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Red Dwarfs, ‘Superflares’ and Habitability

Given their ubiquity in the Milky Way, red dwarfs would seem to offer abundant opportunities for life to emerge. But we’re a long way from knowing how habitable the planets that orbit them might be. While mechanisms for moderating the climate on tidally locked worlds in tight habitable zones continue to be discussed, the issue of flares looms large. That makes a new survey of 12 young red dwarfs, and the project behind it, of unusual interest in terms of astrobiology.

What jumps out at the reader of Parke Loyd and team’s paper is the superflare their work caught that dwarfed anything ever seen from our own Sun, a much larger star. It was enough to set Loyd, a postdoctoral researcher at Arizona State University, back on his heels.

“When I realized the sheer amount of light the superflare emitted, I sat looking at my computer screen for quite some time just thinking, ‘Whoa.’” He adds: “With the Sun, we have a hundred years of good observations. And in that time, we’ve seen one, maybe two, flares that have an energy approaching that of the superflare. In a little less than a day’s worth of Hubble observations of these young stars, we caught the superflare. This means that we’re looking at superflares happening every day or even a few times a day.”

Image: Violent outbursts of seething gas from young red dwarfs may make conditions uninhabitable on fledgling planets. In this artist’s rendering, an active, young red dwarf (right) is stripping the atmosphere from an orbiting planet (left). ASU astronomers have found that flares from the youngest red dwarfs they surveyed — approximately 40 million years old — are 100 to 1000 times more energetic than when the stars are older. They also detected one of the most intense stellar flares ever observed in ultraviolet light — more energetic than the most powerful flare ever recorded from our Sun. Credit: NASA, ESA, and D. Player (STScI)

Loyd’s work is under the aegis of a program called HAZMAT, which stands for HAbitable Zones and M dwarf Activity across Time (ASU’s Evgenya Shkolnik is principal investigator for this project). The issue of time is significant, for HAZMAT will survey young, intermediate and old M-dwarfs using data from the Hubble Space Telescope, and this initial paper focuses on stars that are roughly 40 million years old, mere infants given that this category of star can burn for as long as a trillion years.

As to that superflare, it’s easy to see why it gave Loyd pause. His team detected 18 flares from its 12 target stars, but the superflare swamped them all, emitting 1032.1 erg in the far ultraviolet. That exceeds the most energetic flare from an M-dwarf previously observed by Hubble by a factor of 30.

Have a look at the paper’s description of what the authors call the ‘Hazflare’ in comparison to other flare observations from the past:

This observation is of particular value because superflares are common on stars (e.g., Davenport 2016), yet spectrophotometry of such flares in the UV, the band most relevant to planetary atmospheric photochemistry, is rare. Superflares are estimated from Kepler data to occur on M0–M4 dwarfs at a frequency of a few per day (Yang et al. 2017). Photochemical models exploring the effects of flares on planetary atmospheres have thus far relied primarily on observations of the 1985 Great Flare on AD Leo (Hawley & Pettersen 1991; Segura et al. 2010; Tilley et al. 2017), a flare estimated to emit a bolometric energy of 1034 erg. The Great Flare also showed a clear continuum in FUV emission, and overall the continuum was responsible for at least an order of magnitude more overall energy emitted by the flare than lines, consistent with the Hazflare. However, the Great Flare observations, made with the International Ultraviolet Explorer, saturated in the strongest emission lines, degrading their accuracy.

Image: Observations with the Hubble Space Telescope discovered a superflare (red line) that caused a red dwarf star’s brightness in the far ultraviolet to abruptly increase by a factor of nearly 200. Credit: P. Loyd/ASU.

So the HAZMAT observations prove valuable indeed. Superflares like the one described in this paper are far more common in young dwarfs, which can erupt up to 1,000 times more powerfully in their youth than after they have aged, and the Kepler data on their frequency (above) are noteworthy. The mechanism: Strong magnetic fields twisted by the churning atmosphere of the young star, causing them to break and reconnect, a process producing huge amounts of energy. Such violent activity is associated with M-dwarfs in the first 100 million years of their lifetime.

You would think we could just wait out flare activity and assume that older M-dwarfs were our best bet for habitable conditions, but a key question is what kind of damage will already have been done. Pummeling from flares like these could cause atmospheric damage, perhaps even stripping the atmosphere from what might have been promising worlds in the zone where liquid water could exist on the surface. Ultraviolet and X-ray radiation, even if it leaves the atmosphere more or less intact, would also be a huge factor in determining the emergence of surface life, possibly acting as an evolutionary spur or conceivably preventing it from appearing at all.

We also have to learn what processes can replenish the atmosphere of a planet if it undergoes a period of intense UV bombardment followed by a gradually calmer stellar environment. The sheer longevity of red dwarfs gives reason to hope that life could eventually emerge, but we won’t know until we can make the kind of atmospheric observations that will be coming our way through missions like the Transiting Exoplanet Survey Satellite (TESS), the James Webb Space Telescope and the European Space Agency’s ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) observatory. The latter, to be launched in 2028, will make a large-scale survey of the chemistry of exoplanet atmospheres using transit methods.

But back to HAZMAT, which will move on next to intermediate-age red dwarfs some 650 million years old, followed by analysis of the radiation environment around much older M-dwarfs. The evolution of that environment will help us refine the target list for the above missions as we focus on star systems more likely to have life. Given that most of the habitable zone planets in the galaxy will have had to withstand high flare activity, we need to make modeling the effects of flare erosion on atmospheres a high priority task.

The paper is Loyd et al., “HAZMAT. IV. Flares and Superflares on Young M Stars in the Far Ultraviolet,” accepted for publication at the Astrophysical Journal (preprint).


{ 17 comments… add one }
  • Thomas Goodey October 18, 2018, 14:20

    I see on Wikipedia the amusing remark, “An erg is approximately the amount of work done (or energy consumed) by one common house fly performing one “push up,” the leg-bending dip that brings its mouth to the surface on which it stands and back up.”

    • Coolstar November 1, 2018, 19:19

      And an electron volt is approximately the energy a housefly uses when it THINKS about sneezing…….

  • Thomas R Mazanec October 18, 2018, 15:48

    Well, we know G dwarf stars can support life (Sol b is an existence proof) and K dwarfs are intermediate between M and G dwarfs in characteristics and abundance, so how about looking at some of those?

  • Daniel Högberg October 18, 2018, 16:55

    How about a habitable moon circling a big gas giant in orbit around a old calm old red dwarf? The moons atmosphere continuously being replenished by internal processes and the massage of gravitational pull and magnetic field of the gas giant? Well well, who said I cannot dream?

  • Christian G October 18, 2018, 18:43

    Or it could blow away excess atmosphere that might have produced a Venus or Neptune, and leave something more suited to life as we know it. Earth is certainly a Goldilocks planet, but we shouldn’t assume the sequence of events that DID produce it is the only one that CAN.

    (Also, aren’t we Sol d? Or does it go in discovery order?)

    • Lepton October 19, 2018, 13:40

      On discovery order, Earth should be Sol d indeed.

      • ljk October 22, 2018, 8:55

        Jimi Hendricks said we were the third stone from the Sun.

  • Alex Tolley October 18, 2018, 18:48

    …we won’t know until we can make the kind of atmospheric observations…

    We will be able to end a lot of speculation once we have some hard data on HZ planets around stars, Just showing there are atmospheres, or not, will be important in determining whether life can potentially exist on these worlds. Only then will we have some facts to constrain our models and speculations, as well as directing our searches for life.

  • Nicky October 18, 2018, 21:40

    It is generally believed that the electromagnetic radiation from flares themselves does not pose a threat to planetary atmosphere. Atmospheric chemistry model has calculated that the depletion of ozone in an Earth-like oxygen-rich atmosphere is negligible even without the protection of magnetic field, because the dissociated O recombine rapidly as soon as the flare ends.
    SEE: The Effect of a Strong Stellar Flare on the Atmospheric Chemistry of an Earth-like Planet Orbiting an M dwarf
    SEE: Modeling Repeated M Dwarf Flaring at an Earth-like Planet in the Habitable Zone: Atmospheric Effects for an Unmagnetized Planet

    What we really concerned about is the coronal mass ejection (proton event) accompanied by the flares, which can lead to complete destruction of ozone.
    If a solar flare exceeds a certain energy level (for sun is 10^28 erg), it often would be followed by a CME. Based on the relationship of solar flare and CME, some scientists extrapolate it to low-mass stars and find a much higher rater of CME due to stronger flare activity.
    However, there is no confirmed CME other than the sun’s, and M-dwarfs probably behave differently, in which case a simple extrapolation would be unphysical. Observational evidence rather shows active stars have much lower mass-loss rate per area unit and less frequent CME.
    A strong overlying magnetic field is a proposed as a mechanism by which confines the plasma within the corona, thus reducing CME, and a strong stellar magnetic field is indeed very common among the M-dwarfs.
    SEE: Implications of mass and energy loss due to coronal mass ejections on magnetically active stars
    SEE: Stellar coronal mass ejections – I. Estimating occurrence frequencies and mass-loss rates
    SEE: Suppression of Coronal Mass Ejections in active stars by an overlying large-scale magnetic field- A numerical study.

    The habitability of M-dwarfs perhaps does not seem as pessimistic as we once thought.

  • Rob Flores October 19, 2018, 0:05

    This bodes ill for M stars habitability. If the Atmosphere is stripped
    entirely, is there a mechanism to replenish it.

    What gasses from volcanic activity can over time yield a new large
    nitrogen component. Water may be replenish from deep in a planets
    mantle, but nitrogen, not sure.

    From cometary bombardment, we can expect water to be delivered there, but what percent is ammonia and cyanide which would yield nitrogen

    There must be a dichotomy on a planet’s hydrosphere, around young
    red dwarfs.

    Either the water sources are enough to carry the planet to a more
    mature time when the Flaring is milder, and keep it wet. Or the
    hydrosphere is overwhelmed and a bone dry irradiated planet with volcanoes/unstable crust due to no plate tectonics (requires water) , is the result ( something like Io,)

    • Nicky October 19, 2018, 11:59

      Replenishment would be difficult or even impossible if the initial surface water is completely desiccated.

      Mantle outgassing is often imaged to replenish water and other atmospheric gases, but the prerequisite is that the planet must have kept some amount of water in atmosphere and surface to drawdown CO2 after the early water loss and magma ocean crystallization. In the case of runaway greenhouse during the superluminous period of M-dwarfs, the planet is in magma ocean stage outgassing a massive atmosphere mainly composed of CO2 and H2O up to hundreds of bars. On a planet with atmospheric H2O not stripped entirely, H2o will precipitate in the habitable zone to form lakes/seas/oceans, and surface weathering induced by the hydrosphere will drawdown surplus CO2 and equilibrate in a temperate range. On a planet with a completely desiccated surface, weathering will not operate without water and tens to hundreds of bars of CO2 will remain maintaining permanently high surface temperatures even after the end of the greenhouse phase, much like today’s Venus.

      In the later case, even a late delivery of water by comets or outgassing by volcanism may be unable to restore the planet’s habitability.

  • Abelard Lindsey October 19, 2018, 12:09

    I agree. K’s are good stars to be looking at. There are more of them than there are G’s and they don’t have the problem of M’s, like tidally locked planet for one. Epsilon Eridani is a good one to start with. Who knows, we might find something better than Yellowstone there.

    • Matt M. October 21, 2018, 11:53

      Epsilon Eridani is too young, just under 1 billion years. There’s an article about that: “Early System Evolution: The Disks around Epsilon Eridani”. Much older K-type stars would be more suitable.

  • Ashley Baldwin October 19, 2018, 13:55

    And this is after an even more active period before the star has entered the main sequence , all the more longer ( billion years) for later M dwarfs. The question is not so much how quiescent an M dwarf becomes as it matures ( and there is evidence of plenty such examples) but as to whether any erstwhile ” habitable zone” planets ( a zone that will fluctuate back and forth from pre main sequence to post main sequence ) can maintain a reservoir of all important volatiles long enough to form a terrestrial atmosphere when that tine arrives. Current modelling suggests not but it is based only on modelling and little or no observational data. Hubble and Spitzer observations have hinted that the potentially unfavouarbke M8 Trappist-1 planets have shown evidence of having retained atleast a significant volatile component . So I’m still optimistic. JWST and the E-ELT Metis instrument will finally provide decent observational data within a decade via transit spectroscopy , direct imaging and hopefully combining the two through high dispersion spectroscopy . All to significantly refine simulations of nearby M dwarf planet atmospheres which are currently based almost entirely on educated guess work based around stellar physics -however clever . It’s a frustrating time for the exoplanet community , as TESS , Kepler and ground based RV studies continue to rachet up the number of exoplanets . But with little or no capability ,beyond limited and increasingly precious Hubble and Spitzer use, to assess terrestrial planetary atmospheres. Other than ground based observations of a few close in hot Jupiters . Patience the name of the game , but as with astronomy as a whole and even exoplanet science I am certain that once the data starts coming in we will all be in for some surprises.

  • Geoffrey Hillend October 19, 2018, 16:56

    We will have plenty of time to study the TESS data since the James Web Space Telescope has been delayed even longer until March 30, 2021.

    Ultra violet light becomes a problem for the tidally locked exoplanet around an M dwarf star which leaves only the night time and after sunset sky for unexposed areas. The day side always gets irradiated with UV and EUV which is not a problem for the life belt in G class stars since exoplanet in them are not tidally locked. Also the hill radius for a tidally locked M dwarf exoplanet makes it impossible for it to have a moon and without a Moon there most likely is no magnetic field to stop the solar wind stripping and continual loss of atmosphere.

  • Michael Fidler October 19, 2018, 17:20

    Well, when I was four days old I was pretty loud too!

  • ole burde October 20, 2018, 15:11

    How deep into an ocean would this ´´superflare penetrate ? life started on earth very quickly in an ocean where anything in the top layer (perhaps the first 50m) was killed by UV radiation ….so how much of aproblem would it be if this dead-zone went a little deaper?….probably no big deal ….anyhow local variations in particulate matter could change the size of the deadzone with a factor of ten or more

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