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Enter the ‘Belatedly Habitable’ Zone

The most common objection I hear about what we call the ‘habitable zone’ is that it specifies conditions only for life as we know it. It leaves out, for example, conceivable biospheres under the ice of gas giant moons, examples of which we possibly have here in the Solar System. But there is another issue with defining habitability in terms of atmospheric pressures that can support liquid water on the surface. As Jason Wright and Noah Tuchow (both at Penn State) point out in a recent paper, the classic habitable zone concept does not take the evolution of both planet and star into account.

It’s a solid point. A planet now residing in the habitable zone could have remained habitable since the earliest era of its formation. Or it could have become habitable at a later time. Thus Tuchow and Wright make a distinction between what they refer to as the Continuous Habitable Zone (CHZ) and a class of planets they refer to as ‘belatedly habitable.’ These worlds may benefit from changes in the location of the habitable zone as stellar properties change, or they may enter the habitable zone through planetary migration. They may represent a substantial fraction of planets in the habitable zone. But are they truly habitable?

As the authors see it, there is not a single belatedly habitable zone (let’s refer to this as the BHZ), but rather two. The outer consists of the planets whose stars become more luminous over time, thus moving the habitable zone outward. The question here would be whether planets like this can successfully thaw and become habitable. I like James Kasting’s term for these worlds, coined as long ago as 1993. He calls them ‘cold start’ planets, and they represent a lively area of current research.

The inner belatedly habitable zone holds stars around which the habitable zone moves inward as the star dims. These inner BHZ planets are an intriguing lot because they orbit a wide range of lower-mass objects. Both brown and white dwarfs dim with time as they cool, making previously uninhabitable worlds more clement, though the authors note that these may lose many of their volatiles before achieving temperate conditions.

And because of their ubiquity in the Milky Way, we should pay special attention to M-dwarf planets. These worlds may spend millions of years in a greenhouse phase, with the possible loss of water, before their host star has finished the contraction that will eventually place it on the main sequence, dimming enough for habitability.

Given these distinctions, the liquid water habitable zone is actually a combination that includes the Continuous Habitable Zone as well as the inner and outer belatedly habitable zones, and as the authors point out, at any specific time in a star’s history, these regions will have different sizes and as the star evolves, may disappear entirely.

Image: This is Figure 1 from the paper. Caption: Habitable zone evolution for a 0.5 M⊙ M dwarf (left) and a 1.0 M⊙ solar analog (right). Continuous habitability is considered to start at the dashed vertical line, roughly representing the planet formation timescale. The green regions on the plots represent the continuously habitable zone, while the orange and blue regions represent the inner and outer belated habitable zones respectively. Credit: Tuchow & Wright.

To consider what the authors call ‘belated habitability,’ the star’s evolutionary history must be considered along with the presence of volatiles and their origins, the rates of cooling and outgassing as a young planet evolves, its related geophysical processes and more. Thus the complexity of the habitable zone deepens, taking the edge off quick claims for habitability in any given system. The fact that a planet is in the habitable zone today does not necessarily mean that liquid water exists on its surface:

A large portion of exoplanets that we find in the habitable zones of other stars will lie in the belatedly habitable zones, and future missions will greatly benefit by considering belated habitability and not assuming these planets are habitable. For example, in a search for biosignatures, the target stars and the search strategy will be affected by whether or not one considers the habitability of these planets. While the special circumstances of their habitability have been overlooked in the past, belatedly habitable planets could have major implications for future mission design and warrant future study.

I think these are useful distinctions that should come into play as our new generation telescopes come online. It’s certainly true that the press often exaggerates new discoveries of ‘habitable zone planets’ (and our friend Andrew Le Page is a shrewd judge of such claims), but from the standpoint of creating a catalog of best targets for further investigation, we need to be able to winnow the list efficiently and accurately. The study of ‘belated habitability’ should prove a productive research path.

The paper is Tuchow & Wright, “Belatedly Habitable Planets,” Research Notes of the AAS,” Volume 5, No. 8 (August, 2021). Full text.


Comments on this entry are closed.

  • Mike Serfas August 24, 2021, 9:25

    The bump in the Sun’s curve is surprising to look at. I suppose from http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/herrus.html that when the Sun started to do nuclear fusion … it made it colder? I remember seeing papers about a warm young Mars ( https://arxiv.org/abs/1810.01974 ) but I don’t remember them mentioning this as a reason.

    The authors here write that they ignore belated habitability from planetary migration, but they don’t give themselves enough credit. They also ignore differences in habitability depending on atmospheric composition and tidal locking around smaller stars. Planets can be habitable, but zones? The waters of Neptune are too hot for life and the dark side of Mercury, if it existed, would be much too cold. Even Earth’s habitability is subject to revision based on our plans for greenhouse emissions.

    • Gary Wilson August 24, 2021, 13:20

      To follow up on your comment a bit Mike, yes, habitability can be a constantly changing set of parameters. What is the maximum average global temperature we can “achieve” if we burn all of Earth’s carbon resources (primarily coal, oil and natural gas)? I think it could be 10 C hotter or more than the average at 270 ppm CO2. Let’s see (or please let’s not) what that does for overall habitability on Earth. There are many ways to remove a planet from the habitability index including an extremely short term thinking sentient species running amok.

      • Mike Serfas August 24, 2021, 17:36

        Burning CO2 is a slow and uncertain way to do it, but emissions of fluorocarbons and SF6 can be much more potent. It is conceivable that doomed last stands, such as the Panjshir Valley, might one day learn to produce such gases as a means to coerce international support… In the meanwhile, we might also propose using them for terraforming cold worlds. There are apparently minerals with 0.4% w/w fluorine on Mars ( https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014GL062742 ), and if you can manage to collect that I suppose carbon or sulfur should be fairly easy.

        • wdk August 25, 2021, 15:34

          Off stage in the discussions about HZs is the effect of established
          negative feedback mechanisms. Like machines with automatic control systems, this particular planet has feedback mechanisms
          that overcome a tendency for an environmental point to wander away. The reasons could vary, but in many of them the sun would remain with the same flux and the orbit might remain the same as well. Life’s equilibrium point as it is would be very difficult to re-establish if all life’s feedback mechanisms were removed. Yet you could look at charts with radii at distance, albedo, etc. and argue we/they are in a habitable zone. Correspondingly, the feedback mechanisms could allow life to continue in a zone where it could not have been established in the first place.

          With stellar histories of varied mass, we have a formation sequence that speeds up with increased mass. But correspondingly, the planetary formation processes do not necessarily adjust. In G stars we think of circumstellar disks
          rolling back the drapes on planets in about ten million years based on mechanisms for stars of that mass. We know we’ve got planets in lesser or more massive stars, but I don’t think there has been as much timeline examination…

          …It’s nice to have these discussions.

      • Paul Wilson August 27, 2021, 19:45

        Humans have been putting the brakes on carbon emissions for the past 30 years. Gasoline barely had it heyday before engineers were busy making it cleaner and quieter and more efficient. Now internal combustion engineers are being re-assigned to new projects.
        Coal is totally on the way out. The market is totally positioned to jettison its use.
        Natural gas is a pretty darn clean fuel, but its going down too, perhaps even before it needs to.
        Humans picked the energy low hanging fruit, of course they did, and now they are focused on evolving to the next level.

  • Alex Tolley August 24, 2021, 10:32

    Dr. Ramirez wrote a post a while back about the issue of stellar luminosity over time and how that might change the HZ with planets moving into and out of the HZ over time.

    To consider what the authors call ‘belated habitability,’ the star’s evolutionary history must be considered along with the presence of volatiles and their origins, the rates of cooling and outgassing as a young planet evolves, its related geophysical processes and more. Thus the complexity of the habitable zone deepens, taking the edge off quick claims for habitability in any given system. The fact that a planet is in the habitable zone today does not necessarily mean that liquid water exists on its surface

    Planetary conditions and habitability arguments seem all over the map, much as the speculation concerning life itself.

    We have had claims that most planets are more likely Venus-like or desert worlds with little to no liquid water despite being in the HZ. Conversely, there have been claims that water worlds may be very common. These claims imply Earth-like worlds may be very rare.

    If any of these claims are correct, the Drake Equations term n_sub_e is going to be very much smaller than expected, impacting all the subsequent terms.

    We are precisely in the age when speculation can be rife. Hopefully this period will close as we get far better observational data on exoplanets, both to characterize them and to determine which have biosignatures.

    As for the solar system, the idea of a belated HZ was suggested by Olaf Stapledon in Last and First Men. The last, 18th men, lived on Neptune while the sun was in its red giant phase.

    All these speculations about the HZ assume that the exoplanets’ orbits are nearly circular, like Earth’s. Relatively few consider exoplanets with more eccentric orbits. Aldiss’ Helliconia trilogy has a world with very long seasons possibly due to a more eccentric orbit, whilst Hoshino’s manga series, 2001 Nights has a story “Night 15: An Hour’s Song in a Birdless Sky” where the birds have evolved to migrate in spacetime to avoid the period of extreme heat and radiation of their sun as the planet passes perihelion.

    Mars has greater orbital eccentricity than Earth, but just how many exoplanets have even greater eccentricity? Some may stay in the [C]HZ like Helliconia, others may pass into and out of the [C]HZ. What impact would that have on surface conditions and any life that does manage to evolve on such worlds? The universe is likely to generate far stranger worlds than we currently imagine, many in what we currently think of as being in the HZ and therefore potential living.

  • Geoffrey Hillend August 24, 2021, 16:39

    I agree with Alex Tolley and Dr. Ramirez on the potential for a lack of water and the moving of the HZ over time is not enough. The size of the moon or planet matters. Moons like Europa and Enceladus don’t have enough gravity to keep a thick atmosphere. Consequently, there will be no vapor pressure and therefore no liquid water on the surface whether or not they are habitable zone.

    • Kamal Ali August 25, 2021, 5:39

      People forget our Moon is in the Habitable zone but decidedly not habitable! Yes: mass matters. I bet many other factors (eccentricity, water budget, …) too. Can’t wait for JWST to start collecting data so we can start to move beyond speculation.

  • Michael Fidler August 24, 2021, 21:29

    Well now that we find that the M dwarf planets are not destroyed by Super Flares (Can M-Dwarf Planets Survive Stellar Flares?) we have the Greenhouse Doom!
    They forgot that these planets orbit in 10 days and comets and asteroids flyby at huge numbers. They tic tok like a watch that is over 36 times faster then ours! I’m afraid the development of these planets and the volatile elements needed for life will be replenished long after the greenhouse phase. But there is more, these stars outnumber the the rest 4 to one so how many icy deep freeze comets are they going to steal from the 20 percent giant solar systems like ours. The continuous habitable zone is going to last throughout their lives with the bonus of a much faster evolution of life, think dinosaurs…
    Four times as many M dwarfs with three times as many planets in the continuous habitable zone and then you see why they are not so far away and why Avi Loab is really looking for them.

  • Alex Tru August 25, 2021, 2:10

    At first we need to collect more information about all possible worlds that we can or will be able to observe in the future, we do not have almost any i formation about habitability of other planets, so what we can make today , most probably – to build false theory, if we will use false theory as filter for further data collection, we will get biased results, that will reflect our false theory, but not reality…
    So every “what-if “, probably , can be good training for our imagination, but very bad filter for scientific research.

  • DCM August 25, 2021, 4:10

    We will need to be careful to recognize life that doesn’t resemble ours so we won’t unwittingly damage it.

  • Geoffrey Hillend August 25, 2021, 16:00

    I wouldn’t rule out the M dwarf planets since they are tidally locked and there is no rotation with a permanent nightside. Also there is locations where the Sun is just below the horizon and areas behind mountains which aren’t illuminated when the Sun is just above the horizon and the surface might not be too hot from a greenhouse, so the Jury is still out on M dwarf exoplanets.

  • Ronald August 27, 2021, 6:24

    This just out:
    Ocean planets with a dense warm H2 atmosphere can be suitable for life.
    The link to the paper doesn’t seem to work.
    I have my doubts about the claim anyway, at least with regard to higher life.

    • Paul Gilster August 27, 2021, 7:16

      I review the paper in the post coming out later today.