“Tidal Venuses” May Have Been Wrung Out To Dry


Earth-sized exoplanets within a distant star’s habitable zone could still be very much uninhabitable, depending on potential tidal stresses — either past or present — that could have “squeezed out” all the water, leaving behind a bone-dry ball of rock.

New research by an international team of scientists suggests that even a moderately eccentric orbit within a star’s habitable zone could exert tidal stress on an Earth-sized planet, enough that the increased surface heating due to friction would boil off any liquid water via extreme greenhouse effect.

Such planets are dubbed “Tidal Venuses”, due to their resemblance to our own super-heated planetary neighbor. This evolutionary possibility could be a factor in determining the actual habitability of an exoplanet, regardless of how much solar heating (insolation) it receives from its star.

The research, led by Dr. Rory Barnes of the University of Washington in Seattle, states that even an exoplanet currently in a circular, stable orbit could have formed with a much more eccentric orbit, thus subjecting it to tidal forces. Any liquid water present after formation would then have been slowly but steadily evaporated and the necessary hydrogen atoms lost to space.

The risk of such a “desiccating greenhouse” effect would be much greater on exoplanets orbiting lower-luminosity stars, since any potential habitable zone would be closer in to the star and thus prone to stronger tidal forces.

And as far as such an effect working to create habitable zones further out in orbit than otherwise permissible by stellar radiation alone… well, that wouldn’t necessarily be the case.

Even if an exoplanetary version of, say, Europa, could be heated through tidal forces to maintain liquid water on or below its surface, a rocky world the size of Earth (or larger) would still likely end up being rather inhospitable.

“One couldn’t do it for an Earthlike planet — the tidal heating of the interior would likely make the surface covered by super-volcanoes,” Dr. Barnes told Universe Today.

So even though the right-sized exoplanets may be found in the so-called “Goldilocks zone” of their star, they may still not be “just right” for life as we know it.

The team’s full paper can be found here.

23 Replies to ““Tidal Venuses” May Have Been Wrung Out To Dry”

  1. I wonder how this may play out with regard to planets around red dwarfs? Low luminosity stars implies higher risk for tidal induced dessication. Coupled with the large stellar flares typical of red dwarfs, it seems to me that that even though these types of stars are the most numerous in the universe they may not be so friendly to earth-size planets as we imagine.

  2. I wonder how this may play out with regard to planets around red dwarfs? Low luminosity stars implies higher risk for tidal induced dessication. Coupled with the large stellar flares typical of red dwarfs, it seems to me that that even though these types of stars are the most numerous in the universe they may not be so friendly to earth-size planets as we imagine.

  3. Funny you should say so.

    An alternative hypothesis suggests that our sun may have formed as a binary pair that merged in a luminous red nova (LRN) at 4.567 Ga, and Venus may have been tidally locked to the close binary pair. Then when the pair merged, a small portion of the central mass was ejected by the LRN shock wave causing a slight increase in the semi-major axes of the planets. This may have caused formerly tidally-locked Venus to rotate in a very slow retrograde direction.


    1. That would be quite a bombshell. Is the hypothesis being considered by anyone else or does it exist only on your blog?

      I’m tempted to take a skeptical position if nobody else is discussing it.

    2. Impossible, all planets orbit almost in the same plane, almost circular and in the same direction. Any big object crossing the solar system would have resulted in all orbits being far more elliptical random oriented and maybe even ejected some planets out of the solar system.

      1. Just pointing out, Olaf, that I don’t think he mentioned anything about any big object crossing the solar system. The original Sun was simply two. Both in the centre and both the centre of all their planets’ orbit. Careful.

      2. You may be right, but the word close can mean close within Mercury’s orbit or close as in outside Pluto’s orbit. The question what does he mean with close.

        I have doubt that a close as Mercury distance can be stable for very long. They probably merge before any of the planets formed.

      3. Snowball Solar System (SSS), an alternative solar system model:

        By the way, thanks for the civility. This thing has gone through a dozen revisions already, and if it’s got glaring flaws, I appreciate knowing about them, thanks. And thanks to the moderator for not pulling me down.

        Yes, the central binary pair orbited around a common barycenter inside the orbit of Mercury before decaying to merge in the LRN, and all 8 planets were fully formed at that time, but the contact binary phase of the central binary pair cleared the inner solar system of its protoplanetary disk, ending planet formation, leaving the asteroid belt as a failed protoplanet around the ‘shepherding’ resonances of Jupiter and the Kuiper belt as a failed protoplanet around the strongest resonances of the binary companion Nemesis.

        Nemesis went on to fuel its own solar orbit inflation at the expense of its own close binary pair (SSS suggests our solar system was a quadruple star system with two close binary pairs). At present, Nemesis may be around 20,000 AU.

      4. Maybe you should give some maths that proves that a close binary can actually survive that long until the planets formed.

        And a close formed binary star would give issues like the biggest one would start nuclear fusion first and blow the other gas out of the solar system never giving a chance to form a second star that close.

        Second, a Nemesis at 20,000 AU would indicate that it is tiny, not even a dwarf star. If it was big the it would show up on the redshift – blueshift of surrounding stars since the barry center of the complete solar system would not be at the Sun.

      5. The quadruple star system originally formed with interplay, and a wide binary pair emerged in a hierarchical system in a 4-star version of core collapse that drove the two largest stellar components together through secular perturbation. They didn’t form that way originally, but no, I can’t do the math, I’m strictly conceptual.

        I’ve been meaning to calculate the velocity of a hypothetical companion at 20,000, which comes to .2 km/sec. If the binary companion is 1/10 the mass of the sun, the sun would be reacting by something like .02 km/sec. Our sun’s velocity around the center of the galaxy is 220 km/s so the barycenter effect would be about 1 part in 11,000 which probably a lot less than the known precision of our galactic orbital velocity.

      6. A star 1/10th the sun would be bright and detectable. That is a 104 Jupiter mass and not a brown dwarf anymore.

      7. May I propose you do a simulation by using “Universe Sandbox” or even better Astrograv? Maybe there are some other tools you can use to test the basics of your theory.

        I doubt that a close by double star near the sun can stay long enough for planets to completely form. It would lose orbital speed because the other proto-planets drag on it making the other proto- planets move further away from the Sun and this second star fall into the Sun.

      8. Part II of SnowballSolarSystem, an alternative solar system model:

        Our sun has a binary companion (Nemesis) at 20,000 AU that orbits establishes the invariable plane that formed the outer planets Jupiter – Kuiper belt around its strongest 2:1 to 3:1 resonances. Then secular perturbation caused it to feed off its binary pair, raising its solar orbit out to the present day 20,000 AU, forming Oort cloud comets along the way in its resonances.

        Jupiter in turn formed the inner planets Mercury – asteroid belt around its own 2:1 to 3:1 resonances, and orbit inflation of the planets came from secular perturbation of the central close binary pair that merged in an LRN.

        The asteroid belt is in the invariable plane just as are the planets and the inner Oort cloud comets.

    1. But the further orbit would reduce tidal stresses. So maybe not. Personally, I think this is just a modelling conjecture and not really worth much heed. The variability of planet structures, compositions and dynamics makes this far too complex to easily generalize into liklihood numbers.

  4. This does raise questions about prospects for biologically active planets around M-class dwarf stars. A planet in the so called habitable zone is tidally locked, and early in its evolution the tidal forces on the planet induced this locking. This might have heated up the planet enough to drive off water.

    M-class dwarf stars have other problematic elements, such as flaring. The planetary systems around these stars have small orbital radii, which mean these planets gravitationally interact with each other much more strongly than planets in our solar system. So a planet in the habitable zone at one time might be perturbed out of that zone in a comparatively short period of time.

    Finding biologically active planets around M-class stars is not going to be easy. I suspect a pretty small percentage of these planets are biologically active. This may not mean that none of them are, but I am certain that it is a very small percentage which are.


    1. I don’t see how we can constrain the problem in that way. There are bound to be constraints to surface habitability aside from insolation, in the same way that there will be constraints to volume habitability (say, ice worlds) other than tidal heating (say, radioactive heating). That said, they will be mostly marginal,* and we already know there is a wide variety of worlds.

      Given that M stars are the most numerous, will have the highest frequency of potentially habitable terrestrials, and will remain habitable for the longest time, they are still as a class excellent candidates for astrobiology. The only reason to go after G stars and Earth massed planets first would be that the known example of biosphere is there. Second of interest will be M stars as they will have the potentially most frequent habitables.
      * Specifically as for the gravitational interaction scenario, the known system with many planets seem to imply we lack knowledge of how systems form. No doubt there will be rejects from multiple planet systems, unless they settle nice and tight first as it seems from the examples seen.

      1. Of course we are dealing with unknown unknowns here. Your argument is the numerical preponderance of M class stars weights the statistics for habitable planets. That might be. Of course I suspect we may have to search long and hard to find a bio-active planet around an M class star.


  5. This can’t be a very generic process. It didn’t happen in 3 out of 3 viable planets in our own system. (Assuming Venus lost its hydrogen in the same way it is still loosing it.)

  6. I was curious and looked it over.

    Two immediate problems jump out:

    – They have to assume high eccentricities ( > 0.3) for high enough warming to be a constraint. Yes, such planets can be found but a) a quick check in the exoplanet.eu database says that most found are much lower b) there is bound to be a bias due to detection methods (radial velocity) and sensitivity (large, single planets promoted as of yet).

    – They have to assume hydrodynamical escape for small enough timescales to be a constraint. It is, I think after a quick googling, putatively confirmed for HD 209458b. But it would be good to see more robust testing of that.

  7. The authors acknowledge the possibility that eccentric-orbit planets could have a more distant HZ, but then say that those planets will have supervolcanoes. I think it’s not clear why supervolcanoes would be a problem for habitability (although I could imagine they’d be more likely to have mass extinctions).

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