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How a twin Earth could detect Earth

A research team led by Dr. Sofia Sheikh of the SETI Institute, in collaboration with the Characterizing Atmospheric Technosignatures project and the Penn State Extraterrestrial Intelligence Center, set out to answer a simple question: If an extraterrestrial civilization existed with technology similar to ours, would they be able to detect Earth and evidence of humanity? If so, what signals would they detect, and from how far away?

Researchers used a theoretical, modeling-based method, and this study is the first to analyze multiple types of technosignatures together rather than separately. The findings revealed that radio signals, such as planetary radar emissions from the former Arecibo Observatory, are Earth's most detectable technosignatures, potentially visible from up to 12,000 light-years away.

The research is in The Astronomical Journal.

Atmospheric technosignatures—such as nitrogen dioxide emissions—have become more detectable than they were a decade ago, thanks to advances in instruments like the James Webb Space Telescope and the upcoming Habitable Worlds Observatory (HWO). With HWO we could detect these emissions from as far as 5.7 light-years away, just beyond our closest stellar neighbor, Proxima Centauri.

Finally, as you get closer and closer to Earth, you would detect more and more human-made signatures simultaneously, including city lights, lasers, heat islands, and satellites, offering a comprehensive view of our technological presence.

"Our goal with this project was to bring SETI back 'down to Earth' for a moment and think about where we really are today with Earth's technosignatures and detection capabilities," said Macy Huston, co-author and postdoc at the University of California, Berkeley, Department of Astronomy. "In SETI, we should never assume other life and technology would be just like ours, but quantifying what 'ours' means can help put SETI searches into perspective."

"One of the most satisfying aspects of this work was getting to use SETI as a cosmic mirror. What does Earth look like to the rest of the galaxy? And how would our current impacts on our planet be perceived?" said Sheikh. "While of course, we cannot know the answer, this work allowed us to extrapolate and imagine what we might assume if we ever discover a planet, with, say, high concentrations of pollutants in its atmosphere."

SETI scientists search for advanced alien civilizations by looking for signs of technology—signals or patterns that cannot be explained by natural phenomena that may indicate intelligent life. These signals are called technosignatures and come in various forms. Radio telescopes are the most commonly used tool for SETI searches. Researchers also use optical telescopes to scan for laser pulses that could indicate communication or propulsion patterns.

Another approach involves studying the atmospheres of exoplanets in habitable zones around stars to look for chemical signatures that might suggest life or industrial activity. SETI scientists also consider technologies far beyond those currently invented on Earth, such as Dyson spheres, but these far-future technologies were not considered in this study.

This study demonstrates how Earth's technosignatures can provide a multi-wavelength framework for understanding the detectability of technology on other planets and shaping our search for intelligent life beyond Earth. Future telescopes and receivers could enhance our detection sensitivity or enable us to identify new types of technosignatures, such as other atmospheric signatures of pollution. Repeating this type of study over the years as astronomical technology advances and the human impact on the planet evolves could provide fresh insights and refine our approach to discovering extraterrestrial life.