We think of Earth as our standard for habitability, and thus the goal of finding an ‘Earth 2.0’ is to identify living worlds like ours orbiting similar Sun-like stars. But maybe Earth isn’t the best standard. Are there ways planets can be more habitable than our own, and if so where would we find them? That’s the tantalizing question posed in a paper by Iva Vilović (Technische Universität Berlin), René Heller (Max-Planck-Institut für Sonnensystemforschung) and colleagues in Germany and India. Heller has previously worked this issue in a significant paper with John Armstrong (citation below); see as well The Best of All Possible Worlds, which ran here in 2020.
The term for the kind of world we are looking for is ‘superhabitable,’ and the aim of this study is to extend the discussion of K-class stars as hosts by modeling the atmospheres we may find on planets there. While much attention has focused on M-class red dwarfs, the high degree of flare activity coupled with long pre-main sequence lifetimes makes K-class stars the more attractive choice, although less susceptible to near-term evaluation, as the paper shows in its sections on observability. It’s intriguing, for example, to realize that K-class stars are expected to live significantly longer than the Sun, as much as 100 billion years, and because they are cooler and less luminous than G-class stars, their habitable zone planets produce more frequent transits.
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 50 to 100 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 8% 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. Image credit: NASA, ESA, and Z. Levy (STScI).
Let’s dig into this a little further. The contrast in brightness between star and planet is enhanced around K-dwarfs, and spectroscopic studies are aided by lower levels of stellar activity, which also enhances the habitability of planets. While an M-dwarf may be in a pre-Main Sequence phase for up to a billion years, K stars take about a tenth of this. They emit lower levels of X-rays than G-type stars and are also more abundant, making up about 13 percent of the galactic population as opposed to 8% for G-stars. With luminosity as low as one-tenth of a star like the Sun, they offer better conditions for direct imaging and their planets are far enough from the host to avoid tidal lock.
So we have an interesting area for investigation, as earlier studies have shown that photosynthesis works well under simulated K-dwarf radiation conditions. The authors go so far as to call these ‘Goldilocks stars’ for life-bearing planets, and there are about 1,000 such stars within 100 light years of the Sun, Thus modeling superhabitable atmospheres to support future observations stands as a valuable contribution.
The authors model these atmospheres by drawing on Earth’s own history as well as astrophysical parameters, finding that a superhabitable planet would be somewhat more massive than Earth so as to retain a thicker atmosphere to support a more extensive biosphere. Plate tectonics and a strong magnetic field are assumed, as are elevated oxygen levels that would “enable more extensive metabolic networks and support larger organisms.” Surface temperatures are some 5 degrees C warmer than present day Earth and increased atmospheric humidity supports the ecosystem.
The paper continues:
In terms of the atmospheric composition, key organisms and biological sources affecting Earth’s biosphere and their atmospheric signatures are considered. A superhabitable atmosphere would have increased levels of methane (CH4) and nitrous oxide (N2O) due to heightened production by methanogenic microbes, as well as denitrifying bacteria and fungi, respectively (Averill and Tiedje 1982, Wen et al. 2017). Furthermore, it would have decreased levels of molecular hydrogen (H2) due to higher enzyme consumption (Lane et al. 2010, Greening and Boyd 2020). Lastly…molecular oxygen (O2) levels could increase from present-day 21% by volume on Earth to 25% to reflect a thriving photosynthetic biosphere (Schirrmeister et al. 2015).
Given these factors, the authors deploy simulations using three different modeling tools (Atmos, POSEIDON and PandExo, the latter two to examine observability of transiting planets). Using Atmos, they simulate three pairs of superhabitable planets in differing locations in K-dwarf habitable zones, varying stellar radii and masses and star age. They focused on organisms and biological sources that had influenced Earth’s biosphere, including O2, H2, CH4, N2O and CO2 at a variety of surface temperatures.
The results offer what the authors consider the first simulated data on superhabitable atmospheres and assessments of the observability of such life. What stands out here is the optimum positioning of a superhabitable world around its star. Note this:
We find that planets positioned at the midpoint between the inner edge and center of the habitable zone, where they receive 80% of Earth’s solar flux, are more conducive to life. This contrasts with previous suggestions that planets at the center of the habitable zone—where our study shows they receive about 60% of Earth’s solar flux—are the most favorable for life (Heller and Armstrong 2014). Planets at the midpoint between the center and the inner edge need less CO2 for temperate climates and are more observable due to their warmer atmospheric temperatures and larger atmospheric scale heights. We conclude that a superhabitable planet orbiting a 4300K star with 80% of the solar flux offers the best balance of observability and habitability.
Image: An artist’s concept of a planet orbiting in the habitable zone of a K-type star. Image credit: NASA Ames/JPL-Caltech/Tim Pyle.
Observability presents a major challenge. Using the James Webb Space Telescope, a biosignature detection at 30 parsecs requires 150 transits (43 years of observation time) as compared to 1700 transaits (1699 years) for an Earth-like planet around a G-class star. That would be a mark in favor of K-stars but it also underlines the fact that studies of that length are impractical even with the anticipated Habitable Worlds Observatory. The JWST is working wonders, but clearly we are talking about next-generation telescopes – or the generation after that – when it comes to biosignature detection on potential superhabitable planets.
So what we have is encouraging in terms of the chances for life around K-class stars but a clear notice that observing the biosignatures of these planets is going to be a much harder task than doing the same for nearby M-class dwarfs, where extremely close habitable zones also give us a much larger number of transits over time.
The paper is Vilović et al., “Superhabitable Planets Around Mid-Type K Dwarf Stars Enhance Simulated JWST Observability and Surface Habitability,” accepted at Astronomical Notes and now available as a preprint. The earlier Heller and Armstrong paper is “Superhabitable Worlds,” Astrobiology Vol. 14, No. 1 (2014). Abstract. Another key text is Schulze-Makuch, Heller & Guinan, “In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World,” Astrobiology 18 September 2020 (full text), which looks at candidates. Cuntz & Guinan, “About Exobiology: The Case for Dwarf K Stars,” Astrophysical Journal Vol. 827, No. 1 10 August 2016 (full text) should also be in your quiver.
“They emit lower levels of X-rays than G-type stars”
Shouldn’t that be ‘higher’?
No, I don’t think so. The spectral distribution of K-dwarfs shifts toward longer, infrared wavelengths compared to G-class stars. That should produce lower X-ray output, according to the authors.
Are we talking about stable emissions or about flares? EUV and X-ray emissions from flares of smaller stars can be quite a lot stronger than from solar flares. From what I’ve read, M dwarf flares can somewhat enhance optical and NUV emissions. At least that’s my understanding.
I don’t know the answer to that one. This paper does not discuss flare activity. My guess would be that flares are more common on K-dwarfs, but I don’t have anything quantitative on that. Maybe one of the readers can chime in.
Habitability, or at least for large organisms, depends upon TWO long-term conditions: a star which does not brighten and leave the main sequence too quickly AND continental drift with volcanoes to recycle CO2. Does that sound right?
Plate techtonics depends upon internal heat, which comes mostly from radioactive decay of U and Th. I read somewhere that as the supply of those elements declines, plate techtonics on earth will stop in another billion years or go. Goodbye CO2. It will it will go from the atmosphere into the oceans and be buried as limestone, which will not be recycled.
A star which will be fairly stable for 10 or 20 billion years has no advantage over a more massive star which will get too luminous in only 5 billion years.
There’s a lot of assumptions here that may not hold true outside of earth (or even on earth).
Limestone is a biological product, and is formed by a very limited number of earthly species. There is no meaningful abiotic production of limestone on earth. Ergo, there is no guarantee that limestone will be produced by alien life, or abiotically on an alien planet.
Secondly, limestone spontaneously breaks down into carbon dioxide, as is seen during weathering of limestone. Plus, on earth, there are numerous species of bacteria and fungi which can acquire carbon by “digesting” limestone. In other words, volcanoes are needed to recycle limestone that is pulled beneath the planets surface – something which would cease once plate tectonics ends – surface-exposed limestone recycles via abiotic and biotic processes which would/could continue post-tectonics. And again, that assumes that the planet evolved limestone-generating life to start with.
@Michael, @ Bryan
Both of you are making claims that are not true, AFAIK.
Michael, carbon recycling is not purely abiotic as you state. Carbon is most rapidly recycled by biotic processes – primary production fixes carbon, and metabolism by organisms living on the primary producers releases the carbon as CO2. We can see that in the annual fluctuations in atmospheric CO2. The geologic cycle is far longer. Should the geological cycle end, biological recycling will continue.
Bryan, limestone is primarily a biological product during the last half billion years, but it is also produced abiotically. The dense primordial atmosphere with far higher concentrations of CO2 was reduced by geology, not life, say in the Hadean and probably much of the Archaean. Also don’t forget that other elements, e.g. magnesium, can remove CO2 via precipitation.
Overall, I would agree with Bryan that once life gets started, assuming it is not extinguished, biology can maintain the carbon cycle even if the heat driving plate tectonics declines. Plate tectonics can end for other reasons, e.g. Venus only has volcanoes as the crust is now a lid. Yet clearly Venus has a dense CO2 atmosphere that could be reduced by life fixing the carbon and sequestering it as organic carbon rather than carbonates as happened during our Carboniferous age.
Every rocky planet will experience tectonics, the geological activity driven by cooling. Plate tectonics is a type of tectonics. Venus’s single-lid tectonics is another. I am not sure if Pluto’s cryo-vulcanism is considered tectonics. Afaik, the biotic, fast carbon cycle is likely to permanently sequester carbon and thus unlikely maintain atmospheric carbon for geologic timescales. Tectonics, not necessarily plate, is either required or advantageous.
Why? If plate tectonics ends, then the erosion of high ground to low, ends, and carbon will not be buried. Are you arguing that CaCO3 in shells and skeletons will never be recycled? Or perhaps dead plants and animals will just build up and not be consumed? What evidence is there that this happens in the absence of deposition of sand or mud to bury animal and plant remains?
The only thing I can think of is coral reefs, although wave erosion will eventually turn them into sand that can be recycled by organisms. Not my field, but is there any evidence of “fossil” reefs that remain after the coral organisms died and a few million years of physical and chemical forces failed to erode it away?
Tectonic activity never went through a plate regime on Mars and has slowed to a crawl. Mountains and valleys remain. On a perfectly smooth planet winds would still bury animal and plant remains. The absence of mechanisms to bury and mineralize biology sounds unrealistic. I would expect even the buried oceans of icy moons would have mechanisms.
Biology may persist deep underground on Mars and oceans are a source of atmospheric carbon. However, the loss of tectonics leads to the loss of oceans and surface water. I am absolutely willing to leave open the possibility of life without tectonics but only for the sake of argument.
Life on Earth potentially appeared before the crust formed and plate tectonics became global 2.5 bya, well after life was flourishing. I think we can both agree that plate tectonics isn’t crucial.
Life can surely exist, for a time, without plate tectonics. However, you are that carbon will ultimately be sequestered so that the available carbon will diminish over time. Without some way to restore that carbon (assuming cooling of the mantle ends volcanism), you are arguing that it is inevitable that the biological cycling of carbon will wind down. If true, idk how long that would take, but presumably, this would imply that stars with long lives but with rocky planets whose plate tectonics either never start or have ended, that life may only have a limited time to exist, returning the planet to a sterile condition.
This was not a condition that was considered for the random events’ impact on life. However, if carbon sequestration becomes inevitable as a planet cools and plate tectonics ends, then this becomes a factor for long-lived stars and life-bearing worlds. Is this eventually a testable hypothesis, that stars with ages exceeding T billion years will likely have no biosphere due to a lack of carbon?
The possible outcome is that these stars may have intelligent technological life to extract needed elements to extend the life of their biospheres, mitigating the sequestration of carbon. [Or could non-intelligent life evolve to provide the means to reverse the sequestration?]
The first snowball Earth period would be a good example which was 2.4 to 2.1 billion years ago. It was followed by a hot house period where it took millions of years for the atmospheric carbon dioxide to reach a high enough level to melt the ice. There might be less volcanism without plate tectonics and it might take a longer time. There might be carbon dioxide replenishment problems due to life and photosynthesis without plate tectonics. Venus is a good example. Also a planet with a large Moon like our Moon increases the probability or efficiency of plate tectonics.
I will propose that a planet around a K class star won’t have indigenous intelligent life like us, only lower forms of life and microbial life since the star is formed from a smaller gas cloud and with less gravity such a K star solar system might be missing some important contingencies that our larger solar system has like more gravity and therefore more planets and probability of collisions for Moon forming collisions like the giant impact hypothesis which gave us our Moon.
Recall that a planet can’t have a magnetic field unless it has a fast rotation like Earth which got it’s angular momentum from it’s collision with Thea. The Earth was spinning four times faster after that collision and has slowed down over 4.3 billion years. Without a magnetic field, a planet losses atmosphere due to solar wind stripping, etc. A Earthlike planet in the life belt around a K class star might be tidally locked which is a problem for it having a Moon.
In the case of M dwarf vs K stars , if we look at the in terms of black body flux, there should be considerably less x-ray flux from the former, as both should have much less than our sun. So it seems a question of how and where the main sequence spectral classes depart from the black body model – and why? To be straight on this, I should say I don’t know why, but one thing I do notice different between M dwarfs and G’s like the sun.: Below the surface the convection zone in a red dwarf is a larger fraction of the interior. Usually this is cited as a thermodynamic mechanism that assists a red dwarf to extend its lifetime out 100s of billions of years or more: the core is enriched with more and more hydrogen than its more massive brethren. Now is the this transition in the extent of convection abruptly changed beyond M – or does it extend somewhat into K’s?
Or does it have any connection with x-ray or other surface outbursts? Not a pressing question, perhaps, until one looks more closely at habitability…
Larger stars have more fuel and bigger, hotter furnaces, and as a result emit more energetic (=higher frequency, shorter wavelength) radiation.
Hi Paul
A very interesting read comments and link to a paper.
the amount of research papers on these K stars that I have saved and read is a fraction of the ones about M dwarfs.
These Stars are ideal places to host planets with life, from memory from past studies, its been calculated the ideal star is some 80% as massive as the Sun with a slightly larger.
The readers here have asked some great questions too, that I have been asking myself. To help understand these and M stars Ive done my own sort of Excel list of mass and various features one mistake is not referencing the paper when I noted down the properties. If anyone is interested I can post it up as a comment.
Thanks Edwin
@ Edwin.
Looking forward to seeing a summary of your studies of some kind.
A few entries above, I remarked on the difference in interior extent of convection zones in M-Dwarfs in comparison to the sun. Looking at it further, I note that in diagram form, the convection zone runs from near the “surface” to about 1/4 the radial depth. So when we see cell structure on the surface, we see both structure and a measure of turbulence. But as noted, M-Dwarfs have convective patterns that reach to the core stoking the fusion process with a larger proportion of the star’s hydrogen fuel. Thus the reactive core is not cut off from hydrogen.
But apparently this convective process in M-Dwarfs provides a surface cap which is much richer in high energy emissions and E-M flares or “storms” than than our sun or Main Sequence G stars generally. Apparently K stars have not drawn much attention about their analogous flare environments. But I suspect that the interior models, the extent and location of radiative and convective zones, would give a clue as to what to expect: whether K’s resemble G’s or M’s more in this respect.
We often talk about which is the best stars for habitablity but what about atmospheres. How about an extra thick atmosphere say 5 and 10 bars, the thicker the atmosphere the more 3D of a habitat the planet would become. There are so many complexities of habitablity it boggles the mind !
Hi WDK
they are saved on my home PC so I will have to have a look tonight, I made a series of notes to help me understand these stars and planets, when new discovery’s are made its always good to refer back to.
I’ll try to post it up tonight (Late overnight EST)
Cheers Edwin
Hi folks
To help understand the habitability of stars I’ve made the following notes.
This assumes an Earth mass planet with an Earth like magnetic field and atmosphere.
Below assumes
STARS
Mass Radius Key feature
1.20 Maximum size of a habitable star, any larger and the UV and short lifetime make it increasing unlikeley to have habitiable planets
0.84 planets orbiting stars with masses around 0.84 M1 have the longest habitable lifetimes
0.45 Above this steller mass a planet experiences minimal tides
0.42 A planet is far enough away that when receiving the same flux as the Earth is unlikely to be tidally locked.
0.40 Exomoons orbiting giant planets around stars smaller than this mass will experience more tidal heating.
0.38 A planet is far enough away to be safe from losing its atmosphere from flares and the steller wind.
0.35 Fully Convective (the interiors are fully convective mixing throughout perhaps up to .40 solar masses
0.30 Solar Thermal tides in the Atmosphere can be strong enough to prevent tidal locking above this steller mass range
0.30 Planets in Eccentric orbits are likely to experience tidal heating and flexing, this might be helpful for small planets
0.25 Very unlikely for a moon or Planet to hold an atmosphere if orbiting a star below this mass
0.20 Below this limit there might not be enough photons for Earth like life https://arxiv.org/abs/1901.01270
0.20 Habitable Moons possible above this Steller mass
0.16 Never evolve to red giants at the end of their life’s
0.08 Smallest Star possible
Steller Flux relative to earth
.08 to .50 M dwarf classification mass range
1.44 Tidally locked Maximum Steller Flux for an habitable planet around a red dwarf star
0.91 Maximum Steller Flux for an habitable planet around a red dwarf star
0.22 Minimum Steller Flux for an habitable planet around a red dwarf star
1.11 Maximum Steller Flux for an habitable planet around a Sun like star
0.36 Minimum Steller Flux for an habitable planet around a Sun like star
PLANETS
Mass Radius Key feature
3.00 1.50 Above this mass there is a high probability there will be an extensive atmosphere, making a planet unlikleyu to be habitiable.
2.50 1.40 A rocket would be unable to launch from a planet above this mass
2.00 1.20 As mass increases from this point the probability of becoming a mini Neptune increases
1.50 1.10 Ideal maximum mass to be habitable
0.50 0.70 Ideal minimum mass to be habitable
0.30 0.45 Ideal minimum mass to be habitable, Any smaller and a planet would not be able to hold onto an atmosphere and have a strong enough magantic field.
@Edwin
What are the assumptions in this statement?
All possible rocket propulsion?
Chemical rocket with maximum Isp of ??? s.
Chemical rocket with maximum M0/M1 ratio?
Chemical rocket with LH2/LOX with the best current technology?
I assume this does not rule out other technologies to reach orbit, such as air-breathing or hybrid propulsion, external propulsion energy, nuclear energy, etc.
Hi Alex
All the above notes were just a summery of the key properties, I realize now when taking the notes down I should have referenced every paper they were taken from. (Ive also been doing this with my photography too).
These notes were a guide to help me understand any new discovery’s and planets as they were made, I could refer to this chart as a quick reference.
Alex I don’t have a connection with you on social media or your email address, I’m at work at the moment but I can try and send you the paper that note was from.
From memory a planet above a certain mass a rocket would not be able to launch into space from its surface. It was from a paper looking at Habitability of Super Earth planets.
If your able to send me your email address I can email the papers to you.
So the motto if a little off topic here folks is make sure to take notes on your work and backup your files too :)
Thanks Edwin
Hi Alex
Here is the paper you need to read and refer too
Spaceflight from Super-Earths is difficult
https://arxiv.org/abs/1804.04727
I wasn’t sure if if it was saved in my Super Earth’s folder or spaceflight folder, found it easy enough in my Super Earths folder
Hello, Edwin.
Want to thank you for your follow up on that suggestion. And I would say that the list you compiled is much akin to our present particular situation, which is debating or researching whether K stars have significant x ray and other high energy outbursts. So don’t be surprised if (eventually) comments come back arguing pro or con on the points cited.
In parallel, I would like to report that my approach to the issue was to start looking at descriptions of individual, well-know K stars to see if I could find any remark on
X-ray flares. From my files I found a Scientifc American poster size Hertzsprung Russell diagram with hundreds of labeled stars, several dozen in an orange section of the Main Sequence representing the K stars ( ranging end to end 4000 to 5000 Kelvin. Since the entries were well known, the stars rated a Wikipedia article which might address any x-ray eruptions. Starting at the lower end with 61 Cygni and Epsilon Eridani, I have not seen any so far. In some cases there are remarks on planets, surrounding dust and their binary nature, which for some reason K stars listed seem to tend toward binaries.
But aside from the fact that I have not found a Main Sequence K with x-ray spectra, regular or irregular, it might be worth making a note:
We probably all agree about the idea of a black body spectral behavior for stars overall. And that would be that their luminosity compiled over wavelength has a curvature close to the theoretical black body curve. And it can be expressed as an analytical function for effective temperature, the distribution of radiated light over the spectrum. But nonetheless, stars vary in that adherence to that mathematical distribution, either temporarily with an outburst – or the way they illuminate their surroundings overall.
I think it fair to say that many red dwarfs or M’s do both. Emit x-rays regularly and also have passing or irregular storms where the x-rays and charged particles exceed that associated with our sun.
So, from what I have NOT been able to find is a lot of K star x-ray activity in our neighborhood of, say, stars with ten parsecs or so.
Since luminosity at the K to M boundary on the HR diagram drops precipitously,
it leads one to suspect that there is significant difference in internal structure such as where the boundaries are for heat transport methods, radiative vs. convective.
Since M dwarfs are constructed to about the tenth the radius of the sun and the majority of their interior is convective, that means that the lower bound is very close to core in terms of genuine yardsticks like kilometers. I suspect that the
lower convection layer is hotter than in K or G s and the hand off to the surface
is a lot less uniformly “black body” when it reaches the surface.
Maybe when the internal structure and thermodynamics change significantly enough to notice, that’s where what we have come to think of the K main sequence got its definition. But if that is true, it seems to have come to pass indirectly. As near as I can tell, spectral analysis was where the K – M demarcation originated. The distinction between dwarf and giant in that spectral region is made more of in the references that I can find. The structural reasons seem somewhat of a mystery, but I suspect it has been well addressed – somewhere.