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Kepler-62f: Models for Habitability

So often planets described as ‘potentially habitable’ turn out to be over-rated — we look deeper into their composition and characteristics only to find that the likelihood of liquid water on the surface is slim. How to make more accurate calls on the matter of habitability? One way may be to combine orbital and atmospheric models, adjusting each with the known parameters of the planet in question. A new study does just that for the interesting world Kepler-62f.

About 1200 light years from Earth in the direction of the constellation Lyra, Kepler-62f has a radius 40 percent larger than Earth’s, which puts it well below the 1.6 RE demarcation line that is increasingly thought to define the difference between Earth-like worlds and planets that are more like Neptune. We’re probably looking at a rocky planet here. It’s also a planet that orbits its K-class primary at a distance that could place it in the outer regions of the habitable zone (as defined, again, by the presence of liquid water on the surface).

kepler_62f_rendering

Image: Kepler-62f, shown here in an artist’s rendering, is far enough from its star that its atmosphere would need a high concentration of carbon dioxide to maintain liquid water on the planet’s surface. Credit: NASA Ames/JPL-Caltech/T. Pyle.

The new work on this world was conducted by Aomawa Shields (UCLA), working with Rory Barnes, Eric Agol, Benjamin Charnay, Cecilia Bitz and Victoria Meadows (all at the University of Washington, where Shields received her PhD). It examines the planet’s atmosphere to consider scenarios that could produce habitability. The team modeled climate possibilities using two different methods — the Community Climate System Model and the Laboratoire de Météorologie Dynamique Generic Model. Informing the climate modeling was the use of another computer model, HNBody, which was used to analyze and adjust the planet’s orbital parameters.

This UCLA news release calls the work the first time astronomers have combined the two different kinds of models to study exoplanet habitability. Says Shields:

“We found there are multiple atmospheric compositions that allow it to be warm enough to have surface liquid water. This makes it a strong candidate for a habitable planet.”

The necessary composition involves a great deal more carbon dioxide that we find in Earth’s atmosphere, necessary to keep the surface from remaining in a deep freeze. Remember, the planet circles a K-class star cooler than the Sun, and at a distance further away from the primary than the Earth from our star. By adjusting carbon dioxide levels in the atmospheric model, the team learned that given various orbital configurations, there were circumstances where habitability was possible. Some configurations work better than others, but the team found that even with an Earth-like level of carbon dioxide, there were rare but possible orbital configurations that allowed at least part of the year to be warm enough for liquid water.

From the paper:

At 41% of the modern solar constant, this planet will likely require an active carbonate silicate cycle (or some other means by which to produce high greenhouse gas concentrations) to maintain clement conditions for surface liquid water. With 3 bars of CO2 in its atmosphere and an Earth-like rotation rate, 3-D climate simulations of Kepler-62f yielded open water across ∼20% of the planetary surface at the upper limit of the stable eccentricity range possible for the planet, provided that it has an extreme obliquity (90°). With 5 bars of CO2 in its atmosphere, a global mean surface temperature similar to modern-day Earth is possible for the full range of stable eccentricities and at the present obliquity of the Earth. This higher CO2 level is therefore optimal, as it is below the maximum CO2 greenhouse limit, and generates habitable surface conditions for a wide range of orbital configurations throughout the entire orbital period of the planet. If Kepler-62f is synchronously rotating, CO2 concentrations above 3 bars would be required to distribute sufficient heat to the night side of the planet to avoid atmospheric freeze-out.

There is a faint chance that with CO2 levels like the Earth’s, Kepler-62f could experience surface melting of ice sheets on an annual cycle. This would depend on a high degree of axial tilt, or obliquity — the angle between the planet’s rotational axis and its orbit. The Earth’s obliquity is 23 percent. An axial tilt of 60° or higher, coupled with summer solstice at a given hemisphere occurring at the planet’s closest approach to its star, could produce this annual effect, allowing surface conditions that were at least periodically habitable.

The paper is Shields et al., “The Effect of Orbital Configuration on the Possible Climates and Habitability of Kepler-62f,” published online by Astrobiology 13 May 2016 (abstract / preprint).

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Comments on this entry are closed.

  • Andrew LePage May 27, 2016, 13:28

    Even for someone like me who is skeptical about the majority of habitability claims that have been made about exoplanets readily admits that Kepler 62f is among the best candidates for being “potentially habitable” among currently known exoplanets. At least in terms of size, I think that only Kepler 442b and the venerable Kepler 186f have slightly better claims:

    http://www.drewexmachina.com/2016/04/17/habitable-planet-reality-check-kepler-186f-revisited/

    http://www.drewexmachina.com/2015/01/08/habitable-planet-reality-check-8-new-habitable-zone-planets/

    • Michael May 29, 2016, 16:05

      There are two things that could aid the habitability of this red dwarf planet, Kepler 186f. (0.765 nm peak emission 3788 K)

      1) The long contraction phase which would have driven off the H/He volatiles so no dense atmosphere, provided the water was not broken up.

      2) The blackbody temperature shifts in favour of longer wavelengths where snow and ice and water are not as reflective so more light is absorbed by the surface.

      http://fatberg.nl/wp-content/uploads/2015/06/snow_reflectance.gif

      • Harry R Ray May 31, 2016, 9:52

        NOT SO FAST! The most RECENT paper on this subject apparently LOWERS the thick hydrogen envelope cut-off from 2 Earth mass(Kipping, Cheng et al)to JUST BELOW 1 VENUS MASS, for planets orbiting MV stars(like Kepler 186f, orbiting at MERCURY-like DISTANCES from early-type MV stars, AND; at TRAPPIST-1b and c-like distances from late-type MV stars like Proxima Centauri. IF THIS IS TRUE, then, to be POTENTIALLY habitable, Kepler 186f MUST have a DENSITY of LESS THAN 3gcm3, and would ALMOST CERTAINLY HAVE AN OCEAN THOUSANDS OF KILOMETERS DEEP! The main problem with that scenario would be the TRANSFER od chemicals from the crust to the ocean, due to the fact of an extremely substantial Ice-7 layre BETWEEN the crust and the ocean. I would apreaciate the opinion of someone more knowledgible than I on this(Andrew Le Page et al), because I HOPE THAT I AM WRONG with my interpretation, because of the potentially HUGE IMPLICATIONS it would have if an EARTH-sized planet in the habitable zone HAS been detected around Proxima Centauri(which, in my OPINION ONLY, is STARTING to appear that it HAS, but I am going to HOLD OFF until LATER THIS WEEK until I give a more DEFINITIVE STATEMENT on this). The GOOD NEWS is that we may ALREADY KNOW whether this NEW PAPER holds any weight at all, doe to HST’ May 4th observations of the TRAPPIST-1 system.

        • Michael May 31, 2016, 14:42

          This is the paper, http://arxiv.org/abs/1603.08614

          It states the cut off at around 1.23Re for the onset of light gas run away accretion which is much more massive than Venus. I can find no study of the effect of the stars zero MS effect on the ability to affect the mass of light gas losses. Perhaps some in depth simulations on how M,K,G stars affect the light gases during this important period is needed. Red dwarfs have long early luminous periods with high UV outputs and would have a greater effect on gas uptake than a G type star would at the same light flux.

          • Harry R Ray May 31, 2016, 18:01

            Michael: The paper you mentioned above is the one I STILL TEND TO BELIEVE< but the NEW paper is as follows: "Habitability of terrestrial mass planets in the HZ of M dwarfs-I. H/He dominated atmospheres" by James E Owen and Subhanjoy Mohanty in the Monthly Notices of the Royal Astronomical Society. What is FRUSTRATING is all that was available was the ABSTRACT(i.e. NO PDF). Also, it JUST dealt with planetary MASS(i.e. NOT planetary RADIUS)! I URGE you, Andrew LePage et al to obtain the entire transcript and read it in its ENTIRETY!

            • Michael June 1, 2016, 3:15

              Is this it?

              http://arxiv.org/abs/1601.05143

              I will have a read through it when I get some spare time.

              • Harry R Ray June 1, 2016, 9:44

                YES! THAT’S IT! Unbeknownst to me, this paper appeared on arxiv BEFORE the Kipping/Chen paper did! I must have just breezed through the abstract at the time, and did not think enough of it to read the PDF, because I PROBABLY did not think it would be accepted because it was SO CONTRATY to the generally accepted view. Having read the PDF THIS TIME, the ONLY THING REALLY CONTRARY is that the authors believe that the “Jeans Escape” mechanism does NOT remove the MAJORITY ot the hydrogen unless, ONE A 2/3rock-1/3iron core planet is less massive than Venus, or; TWO, an Earth mass(or slightly higher than earth mass, as is PROBABLY the case with Kepler 186f)planet has a PREDOMINANTLY ICY CORE! Please read the PDF ASAP and reply. Thanks!

                • Michael June 1, 2016, 18:53

                  I have just glanced through it so I have not completely absorbed it. But I can’t see at the moment why the Earth lost its H/He atmosphere and not an Earth mass object recieving the same amount of light flux around a M type star when there is a powerful luminous pre MS stage as well.

                  Unless the moon formation event had something to do with it.

                  I have often wondered if they ever took into consideration the CO2 effect on the H/He blanket when the IR light hits it as it would tend to absorb the light at the mixed boundary of the light and heavy gases and push it upwards further out of the g field allowing easier escape.

                  It is an interesting article which if true would put a huge damper on large habitable planets around stars but there is also the chance of smaller planets that could avoid the light gas run away effect, say like a bigger Mars.

                  I will read a bit more…

                  • Harry R Ray June 2, 2016, 9:35

                    It may be that the ONLY WAY tostrip an Earth-mass planet of its Hydrogen envelope is for an extremely nearby supernova(or maybe even a hypernova explosion is necessary)explosion to do it at the PRECICE TIME WHEN THE ENVELOPE IS THE MOST INFLATED! The implications of this would be HUGE, and that’s why I am waiting for someone like Andrew LePage to read the paper and report back to ALL OF US with THEIR take on this.

                  • Harry R Ray June 6, 2016, 9:42

                    The May 4’th double transit of TRAPPIST-1b and c apparently gave CONVINCING EVIDENCE that NEITHER planet has a thick hydrogen envelope. To me, this is the MOST INCREDIBLE EXOPLANET OBSERVATION made to date, and it just whets the appetite for what kinds of observations JWST can make! As for what this means, if Owen and Subhanjoy’s theory is CORRECT, BOTH PLANETS MUST HAVE DENSITIES LESS THAN 3g/cm3! We will have to wait for ESPRESSO to pin down the masses!

  • Alex Tolley May 27, 2016, 13:39

    3-5 bars of CO2? Where would this amount of CO2 come from? It is certainly much higher than with the paleo earth. It seems to be hand-waved with high CO2 out gassing and weak geological sinks, with no quantitative support for this scenario.

    In addition, no mention of a primordial CH4 atmosphere that could have made this a more realistic greenhouse scenario.

    Because they assume a water world, plant life cannot much reduce the albedo to aid warming (although dark, floating mats might help in the polar latitudes), and their fixing of carbon must not materially reduce the atmospheric CO2 levels.

    My take home is that this paper contrives to make this potentially planet habitable with rather heroic assumptions. If it is a rocky world, it is more likely to be a frozen snowball.

    Which opens up an interesting question. As there is increasing interest in the icy moons possibly having subsurface life driven by hot hydrothermal vents, could a more Earthlike world, frozen into a snowball, harbor life at deep ocean vents, isolated from the need for surface temperatures?

    [ Here is a Nature letter on early Earth CO2 and temperature: A lower limit for atmospheric carbon dioxide levels 3.2 billion years ago that puts this in some context.]

    • Andrew LePage May 27, 2016, 14:30

      > 3-5 bars of CO2? Where would this amount of CO2 come from? It is certainly much higher than with the paleo earth. It seems to be hand-waved with high CO2 out gassing and weak geological sinks, with no quantitative support for this scenario.

      There is no handwaving involved here whatsoever (other than assuming that Kepler 62f has a volatile inventory within a order of magnitude-ish of that of the Earth). It is widely accepted that the Earth has a CO2 inventory on the order of 60 bars (comparable to Venus’ 90-bar inventory) with the majority of that currently tied up in climatically inert carbonate deposits as a result of the actions of the carbonate-silicate cycle removing excess CO2 out of the atmosphere (aided by biological activity). It is not unreasonable to assume that an exoplanet like Kepler 62f could have 3 to 5 bars of CO2 in its atmosphere assuming that it has even a fraction of Earth’s (or Venus’ original) volatile content and enough geologic activity to support the carbonate-silicate cycle.

      • Alex Tolley May 27, 2016, 21:09

        Inventory is one thing, but you have to get there from the initial conditions. Both Venus and Earth started with moderate CO2 atmospheres, but in Venus’ case, a runaway greenhouse effect resulted in the current high pressure CO2 atmosphere. In Kepler-62f. we have the opposite problem, a world that is too cold and needs to increase its CO2 pressure just to keep the planet from freezing. You seem to be saying that just because there is enough carbonate to create a dense, CO2 atmosphere, that this is sufficient to allow the assumption of such a scenario. However you need a plausible mechanism to release that Co2 and the authors do not provide one beyond a vague suggestion of high source (volcanic emissions) and low sink (weathering).

        • Andrew LePage May 28, 2016, 9:23

          While this particular paper is vague about it, the “plausible mechanism to release that Co2” would be the well-known carbonate-silicate cycle. There is not the room here to review decades of peer-reviewed literature on the subject but I would suggest that you read the book “How to Find a Habitable Planet” by James Kasting who is one of the scientists responsible for developing our modern concepts of Earth-like habitability and the role of the carbonate-silicate cycle. The book, which is still available, is an excellent read and fully referenced in case you wish to dig deeper. As long as Kepler 62f has an appropriate initial volatile inventory and has sufficient geologic activity to maintain the carbonate-silicate cycle (and there are no serious “gotchas” that we do not know about), there is no reason not to expect that this world could have a 3 to 5 bar CO2 atmosphere.

          • Alex Tolley May 28, 2016, 14:32

            Thank you Andrew. I have the book in my library and have read it.

            Venus is our best local model here, as it wasn’t disrupted by the formation of the Moon. If Venus started with a multiple bar CO2 atmosphere, then I accept that the posited model is plausible.

            Here is the relevant Wikipedia piece on Venus’s early atmosphere:

            Studies have suggested that billions of years ago Venus’s atmosphere was much more like Earth’s than it is now, and that there may have been substantial quantities of liquid water on the surface, but after a period of 600 million to several billion years,[61] a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere.[62]

            From the 1987 Kasting paper referenced:

            The background atmosphere assumed in
            most of the calculations was an Earth-like
            N2-O2-CO2 atmosphere without any 03.
            Ozone would presumably be destroyed in a
            warm, moist atmosphere by the by-products
            of water vapor photolysis. Sensitivity
            calculations were performed to determine
            the effect of higher concentrations of CO_,.
            The surface pressure was assumed to be
            given by
            P~ = 1 + pCO2 + pH20, (2)
            where pH20 is the saturation vapor pressure
            of water at the surface temperature T,.
            The relative humidity within the moist
            convective region was assumed equal to
            unity, in accord with my goal of estimating
            upper limits on T~. Although this overestimates
            the amount of water vapor in the upper
            troposphere of the present Earth, it is a
            reasonable choice for an atmosphere in
            which water vapor is a major constituent.
            In most cases of interest here water vapor
            is in fact the dominant atmospheric constituent,
            and the relative humidity could not
            have fallen much below unity without violating
            the barometric law. The water vapor
            mixing ratio in the stratosphere was set equal to its value at the top of the convective zone.

            IOW, Kasting posits an early Earth-like atmosphere, although why he assumes this when O2 was almost absent on a prebiotic Earth I don’t know. However he states that H2O is the dominant gas in the atmosphere compared to CO2.

            Although this overestimates
            the amount of water vapor in the upper
            troposphere of the present Earth, it is a
            reasonable choice for an atmosphere in
            which water vapor is a major constituent.
            In most cases of interest here water vapor
            is in fact the dominant atmospheric constituent,
            and the relative humidity could not
            have fallen much below unity without violating
            the barometric law.

            And:

            Figure 2 shows net upward infrared and
            net downward solar fluxes for a moist
            greenhouse atmosphere with a surface temperature
            of 100°C. The surface pressure of
            this atmosphere is 2 bar, half of which is
            attributable to water vapor and the rest to
            N2 and 02.

            My reading of the paper is that water vapor is the dominant gas in the runaway greenhouse, with CO2 having some effect when around 10 bar after the oceans have evaporated and the weathering process stops the CO2 sink.

            My take home is this:
            1. Kasting’s model for an early Venus is a low bar atmospheric pressure, with greenhouse gases dominated by H2O.
            2. The high bar CO2 atmosphere is a result of a runaway greenhouse, not a starting condition.

            If we look at his book (copyright 2010) he uses essentially teh same data. The paper Figure 4 and the book Figure 6.2 show atmospheric pH20 versus surface temperature. The high pH20 on Venus uis due to the runaway greenhouse due to the high soilar radiation flux. This does not happen on Earth or Mars due to teh lower solar flux.

            This indicates that you cannot get a high pressure CO2 atmosphere on a water world, and that you need a runaway greenhouse to stop weathering to allow CO2 to accumulate, as it did on Venus.

            Back to Kepler-62f. We have a posted water world that is not being subjected to a runaway greenhouse , quite the reverse. It’s atmosphere will probably start out just like a primordial Venus, albeit with a different pressure and likely increase in lighter gases. CO2 will not be at high pressure because of weathering and absorption into the oceans. Claiming otherwise is just hand waving, IMO. That this is a plausible state has to be shown, rather than stated as an assumption. The low solar radiation would mean a snowball world, with a high albedo keeping it in that state. Because it has a high mass, it would not dry out like Mars, but would look rather like the snowball Earth.

            More recent work on early Earth atmospheres suggest that other GHGs are needed to prevent a snowball condition when the sun was cooler, notably CH4, H2. Those might be an option for K-62f, but the authors do not model this.

            Therefore the evidence suggests that K-62f, as a large mass, rocky, water world probably could not have had a dense, CO2 atmosphere to keep its surface warm, nore a dense water vapor atmosphere, as it was too cold to support it. CO2 could not accumulate due to carbonate rock subduction as the abundance of water would maintain a high weathering rate. The author’s need to cite evidence for that claim, otherwise it is as fanciful as claiming Venus is habitable (i…, if…, etc.

            I think the ball is in your court to provide a reference that has a model for a high bar CO2 atmosphere as a primordial atmosphere that would justify the initial conditions for K-62f.

            • Andrew LePage May 29, 2016, 11:22

              Section 3.3 “Outer Edge of the HZ (OHZ)” of Ravi Kumar Kopparapu et al., “Habitable Zones Around Main Sequence Stars: New Estimates”, ApJ, Vol. 765, No. 2, Article ID. 131, March 10, 2013

              • Michael May 29, 2016, 13:15
              • Alex Tolley May 29, 2016, 15:03

                In that paper, Kasting again just assumes a high bar CO2 atmosphere. AFAICS, the assumption is that the frozen surface means a reduced CO2 sink, allowing CO2 to build up.

                Further digging produces this paper by Kasting on the early Earth where he also suggests a high bar CO2 atmosphere if the Earth was an ocean planet. Also here.

                This is mirrored by Léger et al with their paper on ocean planets, which assumes that planet formation from icy planetismals has high CO2 content (5%) and that this results in a dense atmosphere, again due to low weathering to prevent a CO2 sink.

                So, pending evidence to the contrary, I concede that a high pressure CO2 atmosphere is a possible condition for Kepler-62f which under certain conditions outlined in their paper, makes it potentially habitable.

                • Michael May 30, 2016, 12:53

                  From the paper a 10% H2 addition has quite a positive GH effect on Mars. It is more than likely if there is a lot of H/He in the K 62f atmosphere it would likely have a high surface temperature which would drive out all the CO2 as well. I can’t see any way around the nebula H/He been driven away and therefore is more than likely is still there plus the CO2 inventory.

        • Rob Henry May 29, 2016, 20:51

          This is very similar to the problem of Snowball Earth. This possibility was missed by geologists for a long time, despite considerable evidence, simply because it looked impossible to emerge from it.
          http://www.snowballearth.org/end.html

          The answer is, yes Earth can build up levels of atmospheric CO2 to the needed ~0.3 atm partial pressure in just a few tens of millions of years (or we wouldn’t be here). So 5 atm CO2 within a billion of so doesn’t seem that far fetched to me.

    • andy May 27, 2016, 17:22

      Have any negative (stabilising) climate feedbacks been identified for water worlds? As far as I’m aware, all the climate feedbacks are positive: ice-albedo feedback, the temperature-dependent solubility of carbon dioxide in water and the increased greenhouse effect of water vapour would tend to cause the planet to end up in either a snowball glaciation or a runaway greenhouse. Barring a much stronger dependence of the rate of seafloor weathering on surface temperature than seems to be expected, it looks like you need exposed land surface in order to have the negative feedback of a silicate-carbonate cycle.

      • Alex Tolley May 27, 2016, 21:00

        Clouds increasing albedo with greater water saturation. But this is complicated, as it depends on where the clouds are in the atmospheric column, and their type.

        • Eniac June 4, 2016, 0:51

          Photosynthetic organisms removing CO2 from the atmosphere and turning it into organic sediments.

  • Ashley Baldwin May 27, 2016, 13:56

    This is indeed a comprehensive analysis working on Rogers et al 2015 study that sets the “Terran” ,Earth like rocky and Neptune gaseous planet divide at about 1.6Re . This has recently been challenged in a detailed probabilistic analysis by Chen and Kipping in March that suggests thus divide occurs at around a much smaller 1.25 Re or about 2.7 Mass Earth plus or minus a bit. Previously planets orbiting M dwarfs were thought to have their atmospheres stripped by aggressive stellar winds and Coronal mass ejections /stellar flares and such the like the as to these stars unpredictable early and pre main sequence evolution. So much so that the concept of “habitable evaporated cores ” was invented whereby a planet that started life as a mini Neptune has most of its thick atmosphere stripped away to become a “habitable core “.
    This in turn has also been challenged just recently by Owen et Al study from Imperial College and Princeton suggesting that in larger than Earth mass planets then gravity still wins out and they maintain their greenhouse primordial Hydrogen/Helium warming atmospheres and become Venus like , , climate wise anyway in the traditional ” Habitable zone” anyway . Venus sized planets or somewhat smaller become the new Earths instead with both new theories effectively making Earth the new Super Earth. Confused ? I am . Who is right, who is wrong ? No one and everyone .Who knows in this battle of (well meaning) simulation programmes . As good as example of why theory alone , however clever , however well thought out , can never trump direct evidence. It’s time to get looking . TESS should find some nearby M dwarfs planets that stay in the JWSTs sights long enough to allow enough transit data to accumulate over a realistic time ( 3 years say rather than an unlikely ten in the case of the 18 day period of the outermost TRAPPIST planet ) . Then it’s down to JWST and transit spectroscopy followed closely and hopefully by a stretch goal WFIRST that can image and characterisenearby near Earth like /sized ( not necessarily interchangeable ) planets in whatever turns out to be the “habitable zone” and according to which simulation you believe . Theory is vital I should add to create the paradigm in which to interpret observational data, which is just as limited on its own too. At present thought , anything goes . Or doesn’t.

    • Andrew LePage May 28, 2016, 9:49

      > Rogers et al 2015 study that sets the “Terran” ,Earth like rocky and Neptune gaseous planet divide at about 1.6Re . This has recently been challenged in a detailed probabilistic analysis by Chen and Kipping in March that suggests thus divide occurs at around a much smaller 1.25 Re or about 2.7 Mass Earth plus or minus a bit.

      You seem to be misinterpreting the results of these two papers. Rogers work found that the 50-50 divide between rocky and volatile-rich worlds (i.e. half are terrestrial and half mini-Neptunes) occurs at a radius of no greater than 1.6 RE with a 95% probability and that the actual divide was likely to be ~1.5 RE. The recent work by Kipping and Chen found that below radii of ~1.2 RE, virtually all exoplanets are rocky. Rocky worlds with radii greater than 1.2 RE are possible but are increasingly improbable as radius increases while mini Neptunes quickly dominate. The two results are NOT conflicting and rocky planets with a radius of 1.4 RE like Kepler 62f are possible and maybe mildly favored over volatile-rich worlds.

      • Ashley Baldwin May 28, 2016, 17:06

        If “conflicting” is too strong I’ve not illustrated my point correctly . The thing is that there is only so far you can go with simulation which can only really be as good as the observational data fed in. That’s what shifts “probabilities ” of whatever parameter defined regardless of what is allowed under the curve at the extremes . (The general shift over time though does appear to be downward in both mass and radius as suggested , real life admittedly crude bulk density estimates when available do have more extreme examples or outliers of course , call them what you will . ) However reliable “bulk density” is as a concept.
        All these simulations fit or don’t and for everyone stating one thing, another , just as robust , another shifts the paradigm further in a different direction ( ” overlapping” if not conflicting ) . This creates the impression of “improvement ” in observational evidence when in fact it is a different statistical way of reviewing the same data or simply a simulation of simulations or meta analysis or whatever it gets called .
        As is usually case extensive observational data , when it finally arrives , will have an element of surprise in it.( though it will probably fit with the ranges of at least one simulation)
        My message is to be wary of simulation , however laudably done as here in sure , but taken to extremes by feeding raw parameters into a scoring system .
        It’s a frustrating delay without dedicated technology and constantly having to extrapolate from the nearest available or nearest interpretation .
        I personally hope and expect that life can survive in all the manner of situations , radii, atmospheres and perhaps even temperatures invoked however improbable . If only because the elements and molecules that make it up are so ubiquitous regardless of bars of CO2 or Earth radii/mass. Until we see a large observational data set in terms of characterisation though these simulations are limited to the sway of probabilities via simulations . Unavoidable the way things are and necessary as they are as long as their limitations are recognised . This is the danger of arXiv ( wonderful as I think it is ) sometimes I think to none statistical experts . Whether that be the mass and radius at which a planet develops a thick , “greenhouse” atmosphere or even a thicker neptune like gaseous envelope or whatever CO2 pressure that allows suitable temperatures and pressures- Kasting if I recall looked at this in relation to Mars too with a favourable pressure , climate and temperature range reached with just 2 bar of CO2.

        • Mark Zambelli June 1, 2016, 5:19

          “I personally hope and expect that life can survive in all the manner of situations , radii, atmospheres and perhaps even temperatures invoked however improbable”

          Ashley, you are not alone there.

  • Paul Topping May 27, 2016, 15:38

    Perhaps we need classifications like they had on Star Trek. Earth is Class M, for example.

  • DJ Kaplan May 27, 2016, 16:13

    Very interesting!

    I wonder if the term “habitable” is used as over-simplified shorthand for more complex categories. The “HZ” is a zone in which planets would receive a certain range of stellar flux; that’s only one of many factors that would make that zone habitable. And the presence of liquid water on the surface is being used as shorthand for “habitable planet”. The facts are probably more complex.

    IMHO, over-simplification is the bane of science writing.

  • Brett May 27, 2016, 16:18

    5 ATM of CO2 might be good, although that’s also the pressure at which CO2 can liquify out of the air at temperatures below -56 degrees Celsius.

  • Michael May 27, 2016, 16:53

    I feel any world that has water and a rocky interface unlikely to have a lot of CO2 as it combines rather quickly into carbonate deposits. There has to be a source and that is normally volcanoes. This world is certainly more massive ~3 times and therefore has much more internal heat than the Earth but is it enough I am not so sure for 3 bars worth.

  • Michael May 28, 2016, 5:35

    I feel this world has a lot of helium and hydrogen in its atmosphere which makes for a more thermally insulated environment. The mass of ~3 Earths is enough to start grabbing hold onto a lot of light volatiles during the formation process and then keeping them, after all Earth had a light volatile dominated atmosphere.

    For me this world is dominated by hydrogen and helium in the atmosphere which contributes to a global warming effect and is more likely to have liquid water on it surface or at least have an ice free percentage. I am surprised that a hydrogen and helium atmosphere was not looked at as well.

  • Ronald May 28, 2016, 10:29

    (Also in reply to Ashley Baldwin)
    At the same time, it now seems that M dwarfs have poorer prospects for having habitable planets:
    James E. Owen, Subhanjoy Mohanty. Habitability of terrestrial-mass planets in the HZ of M Dwarfs – I. H/He-dominated atmospheres.Monthly Notices of the Royal Astronomical Society, 2016; 459 (4)
    http://m.mnras.oxfordjournals.org/content/459/4/4088

    Quote:
    “Thus, if planets form with bulky H/He envelopes, only those with low-mass cores may eventually be habitable. Cores 1 Me, with 1 per cent natal H/He envelopes, will not be habitable in the HZ of M dwarfs.”

    • Michael May 28, 2016, 14:01

      I suspect this planet to have a thick hydrogen/helium atmosphere maybe hundreds of bars thick with a fairly high surface temperature depending on how old the star system is.

      http://arxiv.org/pdf/1401.2765v1.pdf

      Any liquid water I suspect will be under great pressure from the atmosphere but this can still be habitable, life has been found to survive to around 16000 atm’s! It is the temperature that is the main killer of habitability.

      • Alex Tolley May 28, 2016, 16:57

        Ronald’s referenced paper is for M-dwarfs, not K stars. In this case, for Earth mass worlds, a thich He/H atmosphere does not work:

        In Fig. 16, we plot the evolution of surface conditions for mfrac, i ≈ 10−2. Both ‘hot’ and ‘cold’ start cores end up with very similar surface temperatures and pressures after a Gyr, with [Tsurf, Psurf] ∼ [800 K, 2×103 bar]. Fig. 20 demonstrates that these conditions are too extreme for liquid water; any surface water here can only exist
        as a supercritical fluid.

        From the arxiv paper:

        From our results shown in Table 5, it is not comprehensible why these authors concluded that Kepler-62e and
        Kepler-62f should have lost their primordial or outgassed hydrogen envelopes despite the lack of an accurately measured mass for both ‘super-Earths’. If one assumes that both ‘super-Earths’ with their measured radii of ∼1.5R⊕ have an Earth-like rocky composition, one obtain masses of ∼3–4 M⊕ (e.g., Sotin et al. 007). As one can see from Table 5, depending on the nebula properties and accretion rates, ‘super-
        Earths’ inside this mass range can capture hydrogen envelopes containing between hundreds and several thousands EO_H. Although, Kepler-62 is a K-type star which could have remained longer in the XUV-saturation than ∼100 Myr, from our model results it is obvious that, contrary to the assumptions by Borucki et al. (2013), the XUV-powered hydrodynamic escape rates are most likely not sufficient for ‘super-Earths’ within this mass range to get rid of their hydrogen envelopes if they orbit inside the HZ of their host star.

        As I read it, a retained H2 envelope will result in a high surface temperature, obviating life for a super Earth in the HZ zone.

        • Michael May 29, 2016, 3:27

          Although the temperature is high after many billions years it gets lower, these stars can live 10’s of billions of years longer than ours. Say if one of these SE was outside of the HZ cold region and it had a moon that had developed subsurface life, that life could be knocked off and then land on the nearby SE, but that is a lot of if’s.

          But alas I believe this world to be baking hot under high pressure at the surface.

          We are getting closer, Next world please…

    • Alex Tolley May 28, 2016, 15:10

      Interesting paper. So basically an Earth mass world in the HZ of an M-dwarf is likely to have a dense He/H atmosphere that cannot be removed fast enough for habitable conditions to occur. Only rocky worlds of lower mass than Earth will lose those atmospheres fast enough to allow habitability. And that is apart from the other issues of habitability around M-dwarfs.

      If correct, and apparently the data supports the dense atmosphere issue, then M-dwarfs are not good places to find habitable worlds, even though such systems are very amenable to finding small worlds.

      • Michael May 28, 2016, 17:04

        Some organisms love hydrogen and use it in the metabolism, they could quite happily live on these high pressure worlds so long as the temperature is not prohibitive. There is also the possibility of lightning creating rich organic compounds if there is sufficient carbon about.

        http://www.popsci.com/science/article/2011-08/symbiotic-bacteria-serve-hydrogen-fuel-cells-deep-sea-mussels

        http://www2.cnrs.fr/en/1893.htm

      • Ronald May 29, 2016, 9:30

        Yes, very interesting indeed. And of course I realized that K-62 is a K2 star.

        I was wondering whether the two above matters, M dwarf runaway heating and K-62 runaway cooling, can be combined into one model. After all, the dominant parameters in both cases are planet mass and stellar flux:
        – An above threshold mass planet with HZ stellar flux, results in the retention of a dense H/He atm. leading to very high surface temp.
        – A terrestrial mass planet with below threshold stellar flux results in an atm. with too little GHG, leading to excessive cooling.

        This is, of course, not surprising because these parameters define and delimit the HZ, as so well studied by Kasting and Kopparapu et al.

        However, it seems now that the HZ could be narrower, more limited on the outside, as a result of poor CO2 warming: in the conservative definition of the HZ by Kopparapu et al., the minimum stellar flux is 0.35 solar. K-62 has 0.41 and is a rather big terrestrial planet. (Unless it has massive H/He retention) from what I understand from you, its CO2 warming will probably be deficient.

        I would like to see, some day, a diagram of planet mass versus stellar flux, and in this a HZ zone delimited.

        Which reminds me: there is a website with a graph of gas retention versus temperature and planet mass for various elements, that one can play with. It has been mentioned on CD a few times. Can somebody give me the link?

      • DavidB May 29, 2016, 22:59

        Yes, an interesting paper but I think
        their conclusions are not as general as claimed. First, the M
        dwarf they use in their modeling is AD Leo which
        has a mass of about 0.41 Solar masses which is close to the upper
        limit for M dwarfs. Even assuming
        that other M dwarfs with the same mass behave similarly with
        UV emission, lower mass M dwarfs don’t and there are many more
        low mass M dwarfs than high mass M dwarfs.

        For example, the authors mention that for AD Leo, the duration
        is a few hundred million years for the pre-main sequence period
        where the UV emission is elevated. The lower the mass of the M dwarf, the
        longer is this duration of elevated UV emission which for the 0.08 Solar
        mass TRAPPIST 1 is something like 1 Billion years. Is a Billion years
        sufficient time to help get rid of the H/He envelope around low mass
        planets in the HZ of stars like TRAPPIST 1 ? I don’t know, but I do think
        that the paper is not enough to decide one way or another. What I do
        think it is fair to state, is that the paper shows that there is a good chance that
        H/He envelopes may be common around Terrestrial mass planets in the HZ of
        high mass M dwarfs.

      • Harry R Ray June 7, 2016, 18:57

        Sub-earth mass WATER WORLDS with cloudless PREDOMINANTLY H2O atmospheres(which is one of the two REMAINING possibilities for TRAPPIST-1b and TRAPPIST-1c[please read my ABOVE comment on this matter if you have not ALREADY done so])would be potentially habitable if they were IN the HZ(which, alas, TRAPPIST-1b and TRAPPIST-1c are NOT)IF(AND ONLY IF)there were NO ice-7 layre SEPERATING the water ocean from the crust(UNLESS there is some kind of continuoud INTRA-stellar panspermia going on. What we ALL need is a comment(AGREEMENT OR REBUTTAL from an EXPERT(like Andrew Le Page, or, better yet, KIPPING and CHEN on the Owen et al paper!

  • James Stilwell May 28, 2016, 11:40

    Without our two thousand mile diameter Moon I doubt the Earth would have wound up hosting human civilization in this arm of the galaxy…Keep looking for Earth/Moon systems to find civilizations…

    • Alex Tolley May 28, 2016, 15:23

      This theory strikes me as rather anthropocentric. It may have some spin stabilizing effects over geologic time, but not over short periods for the emergence of intelligence and civilization.

      It reminds me of this Douglas Adams analogy:

      Imagine a puddle waking up one morning and thinking, ‘This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn’t it? In fact it fits me staggeringly well, must have been made to have me in it!’

    • Ronald May 29, 2016, 10:11

      James, this is a largely outdated hypothesis. The stabilizing effect of the moon is rather small, and only of some significance over very long time periods (50 – 100 My orso), much longer than other climatic influences. The gyroscopicly stabilizing effect of the spinning Earth itself is probably much greater.

      • andy May 30, 2016, 18:13

        Also, the stellar tides on a planet receiving Earthlike insolation around main-sequence stars of roughly G8-K0 spectral type have the same magnitude as the lunar tide on Earth. I recall studies suggesting that the stellar tide would spin-stabilise a planet in the habitable zone of 55 Cancri, despite the presence of several giant planets in the system (this was before the discovery of 55 Cancri f, which more-or-less put and end to the prospects for habitable terrestrial planets in that system).

  • yelka2 May 29, 2016, 9:00

    “our studies indicate that the ideal stars to support planets suitable for life for tens of billions of years may be a smaller slower burning ‘orange dwarf’ with a longer lifetime than the Sun ― about 20-40 billion years. These stars, also called K stars, are stable stars with a habitable zone that remains in the same place for tens of billions of years”
    This is from Centauri Dreams in 2009. My question is – given that Kepler-62f could be up to 40 billion years old – does this potential great age have any bearing on the possible CO2 levels, temperature, etc. necessary for life?

    • Michael May 29, 2016, 11:52

      The Universe is only around 15 billion yr.’s old, the metals content of the star is −0.209 so is not too low so it could be oldish. Wiki has it around 7 +/- 4 billion years old.

    • Ronald May 29, 2016, 14:01

      40 billion? The universe is almost 14 billion years old, so no star in it can be older. I read an estimated age for K-62 of about 7 billion years.

  • Alex Tolley May 30, 2016, 17:29

    The issue of atmospheric pressure is interesting. There is next to no data to determine this. There is an interesting use of crater sizes as proxies for atmospheric density/pressure which suggests that early Mars had an atmosphere perhaps 3/4 har, or a little higher, much higher than today. There are some attempts to measure the pressure directly from rock samples with air inclusions and also by the size of raindrops in sedimentary rocks which were one mud flats. The planetologists infer very dense atmospheres on formation as it must be composed of primordial matter, and that the carbonate inventory becomes a proxy for the original CO2 in the atmosphere. The problem here is that this will depend on where the planet formed in the protoplanetary disk and whether it migrated or not.

    Once formed, the CO2 will dissolve in the water oceans and precipitate out if basic metal oxides (primarily Ca, Mg) are also present due to weathering, being replenished by heating of the carbonate rock due to plate tectonics. This is further complicated by the size of the planet among other factors.

    So much of this work is really based on inference and speculation with no hard data to support it. To me this just emphasizes the need to collect more data, e.g. spectroscopic, to gain better measures of what the surface conditions are and whether life is there or not. The first confirmed discovery of life on an exoplanet is going to be a major discovery, but will leave the biologists salivating for a probe to analyze samples if the planet is close enough. It is going to be very frustrating to at best observe life through a telescope yet be unable to work with physical samples.

    • Eniac June 4, 2016, 12:35

      This is a good discussion of what is known, Alex, recognizing that there is a lot of speculation and (not that much better) modelling involved in what is published, much of it contradictory.

      The pertinent facts for me are that both Venus and Mars have atmospheres dominated by CO2, and that nowadays the removal of CO2 from the atmosphere of Earth is completely dominated by biological processes (sedimentation of biomass and marine carbonate shells). I know there is a respected theory out there that says the “carbonate-silicate cycle” achieves the same thing without help of biology, but that is not what we actually see. On Earth, carbon sequestration is almost exclusively biological, and on the other planets, it does not happen. So, this carbonate-silicate cycle, to me, is entirely hypothetical and not relevant to what we see, today.

      So, my view is a little different from yours in this way: You ask “Where did the atmospheric CO2 come from?”. I ask “Where did it go?”. CO2 is a standard, default component of any planetary atmosphere, more so than nitrogen, for example. We know it is there in abundance on both Venus and Mars, and its absence on Earth is properly explained by biological fixation, as part of the carbon cycle. That, to me, says it all, without the need for complicated theories and models.

      This, then, naturally leads to the biological thermostasis model, where the constant temperature on Earth during the gradual brightening of the sun is explained by the removal of CO2 from the atmosphere by photosynthetic organisms as soon as it is warm enough for these organisms to thrive. In effect, life regulates its own optimal climate, turning the heat down when it gets warm, using the CO2 knob. On Earth, today, the knob is nearly all the way down, and a climate catastrophy is imminent. If you want to call less than a billion years imminent, that is.

      • Alex Tolley June 4, 2016, 13:54

        As regards Earth, in general, I agree with you. Biology is certainly the major short term recycler of carbon, a fact that is evident in the CO2 level measurements that show annual cycles.

        The question I raise is whether all the carbon in the crust was originally in the atmosphere, as it appears to be on Venus today, or was it part of the mantle so that the atmospheric pressure was much lower than a full carbon inventory would imply.

        There is no data to directly answer that question.

        We can make up all sorts of figures. For example, water is about 10x the mass of carbon, so a 60 bar CO2 atmosphere might be part of a 600 bar steam atmosphere of the very early earth before the water condensed out. With the oceans’ average depth of 4000m this would imply a 400 bar steam atmosphere (ignoring non-oceanic plates, but also ignoring the deep water recently found). So the numbers are in the right ballpark, but did the Earth ever have such a dense atmosphere? At this point I’m willing to concede it may have, rather than a much thinner atmosphere. But clearly the pressure must have quickly declined, and weathering must have been the main reason for the CO2 decline as photosynthesis to fix carbon did not get invented by life quickly, and the great oxygenation event didn’t happen for a couple of billion years. In addition, carbon in the rocks is mainly inorganic carbonate that is not biologic in origin. Biologic carbon is both in the form of oxidized carbon – carbonate -, e.g. chalk, and reduced carbon as coal. Methane is another story with both biologic and non-biologic sources.

        As regards habitability, it seems CO2 alone is not enough to have made the Earth habitable at its birth, as the dimmer sun was insufficient to warm it. Even billions of years later, it underwent huge glaciations that may have covered the planet (snowball Earth). Yet today, with very low CO2 levels, earth is now reaching towards the inner edge of the habitable zone as the sun is brighter than it was. Within a billion years, the required CO2 level will be too low for current photosynthetic plants to survive, and Earth will be pushed out of the HZ.

        All of which means that we have plenty of wiggle room to play with numbers to include planets in the HZ of their stars. Funnily enough, we seem to want to increase the chances of habitable worlds while at the same time are much more circumspect about a possible inhabited world like Tabby’s star. I personally prefer to see more data for both domains, even as I hope that we discover worlds that are not just habitable, but have evidence of life, even single celled life only. Multicellular life would be a fabulous discovery, and I would be an enthusiastic supporter of new telescopes to try to characterize it, even visualize it, as it is likely to be too far away to send probes with journey times within a lifetime.

        • Eniac June 4, 2016, 18:44

          But clearly the pressure must have quickly declined

          This is not clear to me, at all. Can you elaborate?

          In addition, carbon in the rocks is mainly inorganic carbonate that is not biologic in origin.

          That is not what I thought. I understand that carbon exists in rocks almost exclusively as limestone, and limestone is of biological origin.

          Methane is another story

          Here you are right. I think there is some reason to believe that primordial conditions were much more reducing than today, with methane taking the role of CO2. CO2 would have formed from water and methane as hydrogen escaped into space.

    • Eniac June 4, 2016, 12:45

      As for atmospheric pressure, given that the primordial atmosphere was likely dominated by CO2, if you added it back in, it would increase the pressure greatly, to something more like Venus.

      Indeed, it is a good exercise to look at the atmosphere of Venus and simply remove all CO2. What you end out with is something not so dissimilar to Earth’s atmosphere, mostly nitrogen, with just a few bars of pressure left.

      Thus, in my admittedly simple-minded (but relatively free of speculation and complicated models) logic, the primordial atmosphere on Earth must have been Venus-like until photosynthesis arose. I don’t know if there is solid evidence for or against this, that would be interesting to look into.

      • Alex Tolley June 4, 2016, 14:35

        the primordial atmosphere on Earth must have been Venus-like until photosynthesis arose.

        But what about the water? Venus has lost its water, but the Early earth (and Venus) would have had an abundance of water which would have saturated the atmosphere. The CO2 would dissolve in the water and precipitate out.
        A dry, CO2 atmosphere therefore is not a plausible early Earth model. Water would dominate. But then we also have to understand the isotopic ratios, which leads to the speculation of cometary water as the source of Earth’s oceans.

        • Eniac June 4, 2016, 19:04

          Venus has not lost all its water. There is still a significant amount in the atmosphere, although admittedly it is not much.

          CO2 would dissolve in the water, yes, but not all of it, necessarily. It depends on how much there is in the first place. I am not sure that CO2 would precipitate in water without the help of organisms. If it did, and Venus did have water early on, why hasn’t the CO2 all precipitated?

          So, yes, I think a dry CO2 atmosphere is quite plausible on the early Earth. There would be water, but at low temperature it would all be liquid. It would be saturated in CO2, but that does not mean there couldn’t have been lots of CO2 left over for a thick atmosphere.

          • Alex Tolley June 4, 2016, 22:24

            While limestone may be formed by organisms, carbonate rocks initially formed from the reaction of basic oxides from weathering and acidic carbonic acid. If this didn’t occur, carbonate rock deposition would have to awaited life, and so teh weathering process to remove CO2 from the atmosphere could not happen on lifeless planets. Note that teh article on Kepler-62f assumes reduced weathering due to deep oceans preventing exposure of the metals in rock to start this process. Similarly, without carbonate rock deposition, there is no CO2 to be released under heat from volcanism to replenish the atmosphere.

            You are correct that the CO2 in the atmosphere is at an equilibrium with that in the oceans, so a dense CO2 atmosphere would result in a less dense one only.

            I believe Venus lost its water due to photolysis. with the hydrogen rapidly escaping. The rate of photolysis on Earth is low, especially compared to photosynthesis.

            Regarding the pressure. Suppose Earth had a very dense early atmosphere O(10’s bars). Today the total pressure is just 1 bar. Most of that CO2 is now in the rocks. AFAIK the evidence is that the CO2 was removed fairly quickly to around 1% of the atmospheric pressure. If the total pressure was also continuing to decline, wouldn’t that imply a loss of nitrogen and then oxygen too?
            I cannot readily put my hands on a paper with data, but modeling suggests that CO2 couldn’t have been above 0.15 bar 2.6 Ga (gigayears ago) and was most likely a lot less. Unfortunately it says nothing about the status ~ 4 Ga. Kasting suggests that CO2 has to be sub-1 bar to ensure the Earth is warm enough for weathering, but this is just to keep it from freezing. What if the Earth was hot instead and had a dense CO2 atmosphere? I think this is where the weathering becomes rapid and CO2 is “rapidly” removed. We really don’t seem to have much evidence of conditions on the pre-Archaeon Earth. What we do know is that the rocks on the Moon are igneous, so it was formed before the Earth cooled enough to form carbonate rocks and couldn’t form them itself. But the Earth must have become cool enough for life to survive, and the evidence for life seems fairly established at 3 Ga, and possibly older, at which point the CO2 level must have been sub 1 bar and possibly much lower.

            • Eniac June 7, 2016, 11:03

              If this didn’t occur, carbonate rock deposition would have to awaited life, and so teh weathering process to remove CO2 from the atmosphere could not happen on lifeless planets.

              My point exactly. That would explain why it did not happen on other planets.

              Now, mind you, it is possible that there is a slower weathering process going on that is, today, overwhelmed by the biological processes, but I do not see why it is necessary to explain anything, so it seems to be idle and groundless speculation.

            • Eniac June 7, 2016, 12:37

              AFAIK the evidence is that the CO2 was removed fairly quickly to around 1% of the atmospheric pressure.

              That would be interesting, if we could turn up that evidence and it turns out to be solid.

              If the total pressure was also continuing to decline, wouldn’t that imply a loss of nitrogen and then oxygen too?

              No. Why? There wasn’t any oxygen there to begin with, and nitrogen could have been there at 0.8 bar, all along.

              What if the Earth was hot instead and had a dense CO2 atmosphere?

              Yes, that would be the expectation, I think.

              I think this is where the weathering becomes rapid and CO2 is “rapidly” removed.

              Maybe, or maybe not. We don’t really know how well that weathering would work, and whether it is a thing at all. Plus, for all we know, back then there might have been enough water to have no land to whether, at all.

              But the Earth must have become cool enough for life to survive, and the evidence for life seems fairly established at 3 Ga, and possibly older, at which point the CO2 level must have been sub 1 bar and possibly much lower.

              Are you saying any CO2 over 1 bar would become too hot despite the faint young sun? How well do we know that? For that matter, how do we know the ideal temperature is for early life? It might have been quite hot. Remember, high pressure also means higher temperature range for liquid water.

              • Alex Tolley June 7, 2016, 14:22

                This has opened up some side work for me to try to understand what is known about teh early earth. So far it seems that:
                1. The Early Earth was hot and probably had a dense CO2 atmosphere.
                2. The late bombardment heated the Earth and stripped away much of the water and atmosphere.(Hadean period)
                3. Asteroids and comets later replaced some of the ocean water and the atmosphere.

                So the relevant condition we must deal with is the post-Hadean, when the Earth cooled enough to retain liquid oceans.

                As regards temperature. Water is kept liquid at much graeter than 100C in te ocean depths at vents. However the highest temperature that has been found for living extremophile (thermophile) is 121 °C. I would treat that as close to any limit.
                BTW, you might look up Nick Lane and his ideas for the origin of life (as part of his energetics approach to life). He is making some interesting waves in this area and seems to have some persuasive arguments. Bottom line is he is going for alkaline vents as the stable deep ocean site of life formation. So atmospheric pressure is only relevant fot keeping the Earth’s early oceans liquid with a faint sun. As explained in other threads, this does not require a very dense (nulti-bar) CO2 atmosphere, and the evidence is more for a 1 bar atmosphere with other GHG like CH4 to maintain the required temperature.

                As a biologist, it is the origin of life that interests me the most, as this is most relevant as to whether life will occur on any suitable world. If the deep ocean vent hypothesis is true, then it also suggests that icy moons with hot vents are also suitable abodes, making gas giants with such gravitationally heated moons another “HZ” of stars. This makes a plume sampling mission important, as well as probes mapping hot vents by proxy data.

                • Eniac June 8, 2016, 17:48

                  I think both the late heavy bombardment and the water delivery by asteroid theories are under dispute. Read the Zahnle paper, it starts out at the moon-forming impact, not the LHB. I am not sure if the latter is even mentioned. Volatiles delivery by impact is mentioned as a possibility, and if I recall correctly, found wanting.

                  Nick Lane is long on persuasive arguments, but short on an unbiased view.

                  As explained in other threads, this does not require a very dense (nulti-bar) CO2 atmosphere, and the evidence is more for a 1 bar atmosphere with other GHG like CH4 to maintain the required temperature.

                  First of all, I think a multi-bar CO2 atmosphere makes sense even if it is not required for anything, simply because CO2 is one of the favored forms of carbon to condense in the solar nebula. It also is the form of carbon that outgasses from lava, so it would be favored for secondary atmospheres, as well.

                  If, in addition, the surface temperature was to be well over 100 degrees, a multi-bar atmosphere would be needed to keep the water liquid. CO2 is the most likely main constituent of any atmosphere, as it is on Mars and Venus. If it wasn’t CO2, we’d be faced with the more difficult task of explaining the presence of whatever other main constituent you want to postulate, instead. As said, CH4 is another good candidate, but it would only last until the hydrogen escapes.

                  I would go with a cooler version of Venus, with more water. It think it is clear why we need cooler and more water, but I don’t see the need (or evidence) for less CO2. Especially if the CO2 can help explain above-freezing temperatures under the faint young sun.

          • Michael June 5, 2016, 12:38

            The CO2 in the early atmosphere disappeared quite quickly, carbonates can form directly from the interaction of water, rock and carbon dioxide.

            http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2944365/figure/A004895F3/

            Complete article

            http://cshperspectives.cshlp.org/content/2/10/a004895.full.pdf+html

            The reason it has not happened on Venus is that the chemical reaction favours the formation of gas, but drop the temperature a couple of hundred C and the CO2 will act directly with the rock to form carbonates.

            https://www.ipcc.ch/pdf/special-reports/srccs/srccs_chapter7.pdf

            • Eniac June 7, 2016, 12:56

              The image caption you cite states “Here we assume that CO2 reacts with the seafloor and is subducted into the mantle on either a 20 Myr (solid curves) or a 100 Myr (dotted curves) time scale.”. So, this is an assumption, not a fact, as you state it, and they assume two quite different scenarios, indicating that there is not really a good consensus as to how fast this occurs (if at all).

              • Michael June 7, 2016, 14:39

                Abiotic carbonate forms quite readily, the reason there is so much biotic carbonate is that life started using the mineral. Since the carbonate cycle destroyed the orginal abiotic deposits it now looks like life was the only process to form the carbonate mineral deposits.

                And at 3.8 giga years I think any theories are speculative and need to make a few assumptions.

                • Eniac June 8, 2016, 17:59

                  “Abiotic carbonate forms quite readily” I am not sure how we know that. Is that a fact or an assumption? Could you elaborate?

                  If the overwhelming part of carbon mineralisation is biotic today, it follows that any abiotic process you postulate must be a lot slower, so it could not have amounted to much. In other words, carbon mineralisation in the 2 billion years without life would have been negligible compared with the 2 billion years with life.

                  Yes, we must make assumptions, but we should not state them as if they were facts.

                  • Michael June 10, 2016, 3:10

                    ‘If the overwhelming part of carbon mineralisation is biotic today, it follows that any abiotic process you postulate must be a lot slower, so it could not have amounted to much. In other words, carbon mineralisation in the 2 billion years without life would have been negligible compared with the 2 billion years with life.’

                    Carbonate formation from bacterial life can be 10, 20 or more times more efficient than abiotic processes dependant on pH and temperature, but in 100-200 million years an enoumous amount of carbonates can be formed abiotically. Just because life can form carbonates more efficiently does not mean abiotic processes can’t have a significant effect on an atmosphere especially a hot steamy one where life may find it difficult at first.

                    • Eniac June 10, 2016, 21:39

                      That is assuming there are no reverse processes, like redissolving the carbonate at low pH, or subduction and volcanic recycling.

            • Eniac June 7, 2016, 14:00

              That Zahnle paper is very interesting. It mentions that under some models a significant part of the atmosphere might have been CO, and the CO is a possible source of both energy and carbon for (very) early life. Apparently, there is still biological evidence to this day in methanogens that CO may once have been a primary energy source. Presumably the formula for this early metabolism would have been similar to this:

              6 CO + 3 H2O –> COH2 + 4 CO2 + CH4

              where COH2 stands in for biomass.

            • Eniac June 7, 2016, 14:16
        • Eniac June 4, 2016, 19:14

          With regards to cometary water: I believe comets also contain a lot of CO2, so the two would have gone hand in hand.

  • Alex Tolley June 10, 2016, 10:26

    Interesting short piece in American Scientist on habitability:

    The Imprecise Search for Extraterrestrial Habitability

    Talks about gases, weathering and biosignatures.