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Orange Dwarfs: ‘Goldilocks’ Stars for Life?

Our Sun is a G2V type star, or to use less formidable parlance, a yellow dwarf. It was inevitable that as we began considering planets around other stars (well before the first of these were discovered), we would imagine solar-class stars as the best place to look for life, but attention has swung to other possibilities in recent years, especially toward red dwarfs, which comprise a high percentage of all the stars in the galaxy. Now it seems that the problems of M-dwarfs are causing a reconsideration of the class in between, the K-class orange dwarfs.

Alpha Centauri B is such a star, although its proximity to Centauri A may raise problems in planet formation that we have yet to observe. Fortunately, our long-distance exploration of the Centauri stars is well underway, and we should have new information about what orbits the two primary stars here within a few short years. If we were to find a habitable zone rocky world around Centauri B, one thing that makes it interesting is the longevity of such stars.

Unlike our Sun, which is about halfway through its 10 billion year lifetime, orange dwarfs can live for tens of billions of years, offering abundant opportunity for life’s growth and evolution. While not as ubiquitous as M-dwarfs, K-dwarfs appear to be about three times more numerous than G-dwarfs like the Sun. These percentages are always being adjusted, of course, but I’ve seen estimates of G-dwarfs between 3 and 8 percent of the stellar population. A higher population of K-dwarfs, though, gives us plenty of search space for planets possibly bearing life.

How likely are the various kinds of stars to produce habitable conditions around them? M-dwarfs give us fantastically longer lifetimes (into the trillions of years), but at the recent meeting of the American Astronomical Society in Hawaii, Edward Guinan and Scott Engle (Villanova University) described the extreme levels of UV and X-ray radiation through flares and coronal mass ejections that planets in the habitable zones of these stars can receive, with the real possibility of atmospheres being stripped away. “We’re not so optimistic anymore about the chances of finding advanced life around many M stars,” Guinan said.

Guinan and Engle have been engaged in a project called GoldiloKs at Villanova, in which they work with undergraduate students to measure factors like age, rotation rate, and radiation exposure in a sampling of stars ranging across primarily G- and K-class stars. The Hubble instrument, Chandra X-ray Observatory, and ESA’s XMM-Newton satellite are involved in the observations, with Hubble particularly useful for assessing radiation from the K-dwarfs. Lacking the intense magnetic fields powering up X-ray and UV emissions, these stars produce a scant 1/100th as much radiation as would be received by a habitable zone world around an M-dwarf.

To Guinan, these orange dwarfs are indeed the Goldilocks stars:

“K-dwarf stars are in the ‘sweet spot,’ with properties intermediate between the rarer, more luminous, but shorter-lived solar-type stars (G stars) and the more numerous red dwarf stars (M stars). The K stars, especially the warmer ones, have the best of all worlds. If you are looking for planets with habitability, the abundance of K stars pump up your chances of finding life.”

Image: This infographic compares the characteristics of three classes of stars in our galaxy: Sunlike stars are classified as G stars; stars less massive and cooler than our Sun are K dwarfs; and even fainter and cooler stars are the reddish M dwarfs. The graphic compares the stars in terms of several important variables. The habitable zones, potentially capable of hosting life-bearing planets, are wider for hotter stars. The longevity for red dwarf M stars can exceed 100 billion years. K dwarf ages can range from 15 to 45 billion years. And, our Sun only lasts for 10 billion years. The relative amount of harmful radiation (to life as we know it) that stars emit can be 80 to 500 times more intense for M dwarfs relative to our Sun, but only 5 to 25 times more intense for the orange K dwarfs. Red dwarfs make up the bulk of the Milky Way’s population, about 73%. Sunlike stars are merely 6% of the population, and K dwarfs are at 13%. When these four variables are balanced, the most suitable stars for potentially hosting advanced life forms are K dwarfs. Credit: NASA, ESA, and Z. Levy (STScI).

We have about 1,000 orange dwarfs within 100 light years of the Sun, making these interesting targets for future study. Whereas our own planet will face a habitable zone that gradually moves outward as the Sun begins to swell — we’re in deep trouble in a billion years or so — K-dwarfs see much slower migration of the habitable zone, with an increase in brightness by about 10-15 percent over the Sun’s entire lifetime. No wonder Guinan and Engle single out K-star hosts like Kepler-442 and Epsilon Eridani for extra attention. Indeed, Kepler-442 b is a rocky world circling a K5 star that Guinan calls ‘a Goldilocks planet hosted by a Goldilocks star.’

Addendum: I had inexplicably included Tau Ceti above as a K-class star (and yes, I had already had my morning coffee, so I have no excuses). Thanks to readers Alan and Michal Barcikowski for pointing out the error. Tau Ceti is a cool G8 dwarf, with mass about 70 percent of the Sun’s.

All this reminds us of how our views of our own circumstances have changed over time. It was natural enough to believe that in seeking out life elsewhere in the universe, we would look for places like the one we knew supported it. But we’re beginning to ask whether, habitable though it obviously is, the Earth is as ideally habitable as it might be. Let me point you to René Heller (McMaster University) and John Armstrong (Weber State University), who raised similar issues in a 2014 paper in Astrobiology. The duo use the term ‘superhabitability,’ and, although looking primarily at planetary types, also ask about the host stars:

Higher biodiversity made Earth more habitable in the long term. If this is a general feature of inhabited planets, that is to say, that planets tend to become more habitable once they are inhabited, a host star slightly less massive than the Sun should be favorable for superhabitability. These so-called K-dwarf stars have lifetimes that are longer than the age of the Universe. Consequently, if they are much older than the Sun, then life has had more time to emerge on their potentially habitable planets and moons, and — once occurred — it would have had more time to ‘tune’ its ecosystem to make it even more habitable.

Back to Guinan and Engle, whose work over the past 30 years has included X-ray, UV and photometric studies of F- and G-class stars, a corresponding study of M-dwarfs that lasted a decade, and now the collection of similar data for K-dwarfs. My point here is that the K-dwarf work takes place within the context of a robust dataset painstakingly gathered across a wide range of spectral types, giving these two researchers’ conclusions substantial heft.

Is Earth, then, only ‘marginally habitable’ when compared to planets that could exist around stars more benign than our Sun? It’s a fascinating thought that demands we examine our own anthropocentrism while at the same time bolstering our target list for future observatories.

Heller and Armstrong’s paper is “Superhabitable Worlds,” Astrobiology Vol. 14, No. 1 (2014). Abstract available. I’m sure a paper from Guinan and Engle is in the works. For now, however, have a look at Cuntz & Guinan, “About Exobiology: The Case for Dwarf K Stars,” Astrophysical Journal Vol. 827, No. 1 10 August 2016 (abstract).


Comments on this entry are closed.

  • Brett January 13, 2020, 12:17

    The Pre-Main Sequence Luminosity difference is a bit steeper for them than for G-type stars, but it’s probably still within acceptable limits and short enough to not destroy habitability on any planets within the Main Sequence habitable zones of K-type stars. Whereas planets orbiting around M-type stars would probably be super-heated to Venus status or have all their atmosphere and water blown off during the long Pre-Main Sequence phase (especially for the smaller M-type stars like TRAPPIST-1).

  • Tulse January 13, 2020, 12:23

    Isn’t tidal locking still a problem with K-dwarfs? I would think that would complicate the notion of such planets being “super-habitable”.

    • Ashley.Baldwin January 13, 2020, 14:48

      Yes. For later types certainly, some or all of the hab zone falls with the synchronisation area for stars with mass of up to 0.75 Me. Though as has been stated this is no longer the deal breaker it was once thought in terms of habitability.

      Generally the smaller K stars will have larger outer convective layers , a stronger magnetic moment and thus be more active in terms of stellar X ray and UV flux as well as flare activity .

      • Bruce D. Mayfield January 13, 2020, 20:38

        Is there a way of calculating the “synchronisation area” radius as a function of stellar mass? You seemed quite definitive in your answer Ashley.

        • Ashley Baldwin January 14, 2020, 12:35

          Yes, but try :

          ” Asynchronous rotation of Earth-mass planets in the habitable zone of lower mass stars” by Leconte et al , arXiv Feb 2015

          Covers the whole subject with a fantastic “work of art” graph covering all your queries, on page 5. Well worth a read and now excepted as seminal research in this field.

          Ive sent Pul an image of the graph given copyright etc, etc .

          • Paul Gilster January 14, 2020, 12:58

            Yes, and it’s a terrific graph. Will get it up on the site soon.

    • Harold Shaw January 13, 2020, 16:50

      There are atmospheric models that allow tidally locked planets to retain their atmosphere and not have it freeze out on the night side.


      How many planets have the qualities necessary to maintain heat circulation is uncertain. The ones that do could have the type of climate stability that the Moon provides for Earth.

    • Laintal January 13, 2020, 16:55

      For a Planet receiving an Earth like flux orbiting any star above 42% the mass of the sun should put it far enough away.

      I have the links to the papers on my home PC if your interested

    • Michal Barcikowski January 14, 2020, 10:45

      Actually, as currently understood, the habitable zona for synchronous or slow rotators is wider than for fast-rotating planets.

      Source: Kopparapu et al., The Astrophysical Journal, 819:84 (14pp), 2016

      • Ashley Baldwin January 17, 2020, 7:30

        Yes, Yang and Cowan have done work (2014) on tidally locked “hab zone” planets too and shown that the inner edge can be pushed inwards by the build up of a large reflective cloud mass at the substellar point .

        It’s just a question now of whether there are ways such planets can avoid being dessicated by the luminous pre main sequence of M dwarfs , especially the post man in sequence active later types. It’s for this reason that so much time p will be devoted to transit spectroscopy by JWST of the TRAPPIST-1 planets and especially d,e and f. The star is a representative late M dwarf so the discovery of CO2 in particular ( mathematically modelled as just possible within allotted observation time) suggesting a terrestrial style atmosphere – should show that the activity of such stars, pre and post ZAMS , isn’t a deal breaker in terms for any orbiting planets.

        It’s the Pre-main sequence activity that is biggest concern for most geophysicists and astrophysicists at present.

    • Ronald January 15, 2020, 9:26

      This is exactly one of the main reasons why the authors limit the optimum to *early* K-stars, down to about K3, from the article:
      “Another intriguing aspect that also tends to support our main conclusion is the onset of tidal locking, which for planets located in the CLI-HZ (both CHZ and GHZ), pertaining to a timescale of 4.5 Gyr, occurs for stars with effective temperatures close to 4800 (±200) K (i.e., for K3 V stars).”.

      • Ronald January 17, 2020, 10:25

        A bit further on tidal locking and spectral type/HZ, because this is such an important and limiting issue (I am not optimistic about the prospects for higher life on a tidally locked planet);

        Cuntz & Guinan also find tidal locking important, spending a paragraph on it and in fact using it to limit their suitable stellar spectra (also see: Tidal Locking of Habitable Exoplanets, by Barnes, 2017): the authors have taken 4.5 gy as a minimum requirement for a planet to reside in the CHZ. This corresponds to a 4800 K temperature, or about a K3 star.

        I find that very modest, even minimalistic, considering that complex life may need some 4 gy to get really started (e.g. the Cambrian diversity explosion), and we ourselves now being at 4.5 gy. Not exactly super-habitable.

        The overwhelmingly dominant factor in determining tidal locking time (T) is orbital distance (a) of the planet to the host star: if I am not mistaken T relates to the 6th (!) power of a. And inversely: if a planet’s orbital distance is 0.5 , then it’s T will be 1/64 (all other parameters being equal).

        It would be interesting to approach this issue from the other end: calculating how long the tidal locking time (T) would be for an earth-like planet in the CHZ for various spectral type (and hence temperature) solar type stars.
        The optimal situation would be where T is at least as long as the residence time in the CHZ (see table 2 in the abstract by Cunt & Guinan).
        If I find time, I would like to make some ballpark estimates.

        • Ronald January 20, 2020, 8:52

          Ok, I indeed did some back-of-the-envelope calculations, to find the optimum stellar temperature (Teff)/spectral type;

          I made the following assumptions and estimates:
          – I used the CHZ residence times as per the referred article (abstract) by Cuntz & Guinan (table 2).
          – I also used their K3 / 4800 K star as a starting point, and their assumption that tidal locking time (Tlock) for this one would be 4.5 gy.
          – Folowing the previous point, I assumed a planetary orbital distance for that K3 star of 0.5 AU (in other words: at 0.5 AU, Tlock would be 4.5 gy.
          – I then estimated the Tlock for various orbital AU distances.
          – Then I plotted (in an Excel graph) both CHZ time and Tlock (for a planet in the CHZ, earthlike flux conditions) against Teff.

          The result is a kind of ‘X’, where CHZ time goes from high to low with increasing Teff, and Tlock (for a planet in the CHZ) goes from low (exponentially fast) to high with increasing Teff.

          Since CHZ residence time and tidal locking time are *both* limiting, the optimum for long-term habitability would be a stellar Teff where both are at their maximum together: in other words *the crossing point* of the 2 lines.
          Since my guesstimate is rather rough, ánd other factors would also weigh in, I cannot pinpoint it to the K, but my impression is that the optimum for habitability lifespan is somewhere around 5200 K, or about K0 (G8-K1). At that optimum cross-point both CHZ time and Tlock would be between 16 and 22 gy.

  • RobFlores January 13, 2020, 12:37

    Great Work by Guinan and Engle.

    I do agree that Our Sun is not an ideal sun for habitability and for any
    planets to develop advanced life. And even Cooler G5-G9 stars are an improvement on the our sun, habitability wise.
    I think however there is diminishing return on stellar main sequence
    endurance, at least as far a habitability goes. Because the other limit
    is the endurance of internal core size/temperatures of any planets surrounding such stars. After all, Without internal heat, tectonics stops, volcanoes fires extinguish and the end result is no recycling of the crustal materials. For Earth sized planet I think this means 6 billion or so years is the maximum time where there is chance to develop lifeforms, after this time the planet will turn into a dead rock, assuming atmosphere and water is leaked slowly due to a lack of magnetic field.
    Now the exception to this might the Trappist system, since the planetary orbits create friction via gravity and keep the crust hot.
    But the caveat is of course is that Trappist is a very dim M dwarf,
    and there is no guarantee that K class stars would host multiple planets
    with orbits so close to each other, and we have not seen that many compact systems with a passing similarity to it, in the Kepler data.

    • Alan January 13, 2020, 14:27

      I thought that Tau Ceti was a ‘G7-8’ star?

      • Paul Gilster January 14, 2020, 1:08

        Indeed. An inadvertent error I’ll now fix. Thanks.

        • Ronald January 15, 2020, 9:41

          But Paul, despite calling Tau Ceti a K-star, please do not apologize for including it!
          So do the authors:
          *“We are able to conclude that, from a statistical perspective, orange dwarf stars, ranging from spectral type late-G to mid-K are most promising regarding long-term exobiology”.*
          See my much more elaborate comment below.

          • Paul Gilster January 15, 2020, 9:44

            Yes, that’s why I threw it in, as it was in the range covered by the paper, although I couldn’t pull that quotation in time to use it yesterday! Thanks, Ronald.

            • Ashley Baldwin January 17, 2020, 7:35

              Ironically spectral class isn’t just determined by mass. K0.5 40 Eridani has 5% more mass than Tau Ceti whilst G6 82 Eridani has about 9% less and is still significantly more luminous.

              • Ronald January 17, 2020, 10:16

                I have been wondering about that.
                The correlation is (logically) rather sturdy, but not perfect.
                One of my favorite solar type stars: Alpha Mensae, for instance, has an estimated mass about 10% greater than our Sun, but spectral type G6. And B-V color index 0.72, temperature about 5580 K.
                I find that all rather remarkable for such a heavier star.

    • Bruce D. Mayfield January 14, 2020, 17:23

      RobFlores makes a great point about the internal heat requirements for Earth-like planets. Without a sufficient supply of very long lived radioactive elements like U 238 with its ~ 5gy half life the core freezes up, the magnetic dynamo fails and tectonics stops. Therefore, it could be that for planets of K stars to be able to endure as habitable over 10s of billions of years they would need to have formed from more enriched material than the Earth.

      • Ronald January 15, 2020, 9:35

        How about Thorium 232? See my reply to Rob Flores.

      • Ashley Baldwin January 17, 2020, 8:11

        Tidal heating certainly plays a role in maintIning the internal heat load of habitable planets in tight orbits around M dwarf stars.

        The rate at which heat is lost from the core is an important factor too – esoecilly for habitability . Desiccation of any planet by for instance an active stellar flux would likely impact on plate tectonics . This is mooted as requiring water as a lubricant for their “mobile lid” crusts. But the real driver is convection. See what happen to dried out Venus with its “stagnant lid” crust and the difficulty it now has losing its internal heat thanks to suppression of convection throughout the planetary interior . Resulting in little or no magnetic moment. Stagnant lids prevent meaningful convection in the planetary mantle , the process by which heat is transferred way from the core. And the facilitator in chief for core convection. Especially the outer core which needs to be at least partially liquid to be convective itself ( there is evidence that the levels of sulphur play a significant physical role in this too) . For that it needs to be hotter at the bottom than the top to create the necessary thermal gradient. As would occur with mantle convection .

        Regardless of internal heat created by radioactive decay and tidal interactions , planets start with a substantial heat of formation load .

        Barnes and Driscoll explored this for M dwarf planets in 2015 . With two key findings. Firstly that tidal heating plays a significant role in maintaining their core temperature . More importantly still , that convection in a liquid outer core could drive the formation of magnetic dynamos equal or even greater than the Earth – even in the presence of slow synchronous planetary rotation . No need for a moon either if you have a far more massive star nearby to stir up gravitational tides and friction.

        So convection is the name of the game . The Core. The Mantle – and perhaps the star too. For M dwarfs anyway where their large and chaotic magnetic fields heavily impact attendant planets.

    • Ronald January 15, 2020, 9:37

      Thorium 232 has a half-life of 14 gy. A sufficiently large amount of it in the mantle could ensure enough internal heating and plate tectonics for a much longer time period.

      • Michael Fidler January 15, 2020, 10:21

        You have to give our large moon a little credit for some of the heating also. Case in point, we had a very nice total annular eclipse here in the southern Philippines and two weeks later the Taal volcano erupts near Manila. The last time it erupted was in 1977. Planets with mass higher then Earth and super Earth’s will have a much longer active core plus a large asteroid impact would keep plate tectonics going for longer periods.

      • Bruce D Mayfield January 16, 2020, 9:56

        Yes, good point about thorium Ronald. Binary neutron star mergers are the source of both Th and U, so close proximity in space and time to kilonova(s) prior to a K star’s formation would be a help for keeping planetary cores running over the long haul.

        And to add to Michael Fidler’s point re having a large moon, the protoplanet collision that gave us our Moon also gave Earth an enlarged core complete with an extra helping of Th & U. Some Superearth’s may already have plentiful Th & U perhaps due to size?

  • Alex Tolley January 13, 2020, 14:34

    As we don’t yet know what conditions are required for abiogenesis, is stellar type even the right question to ask regarding habitability?

    The analysis implicitly makes teh assumption that the distribution of planetary types is independent of star type, removing a variable that may be more important than longevity and stability.

    On Earth, life evolved very quickly and despite various mass extinctions, it is believed that biodiversity has increased over time (is this real or an artifact of our data and collection limits?). However, we don’t know if there is a biodiversity plateau, and when it might be reached – before or after multicellular life can no longer live on Earth’s surface due to increasing solar output. Cooler stars might live longer but does biodiversity plateau well before solar output renders the planet uninhabitable? Then there is the issue of nutrient and gas cycling longevity. Earth has had continuous plate tectonics during the life era. This seems to be required to weather changing conditions, like ice ages and maintain more stable surface temperature. How long before this cycling stops – as long as teh star can burn, or is this more limited by the energy of the planet, i.e. its formation condition? If so, assuming planet formation conditions are stellar type independent, then is this the more important variable? If so, there could be a large number of cooler stars, many with ages far older than our sun, with rocky worlds still in their HZs, yet effectively constrained to low biodiversity (possibly only microbial) as teh necessary surface conditions could not be maintained as the planet cooled and plate tectonics “froze”.

    For sheer biodiversity, a planet with many isolated regions (e.g. islands) would be best, allowing evolution to create divergent forms in each region and thence different ecosystems.

    My guess is that so many of these questions concerning habitability and inhabitation, we will have to wait until we can collect enough examples to make statistical comparisons to answer these questions. As telescopes improve and we find we can make reliable determinations of the presence of life as easily as we can now detect transiting planets, then we will find out which star types, if any, are the most likely to have inhabited planets.

    Since so much prestige will rest with teh discovery of the first inhabited planet, my guess is that the search will focus on stars that are the easiest to search for such planets, rather than the best candidates based on some theoretical idea of having an inhabited world.

    • Robin Datta January 13, 2020, 23:29

      “so much prestige will rest with teh discovery of the first inhabited planet”
      Indeed. Yet alien megastructures could perhaps be the first confirmation of distant life-equivalent and intelligence. Or for that matter von Neumann probes or stationary robotic outposts. It would be nice if our concepts of biology were supported, but the “unknown unknowns” may or may not do so.

  • Thomas Goodey January 13, 2020, 16:27

    The remarks

    “Higher biodiversity made Earth more habitable in the long term. If this is a general feature of inhabited planets, that is to say, that planets tend to become more habitable once they are inhabited, a host star slightly less massive than the Sun should be favorable for superhabitability… if they are much older than the Sun, then life has had more time to emerge on their potentially habitable planets and moons, and — once occurred — it would have had more time to ‘tune’ its ecosystem to make it even more habitable.”

    sound dangerously like Lovelock’s Gaia nonsense. Why would planets tend to become more habitable once they are inhabited? I think they could equally well tend to become less habitable the longer they are inhabited. It’s just what all the chemicals crashing around amount to in the end – it’s not a super-wise Nature spirit directing the progress of life so that it all turns out for the best!

    • Alex Tolley January 13, 2020, 17:31

      The Gaia Hypothesis is not some woo-woo supernatural idea, but a simple feedback mechanism. “Daisy World” was even used in a recent paper on HZ planets.

      Life does modify the planet and make it more livable for life. This is demonstrated by the colonization of volcanic islands. Bare igneous rock eventually clothed in soils supporting forests full of wildlife. That is why ecosystems emerge and evolve, with more mature ecosystems typically being more biodiverse.

      Plants removing CO2 from the atmosphere and replacing it with O2 via photosynthesis is a temperature regulator. The more CO2 removed and fixed as carbon, the more Earth is kept cool in the face of slowly rising solar output. But equally, add too much O2 and fires start more readily, putting back that CO2 very quickly. The deposition of carbon is the source of our coal and oil deposits which removed CO2 from the atmosphere and helped keep it cooler. Similarly with chalk deposition by marine organisms.

      Ecosystem biodiversity with its many feedback loops creates robustness to perturbations, and different ecosystems allow adaptability by replacing each other in the face of greater perturbations in the local climate due to geological impacts. So we get habitat succession in stable climatic conditions and changes when the climate changes, such as changes in rainfall or temperature.

    • H. Floyd January 13, 2020, 18:57

      Anaerobic archaea: “What he just said.”

      Eukaryote: “Not so fast. Hold my oxygenated beverage.”

      We aren’t mentally equipped to envision K-class timescales. For us, stellar longevity is potentially a great filter that has passed sentence — we wasted Earth’s short youth just deciding what to breathe.

      But for life metabolizing on such vast and relatively stable stellar lifespans, today’s toxin is tomorrow’s food. Several times over, if need be.

    • Project Studio January 13, 2020, 19:32

      Someone could equally well express this as biological ecosystems coming into dynamic equilibrium in such a way that local entropy decreases as low entropy (usable) energy is externally introduced to the open system from a long-life K star. Life is an interesting phenomena not always explainable as ‘chemicals crashing around.’ Otherwise why care about habitability in the first place?

  • Laintal January 13, 2020, 17:01

    Hi Paul

    Wow another very good read, so much interesting news, I need to read these papers, then also catch up on the TOI 700 d papers too. You have set myself and others reading here some homework to complete :)

    • Paul Gilster January 13, 2020, 22:05

      No shortage of interesting papers these days, that’s for sure, particularly with AAS just finishing.

  • Christopher Phoenix January 13, 2020, 17:46

    Fascinating stuff, Paul! I find it extremely interesting that our ideas of planetary habitability have begun changing from the assumption that the Earth and Sun are ideal, to proposing other types of planets and host suns may be even more hospitable.

    It is dangerous to extrapolate from a single data point, and so far we know of only one planet—our own—that supports life. Until we have observed a much larger sample of stars and their attendant planetary systems, we cannot know if our type of planetary system is the most habitable or if it represents a somewhat rarer and less ideal set of circumstances for life.

  • Mike Serfas January 13, 2020, 19:23

    I find I can’t shake the simple-minded view that from a data point of one, we must assume the Earth’s Sun is optimal in every way. Earth has been around for more than a third of the age of the universe, and many earlier systems were short on heavy elements. We seem to be the Goldilocks here, with a long-lived Sun but one that is stable, allowing a large orbit that permits planetary rotation and formation of a large moon. Yes, someday the ravages of age will take their toll, and the forum posters around an orange sun may, very many years from now, make the same argument I am in letters printed against a different color of white – but this is not that day.

    None of this should discourage anyone from looking at orange stars – a point of data is worth more than a volume of philosophy.

    • Robin Datta January 13, 2020, 23:48

      Orange star(s) good?

  • Geoffrey Hillend January 14, 2020, 0:57

    The Gaia hypothesis is supported by the hydrologic cycle, the carbon cycle and nitrogen cycle. The biodiversity of life is also based on the environmental physics, the potentials that can be exploited. It could potentially be that if we remove any one of these requirements which make our Earth a very habitable planet, that the possibility for life perishes, e.g., the need for a strong magnetic field, fast rotation and Moon. The size and gravity of the planet. carbon cycle, not tidally locked. The Gaia hypothesis might be dependent on an exact Earth twin with similar sized Moon, and solar system with large gas giant like Jupiter and outer gas giants. This does not detract from the hardiness and adaptability of life, but only indicates that it’s long term survival must have the most favorable and easily adaptable conditions.

    A good example would be an Earth sized planet without in the life belt around a G class star. Without a rotation and magnetic field there would not only be solar wind stripping, but more radiation as the result of more cosmic rays from it’s own star and interstellar space since there is no magnetic field to repel or trap the charged particles.. Take away a large Jupiter and there might be more asteroid collisions. I am not saying life can’t evolve on a planet in the life belt around a F class star, but more radiation brings more mutations and a more hostile environment. I predict that if one wants to find an exoplanet with intelligent life, that it must have a solar system similar to own and Earth Moon since due to the hill radius, a tidally locked planet can’t have a Moon. I have not considered migrations, and terraforming.

    • Ashley Baldwin January 14, 2020, 12:47

      There are two carbon cycles. The inorganic Carbonate/silicate cycle and the organic photosynthesis/respiration biological based version which underpins the Gaia hypotheses .

    • ole burde January 14, 2020, 13:00

      That IS a good working-hypotesis for the emergence of an advanced biosphere. The next step will then be to investigate the critical values limiting our own earth-life-process : how much smaller could the sun have been without tidal locking happenning for any size of moon ? and how much smaller could the moon have been ?……but for the emergence of simple bacteria-like life in a limited period ,we do NOT necesarrily need much more than warm water , sunligt and a few very common chemical elements. On earth it aparently didnt take more than 100 mil years

    • Ronald January 15, 2020, 10:58

      The importance of our moon for axial stabilization and of Jupiter as an asteroid attractor have been grossly overrated, as research and modelling in recent years have shown.
      And early K stars do not emit (significantly) more radiation.

    • Ashley Baldwin January 19, 2020, 11:21

      Not as bad as you might think. Martinez-Rodriguez ( arxiv 2019) looked at the current sample of exoplanets and found it was possible for synchronously rotating planets in the habitable zones of M dwarfs to possess moons.

      Though in terms of Earth and its Moon, the latter’s postulated role in terms of facilitating habitability is in stabilising the planet’s orbital tilt to just a few degrees . In preventing large changes in this ( as appears to have occurred with Mars for instance ) the theory goes that wild seasonal variations of climate are avoided – changes that might make most of the planetary surface uninhabitably hot or induce “snowball” epochs. Damped down Milankovitch cycles.

      Now as a planet becomes synchronised this process doesn’t just involve rotating at a rate matching its orbital period. Firstly it’s orbital tilt is driven down to zero before secondly ( and much slower ) its orbit is then circularised . Non eccentric. ( further damped down Milankovitch cycles )

      So synchronicity achieves what a large moon does – stabilising planetary orbital tilt. ( whether a zero degree tilt is a good thing is another matter)

      In terms of magnetic fields , Driscoll and Barnes ( arxiv 2015) showed that rapid planetary rotation was not a sine qua non for magnetic field production. Synchronous habitable zone planets around M dwarfs could produce Earth like – or even stronger – magnetic fields, as long as they had a convective liquid iron outer core. This is in turn is dependent on other factors, including inter alia, a convective mantle and the amount of sulphur in the iron core.

      Room for optimism.

  • Geoffrey Hillend January 14, 2020, 0:59

    Excuse me, I meant K class star, not F class star.

  • Michael Fidler January 14, 2020, 4:40

    Well as you all know intelligent life will be found where the planets are, and where they are for the longest time. Where are most of the planets in this galaxy for the longest time??? Around M dwarfs.
    Case closed!

    • Bruce January 14, 2020, 16:46

      No, we don’t all know that at all Michael. Many here have discussed the problems of M dwarf planets losing atmospheres, etc. You’re likely right that the majority of planets will be orbiting M dwarfs, but that in no way leads to an obvious conclusion that M dwarf systems would host the most life.

      • Michael Fidler January 14, 2020, 21:02

        Losing atmosphere is only an issue in the early life of M dwarfs and may be replenished by cometary impacts on a short time scale. The early M dwarfs from M0 to M4 are also not nearly as active flaring as the latter and because of the huge number of these objects should be divided into two seperat groups. Over 3/4 of all stars are in this class and the majority of nearby stars are M and K dwarfs. Tidal locking would create areas near the terimiter of planets that have much less effects from hard radiation. The psychological appeal of finding a earth like planet around a G class star is very strong but I’m afraid that is not where the majority of civilizations exist. The Red dwarf star is the prime example of the human inability to except something that is different from the norm but life and nature does not care what is their, it will find a way to survive.

        • Ronald January 15, 2020, 11:07

          “Well as you all know intelligent life will be found where the planets are, and where they are for the longest time.”

          I am surprised and puzzled by both your comments above: if you read the referred article, you will find that the combination of tidal locking, flaring ánd (long-term) hard UV + X-ray is the problem with M dwarfs and late K stars.

          – The immensely long lifespan of M dwarfs is not yet very relevant in this relatively young universe of ours :-)
          In other words: in this aspect M dwarfs still do not score better than most K stars.
          – The number of M dwarfs may be vast, however, because of their very narrow HZ, the total amount of habitable real estate may actually turn out a bit disappointing.

          • Michael Fidler January 15, 2020, 17:30

            Please read my second comment, I have addressed the issues you mentioned. One of the unusual aspects of these systems is that they are like a watch that is running 10 to 20 times faster then our large planetary systems. These miniature solar systems have comet impacts at a higher rate – Trappist 1 being a prime example, the results on that system from JWST should change many minds…

    • Michael January 14, 2020, 17:07

      Red dwarfs are dangerous for early life BUT look to be very good for intelligent species.

  • Geoffrey Hillend January 14, 2020, 17:36

    I agree. I depends what kind of indigenous life for one looking, the microorganisms, algae, etc or intelligent humanoid ET’s. Migration is a different story.

  • Robin Datta January 15, 2020, 2:48

    The path to intelligence for Home (pauci?)sapiens included the ingestion of an aerobe (now the mitochondrion) by an anaerobe (from the archaea?), the segregation of somatic DNA into the nucleus, the development of eusociality among cells leading to multicellularity. Oxygen-carrying molecules (hemoglobin in Annelida and vertebrates) circulatory systems to transport the oxygen to sites remote from the external milieu, efficient gas-exchange systems as in lungfish and later, allowed for later adequate body and brain size.

    Just the right amount of global water to permit of both land and ocean (with the right orbital distance and relative quiescence of the host star), a moon to stabilize the axis and a Jupiter to sweep away debris may have also contributed.

    Angiosperm forests with continuous canopies allowing brachiation as a means of locomotion, resulting in primates having three-axis movement of the shoulders and binocular vision to accurately judge distance/proximity, allowed for stereoscopic vision when fashioning tools, and the ability to wield and throw objects. Bipedalism freed hominids upper extremities for this; shrinkage of teeth with the control of fire, and other changes to the upper airway with its adaptation to upright posture, allowed for the modulation of sound into speech.

    How’s that for unpacking the biological factors in the Drake equation?Dinosaurs, cetaceans, and others on other tracks did not have the same opportunities. There may be more tracks to technologic intelligence for aliens elsewhere, but for us it is as yet a matter for imagination. Much of science fiction does not address all these issues.

    • Alex Tolley January 15, 2020, 12:31

      I would add to that list of contingent factors the evolution of a body plan that had a notochord and enclosing spine that was the basis of our whole vertebrate lineage, one of the many phyla that appeared in the “Cambrian explosion”. The vertebrate feature was necessary to allow an internal skeleton that allowed for large organisms to colonize the land with the O2 levels we have had since the end of the Carboniferous (an era with large arthropods, both aquatic and terrestrial).

      However, as you note, this is an anthropocentric view. Arthropod analog ETI might well make their own assessment of the factors that led to their species being the highest forms of intelligent life on their world.

      We shouldn’t forget that our raw intelligence has been available for a very long time. It was the “cultural explosion” tens of thousands of years ago that resulted in the flowering of our civilization. We still don’t know why this happened, and it may even be the last great filter behind us.

  • Ronald January 15, 2020, 11:30

    Ok, here’s my longer comment, I took time to study the referred article, which truly fascinated me.

    Great post! Very interesting and relevant, about what is probably my favorite topic, stellar spectral type and (super-) habitability.
    And very well elaborated, also mentioning Armstrong and Heller. Paul, you are like cognac, getting better with time ;-)

    I am pleased to read that what I have argued for years is supported by scientific study and modelling, namely: that there exists a ‘habitable spectral range’. Personally I would expect this range to be approximately from F9 through K2/3, with earlier types being too short-lived for (multicellular) complex life to develop, and later types too problematic with regard to tidal locking and flaring etc. With the optimum around latest G (8/9) to earliest K (0-2). This now seems to be confirmed by this study.

    I think that the most relevant aspect of the referred papers/articles in terms of chances for biological evolution and complex life is *not the total (main-sequence) lifespan of the host star, but rather the “rapidness of stellar evolution for various types of main-sequence stars” (from the article)*.

    More from the article:

    “In this case, we explored when the inner limit of the CHZ/GHZ overtakes the outer limit with the latter recorded at the beginning of the stellar main-sequence evolution. This time of tev, usually referred to as the timescale of the Continuously Habitable Zone (CHZ), describing the region (or duration of time) when a planet can be continuously habitable (i.e., able to maintain liquid water on its surface), is relatively short for early- and mid-F-type stars (…) but very prolonged for the more slowly evolving, cooler, low-mass main-sequence K- and M-type stars”.

    “For example, for K2 V stars, Tev is identified as 22 for the CHZ, which is more than a factor of three longer than for G2 V stars like the Sun.” (in fact, it is over 4 times, as we will see below).

    Table 2 is highly relevant and interesting. It shows the Continuously Habitable Zone (CHZ), not only in terms of orbital distance (AU), but also in time (gy). This is the timespan that a planet can reside within the HZ, while the latter moves outward as a result of the gradual brightening of the host star.

    We see here that for a G2 star like our sun CHZ time is 4.8 gy; note that this is only 0.3 gy beyond our present age! I.e., we still have only some 300 million years left for complex life on earth, which is also shown by the inner edge of the CHZ being so near (0.99 AU in this table). This is, because a G2V star brightens by about 10% per gy, pushing the HZ outward so rapidly.

    We also see, that for a G5 star the CHZ is already 3 gy longer (7.8 gy), for a G8 star it is almost 12 gy, for a K0 star 16 gy, and for a K2 star almost 22 gy. Toward the brighter end, for a G0 star CHZ time is only 3.7 gy and for a F8 star only 2.8 gy.

    This also brings into my memory a much earlier post on CD, closely related and very relevant to this topic:
    This post shows what in my opinion is one of the most important diagrams ever, with regard to planetary habitability and stellar type: Time windows for complex and microbial life on Earth analogue planets orbiting Sun-like stars (F(7), G and K(1) stars) during their main sequence lifetimes.
    This diagram should be updated to show the whole gradual range of spectral types from late F to mid K, but the essence here is: the earlier the type (i.e. the brighter the star), the shorter the window of opportunity for biological evolution, especially for complex life. Which is closely correlated with the CHZ time in the referred article of this post.

    The relevance here is in the time required for a terrestrial planet to ‘ripen’ for complex life, particularly the time needed for the O2 sinks (e.g. crust, oceans) to saturate and O2 to build up in the atmosphere, giving complex multicellular life opportunity to evolve.
    If there exists, as I indeed believe, a sort of ‘clockwork’ mechanism for this, like a natural law, then this implies that complex life requires at least 3 – 4 gy to arise.
    In that case, a G0 star would be the upper spectral limit, with hardly any time left for some evolutionary playtime.

    Back to the article:

    “We are able to conclude that, from a statistical perspective, orange dwarf stars, ranging from spectral type late-G to mid-K are most promising regarding long-term exobiology”.

    Early G-stars like our sun (and F-stars even more so) have a comparatively rapid rate of stellar evolution and hence outward moving of the HZ.
    Latest G and K-stars have longer stable lifetimes and hence slower migration of the habitable zone. They only increase their brightness by about 10-15% during their entire main-sequence life, giving biological evolution a much longer timespan to evolve complex life.
    K-stars later than K3 (and definitely K5 and beyond) have serious issues with tidal locking, flaring and general hard radiation.
    The most promising are latest G and early K stars, from about G8 – K2.

    • Paul Gilster January 15, 2020, 13:20

      Wow, improving with time, like cognac! Many thanks, my friend.

  • Michael Fidler January 20, 2020, 11:04

    I found this on Quora: At least 30% of the 260 red dwarf stars located within 10 parsecs (32 light years) are believed to have flare activity.

    Now with such a long lifetime and such a high clock rate in the mini solar systems there should be a huge number of habitable planets around the older non flaring red dwarf. Higher clock rate = higher replenishment from comets/asteroids of organic compounds and water.
    This means half of the stars are in our galaxy may be habitable red dwarfs with ages of up to 13 billion years or longer.

    • Michael Fidler January 21, 2020, 5:06

      Well, I found some interesting material that may change the viewpoint on Red Dwarfs:

      Newly Discovered Cache of Red Dwarfs Triples the Number of Known Stars in the Universe.
      December 1, 2010

      What is important here is that they are looking at the Initial Mass Function (IMF) which deals with what type of stars made up the initial mass of galaxies after the big bang. What they found was substantial population of low mass stars in luminous elliptical galaxies. These types of galaxies are old and large and form the core of most galaxy clusters. The ratio in these galaxies is 98.33 percent of the stars in them are red dwarfs compared to our galaxy which is around 75 percent red dwarfs. But that’s not all, the age of these stars are from 8 to 13.5 billion years! So like I said there are a lot of none flaring red dwarfs in our universe with many many habitable planets around them and most have been around for over 11 billion years.

      Red Dwarf Discovery Changes Everything!

      A substantial population of low mass stars in
      luminous elliptical galaxies.
      Pieter G. van Dokkum & Charlie Conroy
      Submitted on 29 Sep 2010

      Initial Mass Function Variation in two Elliptical Galaxies using Near-Infrared Tracers.
      R. Elliot Meyer, Suresh Sivanandam, Dae-Sik Moon
      Submitted on 20 Mar 2019

  • Laintal January 30, 2020, 5:57

    The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets