One of the big arguments against habitable planets around low mass stars like red dwarfs is the likelihood of tidal effects. An Earth-sized planet close enough to a red dwarf to be in its habitable zone should. the thinking goes, become tidally locked, so that it keeps one face toward its star at all times. The question then becomes, what kind of mechanisms might keep such a planet habitable at least on its day side, and could these negate the effects of a thick dark-side ice pack? Various solutions have been proposed, but the question remains open.
A new paper from Jérémy Leconte (Canadian Institute for Theoretical Astrophysics, University of Toronto) and colleagues now suggests that tidal effects may not be the game-changer we assumed them to be. In fact, by developing a three-dimensional climate model that predicts the effects of a planet’s atmosphere on the speed of its rotation, the authors now argue that the very presence of an atmosphere can overcome tidal effects to create a cycle of day and night.
The paper, titled “Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars,” was published in early February in Science. The authors note that the thermal inertia of the ground and atmosphere causes the atmosphere as a whole to lag behind the motion of the star. This is seen easily on Earth, when the normal changes we expect from night changing to day do not track precisely with the position of the Sun in the sky. Thus the hottest time of the day is not when the Sun is directly overhead but a few hours after this.
From the paper:
Because of this asymmetry in the atmospheric mass redistribution with respect to the subsolar point, the gravitational pull exerted by the Sun on the atmosphere has a nonzero net torque that tends to accelerate or decelerate its rotation, depending on the direction of the solar motion. Because the atmosphere and the surface are usually well coupled by friction in the atmospheric boundary layer, the angular momentum transferred from the orbit to the atmosphere is then transferred to the bulk of the planet, modifying its spin.
This effect is relatively minor on Earth thanks to our distance from the Sun, but is more pronounced on Venus, where the tug of tidal friction that tries to spin the planet down into synchronous rotation is overcome by the ‘thermal tides’ caused by this atmospheric torque. But Venus’ retrograde rotation has been attributed to its particularly massive atmosphere. The question becomes whether these atmospheric effects can drive planets in the habitable zone of low mass stars out of synchronous rotation even if their atmosphere is relatively thin.
Pressure units in a planetary atmosphere are measured in bars — the average atmospheric pressure at Earth’s surface is approximately 1 bar (contrast this with the pressure on Venus of 93 bars). The paper offers a way to assess the efficiency of thermal tides for different atmospheric masses, with results that make us look anew at tidal lock. For the atmospheric tide model that emerges shows that habitable Earth-like planets with a 1-bar atmosphere around stars more massive than ~0.5 to 0.7 solar masses could overcome the effects of tidal synchronization. It’s a powerful finding, for the effects studied here should be widespread:
Atmospheres as massive as 1 bar are a reasonable expectation value given existing models and solar system examples. This is especially true in the outer habitable zone, where planets are expected to build massive atmospheres with several bars of CO2. So, our results demonstrate that asynchronism mediated by thermal tides should affect an important fraction of planets in the habitable zone of lower-mass stars.
Here is the graph from the paper that illustrates the results:
Image: Spin state of planets in the habitable zone.The blue region depicts the habitable zone, and gray dots are detected and candidate exoplanets. Each solid black line marks the critical orbital distance (ac) separating synchronous (left, red arrow) from asynchronous planets (right, blue arrow) for ps = 1 and 10 bar (the extrapolation outside the habitable zone is shown with dotted lines). Objects in the gray area are not spun down by tides. The error bar illustrates how limits would shift when varying the dissipation inside the planet (Q ~ 100) within an order of magnitude. Credit: Jérémy Leconte et al.
The result suggests that we may find planets in the habitable zone of lower-mass stars that are more Earth-like than expected. Do away with the permanent, frozen ice pack on what had been assumed to be the ‘dark side’ and water is no longer trapped, making it free to circulate. The implications for habitability seem positive, with a day-night cycle of weeks or months distributing temperatures, but Leconte remains cautious: “Whether this new understanding of exoplanets’ climate increases the ability of these planets to develop life remains an open question.”
The paper is Leconte et al., “Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars,” Science Vol. 347, Issue 6222 (6 February 2015). Abstract / preprint available. Thanks to Ashley Baldwin for a pointer to and discussion of this paper.
Not sure it’s just tidal locking. Close enough to the star, and you’d have tidal heating problems as well.
This would seem to rule out Proxima Centauri (0.123 Solar mass) as having a planet that could rotate. So Baxter’s planet in his novel Proxima is likely safely correct in being synchronous.
I note this near the end: “On the other hand, the habitable
zone has been recently shown to be more extended for synchronous planets”.
Doesn’t this mean that the atmospheric reduction of synchronous rotation reduces the likelihood of finding a habitable planet for a red dwarf world?
Thanks Paul. This is a real Earth moving paper and has caused ripples in the exoplanet community. It opens up the possibility of life around M dwarfs ,which are common and also the easiest to view and analyse with missions like yesterday’s Twinkle ( or better) and TESS supported by JWST. The graph has been described as ” a work of art” by the most down to Earth astronomers . JWST might just have the power to spot life on Earth sized planets around M dwarfs . Another paper on M3 dwarfs , almost as exciting and overlapping with this paper, by Houdebine et al was released almost simultaneously and is also available through arxiv . Like this, it is a great read and shows for complex reasons why these stars have low chromospheric activity which makes them easier to view and characterise from Earth . Their tight habitable zone gives lots of deep transits for spectroscopic analysis . We know Sasselov has shown that Super Earths can maintain their oceans for long periods and Kite’s 2008 paper on Geodynamics and volcanism on Super Earths shows this can lead to the extended tectonics needed to maintain secondary atmospheres till long lived M dwarfs settle down. Taken together , all these papers give reason for optimism of historic discoveries in our lifetime.
Brett. You’re quite correct . Clearly non synchronous rotation is not the only factor to consider for life but this mechanism gives a pathway to more Earth like planets when other criteria are met . No atmospheric collapse on the permanently night side. If the orbits remain circular , this will mitigate tidal heating to some extent too.
I have a bit of a quibble with the Science editors. Yes the article is about (slightly) low-er mass stars than Sol, it is not about low mass stars. The fine distinction made in the title of the paper might escape the notice of some readers who are not astronomers.
In fact the mass range discussed is ~ the mass range of spectral class K stars. I like K stars for astrobiology even more than G stars. Also, K stars are ~ 60% more common than G stars. As Alex has noted, this paper has no application to actual low mass M stars like Wolf 359 (mass = 0.09 M?). That is unfortunate, as M stars are ~ 3/4 of the stellar population.
Seems to me the best bet for a habitable planet around a very small star would be a moon of a gas giant. The giants magnetic field might also protect the planet.
I thought I read that the retrograde rotation of Venus was caused by Earth. Was that theory disproven?
One of the unfortunate things we have to live with is the legacy of the M spectral class which is quite a mixed bag. In the present time, it would not have been considered sensible to put Lalande 21185 in the same stellar class as Wolf 359. The larger near neighbor star has 5x the mass, 18x the luminosity, and visually is 6 magnitudes brighter than Wolf 359. Similar stars? Not at all.
@Ashley
I had a look at the Houdebine paper, nice work, so there is observational evidence of transition to convection at M3. If M3 is a sweet spot for astrobiology observations, this would increase the target list considerably as M3 stars are roughly as abundant as K stars.
In the Enterprising spirit, I am pleased to announce that I am taking deposits for habitats to be built in Phase 1 of my proposed property development in the lovely Gliese 687 system. Gliese 687 is a certified stellar-safe M3. Our idyllic system includes a Neptune class planet to grace your night sky. Buy now or be priced out forever! Construction will commence when Phase 1 is fully subscribed. Most major currencies accepted. (sorry no Hryvnias or Quatloos) Operators are standing by!
Don’t count on ALL habitable zone planets orbiting Proxima Centauri-like stars having synchronus rotation. Desert planets will, but Ocean planets MAY NOT, if the recently published paper on the effect of oceans on spin-down rates of planets orbiting close to M stars is correct. The authors of these two papers should now COLLABORATE, with the hope of eventually publishing a NEW paper which deals with this very complicated issue.
JOY: Thanks for the tip about Gilese 687…I hope you can become part of one of the teams to use the Giant Magellan Telescope in 2020…It would be ironic if you find a family of planets around that star…terraforming doesn’t come up much around here…and since no planet will likely be found perfect for human beings…extraterrestrials will live inside giant hollowed mountains, and recess will be under a mile wide transparent dome open to the heavens…We have to start somewhere…
‘Atmospheres as massive as 1 bar are a reasonable expectation value given existing models and solar system examples…’
This may not be correct, the Earth may be an exemption due to the early impact that formed the moon depleting the early atmosphere. If you look at Venus it has much more nitrogen ~2-3 bars worth, so an atmosphere on average around an Earth massed world may be 2 to 3 times higher in pressure and thus aid the anti-tidal lock process down to lower massed stars.
@James Stilwell February 12, 2015 at 12:40
‘It would be ironic if you find a family of planets around that star…terraforming doesn’t come up much around here…and since no planet will likely be found perfect for human beings…extraterrestrials will live inside giant hollowed mountains…’
‘Techaliens’ are more likely to build very large space habitats than terraforming worlds and it these habitats we may find transiting their stars and/or reflecting-emitting light from that star.
@Michael: as regards the larger amount of nitrogen on Venus, as far as I am aware this could be a result of the lack of subduction processes rather than a difference in the initial nitrogen inventory. See e.g. Lécuyer, Simon and Guyot (2000). In addition, the lack of a large satellite does not mean that Venus never had a giant impact: not all such impacts would necessarily lead to the formation of a satellite.
@Michael February 12, 2015 at 12:40
‘It would be ironic if you find a family of planets around that star…terraforming doesn’t come up much around here…and since no planet will likely be found perfect for human beings…extraterrestrials will live inside giant hollowed mountains…’
‘Techaliens’ are more likely to build very large space habitats than terraforming worlds and it these habitats we may find transiting their stars and/or reflecting-emitting light from that star.
I wasn’t very exact…terraforming and living inside a sealed hollowed out mountain on an alien world aren’t the same thing at all…Please read “The City and the Stars” by Arthur C. Clarke for a more accurate reading of my thinking in its dreamy way…A short synopsis of his futuristic city, Diaspar, might also be found on Wiki…
@Michael: Not sure what the moon effect is with regards to nitrogen, but there is also the matter of carbon dioxide, which seems much more material to me. Earth’s atmosphere is depleted of it, and Venus has about 90 bars of it. Exoplanets could be expected to be anywhere in between, thus will tend to have much more than 1 bar of atmosphere.
@andy
Thanks for the paper link.
When we look at the Earth we suspect that there is an oceans worth of water in the mantle primarily due to sub-duction. If we also look at the amount of dissolved nitrogen in the water (15 ppm) which we will say is the major source and that it is pulled into the mantle with the water.
Now a little maths points the way to the fact that there is not enough nitrogen in the mantle, from the sub-duction, to even come close to the observed amount of nitrogen in Venuses atmosphere. I am taking into consideration that the heat on Venus led to significant out gassing and that the moon formation impact also degassed the earth to the same level.
@Eniac February 13, 2015 at 23:41
‘Not sure what the moon effect is with regards to nitrogen, but there is also the matter of carbon dioxide, which seems much more material to me. Earth’s atmosphere is depleted of it, and Venus has about 90 bars of it. Exoplanets could be expected to be anywhere in between, thus will tend to have much more than 1 bar of atmosphere.’
During the moon forming impact the temperature and momentum of the impactor would have removed a significant amount of CO2 and N2. On the Earth the CO2 is mostly locked up in carbonates at around 60 bars worth.
@Michael: I think you are incorrect in assuming that the nitrogen being subducted would mainly be from dissolved nitrogen in water, as ocean floor sediments themselves contain nitrogen from decayed organic matter. The main input of nitrogen to the mantle by subduction would likely be in the form of ammonium, but the behaviour of nitrogen in subduction is not particularly well constrained at the moment. For example, Marty & Daumas (2003) estimate that the nitrogen content of the silicate Earth could be comparable to the atmosphere, but other estimates of the nitrogen input into the mantle vary substantially and some subduction zones appear to be far more efficient than others at transporting nitrogen into the Earth’s interior rather than re-releasing it by volcanism.
@andy
This article below explains a little more than I can. An impact of the size that formed the moon would have an enormous effect on the atmospheric inventory of all volatiles. I am still in favour of the atmospheres of Earth massed planets been thicker than ours even when CO2 has been removed through carbonate formation, this may favour a thermal tide anti-tidal lock process down to lower massed stars.
http://www.hou.usra.edu/meetings/venus2014/pdf/6012.pdf
@Michael: I’m not saying the moon-forming impact did not have an effect, the issue is that multiple giant impacts should be fairly common during terrestrial planet formation. Earth had one set of impacts, Venus received another. Can we actually assume that the Venusian impact sequence is more typical for terrestrial planets than Earth’s? There’s also no particular evidence against Venus experiencing giant impacts that formed moons that were subsequently destroyed (e.g. when further giant impacts changed the angular momentum of the system).
At present there’s not much information at all about how much volatile loss is typical on Earth-mass planets, the number of examples is too small (and that’s before you consider possibilities such as oceans enhancing atmospheric loss during giant impacts, which would tend to favour thinner atmospheres on terrestrial planets located further out). We need data on more planets before leaping to conclusions. It doesn’t help that you’ve also got the complication that the two planets have been undergoing very different geological (planetological?) processes over their history which is going to confuse things even more: different degrees of outgassing, atmospheric erosion and geological recycling are going to have their effects which can mask the evidence for the initial conditions.
It would be interesting to know if the axial tilt would have an appreciable difference on the spin rate or could it even create a chaotic one. But there is still that issue with the long contraction phase of low mass stars depleting a lot if not all of the water of close in planets, that to me is a huge stumbling block greater than the spin issue.
Speaking of moons, if the proposed theory is correct, why is Titan, with it’s thick atmosphere, in synchronous rotation with respect to Saturn? Am I missing something here?
@Harry R Ray: the mechanism is due to thermal tides in the atmosphere, i.e. it requires the primary to be supplying significant amounts of heat to the secondary. So for the mechanism to disrupt Titan’s locking to Saturn, Saturn would have to be a substantial heat source for Titan’s atmosphere. According to my quick back-of-the-envelope calculation, for Titan the heat flux from Saturn is roughly 3 orders of magnitude lower than that from the Sun, which is itself 90 times lower than that received by the Earth from the Sun. Titan’s atmosphere is not strongly heated enough to substantially disrupt the tidal locking effect.
@andy, @Michael: Andy rightly points out how little we really know about what gives a planet the size of Earth or Venus its atmospheric pressure. Consequently, we have to assume that the norm is somewhere in between the two, which would make the expectation value a great deal higher than 1 bar.
My favorite theory is that Earth is atypical, because biological fixation of carbon into limestone and carbohydrates has removed all CO2 from its atmosphere. My expectation is that when we start analyzing the atmospheres of Earth-sized planets in habitable zones, we will find them to typically have an atmosphere similar to Venus’, although with varying amounts of water. I suppose that would mean that planets around red dwarfs could easily avoid tidal lock by the above mechanism.
When we find one that has a thin atmosphere free of CO2, that is when we should really take a closer look…
No we don’t, that would be overinterpreting small number statistics. In fact there’s no particular reason to assume that the terrestrial planets in our solar system are particularly representative of terrestrial planet formation elsewhere. For example the low masses of Mars and Mercury suggest that the initial distribution of material in the inner solar system was rather odd, which may have led to a different growth history than in solar systems that grew from a less truncated initial condition.
@andy February 19, 2015 at 15:09
‘No we don’t, that would be overinterpreting small number statistics. In fact there’s no particular reason to assume that the terrestrial planets in our solar system are particularly representative of terrestrial planet formation elsewhere. For example the low masses of Mars and Mercury suggest that the initial distribution of material in the inner solar system was rather odd, which may have led to a different growth history than in solar systems that grew from a less truncated initial condition.’
Although statistically the number of planets are too low to draw absolute conclusions about atmospheric masses, I am statistically in favour of greater atmospheric masses around increasingly massive planets. I believe the Earths atmospheric mass is lower on average than other Earth massed planets due to the type of impact that it had early on in it’s history as they are quite rare. So we are at a religious opposition, I believe I am right and you believe you are right, now I say let science reveal the truth before all.
In other words we will have to agree to disagree until the matter is settled with better optical systems.
They may have to refine their model as Venus is showing signs of slowing down and it is quite fast ~16 minutes over 16 years, that is a lot! Compare this to the Earths slow down of ~1.2 second over the same time period, that is a whopping ~800 faster.
http://sci.esa.int/venus-express/54064-3-spinning-venus-is-slowing-down/
sorry Paul that should have been 6.5 minutes over 16 years, which is still quite a lot!