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A Tidally Locked ‘Earth’?

Whether or not life can emerge on the planets of red dwarf stars remains an unknown, though upcoming technologies should help us learn more through the study of planetary atmospheres. Tidal locking always comes up in such discussions, an issue I always thought to be fairly recent, but now I learn that it has quite a pedigree. In a new paper from Rory Barnes, I learn that astronomers in the late 19th Century had concluded (erroneously) that Venus was tidally locked, and there followed a debate about the impact of synchronous rotation on surface conditions.

As witness astronomer N. W. Mumford, who in 1909 questioned whether tidal friction wouldn’t reduce half of Venus to a desert and annihilate all life there. Or E. V. Heward, who speculated that life could emerge on Venus despite tidal lock, and wrote in a 1903 issue of MacMillan’s Magazine:

…that between the two separate regions of perpetual night and day there must lie a wide zone of subdued rose-flushed twilight, where the climatic conditions may be well suited to the existence of a race of intelligent beings.

In terms of exoplanets, as Barnes (University of Washington) points out, Stephen Dole was writing about tidal interactions between exoplanets and their host stars in his book Habitable Planets for Man as early as 1964. It was his view, based upon his own calculations, that all potentially habitable planets orbiting stars smaller than 72% of the Sun’s mass would be in synchronous rotation, circling the star just as the Moon does our Earth.

Image: Tidally locked bodies such as the Earth and Moon are in synchronous rotation, each taking as long to rotate around its own axis as it does to revolve around its host star or gravitational partner. New research from UW astronomer Rory Barnes indicates that many exoplanets to be found by coming high-powered telescopes also will probably be tidally locked — with one side permanently facing their host star, as one side of the Moon forever faces the Earth. Credit: NASA.

That would make tidal lock ubiquitous, given the high percentage of stars that are red dwarfs. The work since, beginning with James Kasting in the early 1990s and carrying through until today, has looked at how planets come into synchronous rotation, and just how this situation would affect planetary conditions. We’ve seen a shift from pessimism — such planets could not be habitable — to relative optimism, as new climate models emerged and were adjusted. Barnes’ paper gives all the particulars in a rather fascinating overview of the scholarship.

A brief look through the archives here will show that Barnes’ name comes up frequently and often on matters of tidal effects, giving him an expertise that draws my attention whenever he publishes something new on the matter. The latest paper takes a systematic look at tidal locking to arrive at the conclusion that many exoplanets — and not just those orbiting close to red dwarf stars — will be found to be tidally locked. For it turns out that earlier models used a rapidly rotating early Earth to delve into how a similar exoplanet might become tidally locked.

What Barnes did was to consider the possibility of different initial rotation periods, both slower and faster, examining conditions on planets of different sizes, including those in eccentric orbits. Widening the parameter space suggested that more exoplanets than we once thought could be tidally locked. If Earth had formed with no Moon and its initial rotation period was four days, Barnes’ calculations show one model in which it is tidally locked to the Sun by this point in its evolution. Tidal locking, then, may be a major factor in our analysis of planetary habitability.

Let me quote from the paper:

As astronomers develop technologies to directly image potentially habitable planets orbiting FGK dwarfs (e.g. Dalcanton et al. 2015), they must be prepared for the possibility that planets orbiting any of them may be tidally locked. Such a rotation state can change planetary climate, and by extension the reflected spectra. 3D models of synchronously rotating habitable planets should be applied to planets orbiting K and G dwarfs in addition to Ms. While not explicitly considered here, habitable worlds orbiting brown dwarfs and white dwarfs are even more likely to be synchronous rotators, but their potential habitability is further complicated by the luminosity evolution of the central body (Barnes and Heller 2013).

Thus we extend the quantitative assessment of tidal lock and its effects on habitability to G- and K-class stars as well as M-dwarfs. As the paper notes, “…a systematic survey of the rotational evolution of potentially habitable exoplanets using classic equilibrium tide theories has not been undertaken.”

And it has implications. We are setting about putting assets like the Transiting Exoplanet Survey Satellite (TESS), the James Webb Space Telescope (JWST) and Planetary Transits and Oscillations of stars (PLATO) into space. At the same time, we are working on Earth-based telescopes with apertures in the tens of meters. Our first targets for atmospheric characterization are going to be planets orbiting close to their host star, in the ‘habitable zone’ (or as we said yesterday, ‘temperate zone’) of the host.

The role of tidal locking is thus a crucial factor. Proxima Centauri b is most likely tidally locked, and the worlds around the highly interesting TRAPPIST-1 most likely are as well. We should learn a great deal by studying planetary rotation rates for any temperate exoplanets we find, which should give us clues as to their tidal evolution. Indeed, Barnes simulates the planets the TESS mission will examine and finds that the vast majority of these become tidally locked within a billion years, while about half the isolated (i.e., with no planetary companions) and potentially habitable Kepler candidates could be locked, assuming tidal properties like Earth’s.

A UW news release quotes Barnes on the significance of the findings:

“These results suggest that the process of tidal locking is a major factor in the evolution of most of the potentially habitable exoplanets to be discovered in the near future… I think the biggest implication going forward is that as we search for life on any exoplanets we need to know if a planet is tidally locked or not.”

The paper is Barnes, “Tidal Locking of Habitable Exoplanets,” accepted at Celestial Mechanics and Dynamical Astronomy (preprint). See also this key reference: Kasting et al., “Habitable Zones around Main Sequence Stars,” Icarus Vol. 101, Issue 1 (January, 1993), pp. 108-128 (abstract).


Comments on this entry are closed.

  • David H August 18, 2017, 14:49

    Tidally locked planets might not evolve their own life, but they might make excellent hosts for human colonies. I’d like to run some ideas by the readers here.

    The way I picture it, huge colonies of terrestrial life could comfortably exist on the dark side, under an artificial sky and electric lighting. Obviously, such a structure could be built piece by piece, and potentially level by level. The artificial lighting could be run on the bio-familiar 24 hour cycle.

    Tidally locked planets – and I assume they would be stripped of most of their atmospheres – would have extremes of hot and cold, so pumping heat from the hot side to the cold side would allow for huge and efficient Stirling engines to generate lots of free power – more than enough to power all the lighting, computing, quarrying and building needs of a hemisphere-scale population. The heat sink for the system would be the the inhabited area itself, so the heating system would be free. If there is an atmosphere, there would be incredible winds on the light/dark boundary, easily exploitable through turbines. And of course, if you had to, you could also build solar farms on the sunny side for even more energy generation.

    Because the fragile life would be shielded from the burpy star by the planet itself and by a roof, it would be pretty safe and snug even in a high-radiation environment. Might this not be a pretty nice place to colonize?

    • J. Jason Wentworth August 19, 2017, 19:12

      Mercury, although it isn’t tidally locked to the Sun (it has a 3:2 Hermian day/Hermian year ratio), has conditions that aren’t terribly dissimilar to those of “atmosphere-stripped,” tidally-locked red dwarf exoplanets. In fact, Mercury’s surface conditions–somewhat to my surprise–actually make the planet an attractive world to colonize (see: http://www.google.com/search?source=hp&q=colonizing+Mercury&oq=colonizing+Mercury&gs_l=psy-ab.13..0.1444.9228.0.11520.….0…1.1.64.psy-ab..0.18.1988…46j0i131k1j0i46k1j0i10k1j0i22i10i30k1j0i22i30k1.QWsfMGEZqTE ), and:

      Mercury’s magnetic field, while weaker than ours (1.1% the strength of Earth’s, see: http://en.wikipedia.org/wiki/Mercury%27s_magnetic_field ), deflects the solar wind nevertheless, which would ease the problems of colonization. In addition to being an excellent source of heavy and industrial metals (which could be shot where needed using solar-powered electromagnetic launchers, on the same principle as William Escher’s “Lunatron” launcher [“Hermitron” would be appropriate to Mercury]) and an ideal site for a solar observatory, Mercury would give us plenty of experience that would be useful for settling tidally-locked exoplanets (whenever that became feasible). Since such worlds would likely have infinitesimal or nonexistent magnetic fields, Mercury would give us an easier start in that regard.

      • Michael August 20, 2017, 3:40

        We could increase the strength of the magnetic field by using superconductors around the poles. They could also be used to funnel hydrogen in to be collected for fuel and water.

      • Brett Bellmore August 20, 2017, 8:08

        Used to be some interesting SF located on Mercury colonies, when I was a kid. Indeed, the Solar system is chock full of colonizable niches in the most unlikely places.

    • Christopher Meyer August 20, 2017, 11:29

      I have not seen anyone take into account the constant winds and the long term transport of sediment toward the sub solar point. I would think that this would in time lead to some accumulation of material, much like Antarctica has accumulated a several mile high ice cap. Also would not water vapor be rising and condensing at this point continuously if the planet has water available? I am thinking of a continuous hurricane sized thunderstorm at the planet’s sub solar point as all the Hadley cells converge there, rise, travel to the terminator and descend on the dark side, and then cooled, travel back along the surface to the sunny side of the planet.

  • Charlie August 18, 2017, 15:25

    This would seem to have rather vast impact upon the question about habitability of other planets in other star systems. If it is reasonable to assume that other planets can become tidally locked to their parent stars within a relatively brief period of time of their histories, then that could mean that life might have extreme difficulty getting situated.
    The question might arise as to whether or not the planetary atmosphere could get too warm or too cold, depending on the situation and how that would affect what would presumably be the need for liquid water to produce the genesis of life.
    Since is almost impossible I would think to determined if distant planets to exhibit tidally locked nature’s this might cast some doubt in the long run as to where to successfully a more space probes.
    It also, if factual, my explained why the universe as a whole may be relatively devoid of intelligent beings aside from ourselves.

  • Alex Tolley August 18, 2017, 17:53

    I also think this might be good news for life. Studies showing temperature variation for such planets indicate a wider range of temperatures around the surface, with water and atmosphere transferring heat. Such planets might just be more habitable espec9ially closer to the inner edge of the HZ.

    For stars with high radiation and flares, there is still the issue of atmosphere stripping, but for more stable stars, tidally locked worlds might be better places to support life.

    If nothing else, I would hope for more extensive modeling of such worlds to determine whether tidal lock does, of does not, improve the chances for habitability compared to worlds that rotate.

  • PW August 18, 2017, 19:57

    It’s not just being tidally locked, big negative, it’s also the flare tendancy for the red dwarfs. It’s a one-two punch.
    By all means, lets check it it out.
    I’m personally more interested in sun like stars. Ha, to the extent there are any.

  • Carl Keller August 18, 2017, 20:41

    In my youth, I watched Rod Serling’s ‘Twilight Zone’. In the nineteen-sixties, the accepted “day” of Mercury equaled its year, roughly 88 Terrestrial days, Antoniadi so wrote.

    It was a time of imagination, of possibilities. Some would be inspired by Rod’s fiction.

    Whenever I would hear of “the twilight zone”, I would think of that remarkable land on Mercury where humans could best hope to exist… between the fires of our Sun and the unimaginable cold of the shield of a planet.

  • Brett Bellmore August 18, 2017, 21:09

    The eclipse this Monday, dependent as it is on a celestial coincidence of incredible precision, underscores just how weird Earth really is. Even if life is common in the universe, we originated on a very strange planet.

    Apparently not being tidally locked is just another way it’s strange.

  • Geoffrey Hillend August 19, 2017, 0:56

    I don’t agree with the idea that planets around G and K stars can be tidally locked in their life belts. Venus is not tidally locked. Also I have to conclude that a planet being tidally locked decreases the chances for life on an exoplanet since any magnetic field would be much weaker than Earths without a rotation. If we see spectroscopic biomarkers like methane, nitrous oxide and chloromethane gas I will be surprised. Maybe some kind of microbes evolved and moved and adapted to the night side which seems dependent on oceans still being around..
    An exoplanet has to have plate tectonics though. Otherwise it would not have carbon cycle in balance and the rain would take out all the Co2 out of the air and with it any greenhouse effect. Or maybe volcanism might replenish an atmosphere like on Venus. There is a lot of different possible outcomes including a run away greenhouse effect on the exoplanets near the warmest edge of the life belt. Hope we can get a good enough spectroscopic data to know the answer.

    • Ron S. August 19, 2017, 11:02

      “without a rotation”

      Tidally locked planets do rotate.

  • Michael Fidler August 19, 2017, 3:36

    Since the Astronomers have been finding many exosolar systems that defied the odds on what was expected to exist, I think the biggest implication is they should look at all these exoplanets locked or not, first to see if they have an atmosphere. The capability of the James Webb telescope should be able to analyse many of the near by exoplanets atmospheres and oceans to see if tidaly locking destroys their habitability.

  • Spaceman August 20, 2017, 11:29

    Is Dole’s calculation correct…are most planets in the temperate zone of stars with a mass less than or equal to 72% of the Sun likely to be totally locked? I am no expert, but I thought the value was closer to half a solar mass??

    • Ronald August 21, 2017, 5:56

      See the here cited publication, fig. 6 (and 11); it depends strongly on initial rotation rate en which model (CPL, CTL) is more correct (that’s what this is all about). If the CPL model is correct and initial rotation rates are usually slow (left bottom), then planets up to about 0.75 Msolar will leave the HZ before life can develop much. If the CTL model is correct and initial rotation rates are usually fast (right top), then mainly planets of 0.5 Msolar and below will leave the HZ that soon.

  • LocalFluff August 20, 2017, 14:44

    Venus rotates so very slowly that a year is shorter than a day. It is “more” than tidally locked. Still it has 100 times thicker atmosphere than Earth. Even without any magnetic field to help keep it either, although the Solar wind is about twice as intensive where it is.
    The variation of planetary properties is so great that simple first order forecasts aren’t very useful (other than statistically, assuming all else on average being equal). And then a tidally locked planet could have a moon system that might be habitable, or even keeps its planet from becoming tidally locked. Pluto and Charon are mutually tidally locked and would have no problem with the Sun if they were in the habitable zone.

    So Mercury (eccentricity), Venus (atmosphere), Earth’s Moon (locked to Earth not the Sun) and Pluto (double planet) show that it is hard for a planet to become tidally locked to its star! Even near Sun asteroids and comets don’t become tidally locked to the Sun because of out-gassing, asymmetric heat radiation and such non-gravitational effects. There are so many way they avoid it. Drawing conclusion from a single factor only doesn’t say so much, one has to consider the particularities of the individual planet and its long random history.

  • Geoffrey Hillend August 20, 2017, 16:04

    Oh, I see they are just applying the model of a tidal locked planet to G and K stars but not saying that these stars are tidally locked with planets in the life belt.

  • Ivan Vuletich August 20, 2017, 20:52

    Has anyone done any work on whether 3:2 resonances or Venus day lengths are better or worse for habitability? At least with tidally locked planets, conditions are relatively stable temperature wise.

  • Abelard Lindsey August 21, 2017, 13:59

    Findings over the past 5 years or so seem to be resurrecting Rare Earth, but on different criteria than the Ward/Brownie book. First, the planets starting at sizes slightly larger than Earth on up are likely mini-Neptunes or at least waterworlds with very thick atmospheres. Now, that most G and K star planets in habitable zones are likely tidally locked.