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I have a planet circling an M-Class star (M3/M4). I'm trying to figure out what the effects of flaring on the planet's biosphere might be.

Background: As some of you may know, M-class stars are renowned for their vigorous and powerful flaring (CMEs). It's commonly assumed that this flaring would strip most planets within the habitable zone of their atmosphere, leaving a planet lifeless. However from pouring over research papers, I've come to the conclusion that planets orbiting older M-class stars, particularly of lower classes (M0, M1, M2, M3, M4) would likely experience vastly attenuated flare activity compared to their higher class or younger brethren.

So that offers me a window to create a world with a flourishing biosphere.

The part I'm having the most trouble with is...what will the remaining flare activity mean for life on the planet? The flares are less powerful, sure, but they still pack some degree of radiative wallop (UV, X-rays, etc.) that doesn't typically play nice with life or atmospheres.

My question in full: Given the level of flaring (see below) how much of an impact on the biosphere should I expect? Will the atmosphere suffer periodic damage in some form? Will this and other effects be enough to force adaptive measures during the strongest flares (behavior, anatomical, etc.)? What conditions might result from this flaring?

Details on Flaring/Star/Planet: My star is old. As M-class stars age their rate of rotation slows, inducing progressively weaker magnetic fields which gradually attenuates their flare activity. However, like other M-class stars of this kind, that doesn't mean it has zero flare activity. Far from it. Based on astrophysics papers I've scrounged through, it seems such stars can maintain a reasonable level of flare activity for many giga years. We have flare data for star GJ 4083, so using that as a model for my system and referencing several sources, I've gathered the following data (correct me if I go astray):

First off my planet is generally Earth like: oceans, continents, oxygen/nitrogen atmosphere, the whole shebang. The atmosphere is about 3 bar, and the mass (and hence gravity) somewhat lower. It has tectonics, a carbon cycle, and experiences substantial tidal heating. Magnetic field of my planet would be weak. In the range of 1/8 - 1/3 Earth's. My planet is not tidally locked, but instead in a 2:1 orbital resonance. So all points on the planet experience day/night.

Other specifics are probably not relevant to the question, so I'll leave them out.

Our Sun (for context)

  • A regular "big" flare on Sol (our sun) occurs once or twice every eleven year solar cycle. These might be 1E+32 Ekp(erg)* in power.

  • The 1859 Carrington Flare (one of the most powerful recorded flares from our sun) is estimated to to have been about 5.6E+32 Ekp(erg). That's 5.6x bigger than the solar cycle flare mentioned above, and might occur once-a-century or so.

  • The 774 A.D. Solar Flare (the largest known postulated flare from our sun) is estimated at perhaps 1.6E+34 Ekp(erg). That's 160x more powerful than the flares you normally see at the peak of the 11 year solar cycle for our sun. That might be a once-every-couple millennium flare. From what I've read this may have had appreciable effects on the biosphere, including acute ozone depletion.

    GJ 4083 (My model star)

  • Over a period of several years, the largest recorded flares from GJ 4083 were about 1.6E+31 Ekp(erg) and averaged about one every two months. At the distance my planet orbits my M-class star, that would yield a received flux about 4x more powerful than the 1859 Carrington flare.
  • More frequently GJ 4083 will output flares all the way down to 5E+30 Ekp(erg), which occur about once a month. At the distance my planet orbits, this would yield a received flux slightly higher than the 1859 Carrington Flare.
  • I would guess that GJ 4083 (and my fictional star based on it) emits smaller flares that happen more often (but we can't detect them), and much larger flares that happen at greater time spans. I wouldn't be surprised if GJ 4083 outputs a monster flare every couple centuries or millennia which would rival the 774 A.D. flare earth experienced. There simply isn't data to say one way or the other. If you need a solid answer, assume the star experiences more frequent minor flares and very infrequent super flares just as our sun does.

If it helps, feel free to postulate flare activity/strength somewhat less or greater than GJ 4083. That would certainly be within the realm of possibility.

Ekp (in erg) = Luminosity of the star in the Kepler bandpass, multiplied by its equivalent duration. Unit ergs.

References:

KEPLER FLARES. I. ACTIVE AND INACTIVE M DWARFS – https://iopscience.iop.org/article/10.1088/0004-637X/797/2/121/meta#apj504475s3

The 1859 space weather event revisited: limits of extreme activity https://www.swsc-journal.org/articles/swsc/pdf/2013/01/swsc130015.pdf

https://sohowww.nascom.nasa.gov/hotshots/X17/

Terrestrial effects of possible astrophysical sources of an AD 774-775 increase in 14C production https://arxiv.org/pdf/1302.1501.pdf

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  • $\begingroup$ @Agrajag The OP means an orbital resonance of 2:1. $\endgroup$
    – a4android
    Feb 16, 2019 at 1:10
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    $\begingroup$ @a4android The OP stated "an orbital resonance of 2:1", what he means I've yet to discover - since no other bodies in the system with which a resonance could occur were mentioned. $\endgroup$
    – Escaped dental patient.
    Feb 16, 2019 at 1:16
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    $\begingroup$ Sorry for any confusion. Orbital resonance of 2:1 means 2 planetary rotations per 1 orbit of the star. As a comparison Mercury is in a 3:2 orbital resonance (3 rotations per 2 orbits). Planets in high elliptical orbits are more likely to be caught in integer resonance states than in 1:1 tidal locks. I simply mentioned it to dispel any notion that there would be a "dark" and "light" side. $\endgroup$
    – n_bandit
    Feb 16, 2019 at 1:19
  • $\begingroup$ That's great, what I thought even before googling. Eliptical orbit, did I miss a mention of that in your question? $\endgroup$
    – Escaped dental patient.
    Feb 16, 2019 at 1:21
  • $\begingroup$ @Agrajag Nope. I didn't mention an elliptical orbit. Didn't think it was germane to the question of flares. I've added another detail (Earth like planet) which should provide the needed context. The other relevant info is there: Flare strength + frequency. I think that should be enough? $\endgroup$
    – n_bandit
    Feb 16, 2019 at 1:24

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The problem that I see in your question is the assumption that an earth like planet would exist far away from a red dwarf rather than closer to it, hence the focus on attenuation.

Red dwarfs are larger, but colder, than stars like our own. As a consequence, earth like planets will have to orbit much closer to the sun in order to maintain their temperature. If we say an average of 0.3 AU (or 30% of the distance between the Sun and the Earth), and we go with your weak magnetic field, then the likelihood of life existing is already quite low. The closer to the sun you get, the higher the intensity of the cosmic rays being given off (the cooler sun tempers this a bit, but at 30% distance you still have a problem) meaning that without a stronger magnetic field, life is going to struggle on your planet and the likelihood of your planet being able to maintain an atmosphere is reduced as well. That means that your 3 Bar atmospheric pressure and weak magnetic field are likely incompatible, especially given the age of the star.

So, how would CMEs affect life on this planet? Well, assuming that life can exist there at all (which isn't guaranteed) then the CME is going to be catastrophic, by virtue of proximity.

I'm of the view that;

1) your planet couldn't exist in the first place,
2) if it did, life would be quite fragile on it, and
3) CMEs would likely be the straw that breaks the camel's back.

Remember, that your magnetic field is already struggling against basic solar radiation, so any more strife, like a CME, and you're going to find the magnetic field being overwhelmed.

On the bright side (literally), the aurora at the poles (and possibly over most of the planet bar the equator) are going to be spectacular. Even if life has been killed off, if you have the technology to predict when the next CME is due, you could create a magnetically shielded viewing platform on this planet's version of Europe and sell tickets for the light show across the galaxy.

This planet may struggle to harbour life, but that doesn't preclude it from being a real money-spinner.

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    $\begingroup$ Thanks for the response. I think you are mistaken on a few points, which calls your conclusion into question. If you see this ref en.wikipedia.org/wiki/Red_dwarf you'll see an M3 class star is about 1/3 the size of our sun and 1.5% the luminosity. Ignoring any flare activity, M-class stars give off far less high intensity radiation in their spectrum than a G-class star (like gamma, x-ray, etc.). Also, magnetic fields are not necessarily needed to maintain an atmosphere. For example, Venus has a monstrous atmosphere and an almost negligible field. $\endgroup$
    – n_bandit
    Feb 16, 2019 at 22:19
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    $\begingroup$ Ran out of room.... There seem to be many factors in the retention of a wet Nitrogen/Oxygen atmosphere, only one of theme being photo dissociation of water molecules caused by stellar wind impact. Escape velocity (determined by mass) is probably more critical. Anyway...a planet in the habitable zone of an M3 class star may actually be more hospitable to our form of life: less radiation, longer lifespans, etc. That is, before you factor in any flare activity. Addressing the question of impact on biosphere of modest flaring is the thrust of my post. $\endgroup$
    – n_bandit
    Feb 16, 2019 at 22:23

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