Our recent conversations about the likelihood of life elsewhere in the universe emphasize how early in the search we are. Consider recent work on TRAPPIST-1, which draws on JWST data to tell us more about the nature of the seven planets there. On the surface, this seven-planet system around a nearby M-dwarf all but shouts for attention, given that we have three planets in the habitable zone, all of them of terrestrial size, as indeed are all the planets in the system. Moreover, as an ultracool dwarf star, the primary is both tiny and bright in the infrared, just the thing for an instrument like the James Webb Space Telescope to harvest solid data on planetary atmospheres.
This is a system, in other words, ripe for atmospheric and perhaps astrobiological investigation, and Michaël Gillon (University of Liége), the key player in discovering its complexities, points in a new paper to how much we’ve already learned. If its star is ultracool, the planetary system at TRAPPIST-1 can also be considered ‘ultracompact’ in that the innermost and outermost planets orbit at 0.01 and 0.06 AU respectively. By comparison, Mercury orbits at 0.4 AU from our Sun. The stability of the system through mean motion resonances means that we’re able to deduce tight limits on mass and density, which in turn give us useful insights into their composition.
Image: Measuring the mass and diameter of a planet reveals its density, which can give scientists clues about its composition. Scientists now know the density of the seven TRAPPIST-1 planets with a higher precision than any other planets in the universe, other than those in our own solar system. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).
Because we’ve been talking about SETI recently, I’ll mention that the SETI Institute has already subjected TRAPPIST-1 to a search using the Allen Telescope Array at frequencies of 2.84 and 8.2 gigahertz. The choice of frequencies was dictated by the researchers’ interest in whether a system this compact might have a civilization that had spread between two or more worlds. Searching for powerful broadband communications when planetary alignments between two habitable planets occur as viewed from Earth is thus a hopeful strategy, and as is obvious, the search yielded nothing unusual. A broader question is whether life might spread between such worlds through impacts and subsequent contamination.
What I’m angling for here is the relationship between a bold, unlikely observing strategy and a more orthodox study of planetary atmospheres. Both of these are ongoing, with the investigation of biosignatures a hot topic as we work with JWST but also plan for subsequent space telescopes like the Habitable Exoplanet Observatory (HabEx). The gap in expectations between SETI at TRAPPIST-1 and atmosphere characterization via such instruments highlights what a shot in the dark SETI can be. But it’s a useful shot in the dark. We need to know that there is a ‘great silence’ and continue to poke into it even as we explore the likelihood of abiogenesis elsewhere.
But back to the Gillon paper. Here you’ll find the latest results on planetary dynamics at TRAPPIST-1 and the implications for how these worlds form, along with current data on their densities and compositions. Another benefit of the compact nature of this system is that the planets interact with each other, which means we get strong signals from Transit Timing Variations that help constrain the orbits and masses involved. No other system has rocky exoplanets with such tight density measurements. The three inner planets are irradiated beyond the runaway greenhouse limit, and recent work points to the two inner planets being totally desiccated, with volatiles likely in the outer worlds.
What we’d like to know is whether, given that habitable zone planets are found in M-dwarf systems (Proxima Centauri is an obvious further example), such worlds can maintain a significant atmosphere given irradiation from the parent star. This is tricky work. There are models of the early Earth that involve massive volatile losses, and yet today’s Earth is obviously life supporting. Is there a possibility that rocky planets around M-dwarfs could begin with a high volatile content to counterbalance erosion from stellar bombardment? Gillon sees TRAPPIST-1 as an ideal laboratory to pursue such investigations, one with implications for M-dwarfs throughout the galaxy. From the paper:
Indeed, its planets have an irradiation range similar to the inner solar system and encompassing the inner and outer limits of its circumstellar habitable zone, with planet b and h receiving from their star about 4.2 and 0.15 times the energy received by the Earth from the Sun per second, respectively. Detecting an atmosphere around any of these 7 planets and measuring its composition would be of fundamental importance to constrain our atmospheric evolution and escape models, and, more broadly, to determine if low-mass M-dwarfs, the larger reservoir of terrestrial planets in the Universe, could truly host habitable worlds.
Image: Belgian astronomer Michaël Gillon, who discovered the planetary system at TRAPPIST-1. Credit: University of Liége.
Thus the early work on TRAPPIST-1 atmospheres, conducted with Hubble data and sufficient to rule out the presence of cloud-free hydrogen-dominated atmospheres for all the planets in the system. But now we have early papers using JWST data, and the issues become more stark when we turn to work performed by Gwenaël Van Looveren (University of Vienna) and colleagues. While previous studies of the system have indicated no thick atmospheres on the two innermost planets (b and c), the Van Looveren team focuses specifically on thermal losses occurring as the atmosphere heats as opposed to hard to measure non-thermal processes like stellar winds.
Here the situation clarifies. Working with computer code called Kompot, which calculates the thermo-chemical structure of an upper atmosphere, the team has analyzed the highly irradiated TRAPPIST-1 environment, modeling over 500 photochemical reactions in light of X-Ray, ultraviolet and infrared radiation, among other factors. The results show strong atmospheric loss in the early era of system development, but take into account losses through the different stages of the system’s evolution. It’s important to keep in mind that a star like this takes between 1 and 2 billion years to settle onto the main sequence, a period of high radiation. It’s also true that even main-sequence M-dwarfs can show high levels of radiation activity.
The upshot: X-ray and UV activity declines very slowly in the first several billion years on the main sequence, and stellar radiation in these wavelengths is the main driver of atmospheric loss. Things look dicey for atmospheres on any of the TRAPPIST-1 planets, and the Van Looveren model generalizes to other stars. From the paper:
The results of our models tentatively indicate that the habitable zone of M dwarfs after their arrival on the main sequence is not suited for the long-term survival of secondary atmospheres around planets of the considered planetary masses owing to the high ratio of spectral irradiance of XUV to optical/infrared radiation over a very long time compared to more massive stars. Maintaining atmospheres on planets like this requires their continual replenishment or their formation very late in the evolution of the planets. A further expansion of the grid and more detailed studies of the parameter space are required to draw definitive conclusions for the entire spectral class of M dwarfs.
Image: This is Figure 8 from the paper. Caption: Overview of the planets in the TRAPPIST-1 system and the estimated habitable zone (indicated by the green lines, taken from Bolmont et al. 2017). We added vertical lines at the minimum distances at which atmospheres of various compositions could survive for more than 1 Gyr. Credit: Van Looveren et al.
Note the term ‘primary atmosphere.’ Primary atmospheres of hydrogen and helium give way to secondary atmospheres that are the result of later processes like volcanic outgassing and molecules breaking down under stellar radiation on the planet’s surface. The paper, then, is saying that the kind of secondary atmospheres in which we might hope to find life are unlikely to survive in this environment, although active processes on a given planet might still allow them. The paper ends this way:
Our conclusion from this work is therefore significant for terrestrial planets with a mass that is similar to the Earth’s mass that orbit mid- to late-M dwarfs such as TRAPPIST-1 near or inside the (final) habitable zone. For these planets, substantial N2/CO2 atmospheres are unlikely unless atmospheric gas is continually replenished at high rates on timescales of no more than a few million years (the loss timescales estimated in our work), for example, through volcanism.
I wouldn’t call this the death knell for atmospheric survival at TRAPPIST-1, nor do the authors, but the work points to the factors that have to be addressed in further study of the system, and the results certainly challenge the possibility of life-sustaining atmospheres on any of these planets. The Van Looveren work isn’t included in Michaël Gillon’s paper, which appeared just before its release, but I hope you’ll look at both and keep the Gillon available as the best current overview of TRAPPIST-1.
As to M-dwarf prospects in general, it’s one thing to imagine a high-radiation environment, with the possibilities that life might find an evolutionary path forward, but quite another to strip a planet of its atmosphere altogether. If that is the prospect, then the census of ‘habitable’ worlds drops sharply, for M-dwarfs make up somewhere around 80 percent of all the stars in the Milky Way. A sobering thought to close the morning as I head upstairs to grind coffee beans and rejuvenate myself with caffeine.
The papers are Gillon, “TRAPPIST-1 and its compact system of temperate rocky planets,” to be published in Handbook of Exoplanets (Springer) and available as a preprint. The Van Looveren paper is “Airy worlds or barren rocks? On the survivability of secondary atmospheres around the TRAPPIST-1 planets,” accepted at Astronomy & Astrophysics (preprint).
From the Looveren paper abstract:
This could mean that any life that appeared can only survive in the crustal biosphere, much as we posit may be the case for Mars if life is extant there. Could we even detect any subsurface life, perhaps by some gas emissions by methanogens or similar chemotroph microbes? If this is a general case for M_dwarfs, this would suggest that we should focus biosignature efforts on F, G, and K stars until the time when we can send probes to M_dwarf planets or discover/invent new technologies for remote life detection beneath the surface of planets, including icy worlds with subsurface oceans.
If the “low-hanging fruit” of biosignature detection has been snatched away from M-dwarfs then we should look for them in the HZs of more promising star systems, even if detection is more difficult.
Though disappointing, it’s not unexpected as a lot of evidence has been pointing in this direction. The results also seem to say something about the possibility of late stage habitability as even secondary atmospheres are eroded after ~1Gy, at some point earth sized worlds just run out of volatiles and can’t regenerate atmosphere to take advantage of the far future quiescent stage of M-dwarf evolution. The inhospitability of M-dwarfs does square nicely with the great silence though, if 80% of all stars in our galaxy are excluded as candidates for life that drastically narrows the possible throws of dice for abiogenesis.
Where does the atmosphere go after it leaves one of the planets, and how quickly? With so many planets orbiting so frequently in such a small space, do the outer TRAPPIST-1 worlds have a chance to pick up volatiles that were lost from the inner ones?
In response to (or perhaps anticipation of) Fermi’s Question a Great Silence indeed.
The assumption the paper makes is that a red dwarf and its planets are the same age. Is it possible that this might not be true?
Were Type II civilization were to engineer a stellar system, it would look a lot like Trappist-1.
I can see the earth centered party are jumping for joy. This is a prime example of just how ignorant we really are about exoplanets.
Majority Of Water Hides Deep In The Interiors Of Exoplanets.
https://astrobiology.com/2024/01/majority-of-water-hides-deep-in-the-interiors-of-exoplanets.html
Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars.
https://www.science.org/doi/10.1126/science.abl7164
What about comets replenishing these racetrack planets?
What about this systems passage through the spiral arms of the galaxy causing more comets and larger bodies to be collected.
What about Mimas, the Death star may be the cosmic seed to life in the universe…
https://www.qmul.ac.uk/media/news/2024/se/mimas-surprise-tiny-moon-holds-young-ocean-beneath-icy-shell.html
There is most probably googolplexians of these rouge mini dwarf planets in our galaxy that had enough heat to create life in their internal oceans. When they cool and freeze completely the collision with planets in exosolar systems would create a perfect environment to infect those worlds with life!
Low density worlds of TRAPPIST-1 and just dried out rocks does not fit the picture…
@Michael Fidler
Thanks for that last piece from my alma mater on Mimas. It nicely explains how and why the moon has a subsurface ocean and why it is young. That part was excellent.
It is a pity that they then enter into speculation about Mimas and life that is unwarranted and has no evidence to support the speculation. Mimas is not going to be a “cosmic seed of life” on any time scale worth thinking about.
Glad you liked it. When these dwarf planets like Mimas come into existence they will have enough internal heat and convection to make a nice broth for life to form. It could be an interesting area of research to see how pressure, heat and an organic mix could help life to develope quickly in a deep ocean. This sounds like it may have a higher chance then here since the ecosystem would be more benign. It’s like a giant enclosed lab that has 500 million years to churn out DNA. Then freezes for ever with all the microorganisms ready to return to life when landing in a warm ocean like Earth’s.
Asteroid Bennu sample examined by NASA hints at life on an ancient ocean planet.
“There are indeed similarities between the mineralogy of Bennu and what has been found on Enceladus,” Klenner told New Scientist.
The presence of these materials is significant because, on Earth, they form through exothermic reactions when rock is pushed into a seabed and interacts with water.
This process not only suggests the presence of water but also conditions that could potentially support life.
https://www.earth.com/news/asteroid-bennu-hints-at-the-possibility-of-life-on-an-ancient-planet/
@Michael Fidler
The article seems a little coy at suggesting what planet (or moon?), if any, Bennu was part of, or acquired material from.
If its origin is from, or near, a water-rich planet, that could have been any of Earth, Venus, and Mars. If the surface material is “remarkably similar to that from Enceladus”, then it could also be that plume water adhering to Bennu in the past.
I would think there are a lot of questions about the likely origin of the material, even possibly from Earth (it is an NEA!). Whether it is delivering material or simply acquiring it is a question. If Bennu has this material, other asteroids presumably do too, possibly giving us further clues to its origin.
Keep an eye out for the eventual peer-reviewed paper so we can read what the authors are comfortable saying about this material from the surface of Bennu.
“Magellan revealed that all of the planet’s impact craters are formed within the last 700 million years. This implies that Venus’ surface was completely reshaped by a worldwide volcanic event in its recent geologic past—but exactly what happened is still up for debate.”
Asteroid Strikes May Have Frozen Past Earth Into ‘Snowball,’ Study Argues
https://www.newsweek.com/asteroid-impact-snowball-earth-global-glaciation-1868656
If Bennu’s material age can be pinned down at around 700 million years ago, it may put together some interesting possibilities,,,
@Michael C Fidler Thank you for the update on Mimas. That pocket of water near one of the poles was a less likely place for life to start with, since it was small pond of water compared to global oceans on other moons.
And now with an estimated age of only 5-15 My it can safely be written off as a candidate, life need a stable environment for a much longer timespan to get started.
If that was not the case, new life would get started everywhere literally under our feet in underground aquifiers and we would have numerous alternative starts of life literally coming out of the water taps of our own kitchens!
We still got the Galilean moons at Jupiter, where Europa got an ice sheet that repeatedly crack and where water or geysers might provide a sample.
Excellent presentation on the paper my Michaël Gillon, and here in fig 8 we get a good estimate how far a planet need to be to keep the volatiles. This mean that only the brightest and relatively quiet red dwarf stars might have potentially life bearing planets. That means that among the nearby red dwarf stars only Lacaille 8760 might be a reasonable candidate where the habitable zone might be from 0,28 AU to perhaps as far out as ~ 0,532 AU in the most optimistic estimate from NASA’s star and exoplanet database. But so far no such planet have been detected in that system.
Webb Telescope detects activity within dwarf planets.
A team found evidence for hydrothermal or metamorphic activity within the icy dwarf planets Eris and Makemake, located in the Kuiper Belt. Methane detected on their surfaces has the tell-tale signs of warm or even hot geochemistry in their rocky cores, which is markedly different than the signature of methane from a comet.
https://cosmiclog.com/2024/02/16/webb-telescope-detects-activity-within-dwarf-planets/
@Michael Fidler
This keeps bringing to mind Thomas Gold’s belief that the origin of fossil fuels is primordial methane that is degraded by heat on earth to become oil, and then coal, both contaminated with organisms living in the “deep hot biosphere”. Is it possible that it is primordial methane, rather than geologic processes, or life, that is the transient source of methane on Mars?
I would think the loss of the atmosphere would eventually allow freezing of water on the dark side of these tidally locked planets. Intelligent species could make homes here without much issues I would think.
I’m still inclined to think these planets, at least the ones in the habitable zone outwards, will have atmospheres for two reasons:
i) Their density is less than Earth’s implying higher volatile inventory, especially water. This would mean they would have started out with dense steam atmospheres, which would have diluted the other gases such as Nitrogen and CO2. So when the star reached the main sequence, and any water would have condensed out into oceans, there would still be significant other gases extant.
ii) These planets are packed close together and the size of Earth, so the tidal heating must be many times the value of Io’s, and their volcanism will be phenomenal. Their entire mantels would have been turned inside out. At the very least, I expect a SO2 atmosphere. (This will indicate complete dehydration.)
Note: Trappist 1 is estimated to be 7.2 b.y. old, and it’s current flare energies are in a similar range to the Sun’s; although, higher than the Sun’s on average. The planets are, of course, a lot closer.
Given enough time the CO2 should react with the ground rocks and remain locked up as the carbon cycle grinds to a halt due to the age of the system. However nitrogen should hang around for a long time and it is hard to detect with our current instruments, so these inner worlds may still have nitrogen atmospheres with no water or CO2. I find it hard to believe a total stripping of the planets gaseous contingent occurred.
In textbooks or reviews of stellar formation in the main sequence, beside the corresponding mass luminosity and lifetime charts or tables, there is also a timeline for formation from gas cloud to an ignited main sequence star. Something like ten million years is allocated between cloud fragment becoming a protostar and protostar igniting on the main sequence. Nominally Stage 3 o 5 on an HZ Diagram.
But the example used in basic textbooks is protostar similar to the sun in MS luminosity and mass.
Now if we examine a star about 3 x solar mass such as Sirius, or 0.3x solar mass such as a red dwarf, one would expect the timeline from protostar to MS ignition to be speeded up or slowed in some proportionality to the mass as well. Likely on some power scale.
But when I read articles about circumstellar disks and planet formation, I am yet to see a representative figure other than ten million years as the window allowed for planet formation. Likely a solar emphasis there, but it stretches credibility when one considers we deal with stellar masses between 0.1 and maybe up 3.0 for habitability studies. It could be that the stretched or compressed timelines of protostars of non-solar mass are accounted for in these studies, perhaps implicitly.
But on the other hand, if in the early days of observing circumstellar disks it was thought that there was a narrow window allowing for planet formation ( ten million years and after that would be “strike three”), and the red dwarf systems like Trappist need about 50 million to collapse into stars… and heaven knows how long it takes to form planets…. Possibly, could there be some ground rules still not sorted out before we foresee the HZ planets all fried?
Following up on my above comment: As we are well aware, as a function of mass, both stellar luminosity and lifetime are non-linear functions. The sun on the MS will last 10 billion years but an O star will could leave after ten million years or so.
But in addition, there is a nonlinear function for the march to Main Sequence from out of a nebular cloud. With a mass of about 15 solar units, modeling suggests a time to coalesce into a pre-MS star of about 100,000 years. For 5 solar masses it is about a million years. And for the sun, about ten million. Examining the Looverine et al. report, the answer to what we might have extrapolated lies in the text. The track to the main sequence takes about 2 billion years for a star like Trappist 1.
Here is an excerpt:
After their formation, mid- to late-M dwarf stars evolve very slowly to the phase of stable hydrogen burning. A star like TRAPPIST-1 takes about 1–2 Gyr to settle on the main sequence (Ramirez & Kaltenegger 2014). During this period, the X-ray activity usually stays at a saturated level of ∼10−3 times the bolometric luminosity. Because M dwarfs move vertically down the Hayashi track in the Hertzsprung-Russell diagram, the initial bolometric and X-ray luminosities were therefore higher by between one and two orders of magnitude than after reaching the main sequence. Planets in stable orbits will therefore have been subject to much higher irradiation levels than after the host star reached the main sequence even when they are still at saturation. Consequently, the habitable zone also shrank substantially during the first billion years in the life of such a planetary system; the bolometric luminosity of TRAPPIST-1 decreased by a factor of 40 from an age of 10 Myr, when terrestrial planets have supposedly reached their near-final mass (e.g. Lammer et al. 2020 for the Solar System) to ∼1–2 Gyr when the star arrived on the main sequence (Fleming et al. 2020). This means that the orbital distance of the habitable zone shrank by a factor of √ 40 = 6.3. Bolmont et al. (2017) estimated the inner edge of the habitable zone of a 0.08 M⊙ star to move from ∼0.08 au at an age of 10 Myr to ∼0.012 au at an age of 2 Gyr on the main sequence (their Fig. 1b). This implies that all known TRAPPIST-1 planets (with semi-major axes of ∼0.01–0.06 au) were far inside the habitable zone for the first tens to hundreds of million years when secondary atmospheres are typically assumed to build up. It is likely that potential water oceans would have evaporated, and the TRAPPIST-1 planets would have followed the fate of Venus (Ramirez & Kaltenegger 2014). We note that the loss rates of hydrogen produced by photo-dissociation of evaporated H2O are much higher than the loss rates of the heavy atoms reported in our work.
—-
I should have highlighted this:
the bolometric luminosity of TRAPPIST-1 decreased by a factor of 40 from an age of 10 Myr, when terrestrial planets have supposedly reached their near-final mass (e.g. Lammer et al. 2020 for the Solar System) to ∼1–2 Gyr when the star arrived on the main sequence (Fleming et al. 2020).
It would appear that the planetary formation phase has been fixed to the value assumed for the solar system: about ten million years. Why this particular parameter should remain fixed is NOT obvious to me. Since condensation into a main sequence star is stretched out, it stands to reason to me that the timeline for terrestrial planet formation would be as well. It might be that the condensation of the disk material into planets could not have even started at the beginning of this formation process.
Hi Paul
As a Counter-Point I’d suggest this ApJ paper:
The Carbon-deficient Evolution of TRAPPIST-1c
Katie E. Teixeira1,2, Caroline V. Morley1,2, Bradford J. Foley3, and Cayman T. Unterborn4
The Astrophysical Journal, Volume 960, Number 1
Citation Katie E. Teixeira et al 2024 ApJ 960 44
DOI 10.3847/1538-4357/ad0cec
From the abstract…
A key case study is TRAPPIST-1c, which receives almost the same bolometric flux as Venus. We might therefore expect TRAPPIST-1c to possess a thick, CO2-dominated atmosphere. Instead, Zieba et al. show that it has little to no CO2 in its atmosphere. To interpret these results, we run coupled time-dependent simulations of planetary outgassing and atmospheric escape to model the evolution of TRAPPIST-1c’s atmosphere.
We find that the stellar wind stripping that is expected to occur on TRAPPIST-1c over its lifetime can only remove up to ∼16 bar of CO2, less than the modern CO2 inventory of either Earth or Venus.
Therefore, we infer that TRAPPIST-1c either formed volatile-poor, as compared to Earth and Venus, or lost a substantial amount of CO2 during an early phase of hydrodynamic hydrogen escape. Finally, we scale our results for the other TRAPPIST-1 planets, finding that the more distant TRAPPIST-1 planets may readily retain atmospheres.
…I’d say this current Clash of Models requires a lot more data-collection before we can really say either way.
Good find, Adam. Thanks!
Adam, CO2 sequestration can occur quite readily even at higher temperatures, I think around 200 degrees Celsius is a peak for absorbtion even in the absence of water depending on the rock type. Drop venus’s atmospheric temp by 250 degrees and will start a rapid carbonate formation process directly. These worlds may simply have their CO2 locked up in rocks and have nitrogen atmospheres.
I wonder why SETI did not check at 1.42 GHz, because I thought Carl Sagan in Contact said this was the most likely frequency for everyone else in the universe to broadcast on, because hydrogen, first element bla bla?
In this case, the researchers were looking specifically for communications channels between nearby planets, and these are frequencies we use for communicating with spacecraft.
Maybe they should look for intermittent laser beams between all that flaring…
The recent idea is that ions would work best to receive and transmit superluminal longitudinal waves but both the receiver and transmitter may be based on three ion (Plasma) interferometers.
1) They are capable of penetrating any solid object including Faraday Cages. You can put a transmitter in a box of thick metal and a receiver outside of the box will receive the scalar wave frequency you are pulsing. The potential here is for a transmitter that can penetrate any obstacle or perhaps communicate directly through the Earth from one side of the globe to the other.
2) They are capable of superluminal travel. These waves are claimed not to be electromagnetic, but composed of pure potential energy. Due to this, the speed of light limit does not apply to them. The propagation speed of a scalar wave has been measured as faster than the speed of light and thought by some researchers to be potentially of infinite velocity. (Perhaps SETI – The Search for Extraterrestrial Intelligence – needs a different kind of receiver in order to pick up signals from elsewhere in the universe! Why would advanced ET civilizations even bother using slow transverse waves for interstellar communications?)
I’ve always felt the 21 cm line was too noisy, neutral hydrogen in the galaxy gives off this constant roar that covers up faint signals at that frequency. Still, it is a wavelength astronomically interested listeners will tend to monitor, just as we have.
One compromise might be to broadcast a beacon at some harmonic of that wavelength, say 10.5 or 42 cm. If these frequencies are obscured by other sources of natural emission, then multiplying or dividing the hydrogen line by some constant, (like pi or e) might be worthwhile.
The problem with that is that only SETI obsessed civilizations are likely to be tuned to that spot on the dial. That is one variable that has been left out of the Drake Equation: what is the probability of an intelligent culture actually caring about whether or not there is anyone else “out there”? Perhaps they have determined the galaxy is full of other species they do not particularly want to meet, or that they are so few and far between that its pointless to even try.
Or maybe they have been so successful at SETI already that they have all the correspondents they want.
Referring to Carl Sagan and “Contact” (at least the film version – I cannot recall what it says in the book), the signal they detected was at 4.46 Ghz, which is “Hydrogen times Pi”, as Dr Arroway says. This always made a lot of sense to me – take a ubiquitous emission frequency in the universe, and multiply that by a very fundamental constant, giving a frequency that screams “artificial”. Oh, and broadcast prime numbers on it…. How often do such “significant” frequencies get searched?
Or the fine constant 1/137.035
Regarding the 21-cm line: Maybe I’m wrong on this, but the last time I looked at an illustration of that spectral region, it appeared that the vicinity of 21-cm line is fairly clear of other absorptions or emissions. Hence, if a signal is detected and is not identifiable as an electron flip phenomenon somewhere, a different type of transmission would stand out.
Of course, thus far we don’t have many illustrations of that checking out…