If hostile aliens exist they could detect Earth from 12,000 light-years away

A team of researchers set out to answer a simple question - If an extraterrestrial civilization existed with technology similar to the level of the technology we have on Earth today, would they be able to detect Earth and the evidence of humanity on its surface? And from how far away?

The team was led by Dr. Sofia Sheikh of the SETI Institute, with collaborators from the Characterizing Atmospheric Technosignatures project and the Penn State Extraterrestrial Intelligence Center. Using a theoretical, modeling-based method, the team looked at the technosignatures emitted from Earth, such as radar emissions from observatories, and discovered the maximum distance a hypothetical civilization with technology similar to ours could detect them. What the team found was these technosignatures are detectable from up to 12,000 light-years away.

The team proposed the idea of how Earth would look to the rest of the galaxy through creating a hypothetical "mirror Earth." The team found that different forms of technosignatures have different ranges, such as atmospheric technosignatures, such as nitrogen dioxide emissions have increased in their detectability compared to a year ago, with the team finding instruments such as the James Webb Space Telescope (JWST) is capable of detecting such signatures from as far as 5.7 light-years away.

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"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. 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," said Macy Huston, co-author and postdoc at the University of California, Berkeley, Department of Astronomy

"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," said Dr. Sofia Sheikh