Next Generation Telescopes Could Search for Intelligent Civilizations Directly

We’re still in the early days of searching for life elsewhere. The Perseverance rover is on its way to a paleo-delta on Mars to look for fossilized signs of ancient bacterial life. SETI’s been watching the sky with radio dishes, listening for signals from distant worlds. Our telescopes are beginning to scan the atmospheres of distant exoplanets for biosignatures.

Soon we’ll take another step forward in the search when new, powerful telescopes begin to search not just for life but for other civilizations.

The search for biosignatures is gaining momentum. If we can find atmospheric indications of life at another planet or moon—things like methane and nitrous oxide and a host of other chemical compounds—then we can wonder if living things produced them. But the search for technosignatures raises the level of the game. Only a technological civilization can produce technosignatures.

Technosignatures are simply the effects of technology on an environment. Light from massive cities, particular atmospheric chemicals, and even satellites orbiting a planet are all technosignatures. The granddaddy of all technosignatures is probably the Dyson Sphere. A Dyson Sphere is a hypothetical megastructure surrounding a star and capturing its solar energy output. The idea is that as a civilization grows, its energy requirements will balloon, and the only way to gather the energy the civilization requires is to surround its star with an energy-gathering sphere.

In 2021, the National Academies of Sciences released their Decadal Survey on Astronomy and Astrophysics 2020, called Astro2020. They release one every ten years, and each survey outlines the critical challenges in astrophysics and astronomy for the next decade. Astro2020 contains several recommendations that can advance the search for technosignatures. A NASA working group has released a white paper digging into the technosignature part of Astro2020.

The paper is titled “OPPORTUNITIES FOR TECHNOSIGNATURE SCIENCE IN THE ASTRO2020 REPORT.” It comes from Nexus for Exoplanet System Science (NExSS). NExSS is a multidisciplinary group that includes Earth scientists, planetary scientists, heliophysicists, and astrophysicists. They bring a collaborative and synthesized approach to the search for biosignatures and technosignatures.

“Technosignatures refer to any observable manifestations of extraterrestrial technology, and the search for technosignatures is part of the continuum of the astrobiological search for biosignatures,” the paper says. “The search for technosignatures is directly relevant to the “World and Suns in Context” theme and “Pathways to Habitable Worlds” program in the Astro2020 report.”

The white paper aims to “… demonstrate the relevance of technosignature science to a wide range
of missions…” The NExSS group is urging the larger science community to include the search for technosignatures in the design and implementation of projects like LUVOIR, ELTs, infrared and x-ray observatories, and other similar facilities.

LUVOIR (Large Ultraviolet Optical Infrared Surveyor) is a NASA telescope concept in two proposed sizes. LUVOIR-A is a 15-meter mirror design, and LUVOIR-B is an eight-meter design. Thanks to its multi-wavelength capabilities, it’s a powerful and versatile design with many applications. It would be situated at L2 but would be serviceable like the Hubble was.

ASTRO2020 focuses on the biosignature aspect of LUVOIR in the search for habitable planets, but it mentions technosignatures a couple of times. The authors of this new white paper point out that LUVOIR would be an effective tool in the search for technosignatures. “Industrial pollution represents a class of atmospheric constituents on Earth that could conceivably be technosignatures if observed in the spectra of an exoplanet,” they write. “One example is nitrogen dioxide (NO2), which has large sources on Earth from combustion that are greater than non-anthropogenic sources.”

NO2 makes a good case study in detection scenarios. Elevated NO2 levels in a planet’s atmosphere can indicate industrial activity. But there are natural sources, too, and any detection would have to be studied carefully in case of false positives. This is the same problem biosignature detections face: they need to be unambiguous. But whether a signal is a false positive or not, it first has to be detected.

The white paper authors believe LUVOIR can detect NO2, and to strengthen their case, they cite previous studies showing that LUVOIR could successfully detect NO2 in the atmospheres of exoplanets. “A study by Kopparapu et al. (2021) showed that the absorption features of NO2 … could be detectable with the Large Ultraviolet Optical Infrared Surveyor. Kopparapu et al. (2021) found that a 15 m LUVOIR-like telescope could detect Earth-like levels of NO2 for a planet around a Sun-like star at 10 PC (~33 light-years) with ~400 hours of observation.”

The white paper also addresses how LUVOIR could detect more purposeful technosignatures like laser signals and optical beacons. “Optical beacons could provide a cost-effective means of directed communication between exoplanetary systems, which could be encoded and transmitted through rapid nanosecond pulses,” the paper states. LUVOIR could help with these, too, by placing “… constraints on the prevalence of optical beacons and other pulsed laser signals.”

The authors couple the detection of optical beacons with the detection and characterization of habitable rocky planets and say that LUVOIR is a powerful tool for these detections. “Space missions such as the IR/O/UV telescope could provide detectability constraints on the prevalence of optical beacons in exoplanetary systems,” they write. “… relatively low-powered optical beacons could be detectable with the IR/O/UV telescope for most or all targets where the characterization of rocky planets within the HZ is also possible.”

Extremely Large Telescopes (ELTs) can also play a role in the search for technosignatures. An ELT is a telescope with a primary mirror larger than about eight meters. Eight meters is a design limitation because telescope mirrors larger than that are heavy and deform themselves. ELTs get around that physical limitation with segmented mirrors. The European Extremely Large Telescope (E-ELT) is the most well-known example of an ELT and should see first light in 2027, but the white paper explicitly mentions two other ELTs.

One is the Giant Magellan Telescope (GMT), and the other is the Thirty Meter Telescope (TMT.) Together, the ELTs seeing first light through the next decade will be powerful engines for advancing scientific objectives. The white paper expands on ELTs’ role in the hunt for technosignatures.

“The GMT and TMT are ongoing projects that have been developing for many years,” the paper says. “These ground-based facilities could be capable of characterizing the atmospheres of terrestrial planets discovered by missions like TESS and CHEOPS at optical and near-infrared wavelengths.”

Red dwarfs are the most prevalent type of star, but their light is dimmer, which makes them more challenging targets. The GMT and the TMT should be able to study the atmospheres of exoplanets around red dwarfs. “Possible spectral technosignatures such as atmospheric pollution and optical beacons … could likewise be constrained with observations of exoplanetary systems by ELTs.”

The white paper also talks about far-InfraRed (FIR) probes. There are gaps in our observing capabilities, and FIR probes are one of those gaps. They have to be space-based facilities to be effective, and they could play a crucial role in the search for technosignatures. “… the aforementioned wavelength range is exciting for the so-called artifact SETI, of which the best-known example is Dyson Spheres, the energy-harvesting megastructures conceived by Olaf Stapledon and formalized by their eponym Freeman Dyson.”

In 1960 Dyson published his paper “Search for Artificial Stellar…