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