SETI study: From what distance can extraterrestrials locate us?
SETI normally listens for extraterrestrial signals. A SETI team has switched roles and considered where aliens can hear us from.
Arecibo Observatory: detectable 12,000 light years from Earth
(Image: Dennis van de Water/Shutterstock)
Is someone out there? For over 60 years, the Search for Extraterrestrial Intelligence SETI program has been listening for signals that indicate extraterrestrial intelligence. In a recent study, a team looks at the other perspective. If there is a SETI program out there somewhere listening for signals from us, how far away from Earth can it be in order to receive our signals?
The starting point is a possible extraterrestrial civilization that is at a similar technological level to us. Could they use it to detect so-called techno signatures and use them to discover Earth and humanity? What signals would it recognize, and from what distance?
Technosignatures include, for example, the signals emitted by large radio telescopes, such as the Arecibo Observatory, which collapsed in 2020. Other signatures include lasers, light and heat emitted by cities, or nitrogen dioxide as an indication of industrial activity.
Which technosignatures can be detected?
The SETI team led by Sofia Sheikh has analyzed which technosignatures can be detected with which technology and from what distance. This is the first time that several technosignatures have been examined together rather than separately, the team writes in The Astronomical Journal.
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The assumption is that the alien civilization is sending a spaceship in our direction. When will it pick up which signals? The first thing it can receive are signals from a powerful radio telescope. These can be detected at a distance of 12,000 light years from Earth. That is how far away HR 5171 A in the constellation of Centaur, the largest yellow giant star discovered to date, is from us. It is one of the ten largest known stars and should just be visible to the naked eye from Earth (in the southern sky).
To be able to detect the nearest technosignatures, the spacecraft must be much closer: The signals from NASA's Deep Space Network radiate 65 light years away. The first optical signal that can be detected comes from the laser-based communication system DSOC (Deep Space Optical Communications), at a distance of 5.9 light years.
LTE radiates four light years away
Nitrogen dioxide emissions are detectable from 5.7 light years. By comparison, our nearest stellar neighbor Proxima Centauri is around 4.2 light years away. If there were such emissions there, the James Webb Space Telescope would be able to detect them. Just beyond Proxima Centauri, about 4 light years away, the spacecraft crew could receive LTE mobile communications.
Once the spacecraft has passed the Oort Cloud at the edge of the solar system, it will be able to detect the light signatures of cities on Earth. After the flyby of the dwarf planet Pluto, these are also recognizable as desert islands.
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The most interesting aspect of this work was "using 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 study leader 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."
(wpl)
