Centauri Dreams regular Dave Moore just passed along a paper of considerable interest for those of us intrigued by planetary systems around red dwarf stars. The nearest known exoplanet of roughly Earth’s mass is Proxima Centauri b, adding emphasis to the question of whether planets in an M-dwarf’s habitable zone can indeed support life. From the standpoint of system dynamics, that often comes down to asking whether such a planet is not so close to its star that it will become tidally locked, and whether habitable climates could persist in those conditions. The topic remains controversial.
But there are wide variations between M-dwarf scenarios. We might compare what happens at TRAPPIST-1 to the situation around Proxima Centauri. We have an incomplete view of the Proxima system, there being no transits known, and while we have radial velocity evidence of a second and perhaps a third planet there, the situation is far from fully characterized. But TRAPPIST-1’s superb transit orientation means we see seven small, rocky worlds moving across the face of the star, and therein lies a tale.
The paper Dave sent, by Cody Shakespeare (University of Nevada Las Vegas) and colleague Jason Steffen, picks up on earlier work Shakespeare undertook that probes the differences between such scenarios. We know that conditions are right for a solitary planet, unperturbed by neighbors, to orbit with a spin rate synchronous with its orbital rate, the familiar ‘tidal lock.’ On such a world, we probe questions of climate, heat transport, the effects of an ocean and so on, to see if a planet with a star stationary in its sky could sustain life.
Image: This illustration shows what the TRAPPIST-1 system might look like from a vantage point near planet TRAPPIST-1f (at right). Credit: NASA/JPL-Caltech.
But as TRAPPIST-1 shows us in exhilarating detail, multi planet systems are not uncommon around this type of star, and now we have to factor in mean motion resonance (MMR), where the very proximity of the planets (all well within a fraction of Mercury’s orbit of our Sun) means that these effects can perturb a particular planet out of its otherwise spin-orbit synchronization. Call this ‘orbital forcing,’ which breaks what would have been, in a single-planet system, a system architecture that would inevitably lead to permanent tidal lock.
The results of this breakage produce the interesting possibility that planets like TRAPPIST-1 e and f may retain tidal lock but exhibit sporadic rotation (TLSR). Indeed, another recent paper referenced by the authors, written by Howard Chen (NASA GSFC) and colleagues, makes the case that this state can produce permanent snowball states in the outer regions of an M-dwarf planetary system. What is particularly striking about TLSR is the time frame that emerges from the calculations. Consider this, from the Shakespeare paper:
The TLSR spin state is unique in that the spin behavior is often not consistently tidally locked nor is it consistently rotating. Instead, the planet may suddenly switch between spin behaviors that have lasted for only a few years or up to hundreds of millennia. The spin behavior can occasionally be tidally locked with small or large librations in the longitude of the substellar point. The planet may flip between stable tidally locked positions by spinning 180°so that the previous substellar longitude is now located at the new antistellar point, and vice versa. The planet may also spin with respect to the star, having many consecutive full rotations. The spin direction can also change, causing prograde and retrograde spins.
Not exactly a quiescent tidal lock! Note the term libration, which refers to oscillations around the rotational axis of a planet. What Shakespeare and Steffen are analyzing is the space between long-lasting rotation and pure tidal lock. Indeed, the authors identify a spin scenario within the TLSR domain they describe as prolonged transient behavior, or PTB. Here the planet moves back and forth in a ‘spin regime’ that is essentially chaotic, so that questions of habitability become fraught indeed. Instead of a persistent climate, which we usually assume when assessing these matters, we may be looking at multiple states of climate determined by present and past spin regimes, and their necessary adaptation to the ever changing spin state.
Such global changes are reminiscent, though for different reasons, of Asimov’s fabled story “Nightfall,” in which scientists on a world in a system with six stars must face the social consequences of a ‘night’ that only appears every few thousand years. For here’s what Shakespeare and Steffen say about a scenario in which TSLR effects kick in, a world that had been tidally locked long enough for the climate to have become stable. The scenario again involves TRAPPIST-1:
Such a planet in the habitable zone around a TRAPPIST-1-like star could have an orbital period of around 4-12 Earth days – the approximate orbital periods of T-1d and T-1g, respectively. Due to the TLSR spin state, this planet may, rather abruptly, start to rotate, albeit slowly – on the order of one rotation every few Earth years. The previous night side of the planet, which had not seen starlight for many Earth years, will now suddenly be subjected to variable heat with a day-night cycle lasting a few years. The day side would receive a similar abrupt change and the climate state that prevailed for centuries would suddenly be a spinning engine with momentum but spark plugs that now fire out-of-sync with the pistons. In this analogy, the spark plugs and the subsequent ignition of fuel correspond to the input of energy from starlight. The response of ocean currents, prevailing winds, and weather patterns may be quite dramatic.
Not an easy outcome to model in terms of climate and habitability. The authors use a modified version of an energy release modeling software called 1D EBM HEXTOR as well as a model called the Hab1 TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) Protocol as they analyze these matters. I send you to the paper for the details.
Science fiction writers take note – here is rich material for new exoplanet environments. Notice that the TLSR spin state is different from the one-way change that occurs when a rotating planet gradually becomes tidally locked over large timescales. This is a regime of sudden change, or at least it can be. The authors consider spin regimes lasting less than 100 Earth years, with the longest regimes (these are classified as ‘quasi-stable’) lasting for 900 years or more and perhaps reaching durations of hundreds of thousands of years. The point is that “TLSR planets are able to be in both long-term persistent regimes and PTB regimes -– where frequent transitions between behaviors are present.”
We learn that all tidally locked bodies experience libration to some degree even if no other bodies are found in the system being examined. Four spin regimes are found within the broader spin state TLSR. Tidal lock with libration can occur around the substellar point, as well as around the substellar or antistellar point, or as noted a planet may be induced into a slow persistent rotation. Much depends upon how long any one of these ‘continuous’ states lasts; given enough time, a stable climate could develop. The chaotic behavior of the fourth state, prolonged transient behavior (PTB), induces frequent transitions in spin. Such transitions would be expected to produce extreme changes in climate.
The spin history of a given system will depend upon that system’s architecture and the key parameters of each individual planet, an indication of the complexity of the analysis. What particularly strikes me here is how fast some of these changes can occur. Here’s a science fiction scenario indeed:
The more extreme change is in the temperature of different longitudes as the planet transitions from a tidally locked regime to a Spinning regime or after the planet flips 180 and remains tidally locked. Rotating planets experience temperature changes at the equator of 50K or more over a single rotation period. The exact effects require more robust climate models, like 3D GCMs [Global Circulation Model], to properly examine. However, using comparisons with climate changes on Earth, it is likely that erosion of land masses would increase and major climate systems would experience significant changes.
As if the issue of habitability were not complex enough…
The paper is Shakespeare & Steffen, “Day and Night: Habitability of Tidally Locked Planets with Sporadic Rotation,” in process at Monthly Notices of the Royal Astronomical Society and available as a preprint. The Chen paper referenced above is “Sporadic Spin-Orbit Variations in Compact Multi-planet Systems and their Influence on Exoplanet Climate,” accepted at Astrophysical Journal Letters (preprint).
It would be interesting to find out which forms of life could adapt to these very different changing conditions. Many organisms rely on daily, or seasonal environmental patterns. Adapting to a TL case might prove catastrophic when the planet tips into a SR condition. For photosynthesizers, a slow rotation after many millennia of a TL state might cause a mass die off. Similarly, organisms adapted to a temperate twilight around a terminator might die off when presented with a daily alternation of extreme sunlight and then night.
One argument for the successful evolution of complex life on Earth is the axial tilt stability due to our large moon maintaining regular seasons. If the Moon was suddenly lost and the Earth’s tilt became much more variable, like that of Mars, what adaptations would adapt to those conditions? The TLSR issue might have a very similar destabilized environment, breaking the organisms’ adaptation.
Bacterial life in an ocean’s depths or in the crust would likely be fine. But as life inhabited more light, and therefore potentially unstable conditions, these forms would have to be able to survive the new conditions or go into a dormant stage until the conditions returned to those adapted to. Bacteria certainly can do this, as can many plants with seeds. Can any terrestrial invertebrates with eggs that can become dormant, such as Artemia species, survive for millennia in a dormant state before being reanimated?
For a sci-fi setting, this would make the long cycles in Aldiss’ “Helliconia” trilogy seem a piece of cake by comparison.
I was thinking about Helliconia as well. Wonder what Aldiss would have done with all this.
“Bacteria certainly can do this, as can many plants with seeds. Can any terrestrial invertebrates with eggs that can become dormant, such as Artemia species, survive for millennia in a dormant state before being reanimated?”
As long as cells stay metabolically active, even in the slightest degree, they will need nutrients and waste disposal. Freezing temperatures associated with cytoplasmic constituents that prevent water from crystallizing with sharp points and edges from disrupting intracytoplasmic membranes can stop metabolism altogether allowing revivification after indefinitely long periods.
Otherwise, spores, cysts, seeds and eggs all will become non-viable in due course. The longest survival without freezing that I am aware of were of the germination of some variety of date palm seeds in Israel, around a millennium old. As organisms are adapted to their environments, one would not expect tropical fruits for instance, to do the same.
Remembering a recent post and discussion… These tidal swings could be excellent drivers for evolution of intelligent species, given right conditions (like atmosphere dense enough not to be driven to extremes by changing rotation regime). We humans could have evolved intelligence when forced to shift back and forth between savannas and forests, in sync with glacial cycles. On TLSR worlds, the slow and chaotic shift of habitats across the planet could stimulate evolution even more. Not wiping species but forcing all kinds of migrations, demanding a very complex behavior. Reminds also Trisolaris with it’s Stable and Chaotic Eras, punctuated by occasional cataclysms induced by rare but dramatic events.
Not all compact multi-planet systems could be prone to TLSR, though. Galilean moons have even tighter resonant orbits, and their rotation seems to be stable (at least, I don’t remember seen any single article on possibility of chaotic rotation there, in the past or in the future)
Hi Paul
Yes count me as one of your readers who is “intrigued by planetary systems around red dwarf stars” In fact all your interesting posts on the subject have made me a keen follower of your postings. Lawrence also suggested reaching out a bit more too.
Yes a very interesting paper to study, I’m assuming the the other planets affect the tidal locking of the others.
Anyone have updates on the Webb results?
Thanks Edwin
I was thinking more like “Winter is coming”. Although in this case it would be more like “Nightfall is coming”, but at some incalculable chaotic future date.
As with a previous simulation that found most planetary systems are unstable, I wonder whether the starting conditions are fully realistic. Drop a pencil on a desk, and the odds are it will roll off; but find a pencil on a desk, and there is a good chance it will stay put. Though even our own moon systems do exhibit chaotic rotation in the wild, here I’d like to see some method for extracting surplus energy out of the simulated orbital system over time by friction (tides, etc) rather than using purely a rigid N-body calculation. I think the authors say something similar on page 11.
I was a bit dubious about this phenomenon considering Jupiter’s moons, but there is some evidence for this in our own solar system, most notably Saturn’s moon, Hyperion. Also Pluto’s moons, Nix and Hydra are chaotic rotators.
Jupiter’s Moons are an interesting analog. While as far as known they are rotating synchronously, and as moons they are analogous in dimensions to the Earth’s but the static tidal forces are much larger due to Jupiter’s mass about 300 times Earth ( e.g. Io at about the same distance). But what appears to be responsible for surface disturbances ( volcanoes, ice fissures) is the resonant passage of other moons in the vicinity of the inner most, kind of like plucking on chords. Callisto shows hardly evidence of this – and appears to have the oldest looking surface. Io, shall we say, evolves like crazy.
Now, on the other hand, in cases of orbital operations, it has often been attempted ( or has succeeded) in obtaining local horizontal – local orientation of cylindrical satellites by “passive damping”. If a rod or a long ellipsoidal object were placed in orbit, it might rock chaotically or more like a pendulum, depending on the ratios of its principle axes of inertia. To make a long story short, a rod like a pencil or pen with the eraser or ball point pointing radially down to the Earth is fairly stable, but it can be tipped so the alternate point would point downward. But in principle this phenomenon applied to natural satellites too. No coincidence which face of the moon faces Earth. As much as the moon is oblong, its minimum principal axis is so aligned…
OK. So what if a planet around a red dwarf has fluids on its surface and an atmosphere? And not necessarily 14.7 psi at the surface. Thermal features might cause a transfer of significant amounts of mass ( e.g., ice or mud or volcanic dust ) to collect on the opposite side or unpredictably. And a whole new episode of geologic history would begin after the planet settled into another semi-stability.
And years ago, caught the Brian Aldiss stories about the Helliconia. Memorable stories like that always seem to catch me when I had the flu and a fever … But I could never figure out a reasonable picture for the two binary stars that were supposed to be responsible for the repeated cycles. Now and then it looked like there were elements that he was going to look at later. And I saw some evidence there was an Oxford or Cambridge effort to contribute to the story. But for example, on loose ends, there seemed to have been reference to a hidden scientific monitoring station in the first book – but Aldiss appeared to have abandoned that idea in later stories, what with one of the orbital crew members opting to go to the surface and eventually being felled by a local virus. As he was warned.
In the above comment about gravity gradient stabilization of artificial satellites as an analog for what was discussed with regard to Jupiter’s moons and perhaps the planets of Trappist-1, some elaboration or clarification might be in order.
Especially since natural satellites and planets are nearly spherical; or else they possess only slight differences in the inertias about their 3 orthogonal principal axes.
Speaking of principal axes, for a nearly spherical object, we can identify the first principal axis as the one around which it rotates – and then two more perpendicular to each other at the object’s equatorial plane. In the case of a cylindrical spacecraft the “minimum” inertial would be along the narrowest dimension, like the width of the Shuttle External Tank – or the minimum diameter of an ellipsoid. Pointing the longest dimension parallel to the radius vector gives the best hope of stabilization. Detailed analysis identifies configurations that are stable in this condition. An extended yo-yo or a tethered spacecraft would tend to spin up. And this can be tried at home.
Now planets are somewhat rigid, but we have also observed that rotational dynamics cause them to become oblate: radial dimensions wider at the equator than at the N-S pole. But if an inherent rotational rate is dissipated by the gravitational interaction with a primary, then one could expect that the residual differences in the above three inertial axes, would cause the satellite or planet to come to “synchronous” rest with the minimum axis alignment in this gravity gradient mode.
Would this be the end of geologic history? Likely, “No.”
At mentioned above, if the interior is molten, if the body has seas or thick atmosphere, there could be continued re-distribution of mass. When I inadvertently mentioned mass at sun side redistributed to the dark side, in this scenario, possibly there would be no change in the hierarchy of principal axes.
But plate tectonics, oceans, snow fall, lava flows, glacier buildup … Whatever might be happening to change the mass distribution and the stable axes – will not necessarily pile up on the dark side in uniform manner. Ice might precipitate as soon as it reaches the shadow regions rather than waiting to collect at the center of the dark side.
And meanwhile, each of the outer satellites or planets are tweaking the inner ones.
In the case of the Galilean satellites, it might be possible to examine this hypothesis, though with varying evidence among the four. Io is so dynamic that it might be hard to discern where its polar regions might have been a million or ten million years ago. The icy surface of Europa might be more static, but cryptic. And Ganymede… There might be a literature on this subject – but many not. Some satellites have differences in reflectance that appear to be due to whether a surface faces the direction of orbit or aft, probably contact with charged particles slapping down on the aft side rather than front.
But the geological discussions, I suspect, seldom examine whether the four had different orientations millions of years earlier.
Polar wandering, I think has been examined with Mars, but usually, external factors such as changing orbital parameters ( e.g., northern winter at aphelion or perihelion, precession…]. Still, it is argued that the Earth-Moon system insures Earth axis rotation obliquity with respect to the ecliptic plane. Mars might have more in common with the Trappist 1 hypothetical cases after all.
It just might be a question of identifying the current geological evidence.