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Technosignatures: Looking to Planetary Atmospheres

While we often think about so-called Dysonian SETI, which looks for signatures of technology in our astronomical data, as a search for Dyson spheres, the parameter space it defines is getting to be quite wide. A technosignature has to be both observable as well as unique, to distinguish it from natural phenomena. Scientists working this aspect of SETI have considered not just waste heat (a number of searches for distinctive infrared signatures of Dyson spheres have been run), but also artificial illumination, technological features on planetary surfaces, artifacts not associated with a planet, stellar pollution and megastructures.

Thus the classic Dyson sphere, a star enclosed by a swarm or even shell of technologies to take maximum advantage of its output, is only one option for SETI research. As Ravi Kopparapu (NASA GSFC) and colleagues point out in an upcoming paper, we can also cross interestingly from biosignature searches to technosignatures by looking at planetary atmospheres.

Biosignature science is the more developed of the two fields, though we’re seeing a lot of activity in technosignature work, the robust nature of which can be seen in the extensive references the Kopparapu team identifies. As applied to atmospheres, a search for technosignatures can involve looking for various forms of pollution that flag industrial activity.

To my knowledge, most work on atmospheric pollution has targeted chlorofluorocarbons (CFCs), a useful choice because there is no biological source here, although our own use of CFCs occurred in a fairly brief window and for a specific purpose (refrigeration). The NASA work targets the much more ubiquitous nitrogen dioxide (NO2), which can be a by-product of an industrial process and in general is produced by any form of combustion.

As Kopparapu notes:

“In the lower atmosphere (about 10 to 15 kilometers or around 6.2 to 9.3 miles), NO2 from human activities dominate compared to non-human sources. Therefore, observing NO2 on a habitable planet could potentially indicate the presence of an industrialized civilization.”

Adds Giada Arney, a co-author on the paper and a colleague of Kopparapu at GSFC:

“On Earth, about 76 percent of NO2 emissions are due to industrial activity. If we observe NO2 on another planet, we will have to run models to estimate the maximum possible NO2 emissions one could have just from non-industrial sources. If we observe more NO2 than our models suggest is plausible from non-industrial sources, then the rest of the NO2 might be attributed to industrial activity. Yet there is always a possibility of a false positive in the search for life beyond Earth, and future work will be needed to ensure confidence in distinguishing true positives from false positives.”

Image: Artist’s illustration of a technologically advanced exoplanet. The colors are exaggerated to show the industrial pollution, which otherwise is not visible. Credit: NASA/Jay Freidlander.

This is evidently the first time NO2 has been examined in technosignature terms. The scientists deploy a cloud-free 1-dimensional photochemical model that uses the atmospheric temperature profile of today’s Earth to examine possible mixing ratio profiles of nitrogen oxide compounds on a planet orbiting several stellar types, one of them being a G-class star like the Sun, the others being a K6V and two M-dwarfs, one of these being Proxima Centauri. The authors then calculate the observability of these NO2 features, considering observing platforms like the James Webb Space Telescope and the projected Large UV/Optical/IR Surveyor (LUVOIR) instrument.

Usefully, atmospheric NO2 strongly absorbs some wavelengths of visible light, and the authors’ calculations show that an Earth-like planet orbiting a star like the Sun could be studied from as far as 30 light years away and an NO2 signature detected even with a civilization producing the pollutant at roughly the same levels we do today. This would involve observing at visible wavelengths over the course of at least 400 hours, which parallels what the Hubble instrument needed to produce its well-known Deep Field observations.

But adding yet more interest to K-class stars, whose fortunes as future targets for bio- and technosignature observations seem to be rising, is the fact that stars cooler than the Sun should generate a stronger NO2 signal. These stars produce less ultraviolet light that can break down NO2. As to M-dwarfs, we have this:

Further work is needed to explore the detectability of NO2 on Earth-like planets around M-dwarfs in direct imaging observations in the near-IR with ground-based 30 m class telescopes. NO2 concentrations increase on planets around cooler stars due to reduced availability of short-wavelength photons that can photolyze NO2 . Non-detectability at longer observation times could place upper limits on the amount [of] NO2 present on M-dwarf HZ planets like Prox Cen b.

Where work will proceed is in the model used to make these calculations, which will need to be more complex, as the paper acknowledges:

…when we prescribe water-ice and liquid water clouds, there is a moderate decrease in the SNR of the geometric albedo spectrum from LUVOIR-15 m, with present Earth-level NO2 concentration on an Earth-like planet around a Sun-like star at 10 pc. Clouds and aerosols can reduce the detectability and could mimic the NO2 feature, posing a challenge to the unique identification of this signature. This highlights the need for performing these calculations with a 3-D climate model which can simulate variability of the cloud cover and atmospheric dynamics self-consistently.

The authors consider biosignatures and technosignatures to be “two sides of the same coin,” a nod to the fact that we should be able to search for each at the same time with the next generation of observatories. Finding the common ground between biosignature research and SETI seems overdue, for a positive result for either would demonstrate life’s emergence elsewhere in the universe, and that remains question number one.

The paper is Kopparapu et al., “Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology,” accepted at the Astrophysical Journal. (Preprint).


Comments on this entry are closed.

  • ljk February 18, 2021, 13:09

    How do we know what is alien life?


    • Alex Tolley February 18, 2021, 16:50

      Is this the first good science article I have ever been exposed to on Mashable?

      “Unless there’s a rabbit jumping in front of a rover — and we have to make sure it’s not a robot — it’s going to be difficult,” said Nathalie Cabrol,

      But what a technosignature!

      I have a old-ish book on Mars life- Malcolm Walter: “The Search for Life on Mars” (pub. 1999). He writes quite extensively on the ALH 84001 meteorite and the controversy over the biosignatures. The best part of this slim volume is the strategy for where to search for life on Mars based on the experience of scientists working on Earth searching for microbial paleolife in the geology, such as stromatolites. It is much more complex than it seems to non-specialists.

  • Alex Tolley February 18, 2021, 16:14

    Just as with CFCs, I see NO2 as having a narrow window as an indicator of technological civilization. The bulk of terrestrial NO2 is due to internal combustion engines, especially diesel engines. These appeared at the end of the 19th century and are already looking to be phased out in some classes of engines within the next 50 years as it is a noxious gas that has adverse health effects.

    In cosmic time, ETI will either be in the stone age or in a stable, advanced technology age. If civilization collapses, it may result in a return to a pre-industrial age. IMO, therefore the window to detect such gas is but a sliver in the lifetime of any civilization. Any NO2 detection will be from natural processes.

    It might be worth looking for such gases, although if it is for the purpose of looking for techno-signatures, it seems to be more like looking for your lost keys under a lampost – because that is where the light is, not where you dropped them.

    Whether ETI is biological, machine-based, or both, I think looking for structures, especially in space is the best approach. How you detect them will depend on the technology we have available. Whether that technology is remote detection with interstellar probes, we will have the capabilities to achieve those needed technologies within the next 1000 years, assuming our civilization retains the capabilities and desire to do so.

    • Andrei February 18, 2021, 19:39

      I fully agree with your thoughts here, many countries actively work to phase out diesel engines. And some have set up the ambitious goal by not allowing any but electrical cars to be sold within the coming decade. There’s countries that might reach that goal even without having to legislate on the matter, such as Norway as most sold cars are electric already. While that country have a huge untapped clean power potential, they build windmills instead. Such will not help in rogue nations that are so insane they use fossil fuel to produce electricity – but I digress. Any sensible civ would go down a similar path, meaning that NO2 or any pollutant is unlikely to be a technosignature for any biological civ. It’s harder to tell how a robotic/machine culture would act, if they consider their ancestors to be kept they might create a zoo, while they consider that horrible oxygen atmosphere to be a hindrance – as it make them rust and corrode. And then replace the atmosphere with more sensible gas, then we would not recognize that world as habitable.
      This would not mean that we would be unable to detect a civ with remote means, beside the more obvious one of mega projects in space, a planet with high power production / use would be in thermal imbalance. And with higher resolution studies, not necessarily sharp enough to see details – still might be able to spot if a night side is brighter than it’s supposed to be. But it’s definitely a harder task than looking for unusual gas in a planetary atmosphere.

  • Robin Datta February 18, 2021, 23:40

    In the latest episode of Event Horizon, in which John Michael Godier interviews Dr. Sara Seager
    Aliens are Hard to Find with Prof. Sara Seager
    the discussion includes includes planetary atmospheres and CFCs.

  • James Benford February 19, 2021, 16:40

    I continue to be skeptical that CFCs can be detected at interstellar distances when their < 1 ppb density here can barely be detected from LEO!

  • Geoffrey Hillend February 19, 2021, 17:23

    I thought we could detect carbon monoxide from car exhaust on an exoplanet, but after looking at the Aliens are Hard to Find with Prof. Sara Seager I realize it is only a trace gas and probably not detectable because there is not enough of it and the signal would be to small. CFC’s could be ambiguous.

    I was thinking about the false positive of oxygen and wondering what are the ideal conditions for an FP which is probably a lot of H2O and it’s photolysis? If there is no oxygen, then there won’t be any industrial NO2 or even any Nitrogen cycle without life or no biotic NO2.

  • ljk February 19, 2021, 18:32

    I wonder if the environmental havoc created by the humans inhabiting the existence of Silent Running could be detected at interstellar distances:


  • ljk February 24, 2021, 17:42
  • ljk March 9, 2021, 11:04

    Hat Creek Observatory gets upgrade to comb galaxy for extraterrestrial life

    Jessica Skropanic

    March 5, 2021

    Hat Creek Observatory in eastern Shasta County is getting an upgrade to make its radio telescopes better able to explore deep space.

    The purpose is to help astronomers search for signs of extraterrestrial life throughout the universe, starting with our own galaxy.

    Making a detailed survey of the Milky Way is the first of many projects SETI Institute scientists plan at the observatory once upgrades are completed, said Wael Farah, the observatory’s resident astronomer. Scientists will divide the sky into a grid and listen for radio waves square by square.

    Full article here:


    To quote:

    The SETI Institute already owned the 42 radio telescopes at Hat Creek — the Allen Telescope Array — when it took ownership of the observatory from another non-profit (SRI International) in 2020.

    When it took over, SETI — short for Search for Extraterrestrial Intelligence — got to work immediately, funding upgrades with a $1.2 million grant from Qualcomm technology company co-founder Franklin Antonio, Farah said.

    “We’re currently upgrading the (telescopes’) feed system,” he said. This will make the telescopes able to sense radio waves over a wider range of frequencies.

  • ljk March 9, 2021, 13:43


    Longevity Is The Key Factor In The Search For Technosignatures

    Source: astro-ph.IMPosted March 4, 2021 11:41 PM0 Comments

    Each row shows the unconditional probability distribution of longevities τ (left) and time of appearance ti (right) and the probability distribution of the same quantities conditioned to detection. In both rows, the unconditional probability distribution for τ is a log-uniform, while ti is uniformly distributed in the upper row (Model 1 in the text) and normally distributed in the lower row (Model 2 in the text).

    It is well-known that the chances of success of SETI depend on the longevity of technological civilizations or, more broadly, on the duration of the signs of their existence, or technosignatures.

    Here, we re-examine this general tenet in more detail, and we show that its broader implications were not given the proper significance. In particular, an often overlooked aspect is that the duration of a technosignature is in principle almost entirely separable from the age of the civilization that produces it. We propose a classification scheme of technosignatures based on their duration and, using Monte Carlo simulations, we show that, given an initial generic distribution of Galactic technosignatures, only the ones with the longest duration are likely to be detected.

    This tells us, among other things, that looking for a large number of short-lived technosignatures is a weaker observational strategy than focusing the search on a few long-lived ones. It also suggests to abandon any anthropocentric bias in approaching the question of extraterrestrial intelligence. We finally give some ideas of possible pathways that can lead to the establishment of long-lived technosignatures.

    Amedeo Balbi, Milan M. Ćirković

    Comments: Accepted for publication in The Astronomical Journal

    Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP); Popular Physics (physics.pop-ph)

    Cite as: arXiv:2103.02923 [astro-ph.IM] (or arXiv:2103.02923v1 [astro-ph.IM] for this version)

    Submission history

    From: Amedeo Balbi

    [v1] Thu, 4 Mar 2021 10:08:25 UTC (405 KB)


    Astrobiology, SETI

  • ljk March 11, 2021, 13:22


    Ideas For Future NASA Missions Searching For Extraterrestrial Civilizations


    Posted March 10, 2021 at 10:59 PM

    A researcher at the Instituto de Astrofísica de Canarias (IAC) is the lead author of a study with proposals for “technosignatures” -evidence for the use of technology or industrial activity in other parts of the Universe- for future NASA missions.

    The article, published in the specialized journal Acta Astronautica, contains the initial conclusions of a meeting of experts in the search for intelligent extraterrestrial life, sponsored by the space agency to gather advice about this topic.

    In the article, several ideas are presented to search for technosignatures that would indicate the existence of extraterrestrial civilizations, from the most humdrum, such as the presence of industrial pollution in the atmosphere or large swarms of satellites, to hypothetical gigantic space engineering work, such as heat shields to fend off climate change, or Dyson spheres for optimum use of the light from the local star.

    Some of the proposed searches look very far in space, across our galaxy and even beyond, while others aim at scanning our own solar system in search for probes that might have been sent here in a distant past. In addition, a study is included of a new way of classifying the technosignatures as a function of their “cosmic footprint”, a measure of how conspicuous they are at large distances.

    “We have no idea whether intelligence is something very common in the Universe or, on the contrary, whether it is extremely rare”, explains Hector Socas-Navarro, an IAC researcher, the Director of the Museum of Science and the Cosmos, of Museums of Tenerife, and the first author of the article. “For that reason we cannot know whether these searches have any chance of success. There is no choice but to search and see what we find, because the implications would be tremendous”.

    “The idea of searching for technosignatures draws upon the technology we have on Earth today and possible extensions of our technology into the future”, notes Jacob Haqq-Misra, a coauthor of the article and chairman of the TechnoClimes 2020 organizing committee. “This does not necessarily mean that any extraterrestrial technology must be like our own, but imagining plausible extensions of our own future is one place to begin thinking of astronomical searches we could actually do to look for possible technosignatures”.

    The search for technosignatures

    In 1993, NASA abruptly terminated its initial SETI programme for the search for intelligent extraterrestrial life, when it had hardly started. It comprised two complementary ambitious projects, one using the giant radio telescope at Arecibo, Puerto Rico, and the other with the antennas of the Deep Space Network in California. Now, nearly 30 years later, things have changed and the Agency wants to re-start its search effort.

    In the past decade great advances in astronomical instrumentation have been made, leading to a revolution in the science of discovery and study of exoplanets. The new telescopes, and projects on future space missions will for the first time allow the search for so-called biomarkers, evidence for life on other planets. Many experts consider it plausible that in the coming years we will discover extraterrestrial life, even though it is most likely to be life in very simple form.

    Given present and future technological advance there will be new opportunities to search for technosignatures. That is why NASA has decided to get involved again in the search for extraterrestrial intelligence, taking advantage of the possibilities of the current and proposed future space observatories.

    These subjects, among others, were on the agenda of the meeting TechnoClimes 2020 under the auspices of NASA at the Blue Marble Space Institute of Science (Seattle, USA). With scientists from all over the world, its aim was to propose new developments making way for future advances.

    Finally, due to the COVID-19 pandemic, the meeting was held virtually via videoconference, in which 53 researchers from various disciplines coming from 13 countries discussed a range of aspects of the search for other intelligent species.

    Article: Héctor Socas-Navarro, Jacob Haqq-Misra, Jason T. Wright et al. “Concepts for future missions to search for technosignatures”, Acta Astronautica, 2021:


    – DOI: https://doi.org/10.1016/j.actaastro.2021.02.029
    – Arxiv: https://arxiv.org/abs/2103.01536

  • ljk March 12, 2021, 13:50


    Triggering A Climate Change Dominated “Anthropocene”: Is It Common Among Exocivilizations?

    Source: astro-ph.EP

    Posted March 11, 2021 11:09 PM

    We seek to model the coupled evolution of a planet and a civilization through the era when energy harvesting by the civilization drives the planet into new and adverse climate states.

    In this way we ask if triggering “anthropocenes” of the kind humanity is experiencing now might be a generic feature of planet-civilization evolution. In this study we focus on the effects of energy harvesting via combustion and vary the planet’s initial atmospheric chemistry and orbital radius. In our model, energy harvesting increases the civilization’s population growth rate while also, eventually, leading to a degradation of the planetary climate state (relative to the civilization’s habitability.)

    We also assume the existence of a Complex Life Habitable Zone in which very high levels of CO2 are detrimental to multi-cellular animal life such as those creating technological civilizations. Our models show that the civilization’s growth is truncated by planetary feedback (a “climate dominated anthropocene”) for a significant region of the initial parameter space.

    Ethan Savitch, Adam Frank, Jonathan Carroll-Nellenback, Jacob Haqq-Misra, Axel Kleidon, Marina Alberti

    Comments: 19 pages, 13 figures

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Popular Physics (physics.pop-ph)

    Cite as: arXiv:2103.06330 [astro-ph.EP] (or arXiv:2103.06330v1 [astro-ph.EP] for this version)

    Submission history

    From: Ethan Savitch

    [v1] Wed, 10 Mar 2021 20:25:28 UTC (9,125 KB)


    Astrobiology, SETI