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The Habitable Zone: The Impact of Methane

The definition of a habitable zone is under constant refinement, an important line of research as we choose which exoplanets to focus on in our search for life. Centauri Dreams regular Alex Tolley today looks at the question as it involves the presence of methane. With planetary warming already known to vary depending on the spectral type of the host star, we now learn that the presence of methane can produce thermal inversions and surface cooling on M-star exoplanets, impacting the outer limits of the habitable zone. The work of Ramses Ramirez (Tokyo Institute of Technology) and Lisa Kaltenegger (Carl Sagan Institute, Cornell University), the paper also suggests a possible biosignature near the outer habitable zone edge of hotter stars, one of several results that Alex explores in today’s essay.

by Alex Tolley

Alien world – still from 2001: A Space Odyssey. Credit: Metro-Goldwyn-Mayer (MGM)

As noted in previous posts on biosignatures, especially in regards to life prior to photosynthetic oxygenation of the atmosphere, methane (CH4) is a gas that appears early due to prokaryotic methanogens producing it at rates far higher than geological processes and sustaining its presence despite the chemical destruction. The presence of CH4 in a primordial N2-CO2-H2O atmosphere has been suggested as a component of a biosignature (Detecting Early Life on Exoplanets).

Which leads to the obvious question. As CH4 is a potent greenhouse gas, does it impact the size of the habitable zone (HZ), particularly its outer edge, which is usually defined by the greenhouse gas CO2? This is exactly the question that Ramirez and Kaltenegger try to answer in a new paper. Using their prior approach of assuming a rocky, Earth-like world around different star types, they approach the problem of modeling the effect of the star’s emission on the planet’s atmosphere with varying mixing ratios of CH4 from 10-100,000 ppm.

The authors have previously shown that the spectra of the star impact the warming of a planet [4, 5]. As the spectrum shifts towards the red, the warming efficiency increases. Because CH4, unlike CO2 and H2O, has strong absorption in both the infrared (IR) and near-infrared (NIR), this spectral shift has an even more pronounced effect when CH4 is an atmospheric component gas. For exoplanets with CH4 in their atmospheres around cool stars, the absorption occurs in the upper atmosphere, causing a thermal inversion and cooling the surface. Figure 1 shows the atmospheric temperature profile with altitude when 1% (10,000 ppm) of CH4 is added to a 3 bar atmosphere of CO2 for three star types: F0, our G-type sun, and M3. For both the F0 and G stars, the CH4 increases the surface temperature.20-30K. However, for the M3 star, the surface temperature is reduced by 30K as much of the energy is already absorbed at high altitude. Unlike most atmospheres, exoplanets around M-stars with a CH4 component have a temperature inversion. This has a direct effect on the outer limit of the HZ.

Figure 1. Changes in temperature profiles for a 3 bar CO2 atmosphere (S/So = 0.33) for a model planet with a surface pressure of about 4 bar orbiting a) an F0, b) a solar-analog, and c) an M3 host star when adding 1% (10,000 ppm) CH4. A temperature inversion forms for all three model planets when CH4 is added to the atmosphere. The surface temperature increases when 1% CH4 is added for the planet orbiting the F0 (18K) and solar analog (29K), but decreases for the planet orbiting an M3 star (31K).

Intuitively we expect that CH4 should extend the outer edge of the HZ for the F0 and G type stars as less energy is required to maintain a surface temperature above freezing (273K). For K3 stars and hotter, their model adding CH4 does indeed extend the HZ’s outer edge, quite substantially, while the inner edge remains fairly similar in distance from the star. However, because of the reduced surface temperature of the cooler stars, especially the ubiquitous M-types, CH4 reduces the range of the HZ, pulling the outer edge of the HZ closer to the star.

Figure 2 shows the effect of different mixing ratios of CH4 on the effective stellar flux (SEFF) needed to maintain a surface temperature above freezing against stellar types. Some cooler star exoplanets at the outer edges of their HZ zones are shown, indicating that some may have frozen surfaces if they have CH4 as an atmospheric gas component.

Figure 2. The effect of adding methane to the outer edge of the classical HZ. Stellar effective temperature versus the effective stellar flux (SEFF) for the outer limits of the Habitable Zone. The traditional outer HZ limit, the CO2 maximum greenhouse limit (dashed), is shown along with the empirical outer edge (solid black) and outer edge limits (solid blue) containing various amounts of CH4: 10 ppm (triangle), 1% (square), and fCH4 = 0.1 x fCO2. Some confirmed planets near the outer edge are included with error bars displayed.

The presence of CH4, therefore, has an impact on target exoplanet life. For Earth, CH4 might help explain the problem of the young, faint sun. When the earth was formed, the lower luminosity of the sun was insufficient to raise the surface temperature above freezing, yet life clearly evolved quite early. CH4 emission by methanogens during the Archean eon may have warmed the Earth’s surface sufficiently to have allowed liquid oceans and prokaryotes to evolve, and certainly ensured that an ice-free surface allowed photosynthetic blue-green bacteria to harvest solar energy.

Because CH4 extends the outer edge of the HZ for our sun, calculations suggest that early Mars was within the HZ if it had a dense CO2 atmosphere with a CH4 component. If so, this would support the theory of liquid seas on its surface and possibility that life also emerged to produce the needed CH4, or hydrogen (H2) to maintain its warmth as the atmosphere initially waned [6]. As the atmosphere was eventually largely stripped, the surface cooled and the seas froze to become the subsurface glaciers and frozen aquifers that we detect today, as well as the polar ice caps of frozen water and CO2. Whether life still exists below the surface is unknown, although there are tantalizing hints that just maybe methanogens are the source of the detected CH4 in the atmosphere, rather than geologic emissions.

The authors suggest that due to the far higher biogenic source of CH4, exoplanets around hotter stars on the edge of their HZ’s with detectable CH4 might well be evidence that they are living worlds. This is further bolstered by their model that shows CH4 levels can create a sort of Gaian “Daisy World” [2] where the surface temperature is maintained by processes that stabilize CH4 levels. Figure [3] shows this. Point P2 is stable as a declining CH4 level increases the surface temperature, increasing the CH4, while levels above that point reduce the temperature and reduce the emitted CH4.

Figure 3. Proposed “Daisyworld” scenario for planets at the outer edge of the methane HZ with CH4 in their atmospheres orbiting hotter (~A – G class) stars. The curved lines illustrate the effect that the CH4/CO2 ratio has on surface temperature whereas the straight lines depict the effect that
temperature has on the CH4/CO2 ratio (adapted from Domagal-Goldman et al. 2008).

However, this is not the case with M-star exoplanets. The authors’ model suggests that for these worlds, the increased CH4 cools the planet resulting in a freezing surface which might well extinguish the development of life. Therefore on a planet at the outer edge of the HZ evolving methanogens will cool, freezing the surface. Life would be forced to retreat to oceanic vents and the lithosphere, precluding the emergence of photosynthesis, and energetic motile, multicellular life.

Another question also remains unanswered. M-star dwarf exoplanets orbit so close to their star that they are likely tidally locked. As we have seen previously, such warm wet worlds are also likely to have high cloud cover and therefore higher albedos [4]. Do those factors influence the results of this model? The authors hint that these conditions do matter quite significantly and will be explored in their next paper. Stay tuned!


1. Ramses M. Ramirez, and Lisa Kaltenegger, 2018, “A Methane Extension to the Classical Habitable Zone,” The Astrophysical Journal Vol. 858, No. 2 (7 May 2018). Abstract.

2. Andrew J. Watson, James E. Lovelock, 1983, “Biological homeostasis of the global environment: the parable of Daisyworld”, Tellus B, vol. 35B, no. 4, pp. 284-289 (abstract).

3. H. Lammer, J. H. Bredehöft, A. Coustenis, M. L. Khodachenko, L. Kaltenegger, O. Grasset, D. Prieur, F. Raulin, P. Ehrenfreund, M. Yamauchi, J.-E. Wahlund, J.-M. Grießmeier, G. Stangl, C. S. Cockell, Yu. N. Kulikov, J. L. Grenfell, H. Rauer, 2009, “What makes a planet habitable?”, The Astronomy and Astrophysics Review, vol. 17, no. 2, pp. 181-249 (abstract).

4. Sarah Rugheimer, Lisa Kaltenegger “Spectra of Earth-like Planets Through Geological Evolution Around FGKM Stars”, The Astrophysical Journal 854(1). Abstract.

5. Ramses M. Ramirez. 2014 “Terrestrial planets under extreme radiative forcings: applications to habitable zones, early Mars, and a high-co2 Earth.” Ph.D. thesis. Pennsylvania State University (abstract).

6. Ramses M. Ramirez, & Lisa Kaltenegger 2016. “Habitable Zones of Post-Main Sequence Stars.” The Astrophysical Journal, 823(1), 6 (abstract).


{ 35 comments… add one }
  • Patient Observer May 11, 2018, 13:01

    Wow! It is counter-intuitive for sure. Thanks the for great explanation.

  • Ashley Baldwin May 11, 2018, 18:36

    Not necessarily counterintuitive, but certainly intuitive and possibly seminal. Definitely the new kid on the block. The product of greatly refined atmospheric modelling algorithms . And hard work as ever. Many congratulations to the authors. Its great to see a new theory which supports the possibility that detectable life , even if “only” eukaryotic , could exist around stars as early as late A spectrum stars with main sequence lives as short or shorter than 2 billion years. Though it is still just theory largely based on the presumed atmospheres of early Earth & to a lesser extent ,Mars , orbiting a “short wave” early G class star . But not necessarily for long,

    All the more reason to employ the ELTs and JWST on late M dwarfs ,as the only such stars conducive to temperate terrestrial planet atmospheric characterisation for the foreseeable future. Capable even of revealing the validity of this theory . Or it’s “long wave” arm anyway. ( I agree with Paul here about the great exoplanet biosignature debate that will then ensue, and which will be good as it will help raise the interest required fo fund the kind of big space telescopes necessary to move things further forward )

    A fine neighbouring “short wave” target would be dear old “forbidden planet” star , A7 Altair. This work potentially places a hab zone terrestrial planet as far out as 4 AU . No good for transmission spectroscopy but a wide orbit in combination with close stellar proximity would give a generous “inner working angle” for a future Habex / LUVOIR space telescope . Mind you, the imaging contrast required from any coronagraph would be stringent , at or above 1e10 for such a short wave dominated spectrum star and ruling out the possibility of employing ground based scopes , however big.

    • andy May 16, 2018, 17:42

      Regarding Altair, A-type stars tend to get overlooked when considering habitable planets, although the later A-types do appear to live long enough for biology to plausibly get started. It could be worth taking them into consideration for imaging missions where the wider separation of the habitable zone would presumably be an advantage, though I’m not sure what effect wins out of wider habitable zone vs increased brightness of the star for the ease of planet detection.

  • Adam Crowl May 11, 2018, 19:02

    Hi Alex & Paul,
    Alex, nice, quick write up of that paper, which only just hit the preprint server. Curiously counter-intuitive results, which does make M dwarfs seem somewhat less likely as locales for biocompatible planets. The sequel will be very interesting indeed.

  • Robin Datta May 11, 2018, 21:06

    Methanogens use nickel in the center of a tetrapyrrole ring, similar to iron in heme in hemoglobin,
    magnesium in ptotosynthesis and cobalt in some other processes.

    Methanogens concentrate the lighter isotopes of nickel,
    which could be a possible biomarker for prior methanogenic activity if physical samples of planetary material (as from Mars) could be obtained.

  • Nicky May 12, 2018, 3:10

    CH4-CO2 atmosphere is actually a theory that was proposed to solve faint young sun paradox, before the Great Oxidation Event. But as soon as the atmosphere is filled with oxygen, it would strongly lower the concentration of CH4.
    As we all know, oxygenic photosynthesis is the most important prerequisite in the emergence of complex life. CH4-CO2 atmosphere would eventually come in conflict with the evolution of life. Briefly, CH4-CO2 atmosphere would only function on a planet that has methanogens being the most common life or a planet that has not yet develop complex life such as plants and metazoans.

    • Michael May 12, 2018, 16:55

      “CH4-CO2 atmosphere is actually a theory that was proposed to solve faint young sun paradox, before the Great Oxidation Event. But as soon as the atmosphere is filled with oxygen, it would strongly lower the concentration of CH4.”

      Methane has a lifespan of around 9 years in our atmosphere now and gives some greenhouse gas potential. 2-3-4 billion years ago not only was there more active volcanoes and life producing methane but there was much less oxygen to break it down with and it was also cooler. Reaction rates are a lot slower when it is cooler as they do not show a linear relationship with temperature. So I can’t see why the methane in the early atmosphere could not keep the Earth warmer even with less light from the Sun as there was simply a lot more of it around.

      • Nicky May 12, 2018, 19:14

        I don’t see where your point is. I have said that CH4-CO2 atmosphere is a potential solution to faint young sun paradox, and I do not deny its greenhouse effect. I totally agree that CH4-CO2 were the main greenhouse gases during Archean and kept Earth surface above freezing-point.
        What I am emphasizing here is after Great Oxidation Event or rise of oxygen in atmosphere on a given planet, CH4-CO2 habitable zone would no longer be applied.

        • Michael May 13, 2018, 11:39

          I was just adding a bit more info on the subject that’s all.

  • AlexT May 12, 2018, 3:44

    If life is widespread property of our Univers, in this case I suppose that every star that has a planet in habitable zone is good candidate for ET life searches.
    I frequently met opinion that tidaly locking of M-star planet is not good for the life, cannot accept this point, because exactly tidilly locked planets will have forever “evening” or “morning” zones where host star’s “solar winds” or high UV radiation will not be a problem, opposite there should be very comfort for the life climatic zones with acceptable UV and solar winds level…

    • J. Jason Wentworth May 12, 2018, 19:30

      Plus, if such M red dwarf planets can hang on to their atmospheres (by being Super-Earths, having strong magnetic fields [if their orbit and thus rotation durations are short], being moons of Hot Jupiters with strong magnetic fields, etc.), the atmospheric winds would equalize the day-side and night-side temperatures, as happened on Venus. Venus’ rotation is so slow (243 days, while the atmosphere circles the planet every 100 hours or so) that for all practical purposes, its rotation might as well be tidally locked.

      • AlexT May 13, 2018, 4:56

        Agree with this addition. I suppose there is possible many more other variants.
        If we are ready to search life on the Mars and Europa, I suppose we must not exclude M dwarfs.

        • J. Jason Wentworth May 14, 2018, 1:05

          Thank you. Also (your mention of Europa reminded me), tidally locked M dwarf planets *would* have the “astronomically legendary hermian twilight zone,” which Mercury was assumed to have until the 1965 radar astronomy observations revealed its true 58.646-day rotational duration (its spin-orbit resonance *did* effectively mislead astronomers, by causing virtually the same hemisphere to be presented when Mercury was at the same point in each orbit). If a tidally locked M dwarf planet had polar crater-shadowed ice and/or starlit but clement twilight limb regions, life would be possible, especially in subsurface micro-climates, and:

          A somewhat more distant (far enough away to have a Europa-like ice-surfaced ocean) planet could have equal day and night ocean temperatures, even if it too was tidally locked so that one hemisphere forever faced its star.

  • Michael C. Fidler May 12, 2018, 7:06

    Great article, how would the planets mass effect this. Could there be an area between super earths and sub neptunes that methane would dominate?

    • Alex Tolley May 12, 2018, 12:58

      I will leave that question to the experts. However, if primordial atmospheres are N2-CO2-CH4-H2O then the moon Titan has a N2-CH4 atmosphere. On Earth, most of the CO2 eventually was locked up as carbonate rocks. Methane was maintained by methanogens until photosynthesis resulted in free O2 that resulted in its oxidation. A guess would suggest that surface temperature would determine the gas mixes, as some gases will freeze out – water, then CO2, then methane and lastly nitrogen.

    • Ramses Ramirez May 12, 2018, 20:40

      Hi Michael. Under the assumptions we made in Kopparapu, Ramirez et al. (2014), it turns out that planetary mass does not have a big effect, at least near the outer edge (slightly bigger effect on the inner edge). The CH4 concentrations in our new study are only at the ~10 ppm – 10% level, so I do not expect that the trends we found in the 2014 paper would change much from the addition of CH4 (it is really an atmospheric pressure effect here and the CH4 pressures considered are comparatively low). That is, the planetary mass effect would really be dominated by the rest of the background atmosphere (i.e. the N2 + CO2 mole fractions).

  • Harold Shaw May 12, 2018, 8:56

    Tidally locked planets orbiting red giants are often described as being too hot for life. Could atmospheric methane make them more temperate?

    • Ramses Ramirez May 13, 2018, 15:26

      Hi Harold..Tidally-locked HZ planets would probably be observed only around M-dwarfs (possibly some K-stars). However, the observable universe is not old enough to host red giant M-dwarfs..yet.. Plus, very cool M-dwarfs bypass the red giant stage entirely, becoming blue and then white dwarfs instead….

      That said, any tidally-locked HZ planets would be pretty close in to the star and likely be consumed by the expanding red giant (as we showed in our 2017 paper) so CH4 would not help in that case.

      • Harold Shaw May 14, 2018, 9:26

        Thank you for responding with such patience. I must have been distracted when I posted the question because I intended red dwarf not red giant. If a planet tidally locked to a red dwarf is able to maintain an atmosphere, winds could circulate heat from the day side to the night side, making the planet more temperate. Would CH4 have a significant impact on how well the atmosphere cooled the planet’s surface?

        • Ramses Ramirez May 14, 2018, 16:58

          Hi again Harold. Thanks for the clarification. For the very dense multi-bar Co2-CH4 atmospheres that characterize planets near the outer edge, day-night atmospheric heat transport should be quite efficient – assuming that the planet is able to get warm enough before CH4 enters the system- and so the result that CH4 cools for planets orbiting M-dwarfs would still be true even if such dynamical considerations are included.

  • Geoffrey Hillend May 12, 2018, 17:19

    There is also the photolysis or photodissociation of CH4 into Carbon and Hydrogen. There is doubt whether abiotic CH4 remained there long enough to to have an impact on life. Co2 yes. Methane only exists on Titan because it is sufficiently far from the Sun to have low enough temperature to keep its primordial methane. If you raise the temperature to a Mars or Earth like environment, Titan would loose most of it’s atmosphere and all of it’s methane. The reason it that Titan is smaller than Mars and has a lower escape velocity which controls how much atmosphere a planet can have at a given temperature. If the temperature is low, then the escape velocity matters less. The escape velocity is dependent of the mass of the planet and it’s surface temperature which is dependent on the amount of Sun light it gets based on distance from the Sun.

    • Nicky May 12, 2018, 19:28

      Two major abiotic sources of methane are serpentinization and volcanism, which can potentially replenished the loss.

  • J. Jason Wentworth May 12, 2018, 19:52

    The results of their modeling suggest that outer-HZ planet life around M class dwarf stars (the very planets that would seem most able to retain their atmospheres) might be more dominated by internal heat sources such as volcanic vents under oceans (if any) and volcanoes, geysers, and hot springs on land (with subsurface micro-climates being possible even if there is little atmosphere). The results for A class stars are positively surprising; taken all together, they suggest that life-seeking would be a worthwhile scientific objective for probes sent to M through A stars.

  • J. Jason Wentworth May 13, 2018, 1:35

    This article inspired a “crazy” thought:

    Could M red giants perhaps form “second generation” planetary systems? Especially if such a star was previously subject to one or more (ordinary) nova detonations soon before entering the red giant phase, maybe the original planets would be vaporized, and the starlight pressure–especially from the red giant–would push this debris far enough outward to re-coalesce into new planets. If so, perhaps some of such “phoenix planets” would be in the red giant’s habitable zone, maybe with enough remaining time for at least microbes to arise on them?

  • ljk May 14, 2018, 10:58

    Red dwarf star systems may not turn out to be great places for native life forms, especially complex intelligent beings, even if these suns are very long-lived.

    However the very idea that they may not be good for life and therefore have either none or just simple organisms could mean they would be ideal realms for ETI with interstellar societies who need resources that are not already “claimed” by others. We already know that red dwarf stars do have planets and no doubt moons, comets, and planetoids as well, so that part at least – the motive – is solved.

    So we should be on the lookout for these advanced beings in red dwarf systems, who if they are not transmitting METI beacons or signals directly at the Sol system, should be making some kind of electromagnetic noise and kicking up dust as they mine the worlds of such systems. They could even be settled there, so we should also be on the lookout for the pollution of a technological civilization such as light, geosynchronous satellites, and actual pollution.

    • ljk May 14, 2018, 10:59

      We should also check red dwarf stars directly for any traces of artificial elements, items not normally found in the spectrums of such suns. ETI may use those stars as dumping grounds for nuclear and other forms of high-tech waste.

    • ljk May 14, 2018, 11:08

      An addendum to my above comment about METI beacons:

      If an alien society with interstellar travel capabilities did want to build and operate a METI beacon for whatever reason, they might want to place it in a stable part of an unoccupied red dwarf system and not their own. The reason being that in case they did attract some unfriendly attention, these recipients would not be led towards their home system.

      Now there is a reason to set up beacons around the galaxy in remote places: To flush out and lure hostile interstellar-capable ETI without risking a direct attack. The beacon makers can then decide what to do about these unpleasant neighbors while still in hiding.

      • AlexT May 14, 2018, 12:25

        Very clever ETI build fly traps for a stupid ETI, it seams that “clever” should be very rich and usually spend resources for fun…

        • ljk May 14, 2018, 13:51

          Indeed, the reverse could be true that an ETI could set up an interstellar beacon to attract and capture any visiting aliens, or detect a species that responds to their beacon and pays them a visit.

          The potential reasons include finding and eliminating any competition for galactic resources. Or grabbing new technologies without having to do the actual work themselves. Or both.

          Check out this science fiction radio program from 1956 titled “Junkyard”:



          • AlexT May 15, 2018, 1:39

            Me too very love to read lot of science fiction , but never mix SF with reality :-)
            Meanwhile we do not how any evidience of ETI existence (no signals, no beacons, no bacons) , the only fact we have is – Fermi paradox …

            • ljk May 15, 2018, 10:44

              Yes, but we can still discuss concepts because not only is it good mental exercise and helps us think about and from various perspectives, but also because saying “We have no proof of alien life” over and over is a boring and futile dead end.

              And funny, but science fiction when done right is an excellent and safe place to play “What if” scenarios.

              • AlexT May 16, 2018, 4:10

                “saying “We have no proof of alien life” over and over is a boring and futile dead end”
                Strange that obvious fact – means “dead end” to you…
                Fact should have some scientific explanation and we should try to build some realistic model of our Univers and physical laws based on this fact.
                There is additional fact – our own civilization, this is god prove that intelegent life is possible in our Univers, as sequence it is hard to beleive that we are alone.
                Now you can swtch on your imagination and try to find explanation to this two facts.
                wandering why those facts from our reality means “dead end” for you.

                • ljk May 16, 2018, 10:14

                  You can keep saying we have no proof of alien life and leave it at that, or we can do something about it. Simple as that.

                  • AlexT May 17, 2018, 1:56

                    Meanwhile on our present state of technology and science level Alien life (not inteligent one) can be found only by direct space exploration by manned or unmanned space ships, distant observation can giva us only some hints of evidence that have to be proved physically, visiting those ET worlds.
                    Sorry , but SETI concenrates efforts for extraterrestrial intelligence, most probably that non intelligent life forms cannot communicate with us throughinterstellar distances.
                    I am sure that extraterrestrial life searches many order more important that SETI, SETI it is only small fraction from this task.

  • Geoffrey Hillend May 14, 2018, 18:07

    I don’t think methane will be a false positive. I depends on the rate of methane loss. Any replenishment won’t hang around more than 10,000 years. It will quickly be lost so that the loss rate has to be slower than the emission or gain rate and it’s probably not. The only controversy will be if we do see all the spectral biosignature gases from an exoplanet around an M class dwarf star. If we don’t or see only Co2 and no O2 or CH4, there won’t be any controversy.

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