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Proxima Centauri b: The Habitability Question

Proxima Centauri b is back in the news, although I’ll confess that in my case, it’s rarely out of my thoughts — I’ve been obsessed with the Alpha Centauri system since my youth. The latest comes through work by Anthony Del Genio and colleagues (NASA GSFC), who describe in Astrobiology their new simulations with regard to potential habitability.

You’ll recall the issues here. A planet this close to its host star may well be tidally locked, with one side always facing the M-dwarf Proxima Centauri. Martin Turbet (Sorbonne Universités, Paris) and colleagues described possible climates on Proxima b in a 2016 paper, using a 3D climate model (GCM) to simulate the atmosphere and water cycle of the planet for its two possible rotation modes, a 1:1 and a 3:2 spin resonance (in other words, gravitational forces could keep Centauri b locked to Proxima or rotating 3 times for every 2 orbits of the star).

The Solar System offers analogues: The Moon is in a 1:1 spin resonance with the Earth, while Mercury is in a 3:2 spin resonance with the Sun. What Del Genio and company bring to the table is a model that incorporates these resonances and a global climate model but also includes the effects of an ocean that can transfer heat from one side of the planet to another.

The addition of this dynamic ocean modeling gets us to an interesting outcome: Rather than, as with the Turbet paper, finding an entirely frozen dark side, with a star-facing side that has at least some potential for a sea, Proxima b may have conditions allowing for an equatorial zone of liquid water even on the dark side, with large open ocean possible elsewhere.

This result improves the odds on habitability, as Del Genio is quoted as saying in an article called The Closest Exoplanet to Earth Could Be “Highly Habitable.” An ocean-covered Proxima b with an atmosphere like Earth’s could have open oceans that extend into the dark side, at least at low latitudes, and this turns out to be true both for synchronous rotation and a 3:2 spin-orbit resonance with a somewhat eccentric orbit.

Image: An image of the closest star to the Sun, Promixa Centauri, and its surrounding field of stars. Note that Centauri A and B both appear as a single ‘star’ here, their light combined in the bright object at the left. The bright star on the right is Beta Centauri, which is not a part of the Alpha Centauri system and is much further away from Earth. Credit: Marco Lorenzi.

But in going through the paper, I think it’s important to note that the researchers also derive much colder temperatures than previously suggested, the result of the transport of oceanic heat around the surface. Let’s look at how the paper treats this outcome with reference to Earth:

Because of its weak instellation, however, Proxima Centauri b’s climate is unlikely to resemble modern Earth’s. “Slushball” episodes in Earth’s distant past with cold but above-freezing tropical oceans (Sohl et al, 2017) are better analogs. The extent of open ocean depends on the salinity assumed. Elevated greenhouse gas concentrations produce some additional warming, but this is limited for any M-star planet by the reduced penetration of near-infrared starlight to the surface, and for Proxima b in particular, by its existence near a possible dynamical regime transition.

Unlike Turbet and team, Del Genio’s paper rules out a single ocean at the substellar area — this is sometimes called the ‘eyeball Earth’ scenario — because their dynamic ocean models show enough heat transport to allow water to exist on a wider basis. But note the effect of salinity, which determines the temperature at which seawater freezes. Modeling different levels of salinity in their various simulation scenarios (which also varied atmospheric composition), the scientists found that at high salinity, broad regions of open ocean occur and little ice.

We may have a Proxima b that is habitable but in terms that are starkly different from Earth:

…if we think more broadly about what constitutes a habitable planet (Cullum et al., 2016), it is reasonable to imagine a cold but inhabited Proxima Centauri b with a very salty shallow remnant of an earlier extensive ocean in which halophilic bacteria are the dominant life form, especially if the ocean is in contact with a silicate seafloor (Glein and Shock, 2010).

In an interesting discussion of other exoplanets at the end of the paper, the researchers consider TRAPPIST-1 e, LHS 1140 b, GJ 273 b and other worlds in the context of their models. The point could not be clearer: We are steadily building our inventory of potentially habitable rocky exoplanets around M-dwarfs. Where this paper points is toward continued refinements to climate modeling as we anticipate our first atmospheric detections and characterizations.

Calling the Del Genio paper “a very exciting result indeed,” Guillem Anglada Escudé (Queen Mary University of London), who led the team that discovered Proxima b, likewise pointed to nearby M-dwarf systems and the likelihood that their planets would be rich in water. By ‘rich,’ we are talking about 10-30% in water content, as opposed to < 1 percent for Earth.

The problems of atmosphere retention through early tidal heating and later erosion due to stellar flares and a strong stellar wind are serious and we cannot rule out the possibility that they would make habitability impossible. Anglada Escudé noted these issues in an email this morning, but continued:

“The paper (necessarily) needs to make a large number of assumptions, but this is very important work as it points towards the observational probes that we should soon be able to apply to these planets (temperature, thermal environment, atmospheres rich in water vapor, or whatever). As usual, we would like to see a few more of these simulations by independent teams to obtain a better perspective on the landscape of possibilities.”

Where to next with this research? Anglada-Escudé adds:

“As a shopping list of things that I would like to see from our modeller colleagues, it would be great to see some of these models coupled with erosive effects from stellar activity on the upper atmosphere, and the resulting exotic atmosphere we might expect. That is, even if large chunks are eroded due to a particularly violent coronal mass ejection or flare, the ocean would possibly outgas a new one quickly.”

The paper adds that we have some analogues in our own Solar System to seas of high salinity like those modeled here. Europa, for example, is thought to contain such a sea, as is Enceladus. Interestingly, we can’t observe life signatures within these bodies without going there, setting up a situation in which we may detect — within a few decades — biosignatures on an exoplanet before we find signs of life beyond Earth in our own Solar System.

The paper is Del Genio, “Habitable Climate Scenarios for Proxima Centauri b with a Dynamic Ocean,” published online at Astrobiology 5 September 2018 (preprint). The Turbet paper is “The habitability of Proxima Centauri b II. Possible climates and observability,” Astronomy & Astrophysics Vol. 596, A112 (December, 2016). Abstract

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  • Gideon Marcus September 17, 2018, 13:13

    And, of course, the hellish radiation from the flares. Those may well strip away an atmosphere, right?

    If not, would a deep ocean offer protection?

    Thank you for this article!

    • H. Floyd September 17, 2018, 19:57

      Ditto here. A terrific article as always, and a great question. My coffee break lay inquiry this afternoon suggested that red dwarf radiation may be a Great Filter, somewhere after single-celled or primitive multicellular life but before complex marine ecosystems.

      Bright-side rationale: Very basic marine life can adapt at the organism level to protect against consistent levels of radiation.

      https://doi.org/10.1130/G37095.1

      https://www.sciencedaily.com/releases/2015/10/151026093045.htm

      In addition to adaptations at the organism level, an oxygen-net biosphere gave Earth its ozone layer. Simple life at Proxima B might also be able to terraform past this Filter.

      https://en.m.wikipedia.org/wiki/Geological_history_of_oxygen

      https://en.m.wikipedia.org/wiki/Chapman_cycle

      Now, on the other hand: While long term adaptations or diversification at a given ocean depth might protect against consistent levels of radiation, the tantrums we expect from a red dwarf could continually reboot or even sterilize oceans before complex life has time to evolve and diversify.

      https://www.aps.org/publications/apsnews/200407/extinction.cfm

      Also: Does feeble rotation mean no magnetosphere? No magnetosphere, no atmosphere. No liquid water. Mars, tidally locked, the norm around red dwarfs. A depressing thought.

      Lastly, optimistic speculation, assuming atmospheres and oceans: Time is a mighty hedge against this Filter. Our system is stable but short lived in contrast. Red dwarf planets, if they can just get a start on life, have buddhic ages in which to begin again. Also, extinction events have a way of preceding radical new steps in evolution. Given enough time and hard lessons around a red dwarf, biology might reach levels of cellular sturdiness that we cannot imagine. (We didn’t need to, and didn’t have time to. Our Cambrian ancestors weren’t selected for fundamentally rugged cellular engineering, but for teeth and speed.)

      Anyway, great question. That’s my coffee cup’s worth. Eager to know what anyone else thinks.

  • Charley September 17, 2018, 13:47

    “Proxima Centauri b is back in the news, although I’ll confess that in my case, it’s rarely out of my thoughts — I’ve been obsessed with the Alpha Centauri system since my youth. ”
    OK, OK, I’ve just got the bite on this one … Just why are you obsessed with this particular star system ??

    • Paul Gilster September 17, 2018, 15:04

      As the closest star(s) to Earth, Alpha Centauri would be the first place we’d be likely to send an interstellar probe. So for me, it always stood in general for interstellar exploration.

    • Harry R Ray September 19, 2018, 9:54

      Talk about “back in the news”! Paul(and everyone else)check this out ASAP! Arxiv: 1809.06389. “Spin Evolution and Cometary Interpretation of the Interstellar Minor Object 1I/2017 `Oumuamua.” by Roman R Rafikov. Here’s the KEY QUOTE from the abstract: “…However, given the highly elongated shape of the object, its torque asymetry is likely higher, implying even faster evolution. This would have resulted in rapid rotational fission of `Oumuamua during its journey through the solar system and is clearly incompatible with the relative stability of its rotational state…”. In laymans terms, this means that if `Oumuamua were an elongated object, it would have BROKEN INTO PIECES! I would be jumping up and down right now, but for a GLARING OMISSION made by the author, which is; there is an ALTERNATE SHAPE for `Oumuamua that ALSO fits the light curves – that of a pancake shaped object that is NOT elongated. To summarize, the author has ELIMINATED many possibilities. To me, all that’s left are TWO! ONE: The above mentioned “pancake” comet. AND TWO: An elongated “Bracewell probe” with an IRREPARABLE thruster issue!!!! Though I hope and pray for the latter interpretation, Occam’s Razor dictates that is probably the former.

      • Harry R Ray September 19, 2018, 11:19

        Sorry! I meant to say AND TWO: An elongated “Bracewell probe with a VERY RIGID HULL that was able to WITHSTAND the extreme torques so it did NOT break apart due to an IRREPARABLE thruster issue.

        • Harry R Ray September 20, 2018, 11:15

          OH BOY! I just read the PDF myself. Way WAY over my head. However, I did find a couple of things of interest. The reason the author did not address the alternative “pancake” interpretation of the light curve data, is that, for “simplicity” reasons, he chose not to incorporate “excited spin state”(proposed in an earlier paper, which lead to the proposed “pancake” alternative)hypothesis. The ONE THING I CAN UNDERSTAND about this paper is the claim of “…clear lack of clear signs of mass-loss from `Oumuamua, despite multiple targeted observations aimed at addressing this issue.” I aaume HE assumes that the non-gravitational acceleration detected would HAVE to result in EASILY MEASURABLE mass-loss if it was outgassing from a comet! Lo and behold, this ALSO APPLIES to a “pancake” comet as well!!!!! FINALLY, the author offered NO EXPLANATION WHATSOEVER for a non-cometary non-gravitational acceleration of `Oumuamua! I am simply AMAZED at the complete lack of news coverage of such a STARK CLAIM(“news” in this case means nothing space news blogs like http://www.portaltotheuniverse.org). Keep in mind, I am still not jumping up and down YET, but my legs are getting kinda twitchy!

        • J. Jason Wentworth September 20, 2018, 13:43

          Comet Giacobini-Zinner (often shortened to GZ, for the same reason Comet Churyumov-Gerasimenko, Rosetta’s and Philae’s final resting place, is often called CG), the first comet to be visited by a spacecraft (ISEE-3/ICE, in 1985), is also thought to have a pancake-shaped nucleus, perhaps having an 8:1 ratio between its equatorial and polar diameters, and:

          ‘Oumuamua would be a good target for a Breakthrough Starshot (or a Sun-diver solar sail starprobe) test flight, being a (relatively) close, yet deep space target for testing the navigation, flight control, and high-speed flyby imaging and instrument measurement systems and procedures. If the probe reached ‘Oumuamua’s computed position, but ‘Oumuamua was a long way off (and perturbations of celestial bodies were accounted for), *that* would lead to some very interesting hypotheses…

      • Alex Tolley September 19, 2018, 16:12

        The last episode of the tv series “Salvation” just ended with the asteroid that was apparently on course to destroy Earth, being clearly a visiting starship. We will probably never know, as I don’t believe this series has been renewed for a 3rd season, given its low ratings and ridiculous plot.

      • Michael C. Fidler September 21, 2018, 2:59

        Harry, Rafiko just put up a revised version today.

        SPIN EVOLUTION AND COMETARY INTERPRETATION OF THE INTERSTELLAR MINOR OBJECT
        1I/2017 ’OUMUAMUA
        Roman R. Rafikov1,2
        Draft version September 21, 2018

        https://arxiv.org/pdf/1809.06389.pdf

        J. Jason, I think the Breakthrough Starshot to Oumuamua is a great idea and should be investigated further. ;-)

        • Harry R Ray September 21, 2018, 10:19

          David Kipping tweeted this 21 hours ago. “I think peer review generally works great for most papers because most papers apply standard methods or incrementally improve what’s come before. Disruptive papers are a different beast and one can forsee many reasons why such papers could get stuck never seeing the light of day.” This could very well be why this paper has received ABSOLUTELY NO ATTENTION on ANY of the astronomy news websites. I worry that this may befall his exomoon confirmation paper as well.

  • John walker September 17, 2018, 15:29

    10%-30% water to weight. Habitable. Can someone explain how? The mantel will expel all but a tiny fragment of that water which means on an Earth sized world a “water crust” or hydrosphere over a thousand km deep. Even on such a tiny mass body that would seal the ocean floor(in the substellar liquid zone) with ice vii. No volcanism. No plate tectonics. No nutrients. Biogenesis?
    Still, it’s just conjecture at this point what the water content is if any. The possible dynamical varients are multifold. I hope real measurements are not too far off.

    On a pedantic side note, I like the authors choice of “instellation”!! While its use doesn’t reflect the word”s traditional definition I welcome neologisms setting new standards for old terran/solar specific forms. Why insolation when it’s not from Sol? Instellation! ^_^ Next out, geology.

    Ahem…

    • Enzo September 17, 2018, 18:17

      I was thinking about this the other day during another discussion but never got around to post anything. While I can see that ice at the bottom could be a problem for small (and possibly not too active) worlds such as Ganymede, some volcanic activity must go on. I mean we have plumes through the mantle, why not ice. I could easily be wrong, but, intuitively, it seems easier then compressed rocks. Also, heat from radioactive decay cannot keep building up : eventually the temperature is high enough to go through anything.

    • Eniac September 17, 2018, 18:28

      As long as there is heat in the interior that wants to get out, there will be some kind of volcanism. New and strange forms, perhaps, when there is water and Ice VII involved, but the heat has to get out, somehow, and conduction is too slow. In any case, in my view, to categorically state ‘no nutrients’ is jumping to conclusions, at best.

      • Nicky September 17, 2018, 22:49

        It is possible to have volcanism on water-rich moons, but on water-rich planets it might be inhibited. The gravity of the planet is significantly larger than moon which leads more pressure at the bottom. The consequences are:
        1) The melting point of silicate rocks is strongly dependent on pressure. The melting temperature increases with increasing pressure.
        2) At pressure above 20 GPa, the density of melt becomes larger than surrounding mantle, causing the melt to sink not to erupt.

        With a 200km thick ocean on a 5 Earth-mass planet (corresponding to radius of 1.55-1.60), melting can be fully suppressed even in the presence of plate tectonics, and it can also happen on a 3 Earth-mass planet (corresponding to radius of 1.35-1.40) with a 250km thick ocean. On Earth-mass planet, this critical depth is at 400km.
        Basically, there is heat flux coming out of planet surface, but not enough to cause melting and volcanism.

        There are a lot of studies done on nutrients transportation in an ocean-covered planet. Nutrients come from dissolution of minerals. On Earth, almost all nutrients come from continental rain and river runoff, deep seafloor produces very little. If you look at the ocean productivity map, you would find the coastal areas are alway the highest. In fact, a week ago The Astronomical Journal just published an article studying bioessential elements on ocean worlds, and the team concluded that without any land, the ocean would be depleted of phosphorus.
        See: Is Extraterrestrial Life Suppressed on Subsurface Ocean Worlds due to the Paucity of Bioessential Elements?

    • HarryR Ray September 18, 2018, 9:43

      In this case two “wrongs” MAY make a “right”! Billions of years of megaflares MAY have whittled the 10-30% ocean down to something more life-suitable. ALSO: The two “mesolensing” events gave data strongly supporting a slightly larger mass for Proxima Centauri, so the minimum mass for Proxima b is now 1.52 Earth-mass instead of 1.27. I also wonder whether Dr. Anglada-Escude has submitted a proposal to USE the completed and tested high amplitude pupil mask with SPHERE/ZIMPOL on one of the four VLT telescopes to try to get a DIRECT IMAGE of Proxima b PRIOR to SPHERE/ESPRESSO’s highly anticipated atmosphere data. Can somebody please find out?

      • Harry R Ray September 18, 2018, 16:10

        To find out more about this “high amplitude pupil mask”, go to ArXiv: 1808.o6819. “A simple optimized pupil mask for attempting to direct image of Proxima b with SPHERE/ZIMPOL at VLT.” by Polychronis Papatis, Jonas Kuhn, Hans Martin Schmid. It would be very interesting if one of the authors could post a comment here(or maybe even a guest post)explaining what could be learned from just one image. A SERIES if images would give you the inclination and eccentricity of Proxima b’s orbit.

      • Nicky September 18, 2018, 17:05

        If we use the gravitational mass of Proxima, minimum mass would be 1.56 +0.064/-0.053.
        10% wt of water would be comparable with 700 Earth oceans. For this much of water, even a lifetime of runaway greenhouse would not make the world any habitable. Ribas et al 2017 assumed Proxima b has experienced 4.8 billion years of hydrodynamic loss and found the water loss is between 15 and 25 Earth oceans.

    • J. Jason Wentworth September 20, 2018, 14:17

      I too hope that “instellation” catches on. Much like “periapsis” and “apoapsis” (which are generic, applicable to any celestial body), “instellation” refers to all stars besides our Sun. (There is also “stellification,” becoming a constellation; an account of Pegasus tells of him flapping his wings, “rejoicing at his stellification.”) Also:

      We have selenology (“geology” of the Moon) selenography (lunar cartography–that is, map-making), and areology and areography for Mars. I have never heard or read such terms for the other planets, their moons, asteroids, or comets. The ones for Venus would be interesting, because the adjectival form is “Venereal,” which–like “Aphrodisial” and “Aphrodisian”–have other connotations (“Cytherean” is used because the goddess Aphrodite first emerged from the sea onto the island of Cythera). Carl Sagan covered this planetary nomenclature controversy–with apparent mirth–in “Intelligent Life in the Universe.” Creating these various forms for all of the named planets, dwarf planets, moons, asteroids, centaurs, comets, Kuiper belt objects–and now, interstellar interlopers–will be interesting and fun (some NASA writers are “sticks in the mud,” regarding nomenclature, referring to “the Mars surface” rather than “the Martian surface”).

      • Mike Lorrey November 19, 2018, 7:13

        Kim Stanley Robinson coined “areophany”, “areology”, etc to apply to Mars, I’ve seen ‘cereal’ and ‘cereology’ for Ceres since the DAWN probe settled into orbit there (I’d love a t-shirt with an image of Ceres and the caption “I’m so cereal!”)

  • Gary Wilson September 17, 2018, 15:57

    It’s just amazing to have a planet orbiting our nearest stellar neighbor and that it is in the habitable zone! Now is it possible there are other rocky planets which we haven’t detected yet around Proxima Centauri or A and B? I know A and B pose special problems because of how close they are to each other but it would be a wonderful learning opportunity if indeed planets orbit them as well.

    • J. Jason Wentworth September 20, 2018, 23:22

      The Alpha Centauri system is similar to the far side of the Moon (before Luna 3 [originally called Lunik 3 in the west]), in that they are both so (relatively) close–being the closest objects of their respective kinds–yet being so infuriatingly inscrutable, and:

      Until the flight of Luna 3, we couldn’t see the hidden 41% of the Moon’s surface. The various types of libration bring 59% into our view, but with great foreshortening at the limbs, which made mapping of the limb regions difficult. A few rays from farside craters also extend onto the nearside, but relatively few craters still have visible rays, and the full extent of the huge Mare Orientale, which straddles the nearside/farside divide, couldn’t be determined from Earth, as only its outermost ring of mountains is visible from here. Likewise, in its own way:

      The Alpha Centauri system contains the nearest known extrasolar stars (unless one or more as-yet-undiscovered, very dim brown dwarf stars lie closer than 4.22 light-years). But despite their relative proximity, we *still* can’t tell–due to their mutual gravitational interactions–if Alpha Centauri A and/or B have their own multi-planet systems (or if Proxima Centauri has more planets than the one we know of), but:

      Only a fly-through probe (or perhaps two, one targeted for Proxima and one for the A & B binary [John W. Macvey’s fictional colonists’ names “Alphauri A and B,” in his 1965 projected history “Journey to Alpha Centauri,” re-issued–with diagrams–as “How We Will Reach the Stars” in 1969, might be good ‘familiar’ names for these stars], could definitely establish how many planets all three stars may have, as well as their types, orbital parameters, and moons). Large telescopes, in Earth orbit or on the Moon, would help, but their necessarily distant view (and ironically, perhaps the glare of these relatively nearby stars), plus the stars’ gravitational interactions, would introduce more “noise” into the planets’ orbital parameters observations, and could allow small planets to be missed.

  • Nicky September 17, 2018, 18:17

    The severity of runaway greenhouse phase (water loss) during pre-Main Sequence on M-dwarfs planets might be overstating. We should treat all the calculated water losses amount with extra caution and as the most pessimistic upper limit.

    1) All the water loss models have assumed (without further explanation) there are enough hydrogen atoms from H2O photolysis in atmosphere for escaping, so the water loss rate is not limited by the amount of available H.
    However, as it was noted in Bolmont et al (2016) study, only a fraction (0.2 and less) of incoming FUV photons contributes to dissociate H2O, because other FUV photons are absorbed and backscattered, and some dissociated H2O molecules (O and OH) actually recombine to form H2O again. These processes lead to much smaller H production, which significantly limits the total water escape.
    Bourrier et al (2017) calculated water loss from TRAPPIST-1 planets with both unlimited hydrogens and limited hydrogen production (0.2), concluding that previous studies have overestimated by a factor of 4 or 5.

    2) Earth has actually shared similar evolutionary path with M-dwarf planets, even the sun has way shorter pre-Main sequence duration. The last stage of formation of Earth is called giant impacts phase, lasting up to 100 Myr. In this stage, Earth collides with tens of Moon-size and Mars-size embryos, and each collision induces magma ocean and runaway greenhouse state with duration up to ~10 Myr. Therefore, Earth should also have spent much of its time in runaway greenhouse phase, and for the first 100 Myr of young sun, the XUV flux and magnetic field were hundred times stronger than today. However, Earth still keeps more than one ocean of water and no clear imprint of atmospheric losses is found.

    See: The extreme ultraviolet and X-ray Sun in Time: High-energy evolutionary tracks of a solar-like star
    See: Terrestrial aftermath of the Moon-forming impact

    3) Planets naturally accrete some hydrogen atmosphere before disk dissipation. This hydrogen envelop protects volatile loss as it is being driven off by stellar activities.

    See: Habitability of terrestrial-mass planets in the HZ of M Dwarfs–I. H/He-dominated atmospheres

    • Ramses Ramirez September 18, 2018, 2:28

      Nicky,

      On your point 1, if the rate of H escape exceeds the diffusion-limit (which it can easily do, especially early on during the pre-main-sequence), then it *will* be energy-limited instead because the escape is not limited by the amount of H at the homopause by definition.

      Nevertheless, many of these atmospheric escape models (e.g. Bolmont, Bourrier, Luger and Barnes, mine’s..etc) do take both regimes into account. The resultant escape can still be quite large for planets orbiting mid- to late M-stars (maximum of tens to hundreds of Earth oceans even if you use conservative numbers).

      On your point 2, Earth’s accretion timescale is similar to the pre-main–sequence lifetime of the Sun. This is a *very* different scenario for M-star systems where the planetary accretion timescales for HZ planets tend to much, much shorter than the superluminous pre-main-sequence phases of their stars.
      Earth was not a fully-formed planet for most of the Sun’s short superluminous phase. It was only in the last few Myr (out of the 50 Myr or so) that the Earth embryo got large enough to be able to accrete most of its water. And by then the stellar insolation at Earth’s orbit was already pretty low (relatively-speaking). Plus, any runaway greenhouse phase on the Earth may have only lasted a couple of Myr in the very beginning of the formation of the solar system (e.g. Ramirez and Kaltenegger, 2014), during which time it was too small to accrete any water anyway.

      In contrast, an M-star HZ planet would have still been exposed to runaway greenhouse conditions for tens of Myr, if not hundreds of Myr (if not billions), after it became a planet. So, during the time it is supposed to amass water, and for many Myr afterwards, conditions would be too hot for the planet to acquire any volatiles.

      If anything, the difference between Venus and Earth should be a stark reminder of the power of early atmospheric escape.. and this is all around a star with much less severe superluminosity than what would be experienced around an M-star.

      On your point 3.. primordial hydrogen just does not last very long at all (~ typically a few Myr or potentially much less), especially on HZ planets close to their stars (e.g. Wordsworth, 2012). If Earth had a primordial H envelope, it did not last very long at all.

      The Del Genio et al. paper has interesting simulations and it is fun to assess model behavior. But, without evaluating the pre-main-sequence phase (particularly for such M-stars), any discussion about the potential habitability of Proxima Centauri b is not very meaningful.

      • Nicky September 18, 2018, 16:15

        Many thanks for replying, Dr. Ramirez!

        1) I am afraid that we might be talking about different limiting concepts here. Diffusion-limit does limit the *available hydrogens in upper atmosphere*, but the amount of available hydrogens for diffusion would not be limited.
        Photolysis-limit will eventually determine the amount of water loss for both diffusion- and energy-limit. In fact, Luger & Barnes neglected this limiting process as they explicitly said “” we further assume that water loss is limited by the escape of H to space rather than by the H2O photolysis rate””.
        Energy- and diffusion-limit have potential to induce loss of several Earth ocean’s worth of hydrogen, but the available hydrogens from H2O photolysis might only have one Earth ocean’s worth of hydrogen, a very vague example to illustrate my point. Indeed, photolysis-limit does happen when H production rate comes below diffusion- or energy-limit escape rate. Better illustration would be fig.9 in Bolmont et al 2016, fig.6 in Ribas et al 2016, and fig.12 in Bourrier et al 2017. Bolmont, Ribas and Bourrier all argued for photolysis efficiency of 0.2 or less.

        2) Protoearth experienced 5 to 10 giant impacts during a period of 100 Myr after disk dissipation, and each impact induced magma ocean (runaway greenhouse) lasting for 2 to 10 Myr.
        The most portion of Earth water was accreted before giant impacts and during sun’s saturated phase, and post-impact water delivery only counted for 10% of total water (Greenwood et al., 2018).
        Indeed, water-rich bodies were scattered inward by the growing Jupiter during gas dissipation not after impacts (Raymond & Izidoroa, 2017).
        Therefore, Earth water should also have spent a large fraction of its first 100 Myr in runaway greenhouse phase.
        Water can be chemically incorporated into silicates in the form of OH, avoiding sublimation when it crosses snowline. Water delivery to M-dwarfs planets is still possible.
        Venus’s distance to sun significantly prolonged magma ocean after impacts. Thermal evolution model has found that Venus is at the critical distance where magma ocean would not solidify until almost the entire steam atmosphere has escaped (Hamano et al., 2013).

        3) Depending on how long and rapid the planet was accreted, the planet could have various fractions of hydrogen envelope. Earth had a slow accretion history, and the solar system gas disk dissipated fast within 4 Myr (Wang et al., 2017). There was no way for Earth to accrete a protective hydrogen envelop. However, observations have found gaseous disk of low-mass stars tend to have longer lifetime to up to 10~20 Myr. Planets that originally accrete more than 1% of hydrogen envelop could have lasted through PMS and got evaporated into a habitable planet (Luger et al., 2015; Owen & Mohanty, 2016).

        • Ramses Ramirez September 18, 2018, 22:07

          Hi Nicky,
          > Many thanks for replying, Dr. Ramirez!
          You are welcome…Interesting posts as usual!

          1) Yes, investigators have suggested for years that the H escape may actually be limited by the photolysis efficiency. I have no issue with this being the case in some circumstances, but I personally don’t find the arguments in Bolmont et al. (2016) to be very convincing (which the other 2 papers you mention just repeat without further demonstration). They just state that the efficiency *must* be less than ~20% because of other physical processes without much quantifiable justification. More complex models will be needed that can actually quantify these individual processes, including also heating efficiencies (which are often low-balled in the recent papers) so that more realistic numbers can be obtained.

          However, we have very strong indirect evidence of the power of atmospheric escape in our own solar system: Venus. With a D/H ratio of well over 100, it must have been a very potent process. The lack of atmospheric oxygen is very telling also. If H escape was as inefficient as these authors claim it may have been, I figure it would be very hard for us to explain Venus.

          You also acknowledge that Venus had a long magma ocean stage, possibly explaining its water loss. Well, the magma ocean stage for M-star HZ planets would have been *much* longer than for Venus (assuming the planet could have acquired enough water to begin with) because of closer proximity to the star, higher stellar activity, and a much longer superluminous phase. It is a matter then of whether M-star HZ planets can really acquire the hundreds- thousands of Earth oceans of water some attribute to them (e.g. Alibert et al. 2017). And even then, would such planets really be habitable? What types of irradiated surface environments might they exhibit and can life really evolve in such environments?

          2) Water-rich bodies were definitely scattered inward during the entire accretion phase, including when the gas disk was still around. However, Earth was little more than a clump of a few small planetesimals (and maybe pebbles) until the late stages of accretion. It would not have been very receptive to that water until its gravitational well was large enough, which would have happened in the late stages (e.g. Raymond et al. 2007). And when I am speaking about late stage accretion here, I am not talking post-impact water deliveries or late veneers. As you mentioned, the current evidence does suggest that Earth acquired *most* of its water during accretion and not after.

          3) I personally don’t think that most of those close in terrestrial planets with big hydrogen envelopes are planets with primordial H envelopes. These planets are too small to retain such large H envelopes over such a long time. There’s no way those H envelopes would still exist if they were primordial in origin. Escape would have removed then. That H must have a source. I think that these are actually molten lava worlds.

          • Nicky September 19, 2018, 21:20

            Hello again Dr. Ramirez! Thanks for taking time to response!

            1) Indeed, I agree with you on the first point. They all lack an explanation for assuming the efficiency of 0.2, but I wonder why they are so confident with that result.
            I believe the low heating efficiencies (0.1 and lower) value adopted in recent literature come from Owen & Alvarez (2015) full 1D radiation-hydrodynamic model and Shematovich et al (2014) Monte-Carlo simulations. I thought this number is more robust than arbitrarily placing it at 30%.
            I still think we can use an even lower efficiency value. Because magma ocean solidification co-occurs with runaway greenhouse, the catastrophic outgassing does not only build a steam atmosphere but also hundred bars of CO2, which should contribute significantly to radiative cooling and slow down the water escape.
            Venus’ case is quite different from that of M-star HZ planets. The later will eventually solidify in HZ after tens or hundreds Myr, and the water if not completely desiccated will condense to form ocean. Venus, however, never solidify until all the water has escaped. It is because Nakajima’s limit which sets the upper bound of outgoing radiation in a water-saturated atmosphere is dominated by incoming stellar radiation, and the heat flux from magma ocean is limited and solidification is prolonged, until the majority of steam atmosphere has been blown off (Hamano et al., 2013).
            The distance where incoming stellar radiation dominates over Nakajima’s limit is the critical distance. M-star HZ planets eventually move out as star contracts, but Venus never did (Hamano et al., 2013).

            2) Earth was actually a planetary embryo with at least Mars-mass already by the time of gas dissipation (Morbidelli et al., 2013). D/H ratio of deep mantle water, which is assumed to be primordial and isolated from water cycling between the surface and the interior, suggests at least some of Earth’s water came from solar nebula, in-situ/early delivery (Hallis et al., 2015; O’Brien et al., 2018).

            3) My apology, I explain poorly on this subject. Some rocky planets formed with a hydrogen envelop, a mini-Neptune, but the envelop was blown away during superluminous phase. However, for the most time in superluminous phase, the envelop protects the volatile. After it was blown away and planet enters HZ, most portion of volatile should have survived runaway greenhouse, and the planet would outgassed a habitable secondary atmosphere.

            • Ramses Ramirez September 20, 2018, 20:57

              Nicky,
              All good points! And my responses…

              1) A 30% heating efficiency is probably too high for most cases. Most studies (aside from the recent Bourrier, Bolmont, and Ribas ones) uses typical values like 10 or 15%. Also, some of these papers suggest that the heating efficiency may be orbital radius-dependent although more complex models will be needed I think to look into this.

              M-star HZ planets will undergo an even more intense early runaway greenhouse phase than Venus did.. The difference is that M-star HZ planets *might* be able to acquire more water perhaps, although this is debatable. The other thing I did not mention, which would be different than on Venus, is that impact fluxes and velocities would be higher for the tighter and closer-in M-star planetary orbits (e.g. Lissauer et al. 2007). This would also favor desiccation with respect to Venus. Again, however, even if M-star HZ planets are able to acquire hundreds-thousands of Earth-oceans of water, whether these worlds are habitable or not under such irradiated circumstances remains to be confirmed. We do not know if a planet that can accrete a whole lot more water than Earth is a good thing… or not. Plus, with just a few Earth oceans (on an Earth-sized planet) on the surface , you can eliminate continents and with a bit more you can shut off volcanism and plate tectonics. We have argued that life may possibly evolve in the absence of a carbonate-silicate cycle on such really wet planets (e.g. Ramirez and Levi, 2018) but our mechanism is very speculative.

              Nevertheless, all of the same physics with the Nakajima limit and magma ocean heat fluxes would still apply to M-dwarf HZ planets. The magma ocean durations (and a runaway greenhouse state), however, would generally still be longer than they were for Venus, following those same arguments. Venus had acquired a lot of CO2, possibly even more than the Earth. And, yet, that did not stop atmospheric escape from completely desiccating its surface.

              2) Yes, but a Mars-sized body being irradiated at Venus (or even Earth’s) distance to the Sun that early on is likely still too small to successfully acquire any substantial water on its *surface*….Ultimately, for habitability, your planet needs to have standing bodies of water that are stable on the surface. Deep mantle water is one thing, which likely gets locked into the minerals (as you have mentioned before) and is isolated from surface processes. This probably happened on Venus. However, it’s another thing altogether for a planet (or even an embryo) to also acquire enough surface water to eventually sustain surface oceans. It is for these reasons like these that atmospheric escape models tend to start their calculation *after* the planet has fully-accreted.

              3) That is a possible hypothetical scenario espoused by recent Barnes/Meadows et al. papers but I find it to be a very convenient and unlikely scenario which requires very special circumstances that have to be *just right*. I’d be very surprised if a large fraction of these worlds are this. Even if a secondary atmosphere can be outgassed, it will likely be too much or too little, leading to conditions which are not habitable.

              A lot of these planets are not even in the HZ (much too close to their stars) so they would not be habitable, like the brand new Pi Mensae planet in press.

              • Nicky September 22, 2018, 1:14

                Interesting discussion with you, Dr. Ramirez!

                The new article (arXiv:1809.07498) on arxiv, accepted for publication in the ApJ, including *Rodrigo Luger as the fifth author*, also used heating efficiency between 0.01 and 0.1 with lower efficiency for high XUV flux and higher efficiency for low XUV flux, which are derived from fig.2 and simulations in Bolmont et al (2016).
                It seems like Luger is also accepting rather low heating efficiency calculated by Owen & Alvarez and Bolmont et al.
                They also obtained water loss of 3-6 oceans for planets e to g, which is larger than Bourrier et al (2017) by a small factor of <5.
                What is your opinion on this study?

                • Ramses Ramirez September 23, 2018, 7:24

                  >Interesting discussion with you, Dr. Ramirez!

                  Same here Nicky! I am glad you enjoyed it.

                  I like that Rodrigo gets complete desiccation for TRAPPIST1-b – d.. And they still get complete desiccation for these planets in spite of inferring very low heating efficiencies.

                  That makes sense to me. TRAPPIST-1b is never even in the HZ for the entire lifetime of the star, so it should almost certainly be desiccated. This is in stark contrast to what the original TRAPPIST-1b mass/radius measurements had implied about it being volatile-rich. As I have stated before, I have little faith that these measurements are correct if they get such wild answers for TRAPPIST-1b).

                  There may be some wiggle room for the more distant TRAPPIST-1 planets but it will really all depend on things that are currently unknowable like the initial ocean inventory and the system dynamics. If these planets had really migrated inward from a more distant icy location, that increases their chances of retaining at least some of their water after losses.

                  • Harry R Ray September 23, 2018, 16:37

                    As if the TRAPPIST-1 system wasn’t already complicated enough! ALONG COMES THIS: Research Notes of the AAS, Volume 2, Number 3: Planet-planet tides in the TRAPPIST-1 system, by Jason Wright

                    • Michael C. Fidler September 24, 2018, 6:39

                      Last year I suggest that planet to planet tides would have a large effect on the evolution of each planet in the Trappist 1 system, but it has taken this long for anyone to look at the possibilities.

                      Planet-Planet Tides in the TRAPPIST-1 System
                      Jason T. Wright
                      (Submitted on 21 Sep 2018)
                      “The star TRAPPIST-1 hosts a system of seven transiting, terrestrial exoplanets apparently in a resonant chain, at least some of which are in or near the Habitable Zone. Many have examined the roles of tides in this system, as tidal dissipation of the orbital energy of the planets may be relevant to both the rotational and orbital dynamics of the planets, as well as their habitability. Generally, tides are calculated as being due to the tides raised on the planets by the star, and tides raised on the star by the planets. I write this research note to point out a tidal effect that may be at least as important as the others in the TRAPPIST-1 system and which is so far unremarked upon in the literature: planet-planet tides. Under some reasonable assumptions, I find that for every planet p in the TRAPPIST-1 system there exists some other planet q for which the planet-planet dynamical tidal strain is within an order of magnitude of the stellar eccentricity tidal strain, and that the effects of planet f on planet g are in fact greater than that of the star on planet g. It is thus not obvious that planet-planet tides can be neglected in the TRAPPIST-1 exoplanetary system, especially the tides on planet g due to planet f, if the planets are in synchronous rotation.”

                      https://arxiv.org/abs/1809.08166

                      Something else that I have heard is that no one else has suggested that the whole Trappist 1 system may be artificial. This could be why they are finding such bizarre extremes in the sizes and densities. A level 2 Kardashev scale civilization should have no problem rebuilding these planets into a stable but usable form for their expanding civilization.

                      On the other hand the tides created by the planet to planet tides could cause these planets to out gas large amounts of volatiles that inflate their atmospheres over relatively short periods. This could also cause rings to form around the planet if the eruptions are violent enough.

                      Either way I’m sure that the Trappist one system has a lot to teach the human race.

                    • Harry R Ray September 24, 2018, 9:35

                      This reply is to Michael C. Fidler’s “K2 civilization” conjecture in the reply BELOW this one. It need not even be “KII” if the radius discrepencies are caused by Clarke Exobelts configured in a way where they sometimes transit TRAPPIST-1 while they are DIRECTLY IN FRONT OF OR BEHIND the planet they orbit and sometimes on either side! Solar power stations orbiting the uninhabitable TRAPPIST-1b can STILL BEAM energy to inhabited planets at conjunction.

                    • Michael C. Fidler September 24, 2018, 20:05

                      The transit photometry data is stacked to give an average of 100’s of transits. I’m just wondering if there is or could be made an algorithm to look thru the transits for curves that may show some anomaly? If not, a good program that can analyse the individual transits and form curves that can be visually inspected for signs of rings, moons and Clarke Exobelts or even changes in size of the transits over time. Could we tell if any type of beaming or other type of transmission peaks were also in the light curves. This would be better if the curves were processed by frequency so anything from the stars light could be rejected.

              • Michael C. Fidler September 24, 2018, 6:46

                Dynamical evolution and stability maps of the Proxima Centauri system.

                Interesting – this model includes the effect of Proxima c!

                https://arxiv.org/abs/1809.08210

  • Eniac September 17, 2018, 18:33

    It is very possible. There could be dozens of Earth-like planets around A and B, and we would have no idea. Data from a next generation of instruments could be hugely exciting.

    • Paul Gilster September 17, 2018, 20:53

      Eniac, what a pleasure it has been to see you back these last few times. Always glad to have your take on the stories at hand.

  • Robin Datta September 17, 2018, 20:36

    Looking for the prerequisites of life (habitability) is a perfectly reasonable first step. If we find life, the big question would be where to go from there.

  • Abelard Lindsey September 18, 2018, 0:24

    The other problem with this planet is even if it has an atmosphere that can hold in that ocean, I read somewhere that the ever-present winds as the atmosphere circulates between the hot side and the cold side of the planet would blow at something like a thousand kilometers per hour or even greater across the terminator. That does not sound like a habitable planet to me.

    • andy September 18, 2018, 14:10

      The equatorial jets in the simulations here seem to get up to around 200 km/h or so. Judging by figure 3 of the paper, the western terminator might be somewhat less bad, but there are strong winds over much of the day side.

      It looks like even at the optimistic end of the scale, a habitable red dwarf world is going to be a pretty horrible place by human standards.

  • Rob Flores September 18, 2018, 0:44

    An exiting a place for exploration prox B is. That is the main attractionfor me. Also decent test destination for future fast probes to more promising yet to be discovered extra solar worlds.
    But as far as sending a future (150Yrs) or so manned mission to
    colonize just because it’s relatively close. If it’s mostly the hydrosphere that is habitable, you might do better colonizing Europa’s oceans or Titan.

    Don’t know if anyone has asked this lately:
    If Prox B has landmasses, and animal lifeforms exist in the
    subsurface, could evolution create a physiology that can widthstand
    solar flares with UV and X rays type EM radiation. There would have to be a huge driver for it. Just as aquatic life adapted to land on Earth due to evolutionary pressures , over time a lifeforms maybe able to adapt. (if there is photosynthesis I don’t think an ozone layer could accumulate w/ so much flaring going on)
    After all Proxima C is going to be around for a while.

  • Samuel Goodman September 18, 2018, 4:19

    First TESS discovery; A super-earth orbiting the superjovian-hosting star Pi Mensae.

    https://arxiv.org/abs/1809.05967

    • Abelard Lindsey September 19, 2018, 17:03

      I was wrong.

      I thought the fact that Saturn has Titan and that Jupiter has 4 moons about the size of Titan suggested that Titan-sized moons represented some sort of an upper limit to their size. Clearly this is not the case.

  • AlexT September 18, 2018, 5:47

    Discussed article is science fiction, dream, speculations , no more.
    We know almost nothing about Proxima Centaury b planetary system , so every scenario is possible there and should not be excluded.
    Without direct exploration (by every possible way) of the planets under question , we can speculate endlessly and without any significant result about possibility of ET life here and there.

    • J. Jason Wentworth September 20, 2018, 14:45

      I too am getting impatient with models (I’m not saying that this one is wrong–we simply don’t know which one is accurate, or the ^most^ accurate). Our models of our neighboring solar planets–and even of the Moon–have turned out to be dead wrong (or in need of major revision–look at how thin Mars’ atmosphere turned out to be, and how deep and dense Titan’s was revealed to be, as well as what Mercury’s and Venus’ rotation rates really were [and we were *certain* that Mercury was in a 1:1 spin/orbit resonance–Occam’s Razor and the Hermian maps failed/fooled us that time]) so many times that the models are a dime a dozen. Also:

      A large space telescope or lunar telescope would help, but only sending cameras and instruments past the planets of the Alpha Centauri system–like our lunar and planetary flyby probes closer to home–will answer our questions accurately and in sufficient detail. If the requirements were relaxed a bit (reaching 5% – 10% of the speed of light, instead of 20% of c), we could send solar sail-propelled probes–powered by a 500 meter wide “Sunbeam” spherical-section mirror in solar orbit (it could be spin-tensioned)–that could reach Alpha Centauri within the researchers’ lifetimes, and that we could start working on now.

      • Alex Tolley September 21, 2018, 12:03

        I suspect there is an analogy between particle physics and exoplanets. Physics came up against teh difficulty and cost of devising machines to gain experimental data, while at teh same time, computational power increased sufficiently to allow theoretical physicists a lot of room to play with models, especially string theories. As a result, one can read about all sorts of theories about the ultimate structure of matter, yet there is next to no evidence to guide the models.

        Exoplanet studies are in a rather similar position today, although at least we can see how we might acquire better data to test and calibrate models.

        Science cannot move forward unless we can test the models with falsifiable predictions.

        • AlexT September 21, 2018, 16:04

          Your analogy between particle physics and present state of exoplanets study is not correct.
          Quantum (particle) physics build models to explain different (“strange”) real life events and experiment’s results, i.e. discover laws of nature using collected data.
          Other words – in the beginning there is some amount of data about “strange” event/effect that can be repeated multiple times in real experiment, only after basic data collection scientists can try to invent different theories and build a computer models, that try to check/prove their theories.
          In connection to this particular article related to exoplanets study – present day we almost do not have any information, statistic or data related to topic. We’ve only just began to collect information about planets distribution in our galaxy, still know almost nothing about planets we found (collected data has huge percent of uncertainty). So before someone can begin to build Exo-Earth models on computers he have to collect data, lot of data.
          So I am sure, present time we must make maximal efforts to explore our Universe , at first – we need to find the solution for light years distance travels (at least unmanned).
          Without important data and statistic about discussed exoplanet – it is mostly science fiction, without any scientific meaning… Most important – present time we do not have any instruments and tools to check speculation or prediction related to exoplanets in discussed article.
          Our Universe is huge, so very probable that every human-imaginable scenario of exoplanet (life?) development is possible here (somewhere). But due to lack of knowledge about exoplanets – this area is described today mostly by ancient myths, religious postulates or by science fiction (or other words – literature).
          By the way this situation with exoplanet speculation reminds to me the “Infinite monkey theorem” :-)

          • Alex Tolley September 22, 2018, 15:03

            I have to disagree with you. There is a lot of theoretical physics that is unanchored in data. We need data to discard the ones that are wrong or going down the wrong explanatory path. String theory is very much in that realm.

            • AlexT September 23, 2018, 5:21

              Sorry, but string theory – is not good example to protect your point of view :-)
              String theory at least can explain some very well known quantum effects, and there is lot of scientific data that string theory can use and explain , the problem of string theory that it has good competitors (alternatives) that can explain the same effects by alternative ways.
              In connection to discused exoplanets we know almost nothing, there is no basic data to begin invent exo-planet alago of “string theory” , now every fantastic idea is equal and is pretty good, we know about exo-earths almost the same information that Giordano Bruno knew in 1600 :-)
              So our fantazy is unlimited due to absence of knowledge, and this area is still science fiction kingdom.

              • AlexT September 23, 2018, 5:25

                word alago in my text – it is misprint of “analog”

      • AlexT September 21, 2018, 16:11

        Exactly, before we can to invent some exoplanets theories, we need more data from direct space objects exploration.

  • Michael C. Fidler September 18, 2018, 6:16

    “Volatiles”

    Could the Earth and Proxima Centauri b have been a giant La Brea Tar Pit?

    Abiotic Oil – Abiogenic Petroleum Origin.
    Inorganic Origin of Petroleum:

    http://origeminorganicadopetroleo.blogspot.com/2011/02/normal-0-21-false-false-false-pt-br-x.html

  • Harry R Ray September 18, 2018, 9:53

    TESS’S FIRST CONFIRMED EXPOLANET! Pi Mensae c: 4.82 Earth-mass, 2.14 Earth-radii, 6.27 day orbital period. Pi Mensae b(previously discovered by radial velocity has a MsineI OF >10 Mj and a very eccentric orbit which should preclude ANY planet short of an ultra-short period one from existing in this system, and thus we have a good mystery to go along with the discovery.

  • Alex Tolley September 18, 2018, 13:26

    I too am a little concerned that habitability only works with an ocean world. For a world that is “Earthlike”, we need continents and shallow seas, as well as oceans. Earth currently has no equatorial girdling ocean, and I am not clear that it ever did, from Pangea onwards.

    A waterworld may not be conducive to the emergence of life for a number of reasons, although I am hopeful they may have bacterial life from either a local biogenesis or chance panspermia. Without shallow, well-lit environments, the rich life that we see on continental shelves and coral reefs will be absent. On Earth, oceanic regions are almost deserts due to low nutrients and lack of habitat. That is not to say that multicellular life could not evolve on such worlds, just that the energetics don’t seem to favor it, as it has on Earth.

    The silver lining might be that colonization might be easier on such water worlds. Colonists could build floating cities using resources elsewhere in the system, and be relatively untroubled by local life forms. Seasteading on a global scale.

  • galacsi September 18, 2018, 18:01

    One think I don’t understand is why we focus on hydrogen and don”t care about oxygen ! If a lot of hydrogen escape after photolysis we get an oxygen rich atmosphere and I see two cosequences : 1. Hydrogen will recombine with oxyen to give back water and oxygen will form ozone a good protection again UV. So my take : is it is much more difficult to get rid of oceans than exposed above.

  • wdk September 18, 2018, 18:05

    Reviewing published data on Proxima b, I notice that the discovery was made with visual band doppler data reduction and that the eccentricity was initially limited to under 0.35. That’s a lot of elbow room for velocity measurement uncertainty. Perhaps it has been reduced since the early days.

    Consider two different types of Solar System analogs planets locked into M dwarf system habitable zones: the Galilean satellites of Jupiter with negligible eccentricities and the eccentricity of Mercury (0.206) and its 3:2 resonance of rotation and orbital period.

    An eccentricity that large will expose a lot of proxima’s surface to the its primary – if not all but the poles. But why not the poles? Is there something to prevent large angular precession or tilts to the orbital plane? If a large impact event occurs, plates shift and volcanoes erupt, there could be shifts in principal axes of inertia enough to cause some lurches. Spacecraft and planets have differences and similarities, but I suspect they both have difficulty finding means of damping. ]

    Another less obvious Solar System analog is Triton and Neptune. Triton illustrates plenty of “geologic” history, yet it currently orbits Neptune without hardly any eccentricity at all – but some inclination to Neptune’s equatorial plane ( about 28 degrees retro) and Neptune has a significant oblateness (bulge) from its own rotation. You can imagine that Triton produces a tidal bulge on Neptune with its criss-cross track as well. And I would go so far as to say that the synchronicity existing there is the retro-direction of Neptune’s prevailing equatorial winds…
    Could something like that happen on an M dwarf? With what consequences?

    So I would maintain that there are a number of potential disturbances
    in an M-dwarf system of planets. The disturbances, perhaps in a majority of cases, will end up being quelled, but they could exist to an extent that environments for life could be significantly different than first glance.

    So, in summary? Do we have any means of determining the orbital plane of Proxima b with respect to its primary? Have the eccentricity bounds been reduced? And can we really discount other resonances than 1:1 or 3:2?

    • J. Jason Wentworth September 20, 2018, 23:39

      Indeed, wdk–in our own solar system, we have a “sideways” planet (Uranus), an effectively upside-down one (Venus, whose rotation is retrograde), and a 3:2 rotation/orbit resonance one (Mercury). In an M dwarf planetary system, where a habitable zone planet would have a year just a handful of Earth days long, any of these rotation (or orbit/rotation resonance) modes could enable the planets to have fairly even instellation, and even pretty strong magnetic fields (even Mercury, with its relatively slow 59 Earth day rotation, generates a surprisingly strong–for the circumstances–magnetic field).

      • AlexT September 22, 2018, 2:49

        I suppose that modern science cannot give the correct answer to the simple question:
        Why some planet (Earth, Venus, Mercury … etc.) in Solar system has the magnetics field strength that we can measure today?
        I suppose that much orders less knowledge (most correctly – nothing) we know about Proxima Centaury b planet(s) magnetic field.
        But magnetic field strength parameter can change everything in discussed exo-earth model… as well as many other parameters that we do not know today about Proxima Centaury b planet(s)…

    • Harry R Ray September 26, 2018, 9:37

      YES!!!!! If you have not done so already, please read my ABOVE comments regarding the fact that VLT has ALL OF THE EQUIPMENT AVAILABLE RIGHT NOW to attempt to IMAGE Proxima b and achieve positive results with a reasonable chance of success. If a SERIES of images were successfully taken with the planet in DIFFERENT positions in its orbit, BOTH the inclination AND the eccentricity could be easily deduced.

      • ljk September 27, 2018, 9:27

        Did you contact the VLT to see if they would/could do this?

        • Harry R Ray September 27, 2018, 13:19

          No. All I have done outside of this website is to post comments on Paul Scott Anderson’s(http://planetaria.ca)and Mark Kaufman’s(www.manyworlds.space)websites in hopes that they might inquire and then post something. I do not believe that they, or anyone else would get a good answer before it goes up on the VLT website(a la SPHERE/Visir Alpha Centauri A-B campaign next year). In order for them to schedule an attempt, they would have to have a PROPOSAL in hand. Hopefully Papatis et al have submitted one, but that would most likely be the Gliese 876b test. My hope is that Guillam Anglada-Escude et al would BYPASS the test and go for an image of Proxima b FIRST! I check their “PALE RED DOT” tweets DAILY. So far: Nothing.

  • Geoffrey Hillend September 19, 2018, 15:54

    My prediction is that Proxima b will still have an atmosphere but the biosignature gases oxygen, and CH4 will not be detected with a spectrometer. I like the idea of tidal heating from Proxima and volcanism to replenish atmosphere lost by solar wind stripping.

  • ljk September 20, 2018, 10:22

    Measuring Rocky Exoplanet Compositions with Webb

    Back in June 2016, I advertised a postdoctoral position between me and Steve Desch at ASU for someone to work on the problem of exoplanet interior compositions.

    Normally, the way we determine the interior composition of exoplanets is a combination of inference, measurement, and guesswork. In the Solar System we can study the surface compositions of planets directly, get the bulk density by dividing masses by radii cubed, and a sense of internal structure by looking at how the surfaces have changed or what a body’s gravitational field says about the interior mass distribution. It’s amazing how much we can determine about, say, the interior of Europa!

    Full article here:

    http://sites.psu.edu/astrowright/2018/08/22/rocky-exoplanet-compositions/

    • Charley September 21, 2018, 15:22

      Now I may be incorrect about this, but my take on the determination of the breakdown of the interior of a given body (one subjected to an extremely close orbit of said object) was accomplished in the following manner.

      Briefly, Doppler analysis of the radio signal being sent was continuously monitored and that data based upon the accelerations experienced was ultimately broken down into a mathematical model using the Laplace equation for gravitational potential from which spherical harmonics were derived and thusly from said data (and inference) structure was derived.

      Or am I incorrect in this?

  • Charley September 20, 2018, 23:13

    Wikipedia
    “Alpha Centauri C (Proxima Centauri) is about 13,000 astronomical units (AU) away from Alpha Centauri AB.This is equivalent to 0.21 ly or 1.9 trillion km—about 5% the distance between Alpha Centauri AB and the Sun. Due to the large distance between Proxima Centauri and Alpha Centauri, it was long unknown whether they were gravitationally bound. and an orbital period of 550,000 years.”
    So… That means within approximately 250,000 years Proxima Centauri WILL NOT be the closest star to our Sun. Just thought I’d throw that out there; then we will have to revise the textbooks because Proxima Centauri will be on the opposite side of its orbit. Right ??

    • Paul Gilster September 21, 2018, 9:47

      Everything is in motion. In 28,000 years, after moving into the constellation Hydra, the Alpha Centauri stars will have closed to less than 3 light years from the Sun. Barnard’s Star will eventually get within less than 4 light years. In 1.3 million years, Gl 710 will brush past the Oort Cloud. I’m not sure what will be closest to us 250,000 year from now, but things will have changed a good deal by then. Here’s a bit more on this:

      https://www.centauri-dreams.org/2015/01/02/stars-passing-close-to-the-sun/

    • ljk September 27, 2018, 9:36

      I will be sure to check the textbooks in 250,000 years, definitely. :^)

  • ljk September 24, 2018, 9:42

    I don’t know if Proxima Centauri B has this “advantage”, but if there are more exoplanets in that system and they are as tightly packed as the ones in TRAPPIST 1, perhaps tidal interaction is helping to keep that world geologically active, which could translate into biologically viable:

    http://sites.psu.edu/astrowright/2018/09/21/planet-planet-tides-in-trappist-1/

    Of course this would be “easier” if PCb had a big moon or two.

  • Michael C. Fidler September 25, 2018, 3:26

    He just put an update out today with a new – Erratum: After publication, I became aware of the work of Lingam & Loeb (2018), who noted the potential importance of planet-planet tides on habitability by showing that planet-planet tides between neighboring planets in
    the TRAPPIST-1 system have similar magnitudes to lunar tides on the Earth. The topic has thus not gone completely “unremarked upon” as I stated in my research note.]

    Implications of tides for life on exoplanets.

    “As evident from the nearby examples of Proxima Centauri and TRAPPIST-1, Earth-sized planets in the habitable zone of low-mass stars are common. Here, we focus on such planetary systems and argue that their (oceanic) tides could be more prominent due to stronger tidal forces. We identify the conditions under which tides may exert a significant positive influence on biotic processes including abiogenesis, biological rhythms, nutrient upwelling and stimulating photosynthesis. We conclude our analysis with the identification of large-scale algal blooms as potential temporal biosignatures in reflectance light curves that can arise indirectly as a consequence of strong tidal forces.”

    https://arxiv.org/abs/1707.04594

    Well written, but mainly dealing with oceanic influences, a cursory report about the geological implications. Ljk, I agree with you that any such geologically active, could translate into biologically viable worlds.
    Further that this would cause a SUBSTANTIAL EVOLUTION OF ALL THESE PLANETS, after 7.6 billion years, into forms that are nothing like what exist in our solar system…