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Habitability: Look to Younger Worlds

A liquid water-defined habitable zone is a way of establishing parameters for life as we know it around other stars, and with this in mind, scientists study the amount of stellar radiation a planet receives as one factor in making the assessment. But of course, not everything in a habitable zone is necessarily habitable, as our decidedly uninhabitable Moon makes all too clear. Atmospheric factors and tectonic activity, for example, have to be weighed as we try to learn what the actual temperature at the surface would be. We’re learning as we go about other contributing factors.

A problem of lesser visibility in the literature, though perhaps just as crucial, is whether a given planet can stay habitable on timescales of billions of years. This is where an interesting new paper from Cayman Unterborn (Southwest Research Institute) and colleagues enters the mix. A key question in the view of these researchers is whether carbon dioxide, the greenhouse gas whose ebb and flow on our world is determined by the carbonate-silicate cycle, can come into play to stabilize climatic conditions.

The carbonate-silicate cycle involves delivering CO2 to the atmosphere through degassing in the planetary mantle or crust, with carbon returned as carbonates to the mantle. The effects on climate are substantial and changes to the cycle can be catastrophic in terms of habitability. If weathering sufficiently draws down the concentration of CO2 in the atmosphere, for example, the planet can tilt in the direction of a ‘snowball’ state. So we need active degassing to keep the cycle intact, and the question becomes, how long can a planet’s mantle maintain this degassing?

Volcanic activity and tectonic processes, in turn, are powered by internal heat, and there we note the fact that a planetary heat budget decreases with time, affected by many things, one of which is the presence of radiative decay. Thorium, potassium and uranium have to be available in sufficient quantities, powering mantle convection, which gives us movement from a planetary core all the way to the crust. And radioactive elements, by their nature, decay with time. They are also not evenly distributed from one stellar system to another. If we’re looking for planets something like our own, in other words, let’s learn what we can about their radiogenic heat.

Not that this is the only factor involved in a planet’s heat budget, but it’s an effect that may account for, the authors say, from thirty to fifty percent of the Earth’s current surface heat flow (because of radioactive decay, the current flow represents only 20 percent of the Earth’s heat budget when it formed four and a half billion years ago).

The authors mention other planetary heat sources, as we’ll see below, but confine themselves in this paper to radiogenic heat. Their method: To estimate the distribution of these heat-producing elements by examining stellar composition as determined by spectroscopic data, using this as a proxy for the composition of planets. They combine this information with chemical evolution models for the galaxy at large. They then produce models of thermal evolution that maximize the cooling rate in a planetary mantle, resulting in what the authors call “a pessimistic estimate of lifetime a rocky, stagnant-lid exoplanet can support a global carbon cycle through Galactic history.”

Seventeen exoplanets are subjected to this framework in the paper, all with measured ages. Seven of these, the researchers predict, should be actively outgassing today. Says Unterborn:

“Using host stars to estimate the amount of these elements that would go into planets throughout the history of the Milky Way, we calculated how long we can expect planets to have enough volcanism to support a temperate climate before running out of power. Under the most pessimistic conditions we estimate that this critical age is only around 2 billion years old for an Earth-mass planet and reaching 5–6 billion years for higher-mass planets under more optimistic conditions. For the few planets we do have ages for, we found only a few were young enough for us to confidently say they can have surface degassing of carbon today, when we’d observe it with, say, the James Webb Space Telescope.”

Image: An SwRI-led study suggests that host-star age and radionuclide abundance will help determine both an exoplanet’s history and its current likelihood of being temperate today. For example, the red dwarf star TRAPPIST-1 is home to the largest group of roughly Earth-sized planets ever found in a single stellar system with seven rocky siblings including four in the habitable zone. But at around 8 billion years old, these worlds are roughly 2 billion years older than the most optimistic degassing lifetime predicted by this study and unlikely to support a temperate climate today. Credit: NASA/JPL-Caltech.

Remember that this is a deliberately pessimistic model. It’s also the case that abundances of heat-producing elements are only one factor that can change the degassing lifetime of a planet, and the authors are quick to point out that they do not include these in their model. Thus we could consider the current study a contribution toward a broader model for planetary heat budget analysis, one that should be expanded through examining such factors as cooling after planet formation, the energy released when core and mantle differentiate, and tidal heating induced by the host star or other planets in the system. As the authors describe their results:

The framework we present here that combines direct and indirect observational data with dynamical models not only provides us with a pessimistic baseline for understanding which parameter(s) most control a stagnant-lid exoplanet’s ability to support a temperate climate but also indicates where more lab-based and computational work is needed to quantify the reasonable range of these parameters (e.g., mantle reference viscosity). As we move to more in-depth characterization of individual targets in the James Webb Space Telescope era, these direct and indirect astronomic observables, coupled with laboratory data and models from the geoscience community, will allow us to better estimate whether a rocky exoplanet in both the canonical and temporal habitable zones has exhausted its internal heat and is simply too old to be Earth-like.

We are positioning ourselves, as highlighted by the ongoing commissioning of the James Webb Space Telescope, to begin the analysis of planetary atmospheres at scales smaller than gas giants, meaning that the kind of computational modeling at work in this paper will increasingly be refined by observation. The interactions between a planet’s surface and its interior then become better defined as markers for habitable worlds, with radionuclides a significant factor in producing climate stability.

The paper is Unterborn et al., “Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets Can Support Temperate Climates,” Astrophysical Journal Letters Vol. 930, No. 1, L6 (3 May 2022). Full text.

{ 17 comments… add one }
  • Alex Tolley May 10, 2022, 11:00

    Under the most pessimistic conditions we estimate that this critical age is only around 2 billion years old for an Earth-mass planet…

    Obviously very pessimistic as the Earth is more than 2x this figure and still has active plate tectonics, volcanism, and CO2 outgassing.

    IIRC, Venus is said to have a stagnant lid, yet it has an abundance of CO2. It can no longer deplete this atmospheric CO2 by weathering.

    If this modeling reflects reality, we should expect older worlds to be depleted in CO2. However, for habitability, there may be advantages, for worlds on the inner edge of the HZ, the declining CO2 may offer a more habitable window to appear before increasing stellar luminosity makes them uninhabitable again.

    CO2 is the main source of carbon for photosynthesis. It is also used by methanogens. Methane is used as a carbon source by chemotroph bacteria. Without outgassing either of these 2 gases (or any others), this implies that all life, including lithospheric life, must disappear from such stagnant plate planets. This is important because we have often suggested that planets around M_dwarfs will stay habitable for a very long time and allow complex life and even intelligent life to evolve. However, if internal heat driven by radioactive decay is a dominant process maintaining outgassing, then this assumption may be unwarranted. Indeed, the increased luminosity of your M_dwarfs may limit the initiation of the evolution of life, and internal cooling may then subsequently snuff it out, assuming it was able to appear at all.

    All this is speculation, and it would be better to have a large sample of worlds with biosignatures to determine what are the most important factors for life to thrive on exoplanets.

  • Mike Serfas May 10, 2022, 16:56

    The inner boundary of the “habitable zone” seems to bob regularly between the aphelion (but never the perihelion) of Venus, and some point around 0.95-0.99 AU that leaves Earth a few shale oil wells away from the fire. ( https://en.wikipedia.org/wiki/Circumstellar_habitable_zone ) It’s a relief to see this latest illustration retreat back to Venus … time to start drilling again! :)

    The “faint young sun paradox” seems to encourage a gradual loss of atmospheric CO2 over time to make up for an ever-brighter star: new champagne in new bottles and old champagne in old bottles.

  • Geoffrey Hillend May 10, 2022, 17:38

    I suspect the Trappist-1 system exoplanets to have atmospheres due to the tidal forces on the them by the red dwarf star. The mass and size of the exoplanet matters since a larger planet might also have more internal heart and a larger, iron core. There could still be volcanism without plate tectonics. With oceans and rain, the CO2 would definitely be continually reduced. Without a lot of volcanism the planet might be frozen. One the other hand a lot of carbon dioxide will remain if there is no photosynthesis to help reduce the CO2 levels on a dead world. For sure, I will predict the JWST’s spectrometer will see CO2 and H2O.

    An exoplanet without a moon in the life belt of a G class star might not be frozen due to the large variation in obliquity. If it had very little CO2, then it would still be all frozen

  • Michael Fidler May 10, 2022, 22:59

    A problem with this is large impactors from stellar explosions and neutron star collisions with still radioactive hot elements can renew dead planets when interstellar collisions occur. The earth or any planet sweeping through the galactic arms may have large impactors like the recent ‘Oumuamua both stir the internal currents and and give rebirth to planets. The earth has several large continent size masses in its interior that may be the result of such impacts. The early large earth impactors are what started the plate tectonics in the first place…


    The huge Pacific ocean is only 270 million years old and snowball earth stripped away much of Earth’s history on the land masses. We may be fooling ourselves into believing that there has been no major catastrophes to earth that have given rebirth to it’s interior heat.

    • Geoffrey Hillend May 11, 2022, 16:11

      The impact of Earth with Thea gave Earth a larger iron core than it would have had without that impact since a lot of Thea’s core went into Earth. The debris formed our Moon. Consequently, the Earth retained more heat which may have helped with plate tectonics.

      • Michael Fidler May 12, 2022, 22:51

        Now you’ve got me thinking. The high Thorium and Uranium on the earth facing side of the moon may be from Thea’s encounter, but what about radioactive hotspots on the lunar highland? A very unusual feature called the Compton-Belkovich Thorium Anomaly may be only 800 million years old.

        Astronomers find mystery moon domes.

        Hotspot Found On Moon’s Far Side.

        Age of the Compton-Belkovich Volcanic Complex.

        Could the earth and solar system have moved through the debris field of a neutron star collision 800 million years ago? This could be what happened to Venus causing its resurfacing and the unusual markings on Mercury. The moon is one of the best places to see the fossil history of our solar system…

  • Robin Datta May 11, 2022, 11:12

    Once chemistry progresses sufficiently to acquire the characteristics of life, through evolution slow-growing extremophiles may come to inhabiting nearly inaccesible sites such as fissures in rocks in the depths thus remaining preserved through many vicissitudes as a testament to an otherwise lost history.

  • Henry Cordova May 12, 2022, 15:43

    I used to think that any planet in the habitable zone (where water is a liquid) around an old and stable star could be a candidate for life.

    Now I understand there are many other contraindications, such as tidal locking, a lack of a large moon to stabilize rotation and produce tides, not having a nickel iron core to generate a magnetosphere, failing to have plate tectonics, lack of vulcanism, a history of collisions with other planets, and so on. No doubt there are others.

    I don’t know how many of these are fully agreed upon as non-starters by the astrobiology community, and which ones may or may not be controversial. Is there a list somewhere I can consult?

    Personally, I think life will arise anywhere except in the most hostile and extreme of environments, although I’ll be the first to admit that’s just my unsupported opinion.

  • wdk May 12, 2022, 22:17

    Granted Trappist 1 is an old system at 8 giga-years, there are some other considerations that might sustain Trappist 1 longer than simply an Earth-sized planet placed in the center of a nominal HZ.

    Like the Galilean satellite system of Jupiter, the Trappist system has resonances. To our knowledge they do not have satellites, but they do have large planets careening by each other.

    Now are we to assume that as tightly packed as these planets are they do not have similar responses to those of Io, Europa and Ganymede? Or should we assume them as static and un-mixed as Callisto, but on a terrestrial scale?

    If this system is as old as determined, I have to wonder how it can subsist in such a precarious concentrated state. One might even wonder if such an arrangement is indicative of techno-signature as much as a Kardeshev Roman numeral.

    Eight billion years old, but reminiscent of Stonehenge. Sure, there are a lot of problems with habitability with Trappist 1 from b to … h? Red dwarf flares and IR black body peak. But how do you get 7 terrestrial planets to sit still in that orbital arrangement for 8 billion years?
    It might be interesting to try to integrate backwards for a while… Or else explain how an 8 body system finds 8 billion years of stability.

  • Adam Crowl May 13, 2022, 5:00

    Hi Paul
    Disturbingly there’s this possibility…
    Volcanoes may have killed Venus with a runaway greenhouse: Implications for Earth-bound volcanism
    Based on this paper: M. J. Way et al, Large-scale Volcanism and the Heat Death of Terrestrial Worlds, The Planetary Science Journal (2022). DOI: 10.3847/PSJ/ac6033
    Large Igneous Provinces seem to appear at random – there’s thus nothing stopping several appearing at once. Such a massive pulse of greenhouse gases could trigger a Runaway Greenhouse like that which (possibly) killed Venus and might yet destroy Earth’s biosphere.

    • Alex Tolley May 13, 2022, 9:48

      While we don’t know exactly how they were triggered, the massive volcanic eruptions during the Permian brought on the Permian extinction, the greatest of all the mass extinctions. Fortunately, we didn’t get any runaway greenhouse, but the temperatures rose and there is evidence that the global climate became very hot.

      I also recall Richard Muller, who believed in periodic extinctions due to some astronomical event (e.g. a Nemesis planet in the deep outer solar system) also suggested that there was a possibility that periodic large eruptions from the mantle might be the cause. We say that we know more about the lunar surface than we do of the deep ocean floor, but we really know even less about the Earth deep below the surface, especially once we get below the crust.

    • Mike Serfas May 15, 2022, 12:07

      Adam: this is an interesting and wide-ranging paper, which reminds me of how little we know about Venus. It sounds very favorable about the notion of finding much older geological information rather than just a resurfaced crust. But as I understand it, the paper is saying that large igneous provinces almost turned Earth into a moist greenhouse, and therefore might do the same on Venus, because Earth itself has already seen seawater temperatures of 60-75 degrees Celsius (!). They cite a paper ( https://www.geochemicalperspectivesletters.org/
      documents/GPL1706_noSI.pdf ) which appears to support this with the data in its Figure 2. But that figure lists the coldest part of the Huronian glaciation (“snowball Earth”) as 30 C! I’m no expert on these things, but until I can understand this part I’m not going to believe the rest.

      • Alex Tolley May 15, 2022, 16:36

        Doing some quick reading I am now thinking that calling lower temperatures during events Gya a “glaciation” is a mischaracterization. They just represent cooler periods in very hot conditions.

        What surprises me is that the early Earth temperatures are very much in the extremophile range of thermophiles. Above 60C (140F) proteins start to degrade without some form of stabilization. Interestingly, the calculated temperatures in figure 2b are just about 60C just as early life fossils have been discovered. If life evolved earlier, the higher surface temperatures would only be suitable for organisms like the archaea living around the hot vents in the abyssal ocean and thermophilic bacteria that also live around the vents and in hot springs.

        • Mike Serfas May 17, 2022, 20:06

          There seems to be quite a bit of evidence for extensive glaciation during the Huronian. See https://www.researchgate.net/
          publication/278713309_72_Huronian-Age_Glaciation Banded iron formations on many continents include “dropstones” that seem to be indicative of glacial/iceberg erosion and deposition of rock. The data is backed up by oxygen-18 measurements. I recall there was some hesitancy to accept that the entire ocean was frozen over – I don’t know where that debate stands now – but I think the consensus is still that a majority was covered in ice.

          • Alex Tolley May 18, 2022, 12:38

            The geochem paper clearly implied that the O16 values were correlated with the cooling Earth. Their only explanation of the values implying high temperatures was that the samples were from deposits near hot vents or springs. We do think the Snowball Earth was present in the Precambrium. For me, the paradox is a hot Earth while the sun was much fainter. A thick, atmosphere with GHGs might well be sufficient to offset the fainter sun, but shouldn’t the increased weathering have reduced this in classic geological temperature control? Coupled with the implied high surface temperatures towards the limits of biology, I am somewhat skeptical of their results. Wouldn’t the peer review have made similar points?

  • wdk May 18, 2022, 11:05

    Going back to the image of the Solar and Trappist-1 habitable zones above, I would like to point out that the image does show the dispersal of habitable zone planets in the two systems; however, the two systems are NOT shown to the same scale. The outer bound for the Trappist 1 system (Trappist-1 h) is 0.06189 AU. Mars is near the posited edge of the Solar HZ at 1.52AU. Thus, there is more scale kinship to the Galilean moon system.

  • ljk May 19, 2022, 12:01

    If the Wow! Signal of 1977 was not some kind of glitch or false reading, did it come from 1,800 light years away?


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