Tidal Heating on Some Exoplanets May Leave Them Waterless


As the number of exoplanets being discovered continues to increase dramatically, a growing number are now being found which orbit within their stars’ habitable zones. For smaller, rocky worlds, this makes it more likely that some of them could harbour life of some kind, as this is the region where temperatures (albeit depending on other factors as well) can allow liquid water to exist on their surfaces. But there is another factor which may prevent some of them from being habitable after all – tidal heating, caused by the gravitational pull of one star, planet or moon on another; this effect which creates tides on Earth’s oceans can also create heat inside a planet or moon.

The findings were presented at the January 11 annual meeting of the American Astronomical Society in Austin, Texas.

The habitability factor is determined primarily by the amount of heat coming from the planet’s star. The closer a planet is to its star, the hotter it will be, and the farther out it is, the cooler it will be. Simple enough, but tidal heating adds a new wrinkle to the equation. According to Rory Barnes, a planetary scientist and astrobiologist at the University of Washington, “This has fundamentally changed the concept of a habitable zone. We figured out you can actually limit a planet’s habitability with an energy source other than starlight.”

This effect could cause planets to become “tidal Venuses.” In these cases, the planets orbit smaller, dimmer stars, where in order to be in that star’s habitable zone, they would need to orbit much closer in to the star than Earth does with the Sun. The planets would then be subjected to greater tidal heating from the star, enough perhaps to cause them to lose all of their water, similar to what is thought to have happened with Venus in our own solar system (ie. a runaway greenhouse effect). So even though they are within the habitable zone, they would lack oceans or lakes.

What’s problematic is that these planets could subsequently actually have their orbits altered by the tidal heating so that they are no longer affected by it. They would then be more difficult to distinguish from other planets in those solar systems which may still be habitable. While technically still within the habitable zone, they would have effectively been sterilized by the tidal heating process.

Planetary scientist Norman Sleep at Stanford University adds: “We’ll have to be careful when assessing objects that are very near dim stars, where the tides are much stronger than we feel on present-day Earth. Even Venus now is not substantially heated by tides, and neither is Mercury.”

In some cases, tidal heating can be a good thing though. The tidal forces exerted by Jupiter on its moon Europa, for example, are thought to create enough heat to allow a liquid water ocean to exist beneath its outer ice crust. The same may be true for Saturn’s moon Enceladus. This makes these moons still potentially habitable even though they are far outside of the habitable zone around the Sun.

By design, the first exoplanets being found by Kepler are those that orbit closer in to their stars as they are easier to detect. This includes smaller, dimmer stars as well as ones more like our own Sun. The new findings, however, mean that more work will need to be done to determine which ones really are life-friendly and which ones are not, at least for “life-as-we-know-it” anyway.

12 Replies to “Tidal Heating on Some Exoplanets May Leave Them Waterless”

    1. yes probably. we maybe going to find planets outside the Habitable zone that will have life, but we can also find planets with in the Habitable zone that do not have life. look at mars. I know that they are still looking for life on mars.

      1. For sure. I get excited about these real variables for life like in the article. I get a little sarcastic about the “habitable zones” talk that frequents fairly often.


      2. The HZ is a first step in a sensitivity analysis around known conditions for cellular life.

        I wouldn’t get hung up on its current use, it will diversify over time. As an example, think of how “planet” used to mean Earth.

      3. The days a long gone when the habitable zone was nice little circular disc with crisp edges. Now it’s more of a probability cloud with fuzzy edges, with all sorts of conditionals (atmospheres, gas giant moons) to mess it up.

      4. Given the range of intensity of the greenhouse effect, I guess Mars, Earth and Venus are all 3 in the habitable zone.

        Without the greenhouse efect, all three would be frozen worlds. Including it, we have a negligible greenhouse effect in Mars, a significative greenhouse effect in Earth and a super-greenhouse effect in Venus.

        Mars it geologically dead (a solid core, no volcanic activity, no plate tectonics) from more than a billion years ago. With so little volacnic outgassing and an enhanced atmospherics loss due to the lack of a magnetic shield, most of its atmosphere was likely lost in space.

        Earth has an active interior: a liquid outer core (that produces our magnetic field) and plate tectonics. Plate tectonics is belived to be a “geological thermostat” because fresh crust suffer weathering that sequester CO2 while volcanic activity emits it, moderating the greenhouse effect.

        Venus has volcanic activity but not plate tectonics. Geophysical modeling by Diana Valencia show that Earth is marginally favorable to plate tectonics. With a slight smaller mass or water content, it would not have plate tectonics. Without plate tectonics, the surface will be weathered and eroded until all land is a flat landscape and no rocks are exposed to weathering. Without weathering, CO2 will began to accumulate (even without plate tectonics there would still be moderate volcanic activity around “hotspots” that emit CO2), until a runaway H2O-CO2 greenhouse effect warm the planet until it became a hell like Venus.

        In short, given a geologically active planet (i.e. with plate tectonics) the “habitable zone” of our solar system include not only Earth but also Mars and Venus. It is known that that Mars was habitable in the past, and probably Venus too.

        So, it is reasonable to consider wide HZ ranges of radiuses, taking our solar system as an example. However, habitability of a planet also depends of tectonic activity, and it is easier on bigger and wet(water content “lubricates” the tectonic plates) planets. A geologically dead planet is likely also a biologically dead planet.

  1. I don’t want to diminish the article, which is an excellent expository, but when I first read about this my reaction was “a conference proceeding” and “a small effect, hunting for a large lever” in that order.

  2. To vaporize most of a planet ocean the heat froduced by tidal heating must be huge.

    Earth receives 341 Watts/m^2 from the Sun. Venus receives almost twice as much solar
    radiation as Earth, but only absorbs almost 170 Watts/m^2 (the rest is reflected by the venusian cloud layer back to space).

    The effective temperatures (without the greenhouse effect) and surface solar radiation of Venus, Earth and Mars are:

    VENUS: flux=163W/m^2; T =-41ºC
    EARTH: flux=239W/m^2; T =-18ºC
    MARS: flux=125W/m^2; T =-56ºC

    (source: http://bouman.chem.georgetown.edu/S02/lect23/IntrotoGreenhouseEffect.pdf )

    Taking these 3 planet as examples, imagine we put it around another, smaller star.

    Using the Stefan-Boltzmann law : ? = ?(T^4) [where ? is the radiation flux in Watts/m^2, T the temperature in Kelvin and ?=5.67*10^-8 Joules(s*m^2*ºK^4] we can find how much geothermal heat (from tidal heating, radioactive decay, accretion,etc)in addition to solar radiation is needed to:

    1) melt water ice (T=0ºC)

    VENUS: flux=151W/m^2;
    EARTH: flux=75W/m^2;
    MARS: flux=190W/m^2;

    2)boil water at a pressure of 1 atmosphere (T=100ºC)

    VENUS: flux=934W/m^2;
    EARTH: flux=858 W/m^2;
    MARS: flux=972 W/m^2;

    3)vaporize water at any pressure (critical temperature= 374ºC)

    VENUS: flux=9770W/m^2;
    EARTH: flux=9700W/m^2;
    MARS: flux=9800 W/m^2;

    These fluxes can be from geothermal heat, from the greehouse effect or from a combination of both.

    It is evident that to boil the oceans of a terrestrial planet, there is needed a lot of heat.

    An extreme greenhouse effect (like in Venus) can produce those downward fluxes of thermal radiation, but…

    are heat fluxes from tidal heating anything similar to that for a planet that orbits a red dwarf inside the “habitable zone” ?

  3. It’s my understanding that if a planet is close enough to its star to experience significant tidal heating (i.e. enough to threaten its water repository), it will soon end up tidally locked to the star anyway, and that certainly wouldn’t do much good for habitability – the atmosphere might very well freeze solid on the night side of the planet.

    1. Tidal locking does not necessarily results in “the atmosphere might very well freeze solid on the night side of the planet” if the atmosphere is thick enough and is made mostly of greenhouse gases like CO2.

      The planet will have intense winds that transfer the heat from the dayside to the nightside, resulting in moderate temperature variations between day and night.

      1. Yes, as I said – it might. I remember reading somewhere that it depends very much on the thickness, gas giants of course wouldn’t have the problem at all, but some small rocky planets could easily end up with a thin atmosphere, especially if it is stripped away from the planet by stellar winds.

      2. “some small rocky planets could easily end up with a thin atmosphere, especially if it is stripped away from the planet by stellar winds”

        mmm… Venus has no magnetic shield, but has an atmosphere 90 times ticker than Earth…resulting in minimal day/night surface temperature variations despite its very slow rotation (Venusian days that last several terrestrial months).

        Using our solar system as an example, small planets like Mars could easily end with thin atmospheres, but Earth-sized planets (like Venus) or bigger could easily have thick atmospheres I think.

        To know for sure we will have to wait for future data from future telescopes that can give us information about the thickess, composition and temperature of the atmosphere of extrasolar planets…

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