Searching for biosignatures in the atmospheres of nearby exoplanets invariably opens up the prospect of folding in a search for technosignatures. Biosignatures seem much more likely given the prospect of detecting even the simplest forms of life elsewhere – no technological civilization needed – but ‘piggybacking’ a technosignature search makes sense. We already use this commensal method to do radio astronomy, where a primary task such as observation of a natural radio source produces a range of data that can be investigated for secondary purposes not related to the original search.
So technosignature investigations can be inexpensive, which also means we can stretch our imaginations in figuring out what kind of signatures a prospective civilization might produce. The odds may be long but we do have one thing going for us. Whereas a potential biosignature will have to be screened against all the abiotic ways it could be produced (and this is going to be a long process), I suspect a technosignature is going to offer fewer options for false positives. I’m thinking of the uproar over Boyajian’s Star (KIC 8462852), where the false positive angles took a limited number of forms.
If we’re doing technosignature screening on the cheap, we can also worry less about what seems at first glance to be the elephant in the room, which is the fact that we have no idea how long a technological society might live. The things that mark us as tool-using technology creators to distant observers have not been apparent for long when weighed against the duration of life itself on our planet. Or maybe I’m being pessimistic. Technosignature hunter Jason Wright at Penn State makes the case that we simply don’t know enough to make statements about technology lifespans.
On this point I want to quote Edward Schwieterman (UC-Riverside) and colleagues from a new paper, acknowledging Wright’s view that this argument fails because the premise is untested. We don’t actually know whether non-technological biosignatures are the predominant way life presents itself. Consider:
In contrast to the constraints of simple life, technological life is not necessarily limited to one planetary or stellar system, and moreover, certain technologies could persist over astronomically significant periods of time. We know neither the upper limit nor the average timescale for the longevity of technological societies (not to mention abandoned or automated technology), given our limited perspective of human history. An observational test is therefore necessary before we outright dismiss the possibility that technospheres are sufficiently common to be detectable in the nearby Universe.
So let’s keep looking, which is what Schwieterman and team are advocating in a paper focusing on terraforming. In previous articles on this site we’ve looked at the prospect of detecting pollutants like chlorofluorocarbons (CFCs), which emerge as byproducts of industrial activity, but like nitrogen dioxide (NO₂) these industrial products seem a transitory target, given that even in our time the processes that produce them are under scrutiny for their harmful effect on the environment. What the new paper proposes is that gases that might be produced in efforts to terraform a planet would be longer lived as an expanding civilization produced new homes for its culture.
Enter the LIFE mission concept (Large Interferometer for Exoplanets), a proposed European Space Agency observatory designed to study the composition of nearby terrestrial exoplanet atmospheres. LIFE is a nulling interferometer working at mid-infrared wavelengths, one that complements NASA’s Habitable Worlds Observatory, according to its creators, by following “a complementary and more versatile approach that probes the intrinsic thermal emission of exoplanets.”
Image: The Large Interferometer for Exoplanets (LIFE), funded by the Swiss National Centre of Competence in Research, is a mission concept that relies on a formation of flying “collector telescopes” with a “combiner spacecraft” at their center to realize a mid-infrared interferometric nulling procedure. This means that the light signal originating from the host star of an observed terrestrial exoplanet is canceled by destructive interference. Credit: ETH Zurich.
In search of biosignatures, LIFE will collect data that can be screened for artificial greenhouse gases, offering high resolutions for studies in the habitable zones of K- and M-class stars in the mid-infrared. The Schwieterman paper analyzes scenarios in which this instrument could detect fluorinated versions of methane, ethane, and propane, in which one or more hydrogen atoms have been replaced by fluorine atoms, along with other gases. The list includes Tetrafluoromethane (CF₄), Hexafluoroethane (C₂F₆), Octafluoropropane (C₃F₈), Sulfur hexafluoride (SF₆) and Nitrogen trifluoride (NF₃). These gases would not be the incidental byproducts of other industrial activity but would represent an intentional terraforming effort, a thought that has consequences.
After all, any attempt to transform a planet the way some people talk about terraforming Mars would of necessity be dealing with long-lasting effects, and terraforming gases like these and others would be likely to persist not just for centuries but for the duration of the creator civilization’s lifespan. Adjusting a planetary atmosphere should present a large and discernable spectral signature precisely in the infrared wavelengths LIFE will specialize in, and it’s noteworthy that gases like those studied here have long lifetimes in an atmosphere and could be replenished.
LIFE will work via direct imaging, but the study also takes in detection through transits by calculating the observing time needed with the James Webb Space Telescope’s instruments as applied to TRAPPIST-1 f. The results make the detection of such gases with our current technologies a clear possibility. As Schwieterman notes, “With an atmosphere like Earth’s, only one out of every million molecules could be one of these gases, and it would be potentially detectable. That gas concentration would also be sufficient to modify the climate.”
Indeed, working with transit detections for TRAPPIST-1 f produces positive results with JWST’s MIRI Low Resolution Spectrometer (LRS) and NIRSpec instrumentation (with “surprisingly few transits”). But while transits are feasible, they’re also more scarce, whereas LIFE’s direct imaging in the infrared takes in numerous nearby stars.
From the paper:
We also calculated the MIR [mid infrared] emitted light spectra for an Earth-twin planet with 1, 10, and 100 ppm of CF₄, C₂F₆, C₃F₈, SF₆, and NF₃… and the corresponding detectability of C₂F₆, C₃F₈, and SF₆ with the LIFE concept mission… We find that in every case, the band-integrated S/Ns were >5σ for outer habitable zone Earths orbiting G2V, K6V, or TRAPPIST-1-like (M8V) stars at 5 and 10 pc and with integration times of 10 and 50 days. Importantly, the threshold for detecting these technosignature molecules with LIFE is more favorable than standard biosignatures such as O₃ and CH₄ at modern Earth concentrations, which can be accurately retrieved… indicating meaningfully terraformed atmospheres could be identified through standard biosignatures searches with no additional overhead.
Image: Qualitative mid-infrared transmission and emission spectra of a hypothetical Earth-like planet whose climate has been modified with artificial greenhouse gases. Credit: Sohail Wasif/UCR.
The choice of TRAPPIST-1 is sensible, given that the system offers seven rocky planet targets aligned in such a way that transit studies are possible. Indeed, this is one of the most highly studied exoplanetary systems available. But the addition of the LIFE mission’s instrumentation shows that direct imaging in the infrared expands the realm of study well beyond transiting worlds. So whereas CFCs are short lived and might flag transient industrial activity, the fluorinated gases discussed in this paper are chemically inert and represent potentially long-lived signatures for a terraforming civilization.
The paper is Schwieterman et al., “Artificial Greenhouse Gases as Exoplanet Technosignatures,” Astrophysical Journal Vol. 969, No. 1 (25 June 2024), 20 (full text).
The goal of terraforming a planet is to make it more habitable for humans. The most ideal planet to terraform would be an Earth sized planet in the life belt of it’s star. We would only have to transport some Earthly forms of life there if it had no life. Smaller planets with lower gravity will be more expensive to maintain due to the inability to hold onto an atmosphere like Mars. The atmosphere would have to be replenished in deep time. We could use heavy gases like CO2 which don’t escape easily.
A terraformed planet’s biosignatures might be indistinguishable from a natural one. I like the idea of the infra red interferometer. Certain technological signatures like carbon monoxide are definitely time limited within a century and a half.
Indeed, if the objective is to make the place conducive to human habitation. But alien species may also be considered…
And an insufficiently advanced technology may be indistinguishable from magic while a sufficiently advanced technology may be indistinguishable from nature.
Move advanced civilizations might radiate less heat since they are more energy efficient
The fluorinated gases chosen all assume that the terraforming is for cold worlds that need warming. This obviates the possibility of warm worlds that need cooling, such as our own planet where both energy generation and the emission of heat trapping gases – CO2, CH4, and N2O – might be quite common for planets that were hosts for the evolution of pretechnological life but possibly not for technological life or the natural increased luminosity of the star.
While these gases are relatively inert, can we be sure they cannot be made abiotically under some planetary conditions and therefore ambiguous?
If heating a planet’s surface to terraform it is needed, is using greenhouse-trapping industrial gases the best way to proceed? We assume it is relatively inexpensive compared to orbiting mirrors or similar to add extra energy to the surface (IR or microwaves rather than visible light?), but may their persistence, difficulty of feedback control, or pollution, make them unsuited compared to other means. For example, Mars needs a thicker atmosphere which requires releasing N2 and O2 by 2-3 orders of magnitude partial pressures. While CFCs might be part of the solution, one side effect is that they destroy any needed UV-protecting O3 (for aerobic organisms requiring free O2 for metabolism), making the surface organisms vulnerable to UV. It might be better to just use CO2, CH4, H2O, and N2O as greenhouse gases.
This doesn’t mean we shouldn’t look for them as “low-hanging technosignatures with hopefully low ambiguity potential, but simply as a caution that terraforming might not use these gases, creating potential false negatives. [Although I think the number of false -ves is likely to be very small, maybe vanishingly so.]
The gases mentioned in this post aren’t chlorofluorocarbons, but perfluorocarbons. These don’t destroy ozone (https://en.wikipedia.org/wiki/Fluorocarbon).
@Daniel. Thank you. I stand corrected.
How toxic are these super green house gasses to terristrial life forms?
@Ivan
in their untouched state, they are not toxic to terrestrial life as they are effectively inert. The problem comes when they are broken up by UV. This releases fluorine which is definitely very toxic (but not in the tiny concentrations needed), but more importantly destroys the ozone layer that protects surface life from the UV which becomes very damaging to exposed organic molecules. The worst types of CFCs were banned globally, but satellites have detected plumes from possible manufacturing sites in Asia. There is also the problem of poor disposal of CFC refrigerants that get released into the atmosphere.
Sophisticated planetary manipulation will consider nth-order effects of actions rather than just the 1st-order ones.
I recently read another of Jack McDevitt’s “Priscilla Hutchins” novels, where a corporate terraforming operation was destroying the native life in favor of its human colonies, and was hiding this destruction from the Earth population. Our current sensibility is to invoke “The Prime Directive”, but that is not a universally accepted protocol. If it should prove that life is ubiquitous in any potentially terraformable world, would that restrict humans from terraforming planets or would we just ignore that restriction for all but a few living worlds?
The UV is the issue that breaks up the molecules into some pretty nasty chemicals, one is actually a very dangerous nerve agent. However building a large thin glass disc with anti UV coatings at the L1 point of a planet would reduce the amount of breakdown to a more sustainable level.
To predict powerful aliens, we really should figure out how hard it is to shrink-wrap a planet. A frisbee falls fairly quickly, but if you have a plastic circle a kilometer in diameter, how long must it take the air to get out from under? If there are 15 psi of pressure above a point, and the membrane weighs less than 0.1 pounds per square inch, the air flow generated should be no worse than a storm, and I could picture trimming the weight much further than that (haven’t even got out the graphene wrap yet). Once the whole planet is wrapped, there’s nowhere for the air to go: you can lay down an insulating blanket, a Highlander-style reflective Shield, or an Earth-like atmosphere above Venus or Saturn with minimal maintenance pumping. That is, if it could be set up in a layer of atmosphere with laminar horizontal flow, and active measures used to suppress unwanted oscillations, with strategically located struts and pre-engineered points of failure…? That seems more a matter of predicting chaos with a computer than brute strength.
I once suggested that a tube of air might be contained in the tube sides stretched above the contained atmosphere. This would allow spacecraft to enter and land without needing any airlock. The tube would have to be resistant to differences in inner and outer air pressures. Purely speculative.
However, why terraform a planet if far greater living space can be constructed using large space habitats? The logic against terraforming doesn’t change. Habitats are incremental, yet can create far more surface area from various resources than a planetary surface. All the correct conditions can be made to suit.
This logic applies to building habitats to preserve ecosystems. Only migratory organisms would need a world, and then the seasons, magnetic fields, and even a lunar substitute would be needed for many organisms adapted to their native planet’s conditions.
A robot civilization wouldn’t need an enclosed ecosystem, just a frame and anchor points. How would we detect that with our current and near term available telescopes?
Carbon dioxide and carbon monoxide are harmful when they take away one’s oxygen. These gases are heavier than air like hydrogen sulfide, one of the gases in Earth’s early atmosphere. They can be dangerous near the ground and low lying areas during volcanic activity. Carbon monoxide comes from gas powered motors. They are not toxic in our atmosphere since they are only in small amounts and the same is true when the carbon dioxide levels were much higher in the past.
CFC’s would be technological signature gases that would not be false positives since we produced so much of them. It is interesting that the same harmful ultra violet radiation helped make the ozone layer by the dissociation of molecular oxygen O2 into monoatomic oxygen O1 which combines with diatomic oxygen to make ozone, O3 which came from cyanobacteria, Earth’s first life.
The heavier isotopes of oxygen are left behind during the history of processes of atmospheric escape on Mars. The heavier isotope of oxygen is in the CO2 molecule, The photo dissociation of CO2 with heavier and light oxygen isotopes is also by ultra violet light. Jeans escape tells us that the heavier gases require more energy or heat to excite these molecules to make them escape. Molecules are propelled higher into the atmosphere by heat where they can be split apart by ultra violet radiation and collisional escape, etc or solar wind stripping. With Jeans escape, the speed of escape is dependent on the atomic weight of the gas and the gravity or the planet which is dependent on it’s size. Planets closer to the star will loose a gas faster than those further away and smaller planets have less gravity and therefore can’t hold onto an atmosphere. The Moon is a good example where the escape velocity is too low and we can’t terraform it.
These all have to be considered in terraforming. The space Mirror solar heating idea will work, but we might have to wait until our propulsion technology is better and it become much cheaper to send large payloads into space.
The prime directive would restrict us, but we might get help if those planets have a surface gravity which is too much lower than ours. Mars is a good example where the gravity is less than half of ours. The jury is still out whether Mars red surface or hematite oxidation was caused by abiotic chemical process or biotic ones. I favor the former until it is proven wrong. We still have to go to Mars and prove it.
I can imagine a biosphere that learned to produce greenhouse gases metabolically in response to long-term steady cooling. Ultra-cool dwarfs are the right setting for this because they have prolonged pre-main sequence stage with decreasing luminosity. Not sure if CFCs could be produced but who knows what are the limits of possibility under strong and consistent evolutionary pressure in the right direction. But likewise, life on steadily-warming worlds could develop reflective foliage and this is much more plausible. More common also because G/F stars firstly give enough time to develop a complex biosphere and then start to slowly turn the heater on.
PS although, why nothing like this happened on Earth in response to Cenozoic cooling and Quarternary glaciation? (not counting us humans :D )
@torque_xtr
You have just described Lovelock’s “Daisy World”. However, the needed tradeoffs – shiny white petals and very pale leaves covering the planet may be incompatible with the need for water, nutrients, falling CO2 levels, and oceanic energy absorption. It is a nice Gaia-inspired model, but is there evidence that reflectivity is effective even if true? [ Obviously the pepper moth evolution during industrialization in the UK is a minor adaptive example of color change, but apart from plants using color to increase the number of visiting pollinators, is reflectivity going to be effective?
>>> Lovelock’s “Daisy World”.
Exactly that! Well, on the inner edge of HZ it’s precarious, at least a tradeoff between cooling effect and energy available for photosynthesis. And the more ocean coverage – the less is the effect (although, it could be covered by white lily-pads). The outer edge looks much more-favorable to Daisyworld-like effects. In example, a tree or grass species could develop darker leaves and create warmer climate over the forests, which then could cause melting of adjacent ice caps and allow forests to reclaim more living space, until glaciers are gone. True, we have no obvious observational or palaeontological evidence for this, and it seems there aren’t many climate modelling researches on these effects. So I really don’t know why Daisyworld effects are at least not common.
But they could mimic natural signatures. Reflective leaves could be expected right in the Venus zone where reflective clouds are common; abnormally dark planets on the outer edge of HZ could be just waterless basaltic worlds.
Fluorinated species are a powerful signature anyway, it’s just that they may be biogenic instead of technogenic, if biosphere somehow adapted to release them in response to a prolonged cooling. But change of reflectivity seems a much shorter way to counter it.
Here is the LIFE – Large Interferometer For Exoplanets website;
https://life-space-mission.com/
A good overview of LIFE;
https://www.aanda.org/articles/aa/abs/2022/08/aa40366-21/aa40366-21.html
We need more dedicated telescopes in space. Look at how popular Hubble and the JWST have become. Public support and funding is what drives these programs and any mention of life or possibly finding planets that support life has great public acceptance.
I would like to see updates on projects such as these and what could be done with the nearly active Space X Starship. It holds a lot of promise and could start a renaissance in exoplanet research and space telescopes…