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Can M-Dwarf Planets Survive Stellar Flares?

We can learn a lot about stars by studying magnetic activity like starspots, flares and coronal mass ejections (CMEs). Starspots are particularly significant for scientists using radial velocity methods to detect planets, because they can sometimes mimic the signature of a planet in the data. But the astrobiology angle is also profound: Young M-dwarfs, known for flare activity, could be fatally compromised as hosts for life because strong flares can play havoc with planetary atmospheres.

Given the ubiquity of M-dwarfs — they’re the most common type of star in our galaxy — we’d like to know whether or not they are candidates for supporting life. A paper from Ekaterina Ilin and team at the Leibniz Institute for Astrophysics in Potsdam digs into the question by looking at the orientation of magnetic activity on young M-dwarfs.

The sample is small, though carefully chosen from the processing of over 3000 red dwarf signatures obtained by TESS, the Transiting Exoplanet Survey Satellite mission. The results are promising, indicating that the worst flare activity an M-dwarf can produce occurs along the poles of the star. If that is the case, then a young planetary system may remain unscathed. Here’s how Ilin describes this:

“We discovered that extremely large flares are launched from near the poles of red dwarf stars, rather than from their equator, as is typically the case on the Sun. Exoplanets that orbit in the same plane as the equator of the star, like the planets in our own solar system, could therefore be largely protected from such superflares, as these are directed upwards or downwards out of the exoplanet system. This could improve the prospects for the habitability of exoplanets around small host stars, which would otherwise be much more endangered by the energetic radiation and particles associated with flares compared to planets in the solar system.”

Image: Small stars flare actively and expel particles that can alter and evaporate the atmospheres of planets that orbit them. New findings suggest that large superflares prefer to occur at high latitudes, sparing planets that orbit around the stellar equator. Credit: AIP/ J. Fohlmeister.

This is provocative stuff, even if only four stars emerged from the TESS data as fitting the criteria the researchers were looking for. To understand how they winnowed these stars out, we need to look at PEPSI, the Potsdam Echelle Polarimetric and Spectroscopic Instrument, mounted at the Large Binocular Telescope (LBT) in Arizona. Feeding polarized light to the spectrograph, scientists using PEPSI have been able to use what is called the Zeeman effect — involving the polarization of spectral lines due to an external magnetic field — to analyze the field geometry of the field.

This earlier work has implied the existence of concentrated magnetic activity near the poles of fast rotating stars like young M-dwarfs, activity that emerges as spots and flares. While the Zeeman technique could reconstruct a stellar magnetic field, no observations of this polar clustering had previously been made — bear in mind that we cannot resolve the surface of the target stars. The Potsdam researchers were able to detect signs of polar clustering by analyzing white-light flares on their target stars, pinpointing the latitude of the flaring region from the shape of the light curve.

This works because modulations in brightness, caused by the young stars’ fast spin as the flare location rotates in and out of view on the stellar surface, carry useful information. M-dwarfs remain fast rotators much longer than stars like the Sun; in fact, the fast rotation enhances their magnetic and flare activity. The researchers were able to determine where on the star these flares occurred. According to the paper:

The exceptional morphology of the modulation allowed us to directly localize these flares between 55◦ and 81◦ latitude on the stellar surface. Our findings are evidence that strong magnetic fields tend to emerge close to the rotational poles of fast-rotating fully convective stars, and suggest a reduced impact of these flares on exoplanet habitability.

The kind of long-duration superflare activity considered most lethal for planetary atmospheres, in other words, occurs much closer to the pole than the weaker flares and spots found below 30◦. On our own mature G-class star, sunspots and flares associated with them tend to occur near the equator. This paper offers, then, a continuing lifeline for those interested in the prospects for life around M-class stars, while also pointing to the need for what the authors call “the first fully empirical spatio-temporal flare reconstructions on low mass stars.” The emergence of such a model will help us draw broader conclusions on the impact of stellar magnetic activity on M-dwarf planets.

The paper is Ilin et al., “Giant white-light flares on fully convective stars occur at high latitudes,” accepted at Monthly Notices of the Royal Astronomical Society 05 August 2021 (abstract / Preprint). Thanks to Michael Fidler and Antonio Tavani for an early heads-up on this work.

{ 16 comments… add one }
  • Patient Observer August 11, 2021, 7:52

    It is simply amazing how much information can be gleaned from photons from what appears to be a point source located many light-years away. It is a testimony to the instrumental builders and the intelligence of those who use the instruments.

    And this paper is good news for prospects for planetary life around M class stars. What a wonderfully complex cosmos we live in full of loopholes that allow life to form and flourish!

  • James Essig August 11, 2021, 8:16

    The possibility of habitable planets around red dwarfs is intriguing.

    Since red dwarfs in theory live from between a few trillion years to perhaps as long as 30 trillion years, and most stars in our universe are red dwarfs, this enables potential civilizations to thrive and evolve for many trillions of years.

    One thing about potentially habitable red dwarf solar systems that would be beautiful is prospects for star sets and rises on orbiting planets. I can imagine how the stars would appear several degrees wide and almost brownish red when setting.

  • Ron S. August 11, 2021, 9:32

    I went through the pre-print and it looks like really nice work to my inexpert eyes. However, it is incorrect to think it is all clear for habitability on suitable planets in these systems. The flare activity is still frequent and intense closer to the stellar equators. Even moderate strength flares are a major threat since the planets are very close to the stars. It is only the rare and most intense flares that may have been reduced as a severe risk to planetary atmospheres and potential life.

  • Michael Fidler August 11, 2021, 10:48

    These stars make up about 37 percent (24) of the stars within 16 light years and if you include G and K stars over 50 percent (10). This increases the number of habitable stars within 16 lightyears by almost 300 percent!

    About half of the M dwarfs are fast rotators and the rest – older ones are slow rotators like Proxima Centauri. Proxima with a 82.6 day rotation and and age of 4.85 billion years is still having super flares. This long rotation makes it difficult to use the technique that the researchers use to find the latitude of the flares, since the flares last a short period. The Zeeman effect or splitting of spectral lines in a strong magnetic field, by using PEPSI may help in the case of Proxima but we need it on a telescope in the southern hemisphere.

    One other problem is the cutoff between M dwarfs that have a small radiative core and the fully convective M dwarfs. Many M dwarfs are bunched up into the M5.5V bin most probably because of lack of information. This also seems to be the area where they become fully convective. There may be issues with influx of material from comets and asteroids such as Lithium that may have large effects on these low mass stars.
    As these stars from M5 to M9 become cooler might we see bands like Jupiter with magnetic vortices near the poles creating the large flares?

    • Edwin August 16, 2021, 6:36

      Michael you might like this paper

      An explanation for the gap in the Gaia HRD for M dwarfs

      • Michael Fidler August 16, 2021, 11:56

        Edwin, thank you, I remember seeing this before but my memory is not as good as it use to be. I find this last comment interesting;

        “We have not considered the effects of magnetic fields. It has been established that magnetic inhibition of convection and/or the presence of dark stellar spots results in main sequence models having larger radii and lower luminosity than non-magnetic models of the same mass. In general, the strongest magnetic fields in M dwarfs are found in those that are rapidly rotating and fully convective. Thus if a large enough fraction of fully convective M-dwarfs are rapid rotators (rotation periods less than a few
        days according to Shulyak.), this could also give or enhance a dip in the luminosity function near the transition to fully-convective.”

        How could this relate to Proxima Centauri and it slow rotation being a fully convective at M5.5. Someone needs to do a really good paper of this transition and the fast and slow rotators. Seems there are two groups, fast and slow, from M3-M9, but why? Planets, age or some other feature we are missing and what of the flares, do the slow rotation still have high latitude flare?

  • wdk August 11, 2021, 11:22

    This is, of course, good news for any of us crossing our fingers that places like Trappist 1 will not be devoid of life or even organic chemistry of consequence. But since the intensity of flares have been observed in red dwarfs for decades and undeniable, this was about the only remaining exit route for the viability of life dilemma.

    Looking at this phenomenon from the other end of the proposition, we might ask why are G and K stars, say, different in characteristics?
    For example, we were led to assume that flares could pop out of M dwarfs in any direction or latitude based on a solar model. Reflecting on other bodies with eruptive behavior, there does appear to be a polar nature, either with magnetic or polar axes – both. The more dense the matter and energy, ( neutron stars, active galactic nuclei), the more likely this is true. I’ll defer to anyone ready to discuss Jupiter class planets and brown dwarfs, but there seems to be evidence there too.

    To my surprise, the authors attribute the polar or high latitude nature of the flares to the fully convective interior of red dwarfs. This is a property that has been discussed on Centauri Dreams and is one of the reasons that red dwarfs survive for so long: concentric layers of nuclear ashes are not forming in the interior; convection routes hydrogen down to where it can be burned. But other examples of “polarized” jets or flares in other celestial objects are not attributed to such a cause. For example, with a neutron star, it couldn’t be.

    The manner of heat transport does have a tendency to create shells, cells or bands. Bands are observed in Jupiter and Saturn’s outer layers and presumed in brown dwarfs, but I have to wonder what a red dwarf would look like close up: bands or cells like the sun? But if you have large regions of turbulent convection, the chimney like behavior for flares could be disrupted either by fluid or magneto-fluid mechanics, likely both.

    So, going back to Trappist 1 for illustration, we could imagine a flare alert ( for local residents or visitors like us) going out from time to time and an eruption heading out from the high latitude regions. It would be quite a sight from a planet. And then it would still have consequences to consider. Suddenly an awful lot of UV and other short wave radiation that wouldn’t be directed away from the orbital plane.

    We will still have plenty of environmental concerns to examine.

    • Alex Tolley August 11, 2021, 15:31

      Do you think there might be any connection between the polar large CMEs of M_dwarfs and the higher velocity of the sun’s solar wind at the poles – about 2x as fast as at the equator?

      • Geoffrey Hillend August 11, 2021, 19:59

        From what I’ve Googled, the CME’s from the Sun come from above the Sun spots and magnetic field lines twisting and breaking and snapping like a rubber band releasing a lot of energy and freeing the gas in the CME. Sun spots frequent the high latitudes on our Sun. I don’t know where the star spots are on an M dwarf, but they would be associated with CME’s and the magnetic fields are the strongest over dark star spots and might hold the largest amount of gas and biggest CME’s? https://spaceplace.nasa.gov/solar-activity/en/


      • wdk August 11, 2021, 23:25

        Sounds like an interesting observation in this context.
        If there is connection, it might be that the steady flow acts like an adequate relief valve whereas in the Red Dwarf case the mechanism could be absent.

        Additionally, while the solar surface has convective cells, at some depth between the surface and the core, a G star reverts to radiant
        heat transfer significantly at larger fractional radius than an M, hence the distinction between red dwarf and larger
        suns in utilizing hydrogen fuel.
        It sounds like answers to these things are in reach. If for no other reason than the HZ issues give a new perspective on structural

      • wdk August 11, 2021, 23:55

        One more remark. Density inside stars is not uniform, but the overall density of an M dwarf of roughly 1/8th the radius of the sun and 1/8th the mass, density is 64 times higher. Surface gravitational acceleration would be about 8 times higher. I don’t have a picture in my mind yet how that would effect mass ejections and flares, save indications of greater stress, but could be something to consider in comparison.

  • Geoffrey Hillend August 11, 2021, 17:21

    The problem with the Zeeman effect is it is harder to detect it in smaller stars like M dwarfs. Spectropolarimetry or molecular absorption lines work better. Astrobites, Cool Stars Have Magnetic Fields, internet article.

    A planet in the life belt around an M dwarf is tidally locked. I wonder how much tidal forces it gets and whether they are enough to increase volcanism and keep an atmosphere with solar wind stripping, and no magnetic field. Even without tidal forces and plate tectonics, it should still have some volcanism over time if it is Earth sized using Venus as an example.

  • Robert Flores August 14, 2021, 23:05

    note that even with lower stellar flare frequency,
    there is still the problems of tide locking. And specifically for the
    Trappist system, the cores of ALL of the
    planets there are probably inactive by now, and have been
    for some hundreds of millions of years.

  • Michael Fidler August 18, 2021, 23:14

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