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M-dwarf Superflares and Habitability

We could use a lot more information about flare activity on M-dwarf stars, which can impact planetary atmospheres and surfaces and thus potential habitability. Thus far much has been said on the subject, but what has been lacking are details about the kinds of flares in question. It’s a serious issue given that, in order to be in the liquid water habitable zone, an M-dwarf planet has to orbit in breathtaking proximity to the host star.

Flares occur through a star’s magnetic field re-connection, which releases radiation across the electromagnetic spectrum. While flares can erode atmospheres and bathe the surface in UV flux, too few flares could actually be detrimental as well, providing as a new paper on the matter suggests, “insufficient surface radiation to power prebiotic chemistry due to the inherent faintness of M-dwarfs in the UV.”

The paper is out of the University of North Carolina, measuring a large sample of superflares in search of a clearer picture of their effect. Flares on the Sun are common enough, as witness the image below, shot in 2012, which shows the kind of coronal mass eruption (CME) associated with stellar flaring. At young M-dwarfs, we can get ‘superflares’ in energy ranges 10 to 1,000 times larger than our Sun provides, bathing a nearby planet in intense ultraviolet light.

“We found planets orbiting young stars may experience life-prohibiting levels of UV radiation, although some micro-organisms might survive,” says lead study author Ward S. Howard.

Or could flares be a driver of evolution? Just how much is too much when it comes to flaring?

Image: A beautiful prominence eruption producing a coronal mass ejection (CME) shot off the east limb (left side) of the sun on April 16, 2012. Such eruptions are often associated with solar flares, and in this case an M1 class (medium-sized) flare occurred at the same time, peaking at 1:45 PM EDT. The CME was not aimed toward Earth. The eruption was captured by NASA’s Solar Dynamics Observatory in the 304 Angstrom wavelength, which is typically colored in red. Credit: NASA/SDO/AIA.

Howard’s team used data from TESS, the Transiting Exoplanet Survey Satellite, coupled with observations from the university’s Evryscope telescope array, to make the study. Previous flare work has homed in on a relatively small number of stars in terms of flare temperatures and radiation flux. Using the largest sample of superflares ever studied in terms of temperature, the new study finds it predictive of the amount of radiation that is likely to reach a planetary surface. A statistical relationship emerges between the size of a superflare and its temperature.

Superflares are not long-lasting events, emitting most of their UV in a rapid peak that may last between 5 and 15 minutes. The TESS data, taken simultaneously with the Evryscope observations, were obtained at a two-minute cadence for 42 superflares from 27 K5-M5 dwarfs. The work extends the range of our observations, while simultaneously playing into the potential target list for the James Webb Space Telescope.

The authors note that flare emissions have usually been approximated by a 9000 K blackbody (a surface that absorbs all radiant energy falling on it). But if superflares are hotter than this, the UV emission may surge by a factor of 10 higher than would otherwise be predicted through optical observation. Only a handful of multi-wavelength observations over short periods of time have been performed, and the TESS/Evryscope work doubles the total number found in the literature, helping us understand how temperature evolves in M-dwarf superflares. 43% of the superflares studied emit above 14,000 K, 23% above 20,000 K and 5% above 30,000 K, with the hottest one observed briefly reaching an incandescent 42,000 K.

We’re dealing, in other words, with a lot of UV flux here, particularly the dangerous ultraviolet C (UV-C) wavelength. From the paper, which takes note of a 2016 superflare observed at Proxima Centauri:

If HZ planets orbiting <200 Myr stars typically receive ∼120 W m−2 and often up to 103 W m−2 during superflares, then significant photo-dissociation of planetary atmospheres may occur (Ribas et al. 2016; Tilley et al. 2019). As a point of comparison, the likely water loss of Proxima b is due to the long-term effects of a time-averaged XUV flux (including flares) of less than 1 W m−2 (Ribas et al. 2016). The median value from the flares observed in YMGs [young moving group stars] is comparable to the ∼100 W m−2 of UV-C flux estimated at the distance of Proxima b during the Howard et al. (2018) Proxima superflare. While Abrevaya et al. (2020) found 10−4 microorganisms would have survived the Proxima superflare, it is presently unclear what effects a 10× increase in UVC flux would have on the evolution and survival of life prior to 200 Myr; it is possible such high rates of UV radiation could drive pre-biotic chemistry (Ranjan et al. 2017; Rimmer et al. 2018), suppress the origin of life on worlds orbiting young M-dwarfs (Paudel et al. 2019), or not impact astrobiology at all if the timescale for life to emerge is longer than 200 Myr (Dodd et al. 2017; Paudel et al. 2019).

Addendum: Alex Tolley makes a solid point about this material in the context I gave it. Let me quote him:

“The extract starting with:

“If HZ planets orbiting <200 Myr stars typically receive
~120 W m-2”

does not make it clear that the flux is for UV, not total output. Only the low values (compared to Earth) and the later inclusion of “UV” allows the reader to infer that this is only the UV flux. In the paper, this section is prefaced by some text that talks about UV and makes the next paragraph clearer. For readers like me, not steeped in stellar knowledge, this could be clearer.”

Exactly so, and I appreciate the note, Alex. — PG

Three of the stars targeted in this study, the aforementioned Proxima Centauri, LTT 1445 and RR Cac AB are already known to host planets. And what of the age difference between young M-dwarfs and their presumably more sedate elder cousins? Should we preferentially target older M-dwarfs? The authors argue that it’s too early to make a call:

Although higher-mass young M-dwarfs may emit more biologically-relevant UV flux as a consequence of frequent superflares than do lower-mass young M-dwarfs, we do not confirm that more UV-C flux from early M-dwarf superflares consistently reaches the HZ. The relative habitability of early versus mid M-dwarf planets is a topic for future work. In particular, the shorter active lifetimes of early M-dwarfs may allow planetary atmospheres to recover as the star ages via degassing (Moore & Cowan 2020).

So we have a useful survey of optical temperature evolution in M-dwarf superflares, demonstrating the predictive quality of flare energy and impulse. The authors intend to continue observations of future flare activity at the same 2 minute cadence, aided by the re-observation of the Evryscope flare targets at its own 20 second cadence.

The paper is Howard et al., “EvryFlare III: Temperature Evolution and Habitability Impacts of Dozens of Superflares Observed Simultaneously by Evryscope and TESS,” in process at the Astrophysical Journal (preprint).

{ 24 comments… add one }
  • Henry Cordova October 12, 2020, 10:56

    Red dwarfs have long and stable lifetimes, and they are by far the most common category of star, but there are other reasons besides their propensity to flare that make them poor choices to search for life.

    Their habitable zones (the range of planetary radii that can be considered likely to allow liquid water to exist on their surfaces) are extremely narrow. Whatever planet-forming process occurs on these stars, the probability that a planet will form, or remain unperturbed, in that narrow an HZ long enough to evolve life seem to me to be negligible.

    All the other contraindications for life, such as tidal locking, seem to me surmountable obstacles considering the sheer number, stability and longevity of these stars available as potential habitats. Primitive microbial life can arise in hostile environments, and can quickly adapt to rapidly changing conditions. But the narrow dimensions of these HabZones, and their questionable stability, seems to rule out multicellular as well as intelligent life.

    I’m aware that many well-informed thinkers on these topics do not necessarily agree with me. Could someone who understands their arguments please explain them to me?

    • Andrei October 15, 2020, 11:48

      While the HZ is indeed very narrow, the close orbits around the star will actually keep planets that circle red dwarf stars quite stable orbits over time. A planet in a more wide orbit will move slower and therefore spend more time in one area being influenced by a second object, a passing star will also affect such a system to a small degree, unless it end up coming very close in a bulls eye approach. Lastly these system often come with several planets, and trough interaction / resonance they do tend to herd each other via gravity into place – even if the system have been disturbed in one way or other.
      This is one reason there’s so much enthusiasm for the Red dwarf systems, the other is of course comparably easy to find both from transits and doppler spectroscopy.
      Now as for the enthusiasm of finding life on such a planet, I cannot but agree I find it less likely than on a world with more moderate conditions.
      I can only guess that the idea is that since such planets are so common, one or other must had a strike of luck and life evolved.
      The main text mention that a changing environment can drive evolution, while this is true, it is then with timescales of 1000 or longer time periods – not minutes!
      But even so such an environment will not be suited for advanced life, on that part we agree, complex life will only be possible under a set of extremely unusual conditions such as the planet got both a big moon that it’s tidally locked to instead of the main star, happen to be located by one unusually quiet red dwarf and still have an atmosphere and magnetic field that provide moderate conditions on the surface.
      If an when such a red dwarf planet is discovered, I will take keen interest in any in-depth study, and if someone else don’t I might even write the research proposal myself! ;)

  • Michael Fidler October 12, 2020, 11:20

    Looking at the nearest stars within 16.26 lightyears we find 83 percent are M Dwarfs. Of these nearby M dwarfs 22 percent are M5.5. The 60 stars within 16.26 lightyears 18.33 percent are type M5.5!

    The reason I bring this up is that we have very little information about even our nearby red dwarfs, their age, type and magnetic properties are not known with any precision. None of the M dwarfs are of naked eye visibility.

  • Alex Tolley October 12, 2020, 13:44

    I am not sure that the UV destroys the needed organic chemistry is correct. I am not saying that this statement is false, but rather that the surface planetary conditions and variability of the UV fluxes mitigate this argument. UV flux even from flares must decline as one moves towards the nightside of a tide-locked planet. There may even be a Goldilocks spot. The variability and intermittency of the flares allows for organic compounds to disperse to “refuges” away from the next high UV flux event.

    In related news, it appears that a very simple reaction can generate the needed compounds to mimic the Krebs energy cycle at the heart of metabolism. New Clues to Chemical Origins of Metabolism at Dawn of Life

    In summary, the UV flux of M-dwarfs from flares may not be the showstopper as often stated (at least for organic chemistry) and that the core respiration of terrestrial organisms may be relatively easily created.
    However, this may be a moot points as:

    I have been more concerned about the very high luminosity of M-dwarfs early in their lives for planets subsequently in the HZ after this early phase ends. For life to potentially evolve during this early period implies that a planet currently in the HZ has survived conditions that should make it a dry world, possibly without an atmosphere, unless the nightside provides a permanent zone to retain water (something a number of commenters have said is not possible).

  • Geoffrey Hillend October 12, 2020, 15:49

    A lot of flux of UVC can also cause the photodissociation of water molecules and water vapor in an exoplanets atmosphere in addition to being harmful to life. There needs to be a lot of ozone and nitrogen in the atmosphere which can block UVC and shorter wavelengths.

  • charlie October 12, 2020, 15:59

    So this brings up an obvious question (at least to me) which is the following: if you have these super flares arising around these red dwarf stars then what is geometrically speaking the likelihood that a particular flare will flare out in the particular direction of the presumably habitable planet? In other words statistically is there a way to find out as to the likelihood that a particular flare will be bursting out in the direction of the planet in which you suspect there may be life?

    If that could be determined in some statistical manner perhaps said planets around said stars might be more likely to harbor life; only occasionally getting bombarded by radiation, which would keep life from perishing but at the same time introducing hopefully useful genetic changes to the population(s) present.

    • Alex Tolley October 12, 2020, 23:02

      Why not assume random directions? Then statistically, the planet will be impacted as a fraction of flare frequency. whether a flare on the surface irradiating the planet, or a CME interacting with teh planet.

      • charlie October 13, 2020, 14:33

        “Why not assume random directions?”
        You can. You certainly can. The directions could be very random, admittedly. My comments simply meant if there was a determination of a somewhat nonrandom irradiation then perhaps that might be a determining factor as to whether or not a particular planet might be more inhabitable versus not.

  • Michael October 12, 2020, 16:25

    I am still astonished with the lifetime of stars !


    Trillions of years and they end up blue ! All an advanced species needs to do is add hydrogen every now and then, a very, very long lived fusion reactor.

  • Michael Fidler October 12, 2020, 22:56

    Here is a very good article dealing with the same exact problem but with the inclusion of the ultracool M Dwarfs from M4 to M10. The analysis finds that the later M dwarfs secondary atmosphere will survive the initial flaring and luminosity-induced escape. The late K and M0-M4 stars are most likely to completely erode both their proto- and secondary atmospheres. The title gives the reason why but it is complicated and I have been spending some time trying to reconcile these two articles:

    Stellar Flares versus Luminosity: XUV-induced Atmospheric Escape
    and Planetary Habitability.
    Space weather plays an important role in the evolution of planetary atmospheres. Observations have shown that stellar flares emit energy in a wide energy range (1030-1038 ergs), a fraction of which lies in X-rays and extreme ultraviolet (XUV). These flares heat the upper atmosphere of a planet, leading to increased escape rates, and can result in atmospheric erosion over a period of time. Observations also suggest that primordial terrestrial planets can accrete voluminous H/He envelopes. Stellar radiation can erode these protoatmospheres over time, and the extent of this erosion has implications for the planet’s habitability. We use the energy-limited equation to calculate hydrodynamic escape rates from these protoatmospheres irradiated by XUV stellar flares and luminosity. We use the Flare-Frequency Distribution of 492 FGKM stars observed with TESS to estimate atmospheric loss in Habitable Zone planets. We find that for most stars,
    luminosity-induced escape is the main loss mechanism, with a minor contribution from flares. However, flares dominate the loss mechanism of ∼20% M4-M10 stars. M0-M4 stars are most likely to completely erode both their proto- and secondary atmospheres, and M4-M10 are
    least likely to erode secondary atmospheres. We discuss the implications of these results on planetary habitability.

    • Michael Fidler October 13, 2020, 23:43

      Nearby star systems within 16.26 lightyears we have 1 A type 1 F type 3 G type and 6 K type stars. From Mo to M3.5 there are 19 systems and from M4 to M10 there are 31 systems. There are 3 times as many stars that may have secondary atmospheres in the M4-10 class as in all the F, G and K classes.
      This seems to be a good indicator that these two separate groups should be look at with the same diligence for habitability, SETI and technosignatures.

    • Ronald October 21, 2020, 9:21

      Good points.
      From the above post and referred paper (and others) I get the impression that in particular the latest K through early M stars (about K5-M5) are prone to superflaring. However, from other sources it is known that much smaller/later M dwarfs, even M9, can be superflare stars.

      My question then is: how is the correlation between stellar mass (and hence spectral type) and superflares?

      A second question: until what age are those flare stars susceptible to superflares? This post mentions 200 my, but I thought it could be much longer than that.

      • Michael Fidler October 22, 2020, 0:15

        Superflares even occur in brown dwarfs, but in M dwarfs between M3.5 and M5.5 there seems to be the transition to a fully convective star from core to surface.
        The main point for stars from M4 to M10 is that UV to X-ray emission do not play a larger role in destruction of the secondary atmosphere but stars earlier then M3.5 it does destroy it.
        Now there several points that may indicate that where the stars form will dictate how much and how long they superflare.
        If they form in clouds that are high in lithium and other metals there may be more and longer flaring, this relates to Lithium 7 ability’s to supplement the hydrogen to helium reaction in the red dwarfs. This also will effect ageing of stars, there is another effect, the in fall from large comets and asteroids in these very low mass stars may have unique reactions. We see in them many molecular clouds yet these should be destroyed in the fully convective atmosphere. There may also be a small core that causes flaring as the magnetic field and high densities are not like our sun.
        Two other ideas that may be causing flaring near the polar region of these stars; Jupiter’s Ion tube from Io and the vorticities we see near the polar region on Jupiter. What is needed are better and longer observations of the nearby red dwarfs to see if there are differences in the type of flares that are being produced.

  • Mike Serfas October 13, 2020, 11:19

    This is a case in which a true tidal lock might be beneficial to the evolution of life, since there would be a gradient of niches for ever more UV-hardy organisms. Orbital resonances or chaotic rotation in densely packed systems might mean life is restricted to more sheltered alcoves for a long time.

    • Alex Tolley October 13, 2020, 22:35

      Or if we consider oceanic life, the effect might be to restrict how close life can live near the surface depending on longitude. The closer to the longitude directly facing the star, the deeper life has to be to avoid the UV damage. Just behind the terminator, or shileded by islands, life may live at the surface as well as the depths, supported by phototrophic organisms in the starlit longitudes. Microorganisms can also live in the lithosphere, protected by very thin layers of rock.

  • Dave Moore October 13, 2020, 15:43

    While atmospheric erosion may be a serious problem for the long term ability to sustain life, a massive UV flux should have little effect on oceanic life. I looked up the extinction coefficient of UV in water, and it rises rapidly from blue light through UV-A and UV-B, until it reaches 10 to the 7 per meter at UV-C. A meter of water blocks all short wavelength UV.

    • Alex Tolley October 13, 2020, 22:27

      So even complex life should be able to exist in an ocean. IDK, but sometimes I get the impression that astronomers seem to think of life as purely surface living on land, and forget that there is life underground and in various aquatic ecosystems. [Dr. Ramirez’ CD post on constraints on complex life primarily focussed on land organisms, although he was cognizant of aquatic life.] Water acts a good radiation shield for all kinds of radiation, em and particles. It might sterilize the surface of the ocean, but there should still be a living photic zone to harvest filtered light from the star.

  • Robin Datta October 14, 2020, 3:02

    The bigger the molecule, the more easily it is disrupted. Small molecules may survive flares. The big ones may need shielding, whether biological, environmental or otherwise. A consideration for such shielding in the course of abiogenesis may help unravel the complexity.

  • Michael Fidler October 14, 2020, 7:33

    Do not forget the shells, from diatoms to seashells even dinosaurs had protective amour. Enough hair or feathers may protect from UVC. Thin skin humans that are black have much less problem with UV radiation, and do not forget the mushrooms!

  • ljk October 16, 2020, 11:21
  • ljk October 21, 2020, 11:55

    Astronomers propose telescope to monitor Betelgeuse dimming

    19 Oct 2020

    An international team of astronomers has proposed a telescope to monitor the bright star Betelgeuse to provide clues about the cause of its sudden drop in brightness. The Betelgeuse Scope concept – which is anticipated to cost about $0.4m – would use twelve off-the-shelf 10 cm-aperture telescopes secured to a radio telescope dish to provide detailed, nightly observations of the supergiant star.

    Betelgeuse’s “great dimming” began late last year and changed the naked-eye appearance of the constellation Orion. With it continuing to enthrall astronomers, theories have emerged to explain why Betelgeuse’s glow has plummeted. A leading contender is that the star’s surface churned out an immense dust cloud that hid some of its famously ruddy light. To closely scrutinize this “mass-loss” activity, researchers will need frequent, high-resolution views of the roiling surface of the star, which are difficult to acquire with most telescopes right now, but possible via interferometry by connecting several telescopes as if they were one instrument.

    “The idea of a dedicated Betelgeuse Scope is a really nice one.” – Graham Harper

    The team is building a prototype of the Betelgeuse Scope that would be placed on a University of Arizona 6.1 m radio antenna and are now seeking funding for the final telescope.

    “If successful, we will bring it to a larger antenna [of] 12 m or more to increase the interferometer array size,” says astronomer Narsireddy Anugu from the University of Arizona, who is leading the project. By using relatively inexpensive instruments affixed to the structure of an already-constructed radio telescope, the final Betelgeuse Scope system should be cheaper than a more complex, conventional arrangement.

    “It also saves money by using the pointing and tracking of the existing radio antenna,” adds Anugu. “So we don’t have to build it for all the individual amateur optical telescopes.”

    Full article here:


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