which we started to send out about 100 years ago.
We already know that some of the stars in this sample have exoplanets, including the Ross 128 system, the Teegarden’s star system, and the Trappist-1 system, with an astounding seven terrestrial-size exoplanets. Ross128 is only about 11 light years away, and is the 13th closest system to ours. From there, an Earth transit would have been observable for 2,158 years, up until 900 years ago. From the Teegarden’s star system we are currently not observable, but we will be starting in 2050, for another 410 years.
Life has existed on our planet for four billon years, and widespread oxygen-breathing life has been here since the Great Oxidation Event about 2.6 billion years ago. So, in principle, many more star systems could have detected life on our planet before humans came along by analyzing the gases in our atmosphere during transit events. And even within our cosmic neighborhood, a substantial number of alien civilizations could have been watching as life evolved on Earth.
In the end, all such speculations come down to the prevalence of life in the Universe. Based on some estimates, life—and perhaps even complex life—could certainly exist on some of the planets that have been swept by radio signals from Earth or may have seen our planet transiting against the Sun. Whether intelligent, technologically advanced life exists on these worlds is much more questionable. But it remains a possibility.