Astronomers hunting for alien civilizations have mostly looked for signs of life. A group of NASA-linked researchers now wants the next generation of big telescopes to look for signs of technology as well, and the best-known target on their list is the Dyson sphere.
A Dyson sphere is a hypothetical megastructure that surrounds a star and captures its energy output. The thinking goes like this: as a civilization grows, its appetite for power balloons, and the only way to feed it is to wrap the star in collectors. Here’s what that means in practice. Anything that big would soak up a staggering amount of energy and give off waste heat, and that heat would show up in infrared light.
Freeman Dyson made that point in a 1960 paper titled “Search for Artificial Stellar Sources of Infra-Red Radiation.”
A white paper with a request
The proposal comes from the Nexus for Exoplanet System Science, or NExSS, a group of Earth scientists, planetary scientists, heliophysicists and astrophysicists. Its white paper, “Opportunities for Technosignature Science in the Astro2020 Report,” responds to the decadal survey the National Academies of Sciences released in 2021. Astro2020 sets out the main challenges for astronomy over the next ten years. Technosignatures, the group says, got a place only in its appendices.
The authors want that changed. They argue that technosignature searches can usually ride along with other observations, without altering a mission’s design or adding cost. “This white paper recommends that all of the missions and facilities discussed above should consider including the search for technosignatures as part of the explicitly stated science case,” they write.
Which telescopes, and what they could catch
NASA’s proposed LUVOIR telescope, in 15-meter and eight-meter versions, gets the most attention. The authors say it could pick up industrial pollution in an exoplanet’s atmosphere, such as nitrogen dioxide. A 2021 study by Kopparapu and colleagues found that a 15-meter LUVOIR-like telescope could detect Earth-like levels of the gas around a Sun-like star about 33 light-years away, with roughly 400 hours of observation. LUVOIR could also look for optical beacons, which the paper describes as a cheap way for two star systems to talk to each other through nanosecond laser pulses.
Extremely Large Telescopes are the other focus. These ground-based instruments use segmented mirrors to get past the roughly eight-meter limit on single mirrors. The European Extremely Large Telescope should see first light in 2027. The paper also names the Giant Magellan Telescope and the Thirty Meter Telescope, and says they could study the atmospheres of planets around red dwarfs, the most common type of star.
Far-infrared probes matter for the Dyson sphere search in a different way. A sphere would not give off much far-infrared light, unlike dust, so a far-infrared instrument could help rule out look-alikes such as protoplanetary disks. The white paper also suggests infrared spectroscopy could find chlorofluorocarbons, which no known natural process produces and which can linger in an atmosphere for tens of thousands of years. The James Webb Space Telescope can detect them in some cases, but it has a lot of other jobs.
The authors treat X-rays as a longer shot. Their example: a kilometer-sized rock hurled onto a neutron star might produce an X-ray pulse of about 10^29 watts, bright enough to detect across the Milky Way.
