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A Biosignature Plus for K-Class Stars

Kepler-62 is a reminder of how interesting K-class stars (like Alpha Centauri B) can be. Here we find two worlds that are conceivably in the habitable zone of their star, with Kepler 62f, imagined in the image below, orbiting the host star every 267 days. Kepler-62e, the bright object depicted to the right of the planet, may orbit within the inner edge of the habitable zone. Both planets are larger than Earth, Kepler 62f about 40 percent so, while Kepler-62e is 60 percent larger.

Image: The artist’s concept depicts Kepler-62f, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun, located about 1,200 light-years from Earth in the constellation Lyra. Credit: NASA Ames/JPL-Caltech/Tim Pyle.

We actually have five planets here, all known thanks to Kepler to transit their star. The two of habitable zone interest may or may not be solid planets — their masses are not well constrained through either radial velocity or transit timing methods, so we are a long way from knowing whether life might actually form on either. Kepler-62e may well turn out to be a gaseous mini-Neptune, based on its radius. As for the host, Kepler-62 is a K-class main sequence star approximately 70 percent the mass of the Sun, and about 7 billion years old.

K-class stars, particularly those closer than Kepler-62, are seeing a flurry of interest as potential homes for life. In fact, Giada Arney (NASA GSFC) sees them as “in a ‘sweet spot’ between Sun-analog stars and M stars,” for reasons that become clear when you compare them to their smaller and cooler cousins. M-dwarfs are ubiquitous, comprising perhaps 80 percent of all stars in the galaxy, but they’re also given to severe flare activity especially in their early years, enough so that there is a real possibility of damage to the atmosphere and loss of liquid water on the surface.

Add to this problems like tidal locking that could afflict planets in the close-in habitable zone around a cool M-dwarf and by comparison, K-class stars have particular advantages. Arney’s analysis of K star habitability and biosignatures appears in Astrophysical Journal Letters, and it makes the case that a biosignature like the simultaneous presence of oxygen and methane will likely be stronger around a K star than a star like the Sun.

To examine the issue, the scientist developed a computer model simulating planetary atmospheres that could be subjected to conditions around a variety of host stars. Simulations of planetary spectra from these atmospheres could then be produced for analysis. Arney’s work shows that a habitable zone planet circling a K star is one that allows methane to build up in the atmosphere because the host star’s ultraviolet does not generate the highly reactive oxygen that destroys methane as quickly as a star like the Sun. With methane lasting longer within an oxygenated atmosphere, our chance of detecting disequilibrium between the gases increases.

We can add in another factor (one that also favors M-dwarfs): The contrast in brightness between the Sun and our Earth would, to a distant observer, be about 10 billion times, making Earth a very tricky world to observe. Whereas the contrast between a habitable zone planet and a K star is closer to 1 billion. That makes nearby K stars interesting places for future biosignature searches, allowing shorter observing times to achieve a given signal to noise ratio. The author thinks we should keep these advantages in mind as we plan future exoplanet observatories.

The paper lists some interesting targets:

These simulations suggest that nearby mid-to-late K dwarfs such as 61 Cyg A, and 61 Cyg B, Epsilon Indi, Groombridge 1618, and HD 156026 may be particularly excellent targets for biosignature searches on exoplanets. In addition to the “K dwarf advantage” for biosignatures, these stars can offer access to a wide range of wavelengths for HZ planets even with IWA [Inner Working Angle] constraints. 61 Cyg A, 61 Cyg B, Epsilon Indi, and Groombridge 1618 provide higher or comparable S/N to Tau Ceti, the closest G dwarf other than the Sun and Proxima Centauri A. In particular, 61 Cyg A and 61 Cyg B, which are at a similar distance as Tau Ceti (3.6 pc), offer S/N that is 1.6–1.7 times better in the same integration time. HD 156026 is at a similar distance as 82 Eridani (6 pc), and it offers 1.4 times better S/N compared to this G6V star.

But there is this challenge, as alluded to above: Habitable zone planets around K stars will orbit closer to their host than comparable planets around G-class stars like the Sun. That could mean that such planets fall inside the Inner Working Angle (IWA) of future observatories. The IWA defines the smallest separation between planet and star at which the planet can be resolved. Direct imaging telescopes, including the future LUVOIR and HabEx may not, then, be able to see the planet at the needed wavelengths. The paper considers starshade and coronagraph designs that could solve this problem.

The paper is Arney, “The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets,” Astrophysical Journal Letters Vol. 873, No. 1 (6 March 2019). Abstract / full text.

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{ 26 comments… add one }
  • Alex Tolley March 13, 2019, 15:00

    The reduced oxidation of CH4 also suggests that this might make Archaean atmosphere gas ratios (N2-CO2-CH4-no CO) biosignatures easier to detect too. As these are likely to dominate based on terrestrial evidence to date, that might be the biosignature to look for.

    As K-stars live longer than G-stars, what modeling has been done to determine how long a planet in the HZ of a K-star can last before the increased luminosity makes photosynthetic life impossible due to very low CO2 ratios to maintain surface temperatures for liquid water?

  • Bruce D. Mayfield March 13, 2019, 15:46

    Two illuminating paragraphs from the paper, showing the advantages of K Dwarf Stars as far as habitability is concerned:

    “Compared to F and G dwarfs, K dwarfs offer certain
    advantages as habitable planet hosts: they are more abundant
    than G and F dwarfs, comprising about 12% of the main
    sequence stellar population (G dwarfs comprise about 8%,
    while F dwarfs comprise a paltry 3%); their lifetimes are longer
    than F and G dwarfs (17–70 billion years for K dwarfs,
    compared to 10 billion years for the Sun); and the planet-star
    contrast ratio is better for K dwarfs than for F and G dwarfs (a
    K2V star is only about one-third as luminous as a G2V star,
    and a K6V star is only about one-tenth as luminous), making
    their planets easier to observe via direct imaging. Many
    advantages of K dwarfs as habitable planet hosts are discussed
    in detail in Cuntz & Guinan (2016).
    Additionally, compared to M dwarfs, K dwarfs are less
    active, and their pre-main sequence phases are shorter
    (<0.1 Gyr compared to up to 1 Gyr for M dwarfs; Luger &
    Barnes 2015). Recently, Richey-Yowell et al. (2019) measured
    the near-UV (NUV), far-UV (FUV), and X-ray evolution of K
    dwarf stars in moving groups aged from 10 to 625 Myr, finding
    that young planets orbiting K dwarfs are subjected to 5–50
    times lower UV and X-ray fluxes compared to planets orbiting
    early M dwarfs, and 50–1000 times lower fluxes compared to
    planets orbiting late M dwarfs. Richey-Yowell et al. (2019)
    also found that K dwarf FUV and X-ray fluxes decrease after
    ∼100 Myr, compared to ∼650 Myr for M dwarfs, which may
    have implications for early habitability and atmospheric
    evolution for planets around these different types of stars.
    The UV environment of a given host star is critical to consider
    when studying planetary habitability and photochemistry.

  • Adam March 13, 2019, 16:19

    Hi Paul
    The stable gas mixes that K stars might encourage is fascinating. The lower EUV and XUV levels might mean that H2 can build up to higher levels before leaking away into space. That’d change the greenhouse effect warming range for biocompatible planets significantly. Additionally it might encourage biospheres that use free H2 gas, of the kind that Baines & Seager have speculated about.

    Then there’s the more fanciful hydrolox possibility – H2 dominated atmospheres with breathable, non-flammable levels of O2. The surface pressure would be >10 bars so the O2 remains at less than 5%. My chemistry knowledge isn’t enough to assess how quickly such a disequilibrium would last, other than guessing it’d need a vigorous photosynthetic process to return free H2/O2 to the atmosphere. Thanks to the enhanced greenhouse effect from the H2, such worlds would maintain liquid water on the surface out to insolation levels of ~1/100th Earth’s.

    • Andrei March 15, 2019, 13:31

      Hydrogen and methane producing bacteria hide from the oxygen in the atmosphere on Earth. Yet grow very fast when conditions are such that they are not exposed from too much oxygen – like on the bottom of certain lakes and underground. So such life do rather not develop in the open atmosphere.
      But let us speculate that an entire biosphere developed around this instead of being found only in a niche.
      All life is dependent on energy to grow, mate, find food or to avoid enemies. If such life developed that produced oxygen in a hydrogen environment, the organism would have a vast benefit from using it for it’s own needs instead of releasing it. Even if it will not do so, other species would soon find an enormous benefit of utilizing the organism – either as parasite, via digestion or perhaps even by creating an umbrella or gasbag to collect the gas.
      It could be fun to speculate of the fastest hunter on such a planet having a biologically developed rocket engine – haha.
      But this is a very unlikely scenario. The gas would be used by the original plant, or any animal that soon develop to use it for it’s own benefit and very little would be released to the atmosphere – just like on Earth.
      The paper by Baines and Seager is interesting and seem plausible, even more so that the end product is methane. The methane in the gas planet atmospheres is thought to be either primordial from the formation of the planet, or made chemically later. But the ratio of Methane is different between the gas giant planets – might there be one that have some kind of free floating airplankton?

      • Alex Tolley March 16, 2019, 14:18

        It could be fun to speculate of the fastest hunter on such a planet having a biologically developed rocket engine

        Perhaps not so impossible. Jet propulsion is used by squid. Chemical reactions to produce a jet of steam is the defensive strategy of the bombardier beetle. It is not impossible to imagine that an organism couples the chemical reaction to create a steam jet with the functional requirement to move very quickly, perhaps even in the air.

        • Andrei March 17, 2019, 14:27

          Many interesting things have evolved naturally, sonar is one used by bats and swallows. And some insects have even developed countermeasures – one butterfly respond by a sound pulse on their own to confuse detection.
          Now a biological rocket engine is possible to imagine, the temperature of a hydrogen-oxygen jet make the idea less likely to happen. In biology there need to be intermediate steps since the goal cannot be known in advance – but the idea of an animal that use a compressed air jet is a possible path.
          So yes it’s not impossible, but would be a rare case.

        • Michael C. Fidler March 17, 2019, 22:53

          Something that has not been discussed before and could be very fast is warm blooded giants. We have the whales and many extinct land dwelling giants, but just how big can a large brain animal like whales exist on land? Could there be super earths with huge size (1000 feet) creature that have a intelligents and civilization? Can the huge brain be cooled like the whales or would it need some other type atmospheric gases to keep the temperature down. Some of the dinosaurs had separate brains and of course the octopus has a central brain or two plus one for each leg, so could a huge creature have separate brains so its reaction time and speed would be very fast??? Could the dinosaur giants eventually have evolved into intelligent creatures?

          • Michael C. Fidler March 17, 2019, 22:58

            Or sub earths with lower gravity that could develop giants, these may be much more common then any other type of life bearing planets since the current instruments have problems finding the sub earths.

  • Robin Datta March 13, 2019, 16:34

    It is a good idea to look where one has a better chance of finding what one is looking for. Unlike the case of the drunk who lost his keys down the alley and looked for them under the streetlight because of better illumination there, in this case it would seem that there both a better chance that the object sought will be where we seek, and the seeking also may be easier.

  • torque_xtr March 13, 2019, 16:58

    Just imagined what a great conflagration could result if both XUV flux and catalytical activity are so small that O2 and CH4 concentrations could build up until explosion. A chemist in my head says it’s kinetically impossible even to have significant amount of stored energy in the atmosphere this way, but nevertheless… it’s a Planetary Nova!

    • Bruce D. Mayfield March 14, 2019, 11:39

      And if you also have water there would also be clouds with air currents separating charges that in turn produces … lightning. The rapid ignition of a planet’s atmosphere could very well be a real thing.

      There could even be life forms with life cycles requiring such conflagrations for reproduction, like several plants that require fires for germination.

  • Enzo March 13, 2019, 18:17

    I personally don’t share all this optimism on K-stars.
    They are smaller than the sun, so earth size planets passing in front should be easier to detect (bigger luminosity drop in %). Their HZ is closer to the star => shorter orbital period => more orbits for Kepler to observe.
    Where are all the K-earths in Kepler’s data (i.e. with solar flux < ~1.1 or thereabout) ?

  • Michael C. Fidler March 14, 2019, 0:09

    This is great news and should be the direction taken since the Earth centered people have such a problem with intelligent life developing around the Red Dwarfs. But something that is surprising is the ratios of M to K to G to F type stars, there are 6.25 time as many M Dwarfs to K Dwarfs but only twice as many K Dwarfs to G Dwarfs. The G Dwarfs are twice as common as F Dwarfs. So why the huge jump from K to M dwarfs, one possibility is that K Dwarfs have been enclosed by Dyson Spheres because they are the most common stars with exterrestrial civilizations.

    Here is a paper from yesterday about a compact companion around an F9VFe main sequence star with an orbital period of 2070 days and a distance from the primary of 3.69 AU. The star, HD~118475, is 107 light years away and a low-luminosity compact object, considered most likely a white dwarf because no direct imaging observations. The object is found to have a minimum mass of 0.445 that of the sun and is likely higher. But as stated there are problems with the white dwarf concept:

    “Therefore, the only way to reduce the companion’s mass
    would be to reduce the primary’s mass. In doing so, the
    contrast ratio between them would still be such that
    the companion would be readily detectable in imaging
    observations at 880 nm.
    Interestingly, the fact that the companion must be a
    compact object in turn means that the system can not
    be edge-on to our line of sight. Given the age of the
    system (∼4.1 Gyr), we can place limits on the minimum
    mass that such a compact companion could have, on
    the basis that such a body must have passed through
    the entirety of its main sequence evolution. We mod-
    eled the evolution of stars of varying mass, assuming
    that the initial metallicity of the companion matched
    that of the primary ([Fe/H]=0.07), using MIST. As a
    function of initial progenitor mass, we noted the age at
    which the post-main sequence mass loss rate declined to
    approximately zero. This age was coincident with the
    beginning of the white dwarf cooling phase. In this man-
    ner, we determined that the lowest mass progenitor for
    a white dwarf companion that would have completed its
    evolution within the 4.1-Gyr age of the system would be
    a 1.38 M ⊙ star. According to the MIST tracks shown in
    Figure 3, such a star would leave a white dwarf of mass
    0.559 M ⊙ . Comparing this mass with the M B sini of
    0.445 M ⊙ allows us to estimate the range of orbital in-
    clinations that are allowed for our solution. We find that
    orbital inclinations between 90 ◦ (edge-on) and 52.8 ◦ are
    excluded by the compact nature of the white dwarf.
    Thus, the system is not old enough to have produced
    such a low mass compact companion. There is obviously
    a chance that the unseen companion could be more mas-
    sive still. A white dwarf can be no more massive than
    the Chandrasekhar limit of 1.4 M ⊙ . If the companion
    is truly a white dwarf, this means that the inclination
    cannot be less than 18.5 ◦ , as such an inclination would
    require the mass to exceed the Chandrasekhar limit.
    This opens up the interesting, but unlikely, possibil-
    ity that the unseen companion is either a neutron star
    or black hole. Our observations do not rule out such
    an eventuality. We note that the theoretical maximum
    mass for a neutron star is of order ∼3 M ⊙ which corre-
    sponds to an orbital inclination for the system of ∼8.5 ◦ ,
    beyond which the unseen companion must be a black
    hole.
    A fascinating aspect of the system is the significantly
    non-zero eccentricity of the compact companion.”

    The other likely aspect is that this is a K Dwarf with a Dyson Sphere around it.

    Discovery of a Compact Companion to a Nearby Star.

    https://arxiv.org/abs/1903.04529

    Here is a list of K Dwarfs within 100 light-years:

    http://www.solstation.com/stars3/100-ks.htm

    Simbad data on HD~118475.

    http://simbad.u-strasbg.fr/simbad/sim-id?Ident=LTT+5276

    Interesting article on CD from 2009 on K Dwarfs before Kepler!

    https://www.centauri-dreams.org/2009/08/12/in-praise-of-k-class-stars/

    • Ronald March 14, 2019, 8:56

      The over-abundance of M dwarfs is somewhat artificial: in spectral class definition, about everything from 0.5 * solar mass downward (to about 0.08, brown dwarfs) is considered M.

      • Michael C. Fidler March 14, 2019, 10:16

        The Lithium test:

        High-mass brown dwarfs versus low-mass stars.

        “Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which reach the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. Fusion of lithium-7 and a proton occurs producing two helium-4 nuclei. The temperature necessary for this reaction is just below that necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is eventually depleted. Therefore, the presence of the lithium spectral line in a candidate brown dwarf is a strong indicator that it is indeed a substellar object.”

        https://en.wikipedia.org/wiki/Brown_dwarf

        But even Brown Dwarfs are capable of having a habitable zone!

        But that is true to some extent, I’ve wondered if the M Dwarf class should be divided into two groups M 1-4 and M 5-9.

    • torque_xtr March 15, 2019, 6:26

      It will blaze like hell in mid-IR if it is a Dyson Sphere. Would be the first thing we’ve noticed on WISE survey at this distance.

      • Michael C. Fidler March 15, 2019, 10:13

        Yes, if the Dyson Sphere was built by humans I suppose it would blaze like hell, but we are looking at technology way advance from 20th century heat engines. Even now we are looking at ways to capture heat and reuse it, so a civilization that has the expertise to build a Dyson Sphere would have figured that problem out long ago. One of the prime goals would be to make yourself invisible in the cosmic jungle and stealth and camouflage is one of earth’s way that nature uses in our jungles.

        • Alex Tolley March 15, 2019, 12:11

          Yes, if the Dyson Sphere was built by humans I suppose it would blaze like hell, but we are looking at technology way advance from 20th century heat engines.

          This sounds like you expect such technology to break the laws of thermodynamics. At best they might redirect omnidirectional radiation from the sphere/swarm so that observers in most directions do not see it. Perhaps that is what you are thinking about “camouflage”?

          • Michael C. Fidler March 15, 2019, 23:01

            No, but we are dealing with technology that is very advanced from our perspective:

            Kardashev scale – Wikipedia
            https://en.wikipedia.org/wiki/Kardashev_scale

            “A Type II civilization —also called a stellar civilization—can harness the total energy of its planet’s parent star (the most popular hypothetical concept being the Dyson sphere—a device which would encompass the entire star and transfer its energy to the planet(s)).”

            If nothing else, convert the heat to steam and generate power to laser/particle beams to propel spacecraft. But this is like we are living some 30,000 years ago and have a high IQ but if transported thru time to today would look at everything created by man as pure magic. Remember, Arthur C. Clarke third law supersedes all others:
            “Any sufficiently advanced technology is indistinguishable from magic.”

            But the idea that a dyson sphere would feed energy to the planet sounds too planet centered, the entire inside of the sphere would be habitable with whatever technology beneath. If large enough heat may not be the problem, but we are just beginning to contemplate how such ideas would develop.

            • Alex Tolley March 16, 2019, 14:31

              For a planetary civilization to maintain an “invisible” civilization, they could remain at a low technological stage, preferably building underground and maintaining no surface artifacts and ensuring all surface activities like crop growing look very random and natural. No straight edges, no flat roads, no large dams etc. All energy use would be from that available on the surface – solar, wind, gravity – but no surface machines to collect it. If em emissions are needed, then some sort of Faraday cage or camouflage net around the planet could hide emissions and any necessary surface signs of technology. I cannot imagine such a fearful civilization developing much technology, and it would be very authoritarian to enforce the required constraints.

              However, I don’t see how a KII civ is going to want to do this on a solar system scale. If the whole point of such a civ is to be energy rich, then the energy must be emitted in some form that is detectable. As Paul451 says, if you can violate thermodynamics, why even build such structures at all?

              • Michael C. Fidler March 16, 2019, 19:42

                Well, our civilization is already that fearful and the authoritarian to enforce the required constraints already exists. All we need is the proof that hostile civilizations exist!

                As for thermodynamics, yes it’s a law but heat can be changed into other forms.

                The whole point is not to be energy rich but is to survive, that is the oldest fact about life! So yes, energy is being emitted but in short burst like in a military exercise.

                Life is cheap here in the Philippines, if someone gets run over by a bus, the company may pay one to three thousand dollars to the family. Now in the USA insurance policies may insure your life for several million dollars. Now just imagine how much the life of an exterrestrial being is going to be insured for.

                Paranoia strikes deep, but it also ensures stability and protection.

                The next level in any civilization will be the ability to protect and stabilize its existence, how many times have we forgotten histories lessons and paid dearly for it!

                It’s not Fermi’s paradox, but actually Femi’s paranoia that is the reason for no ET direct contact.

        • Paul451 March 16, 2019, 9:18

          If you have the ability to violate thermodynamics, then you don’t need a Dyson swarm. You effectively have free energy without all that messy swarming around stars. Build your habitats in deep space between the stars, no external energy needed.

    • Michael C. Fidler March 16, 2019, 10:46

      A couple of points relating to technosignatures:

      The idea that a civilization is going to be playing their BoomBox as load as possible sounds like a real bad idea. The thousand foot monsters in the next star system may be looking for just such a signal to raid them for their technology. The infrared is the first place to look as in our heat seeking weapons.

      What we may be seeing in this case is a artifact that is still functioning but the original owners have left to live in deep space as Paul451 suggest. This object mimics other objects but is unobservable and the only way it can be detected is thru the it’s effects of its gravity on other objects. This takes time to observe it and as stated could be a white dwarf or even a black hole!

      So here is the question I put to all of you; How would we be able to tell if this object is artificial if the original purpose of it is to not let out any signal that would indicate that it was. Essential making it invisible, no reflection, no gases, no EM radiation of any kind, maybe even no eclipses when it passes in front of it primary star (Stealth technology is already there).

      Hiding it effects on other objects, such as gravity and gravitational microlensing, would be very difficult to do at a Kardashev level 2.

      Stealth technology works well when used on a fast moving aircraft or alien spacecraft but what about distortions in a relatively slow moving object crossing a star?

      You would also have the magnetic field from the K dwarf star that is enclosed by the dyson sphere, maybe auroras?

      • Michael C. Fidler March 16, 2019, 10:58

        Just an afterthought, any optical or EM stealth may only work over only a portion of the EM spectrum or even be switch back and forth.
        Some of the contacts with objects in our skies have been familiar with ECM.

        Electronic countermeasure – Wikipedia
        https://en.wikipedia.org/wiki/Electronic_countermeasure
        An electronic countermeasure (ECM) is an electrical or electronic device designed to trick or deceive radar, sonar or other detection systems, like infrared (IR) or lasers. It may be used both offensively and defensively to deny targeting information to an enemy.

  • Michael C. Fidler March 17, 2019, 23:28

    The 2020 Decadal Survey.

    Which Habitable Zones are the Best to Actually Search for Life?

    https://www.universetoday.com/141739/which-habitable-zones-are-the-best-to-actually-search-for-life/

    https://arxiv.org/ftp/arxiv/papers/1903/1903.03706.pdf

    This is very interesting since now they are looking at main sequence type-A dwarf stars – like Sirius A, Altair, and Vega as having the potential for habitable zone planets. The main problem is the shorter lifetime for these stars, but the larger habitable zone which may include more goldilock planets and UV that may help life develop could open the possibility of planets that are a paradise to life. Our only example is Earth and advanced life only developed in the last 750 million years, plenty of time on most of the A dwarfs for intelligent species to develop.

    One other problem is our lack of understanding and knowledge in the planets between 1/2 the size of earth and 4 times the size, the Titan to Neptune class. there may be a huge variety of types associated with these sizes depending on many factors. Composition, distance from star, volcanic and plate tectonics, atmospheres, impact rates, etc. Large planets could have high volcanic activity that would affect the large oceans, ices, atmospheres, even to the extent of large land masses, since we have no examples of such planets in the sun’s habitable zones. These planets in the Neptune to Titan class have many other factors at work then the gas giants that could make for a huge variety and a vastly more evolving type of planets.

  • Ronald March 18, 2019, 6:29

    And another interesting, and concise, study on HZ prediction, by Ramirez et al.:
    HABITABLE ZONE PREDICTIONS AND HOW TO TEST THEM
    https://arxiv.org/ftp/arxiv/papers/1903/1903.03706.pdf

    And a nice overview here on Universe Today:
    https://www.universetoday.com/141739/which-habitable-zones-are-the-best-to-actually-search-for-life/#more-141739

    In the above-mentioned brief study, Ramirez even considers A class stars as reasonably suitable (with reasons), however, with the critical note that total main sequence HZ time is probably too short for anything beyond bacterial life.

    Personally, I strongly lean toward a kind of Goldilocks Zone with regard to spectral class: roughly the range from about latest F (F7-9), through all G, to early K (K0-2).

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