It would come as no surprise to readers of science fiction that the so-called ‘terminator’ region on certain kinds of planets might be a place where the conditions for life can emerge. I’m talking about planets that experience tidal lock to their star, as habitable zone worlds around some categories of M-dwarfs most likely do. But I can also go way back to science fiction read in my childhood to recall a story set, for example, on Mercury, then supposed to be locked to the Sun in its rotation, depicting humans setting up bases on the terminator zone between broiling dayside and frigid night.
Addendum: Can you name the science fiction story I’m talking about here? Because I can’t recall it, though I suspect the setting on Mercury was in one of the Winston series of juvenile novels I was absorbing in that era as a wide-eyed kid.
The subject of tidal lock is an especially interesting one because we have candidates for habitable planets around stars as close as Proxima Centauri, if indeed a possibly tidally locked planet can sustain clement conditions at the surface. Planets like this are subject to extreme conditions, with a nightside that receives no incoming radiation and an irradiated dayside where greenhouse effects might dominate depending on available water vapor. Even so, moderate temperatures can be achieved in models of planets with oceans, and most earlier work has gone into modeling water worlds. I also think it’s accurate to say that earlier work has focused on how habitable conditions might be maintained in the substellar ‘eye’ region directly facing the star.
But what about planets that are largely covered in land? It’s a pointed question because a new study in The Astrophysical Journal finds that tidally locked worlds mostly covered in water would eventually become saturated by a thick layer of vapor. The study, led by Ana Lobos (UC-Irvine) also finds that plentiful land surfaces produce a terminator region that could well be friendly to life even if the equatorial zone directly beneath the star on the dayside should prove inhospitable. Says Lobo:
“We are trying to draw attention to more water-limited planets, which despite not having widespread oceans, could have lakes or other smaller bodies of liquid water, and these climates could actually be very promising.”
Image: Some exoplanets have one side permanently facing their star while the other side is in perpetual darkness. The ring-shaped border between these permanent day and night regions is called a “terminator zone.” In a new paper in The Astrophysical Journal, physics and astronomy researchers at UC Irvine say this area has the potential to support extraterrestrial life. Credit: Ana Lobo / UCI.
The team’s modeling simulates both water-rich and water-limited planet scenarios, even as the question of how much water to expect on a habitable zone M-dwarf planet remains open. After all, water content likely depends on planet formation. If a habitable zone planet formed in place, it likely emerged with lower water content than one that formed beyond the snowline (relatively close in for M-dwarfs) and migrated inward. We also have to remember that flare activity could trigger water loss for such worlds.
Water’s effects on climate are abundant, from affecting surface albedo to the production of clouds and the development of greenhouse effects. They’re also tricky to model when we move into other planetary scenarios. As the paper notes:
Due to water’s various climate feedbacks and its effects on the atmospheric structure, the habitable zone of a water-limited Earth twin is broader than that of an aquaplanet Earth (Abe et al. 2011). But while water’s impact on climate is well understood for Earth, many of these fundamental climate feedbacks behave differently on M-dwarf planets, due to the lower frequency of the stellar radiation.
To perform the study, Lobo’s team considered a hypothetical Earth-class planet orbiting the nearby star AD Leonis (Gliese 388), an M3.5V red dwarf, using a 3D global climate model to find out whether a tidally locked world here could sustain a temperature gradient large enough to make the terminator habitable. The study uses a simplified habitability definition based solely on surface temperature. The researchers deployed ExoCAM, a modified version of the Community Atmosphere Model (CAM4) developed by the National Center for Atmospheric Research and used to study climate conditions on Earth. Their software tweaked the original code to adjust for factors such as planetary rotation.
The results are straightforward: With abundant land on the planet, terminator habitability increases dramatically. A water-rich world like Earth, with land covering but 30 percent of the surface, is not necessarily the best model for habitability here, as we consider the factors involved in tidal lock, with extensive land offering viable options in at least part of the surface. A ‘ring’ of habitability may prove to be a common outcome for such worlds. But it’s interesting to consider how these initial conditions might complicate the early development of biology. Here I return to the paper:
There are still many uncertainties regarding the water content of habitable-zone M-dwarf planets. Based on our current understanding, it is possible that water-limited planets could be abundant and possibly more common than ocean-covered worlds. Therefore, terminator habitability may represent a significant fraction of habitable M-dwarf planets. Compared to the temperate climates obtained with aquaplanets, terminator habitability does offer reduced fractional habitability. Also, while achieving a temperate terminator is relatively easy on water-limited planets, constraining the water availability at the terminator remains a challenge. Overall, the lack of abundant surface water in these simulations could pose a challenge for life to arise under these conditions, but mechanisms, including glacier flow, could allow for sufficient surface water accumulation to sustain locally moist and temperate climates at or near the terminator.
The paper is Lobo et al., “Terminator Habitability: The Case for Limited Water Availability on M-dwarf Planets,” Astrophysical Journal Vol. 945, No. 2 (16 March 2023), 161 (full text).
“Brightside Crossing”, Alan E Nourse
https://archive.org/details/galaxymagazine-1956-01/page/n7/mode/2up?view=theater
Still a great read after all these years!
Interesting! Thanks for the link.
Thanks for the discussion. It’s interesting to imagine a variety of tidally locked worlds supporting life, but with many differences in the details.
Addendum: Can you name the science fiction story I’m talking about here?
If it was a juvenile book maybe it was Isaac Asimov’s Lucky Starr and the Big Sun of Mercury? (Wikipedia entry here: https://en.wikipedia.org/wiki/Lucky_Starr_and_the_Big_Sun_of_Mercury) Clark Ashton Smith’s story “The Immortals of Mercury” also had human settlements in the supposed terminator zone, I think. And Leigh Brackett’s Eric John Stark grew up in such a place, if I remember right.
Have never read the Clark Ashton Smith story, so thanks for that. And I bet you’re right about the Lucky Starr book, as I recall reading those.
The Isaac Asimov series would have been my vote too.
I remembered the name (Lucky Starr),. but it seemed like a stretch.
Because the series also had “Lucky Starr and the Oceans of Venus”.
With these two instances to go on, it begs the question: Just how many planetary situations did Lucky Starr get himself in and then have to get extricated to go on to the next adventure?
Tough question, Wes. My reading of Lucky Starr was a long time ago! But the adventures sure were plentiful, and I particularly remember one involving Saturn’s rings.
I feel like the SF story about habitation at Mercury’s once-supposed terminator ring was mentioned in one of the “about the author” blurbs following a short story in the most recent issue of Analog magazine. I’ll look it up.
Thanks, Orion!
i was close: it’s alluded to in a letter to the editor. their memory is “the coldest place” by asimov, set on the dark side of mercury. the zinger is that between the story being bought and being published, mercury’s rotation was discovered !
this page credits Asimov with coining the phrase “ribbon world” for tidally locked planets, and then Larry Niven with a story called “the coldest place” set on the then-presumed dark side of mercury. (not the terminator). funnily between the story being bought and published, Mercury’s rotation was nailed down. the mention in this bimonth’s Analog was a reader letter, not an author blurb. so i was close!
https://asimov.fandom.com/wiki/Ribbon_World
Wikipedia has a page on “Mercury in Fiction”. Several early stories meet the criteria you mentioned, but my vote is for Asimov, though Anderson or Niven may have written the story you recall.
Randal Stockton
I’m with you in guessing it’s Asimov.
Mercury still has a “terminator” of sorts – with virtually no axial tilt, it has accumulated ice near its poles ( http://public.media.smithsonianmag.com/legacy_blog/Mercury-n-pole-shadow-ice.jpg ). Soil temperatures in the vicinity vary from 50K to >400K ( https://photojournal.jpl.nasa.gov/catalog/PIA19247 ). If you build domes over the water with some well-considered architecture to tap the abundant solar power … while shielding tourists from a nasty sunburn … the polar craters of Mercury might one day surpass those of the Moon as the hottest astronaut real estate in the Solar system. Provided, of course, you can put up with the commute.
There is enough power to build a very large magnetic field as well and coupled with a lens at its near lagrange points enormous power that can be delivered at any point on the surface. Enough power to send interstellar craft out at very high velocities.
I wonder whether the HZ could be quite wide for these worlds. The reason being that the ring of habitability (a ring world!) would slide back toward the dark side for tidally locked planets inside the traditional HZ (very narrow for M dwarfs), and slide forward into the light for those outside the HZ.
That idea should be especially relevant for red dwarfs around 0.35 solar masses, which are believed to slowly fluctuate in luminosity and helium-3 content as they undergo bouts of convection between their convective core and outer convective envelope.
(I just read about the above on page 9 of https://arxiv.org/abs/2209.11160 , which presents the oddity of a planet heavier than Jupiter orbiting such a star only four times its radius. Apparently this stretches planetary formation models, requiring a protoplanetary disk with 10% the mass of the star. We might still keep an eye out for moons tidally locked to such planets, though the need for a solar day seems decreased recently :)
The traditional HZ is just as wide or wider for M dwarfs because the planets orbit closer to the star, that is why Trappist 1 has 4 planets in tbe HZ. The ring could extend that even further depending on depth of atmosphere and terrain in the ring since having billion of years to evolve could make for some very interesting geology. Think biology and thousand foot trees…
These worlds could last for 10 of billions of years in a much higher stability, then earth like planets around G Dwarfs. For ever earth world 20 times as many M and K Dwarf worlds with over double the number of planets in the HZ with 3 times the length of time of our 3 billion years to survive until red giant phase of our sun.
Why do you think the 1 meter tall aliens that visit us have such large eyes, to see in the dim infrared light in the terminator ring. Why do you think they do not make their presence obvious, because they have existed for 10 of billions of years…
“The Terminator”
Interesting read as always – I think the fundamentals for this discussions are are;
1. Energy output of the star
2. Stability of the star
3. Semi-axis major of the planet
4. Mass of the planet
5. Mass of the atmosphere
6. Constituent chemistry of the atmosphere
7. Whether the planet has any natural satellites and their mass
8.Precession effects of other bodies orbiting the planet and/or star
9. Is the star part of a binary or multiple star system
10. Stability of the planetary orbit.
All these will impact any model for the survivability of any life that may arise in any form within any part of any planet where conditions are stable for long periods to allow for the complex chemical processes that ultimately lead to life – even if it is only bacterial life.
We can model the “perfect planet”, but in reality they do not exist, as we are finding from exoplanets, many planetary systems around M class dwarves are in some form of orbital resonance with each other – we see this in the Kepler 80 system, Kepler 223, TOI-178 and even the 7 planets of Trappist 1 all show orbital resonance – the innermost of these are tidally locked to their star as far as we know – but as we have no real analogues in the solar system, we do not know how the resonance impacts the motion of the planets over geological timescales – models give an opinion, but not a definitive answer.
Clearly, life in an terminator zone would be highly specialised, but equally they would likely evolve species that are highly adaptable, possibly able to nip into and out of the extreme zones edges for short periods of time because it is unlikely there would be a sharp edge to any “temperate” zone, rather the borders would trail off with conditions becoming increasingly harsh and possibly unstable – but we see that life on Earth has also conquered such regions, so we should be careful about making assumptions about how far life would extend out from these “safe” areas.
Night Polar bears, Penguins bats on the far side, Shield camels and Dune worms on the eyeball side.
Would a tidally locked world, have any residual axial tilt?
That is awesome especially considering there are somewhere around 10 EXP 23 and 10 EXP 24 red dwarfs in the observable universe. If technologically advanced life forms exist, they may have at least 20 trillion years to evolve into the future.
There may be multiple paths for biology to achieve advanced technology; in our case certain aspects seem important.
Depth of field perception and stereoscopic vision require substantial overlap in fields of vision permitting binocular vision. This evolved with the need for accurate judgement of the distance to the next branch when brachiating: this would necessitate a continuohs overhead canopy of tree branches extending through a forest. A tropical rainforest with competiion for the sunlight.
Brachiation would also provide a prehensile and stereognostic hand. And a shoulder with three axes of movement.
All this before we descended from the trees and assumed an upright bipedal posture which was important in reshaping the larynx, pharynx and oronasal cavities to permit modulation of phonated sound into speech. And speech was important in shaping society and intelligence. And an upper limb free to manipulate sticks and stones.
The terminator zone will have to provide a tropical rainforest (and thereafter a grassland) if a path similar to ours is to be recreated.
Didn’t the climate have to change, and with it the local ecosystems, for human evolution? The drying of the E. African forests forced prehumans to come down from the trees and learn to live in the savannahs, driving bipedalism, and possibly enhanced vocalization. Without these climate changes, prehumans might have taken a path more like that of our ape cousins and cut off our physical, and subsequent cultural, evolution that only in the last few millennia have started us on the road to a hi-tech civilization.
We are the contingent result of our prehistory and history.
One major problem for ribbonworld is where most ribbonworlds likely live, which is close to a M dwarf. The TRAPPIST-1 planets are the closest to us besides Prox.
The wavelengths look red but mostly give off infrared. Chlorophyll absorbs 430 nm (so can catch that) and 662 nm (very dim from a M dwarf). This is why plants look green by the way; that’s what gets reflected.
Basically we might get a habitable-temperature ribbon from a planet tidally-locked to a M … but no plants means no oxygen, unless we’re bringing the sun lamps.
*now, if we can somehow find a planet tidally locked and 0.7 AU from a K star . . .
This is an interesting point to dig into that touches on many disciplines. In a basic sense, the Carnot efficiency of a red dwarf at 2500 K interacting with a planet at 300 K is still quite high (88%); the star continues radiating energy at the planet. But as you say (graph: https://www.electrical4u.com/images/march16/1461574752.jpg ) the bulk of the light shifts into the infrared per Wien’s law. TRAPPIST-1 is a smidge hotter (‘soft white’) than a standard old-fashioned incandescent lamp filament, but a habitable zone must still surely find it short on actual illumination.
Where it gets mysterious is the biology. Plants thrive in the understory of a rainforest, so I wouldn’t count even Earth plants out where a habitable TRAPPIST-1 planet is concerned. But surely they should face competition from plants better adapted to their own world… The star should put out maximum light in the middle of the near infrared. Here’s a movie clip set for you: Brindabellas/Silver Dory put out these images of Australian landscape by near infrared: https://vimeo.com/showcase/3417427 You can find many similar photos with free websearches, though it took me quite a while to find one that wasn’t either spam or 404 when clicked on. (As one expects nowadays… I trust rapid progress in AI will soon help to eliminate those last leftover non-spam links!) What’s striking is that, apart from their trunks, trees seem nearly all white in the sunlight, reflecting (or fluorescing?) their waste heat out to the black sky. (I did a double take before realizing that 1500 nm is not the same as the IR fingerprint region at 1500 cm-1 = 6000 nm, which is why all the organic chemicals in the trees don’t make them black) On another planet, perhaps the plants might be a darker color. Earthly photosynthesis uses two photons to pump I think 10 protons across the thylakoid membrane, so it seems conceivable to design modified photosystems to tap the energy of a combination of several smaller photons while keeping the same pH gradient downstream.
Even on the dark side of the planet, or under an even colder star, a planet that is heated could power a biosphere with “anti-photosynthesis”. The plants would release their heat to the dark sky and extract free energy in the process. See https://scitechdaily.com/anti-solar-cells-thermoradiative-photovoltaic-cells-work-at-night/ and search “thermoradiative” at Arxiv for a few papers.
My guess: life could find a way.
Bacteriochlorophyll uses only red and IR wavelengths. So bacteria can use the star’s energy for growth and reproduction. So chemotrophy is not the only possibility, dependent on geological H2/CH4 emissions for energy.
However, given that chemotrophy was the first metabolic invention, this requires the planet to support the conditions for abiogenesis and produces those gases. In a water-limited world, are those conditions even present? IOW, can such a world even support abiogenesis so that photosynthesis of some variety can subsequently evolve?
Thanks Paul
The article and comments are all very interesting, I was thinking along the lines of James’s post as well.
Cheers Edwin
My reading of the paper suggests that the conditions have to “be just right” to maintain a habitable terminator. Long-term stability is only commented on, no data is provided. The planet has to be water-limited to prevent water accumulation in the farside cold trap (much like the water at the lunar poles?).
But consider our Earth. Over a long period, it has experienced both deep glaciations and hot periods. The Milankovich cycle causes predictable glaciations, of which the next is being offset by our human disruption of the climate.
My sense is that the authors’ climate modeling is somewhat assuming a very stable planet, with no eccentricity, and no experience of potential shocks. Only a slow increase in the M_dwarf’s luminosity changes anything.
One theory for Snowball Earth is the configuration of the land masses. This requires continental drift via plate tectonics. On a “land planet” with limited water, this suggests to me that plate tectonics is absent. If so, does that also imply that the heat loss from the core may not even provide the hot vents that are the current theory du jour as the most probable location for abiogenesis?
Life may be translated to the HZ, but cause it originate there?
Life has changed Earth’s climate over 3.5 bny. Would not life upset the conditions on the tidally-locked planet and cause the end of the terminator HZ?
Hot air rises and cold air falls. This means that the air on the inboard side of the planet will rise and expand outward and the air on the outboard side will fall and expand outwards. This suggests very high winds across the terminator. I believe there was a paper somewhere that postulated wind speeds of around 500 MPH or something across the terminator. This may be OK for bacteria and algae. But its definitely not OK for plants and animals. I would not call such a terminator hospitable by any stretch of imagination.
I imagine an animal that grows cemented in crevices in the rock, which secretes a piezoelectric plume around a central conductor that extends upward into the breeze. From waves of turbulence it could obtain all the energy it needs to power an autotrophic metabolism beneath the water table. Behind the first row of plumes on the nightside, another row rises a little further, produced by animals that need to produce a little less metabolic heat to survive … and so forth, until a great forest of fans juts far out above the twilight zone, tapping every centimeter of oncoming wind turbulence with the greed of an earthly rainforest facing the sun. The katabatic winds of Earth are less reliable, and occur in inhospitable arctic zones, but now I wonder if perhaps before the Ice Age there were ever any organisms that evolved some ability to use them for metabolic energy…
Certainly some recent fictional explorations set in such a world. If one wants to read more from some of the young, up-and-comers – I recommend: The City In the Middle of the Night (2019) by Charlie Jane Anders (note: has a bit of a climate-action-fiction vibe)
“… the far future, on a tidally locked planet called “January”. Humans live in the twilight zone, between the boiling heat of the sun-facing side and the frozen wasteland of the night side. Local inhabitants are mostly divided between two diametrically opposite urban locations…”
Nominated 2020 Arthur C. Clarke Award and 2020 Hugo Award for Best Novel, and won the 2020 Locus Award.
Good to know about these. Thanks, Jer.
How do you determine if an exoplanet near an M dwarf rotates like the moon around the Earth or like Mercury around the sun?
BTW, you have to list:
Diffractive Interfero Coronagraph Exoplanet Resolver (DICER):
https://www.nasa.gov/directorates/spacetech/niac/2023/Diffractive_Interfero_Coronagraph_Exoplanet_Resolver/
and
Fluidic Telescope (FLUTE):
https://www.nasa.gov/directorates/spacetech/niac/2023/fluidic_telescope_flute/
and
Great Observatory for Long Wavelengths (GO-LoW):
https://www.nasa.gov/directorates/spacetech/niac/2023/Great_Observatory_for_Long_Wavelengths/
After giving this paper a quick go-over, I find it somewhat limited. The modeling appears to be simple. There is no mentions of cloud cover, and any consideration of oceanic heat transport, which on Earth is responsible for by far the largest portion the the equator to pole heat transfer. Caltech was producing far more sophisticated models 10 years ago, and they showed that the climate of the dark side of a tidally locked planet is relatively mild, although below freezing for an isolation the same as Earth’s.
The weather pattern on tidally locked planet is as follows: At the sub-stellar point, there is a large blob of tropical convergence, bright white cloud with 600 inches of rain per Earth year underneath it. This is surrounded by a ring of desert, which is the hottest part of the planet. At the terminator, a steady cold, dry wind blows in from the anti-stellar point. Going the other way in the stratosphere is a multi-hundred mile per hour jet stream. This is an idealized case. A mixture of continents and oceans messes this arrangement up considerably.
The smaller the red-dwarf, the closer the planet’s orbit’s need to be for it to be in the habitable zone; therefore, the faster the planet rotates to match its orbital period, so the stronger the Easterly and Westerly bands become relative to the North-South flow and the weather patter begins to resemble Earth’s.
The paper is also limited in that it addresses habitability at just the inner edge of the habitable zone on planets with 1 bar or half bar atmospheres, i.e. thin atmospheres. What this paper does suggest is that a planet is maximally stable against runaway greenhouse when it is configured for maximum climatic extremes as in the case of having little water vapor to aid heat transport.
This is a nice little addition to a series of papers studying planetary runaway greenhouse in relation to axial tilt and orbital eccentricity. The papers showed that the more extreme the climate, the more resistant the planet was to a change of state. It seems that the more even a planet’s climate is, the easier it can flip into either a snowball state or greenhouse state.
The meteorology of an M dwarf exoplanet in the life belt should have strong winds at the terminator like a single large Hadley Cell. The reason being is the cold side is the high pressure and the warm side the low pressure? Winds result from the higher region filling up the lower pressure region or partial vacuum.
Over time I wonder if a torque would be set up because of all the ice build up on the other side of the planet. It would cause a slow rotation of the planet, my guess very slow but i don’t think it would be a huge benefit to say plant life as they would freeze to death.
Have there been studies on the conditions which would result in long rotation periods, like 1000+ hours, a day longer than the year? Or Venus rotating backward? What are the likely conditions? Presumably a planet doesn’t form with its rotation already tidally locked. How long does it take for tidal locking to occur?
Hi Paul,
I came across the following novels that quote ‘Mercury Terminator’ as a place for humans to live there as from 1893 to 1965, it was believed that Mercury was 1:1 tidally locked with the Sun.
Ray Cummings’ 1930 novel “Tama of the Light Country”
Clark Ashton Smith’s 1932 short story “The Immortals of Mercury”
Isaac Asimov’s 1942 short story “Runaround” (later included in the 1950 fix-up novel I, Robot)
Hal Clement’s 1953 novel “Iceworld”
Asimov’s 1956 short story “The Dying Night”
Alan E. Nourse’s 1956 short story “Brightside Crossing”
Poul Anderson’s 1957 short story “Life Cycle”
Kurt Vonnegut’s 1959 novel “The Sirens of Titan”
Eli Sagi’s 1963 novel “Harpatkotav Shel Captain Yuno Al Ha’kochav Ha’mistori” (English title: “The Adventures of Captain Yuno on the Mysterious Planet”)
Larry Niven’s 1964 short story “The Coldest Place” (maybe the last one )
Regards
Ricardo Orsini C Amarante
Thanks, Ricardo. Your mention of the Nourse story “Brightside Crossing” really resonated with me. I know I read that back in the day.