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A Renewed Concern: Flares and Astrobiology

Before the recent American Astronomical Society meeting in Seattle gets too far behind us, I want to be sure to include an interesting story on red dwarfs in the coverage here. The story involves an extrasolar planet survey called SWEEPS — Sagittarius Window Eclipsing Extrasolar Planet Search, which used the Hubble Space Telescope to monitor 215,000 stars in the so-called Sagittarius Window (also called Baade’s Window, after Walter Baade, who discovered it with the 18” Schmidt camera on Mt. Palomar). The ‘window’ offers a view of the Milky Way’s central bulge stars, which are otherwise blocked by dark clouds of galactic dust.

M-dwarfs are by far the most common type of star in the Milky Way, and therefore have major implications for the search for extraterrestrial life. We now know from SWEEPS data that these small stars are given to stellar flares that can have major effects on a planetary atmosphere. Flares have often been mentioned as a serious problem for the development of life on M-dwarf planets, but the new data tell us they may be more dangerous than we had thought, occurring on a regular and frequent basis. This BBC story quotes planet hunter Geoff Marcy:

“Such powerful flares bode ill for any possible biology, life, on any planet that happens to be close to that flaring star. It’s extraordinary to think that the most numerous stars, the smallest ones in our galaxy, pose this threat to life.”

How Red Dwarf Flares Happen

The threat, vividly portrayed in the results presented by Adam Kowalski (University of Washington) at the conference, involves an eruption of hot plasma that happens when magnetic field lines in a stellar atmosphere reconnect and release an amount of energy that can surpass that of 100 million atomic bombs. From the perspective of life on a planet orbiting an M-dwarf, the planetary surface is blasted with ultraviolet light and a bath of X-rays, along with the charged particles of the stellar wind. The SWEEPS study, with observations over a seven-day period, found 100 stellar flares in this largest continuous monitoring of red dwarfs ever undertaken.

You wouldn’t think small M-dwarfs would pack an impressive punch, but it turns out they have a deep convection zone where cells of hot gas can bubble to the surface in a process Rachel Osten (Space Telescope Science Institute) likens to ‘boiling oatmeal.’ It’s within this zone that the magnetic field is generated that produces the flare, a magnetic field stronger than our Sun’s. I learned from reading papers related to this topic that while sunspots cover less than one percent of the Sun’s surface, the star spots that cover a red dwarf can occupy fully half their surface. And it’s not just young, active stars that pose the threat, according to Osten:

“We know that hyperactive young stars produce flares, but this study shows that even in fairly old stars that are several billion years old, flares are a fact of life. Life could be rough for any planets orbiting close enough to these flaring stars. Their heated atmospheres could puff up and might get stripped away.”

Most flares last for only a few minutes, but some have been observed to persist for up to eight hours. Older stars do seem to flare less frequently than younger ones, but this survey, taken from data originally compiled in 2006 as part of an exoplanet hunt, tells us that flares continue to be an issue for M-dwarfs that have moved past their youth. Some of the surveyed stars grew as much as 10 percent brighter in a short period of time, making their flares much brighter than those from our Sun, and a few of the stars surveyed produced more than one flare.

Waiting for Stellar Maturity

I haven’t found the paper on this work, but related papers using other surveys include Hilton et al., “The Galactic M Dwarf Flare Rate,” from the Proceedings of the Cool Stars 16 Workshop (preprint) and Hilton et al., “M Dwarf Flares from Time-Resolved SDSS Spectra,” accepted for publication in The Astrophysical Journal (preprint). The latter gets into the age issue re flares. From the abstract: “We find that the flare duty cycle is larger in the population near the Galactic plane and that the flare stars are more spatially restricted than the magnetically active but non-flaring stars. This suggests that flare frequency may be related to stellar age (younger stars are more likely to flare) and that the flare stars are younger than the mean active population.”

With red dwarfs comprising 75 percent and perhaps more of the stars in our galaxy, the question of life around them may come down to how long it takes a flare star to attain a more sedate existence, with flare activity slowing to less threatening levels. On that score, we have much to learn. Because their lifetimes are far longer than the current age of the universe, we have no senescent red dwarfs to study.

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Comments on this entry are closed.

  • Erik Anderson January 17, 2011, 13:17

    “[How long does it take] a flare star to attain a more sedate existence, with flare activity slowing to less threatening levels[?]”

    Don’t hold your breath. ;-) Nearly all the extremely low metal red dwarfs in the solar neighborhood (i.e., the very oldest ones — visiting the Galactic plane from the thick disk and halo) are still variable stars.

    I’ve never been sympathetic to the idea that red dwarf stars could host habitable planets. The problem of tidal locking would freeze out all but the heaviest planetary atmospheres; throw in some flare activity and the environment becomes utterly unstable. I would not dare try to live on such a place — nor even visit.

  • James M Essig January 17, 2011, 13:20

    Hi Paul;

    This is a very interesting post.

    I wonder if any pre-biological complex organic chemistry that begins in Red Dwarf systems only developes in thermodynamic complexity to evolve into life if the evolved pre-biological conditions are able to continue evolution within any high UV and soft x-ray flux radiation environments.

    It might be the case that evolution of such life forms is kept in check until certain statistical-mechanical, pre-biological states become radiation resistant through self repair or self assembly types of mechanisms.

    For biological systems that have evolved from such precursor compounds, the self repair mechanism would need to be much more robust than Earth-based DNA systems.

    I was pondering just how hard this would be for nature to achieve as I was reflecting on the irradiated ground beef that my mother used to make meatloaf last night for dinner. E-coli in many of its genetic variants is extremely virulant once ingested and it pretty much has to all be “killed completely dead” by the irradiation process even though the ionizing radiation used to kill the bacteria is not intense enough to cook the meat to even a miniscule extent. The irradiated ground beef we buy looks as fresh and bloody as the best of ground beefs.

    The point is that given the sensitivity of bio-chemical molecular machinery of cells, perhaps any life that has developed in M-class star systems would need to devote much more metabolic energy to radiation self-repair, or perhaps be composed of materials having much stronger interatomic and inter-molecular bonds. Some such extreme bonds include carbon diamond bonds, carbon-nanotube bonds, boron-nitride nanotube bonds, and carbon-nitride bonds for which carbon-nitride has been proposed to be harder than pure diamond.

    If we can step back and remove from our mind-set the necessity that life forms in M-class systems would need to be human-like or humanoid, then perhaps planets in such systems supporting highly evolved life would seem more plausible. I would think that such organisms could resemble the ETIs in the movie “Alien” and its sequels.

  • ProtoAvatar January 17, 2011, 13:52

    ~75% of the stars in the Milky Way galaxy (in most/all galaxies?) have been and are lethal to life on the planets around them?

    This considerably strengthens the case for the ‘Rare Earth’ hypothesis.

  • kzb January 17, 2011, 14:10

    James, I don’t think it’s the direct radiation dose that is much of an issue. I guess the main problem is that planetary atmospheres in the habitable zone will be gradually stripped away by the stellar wind, much like Mars has lost its atmosphere in our system.

    Planets in M-dwarf habitable zones will necessarily be tidally locked, therefore the planetary revolution speed will be slow -too slow to give much of a magnetic field, which otherwise might preserve the atmosphere.

    On the other hand, many of these planets seem to be “super-Earths” in terms of mass, which would help them retain atmosphere. After all, Venus has a very thick atmosphere, and that is smaller than Earth.

    When we were discussing the Gliesens a while back, it was said that planetary conditions there would have been constant and unchanging over many billions of years, therefore evolution could have reached a dead end and stopped. This latest article implies that conditions may be more challenging for evolution than we thought :)

  • Eric January 17, 2011, 14:16

    This news makes me a little sad actually. Since M-dwarves are so common, it would be LOTS of wasted real-estate to have them so uniformly hostile to life.

    @Essig. Agreed. I suspect there will be lots of complications in this perspective that may mitigate the impact of flares. Or perhaps some types of complex and interesting chemistry / life can cope with such events. We don’t really understand the space of all possible complex chemistries to really say that M-drawf flares have to be a show-stopper.

  • Dave January 17, 2011, 14:17

    I believe there was a study that suggested that solar flares from red dwarf stars would actually strengthen the ozone layer around earth-like planets to the point that the layer would protect life from the flares. Throw in a magnetic field and you just might have a habitable planet.

  • Eniac January 17, 2011, 14:32

    Let us not forget that life does not have to be at the surface, and in fact from what we know about its origins, it probably wasn’t, originally. A few feet of water or rock is all that’s needed to shield such flares, and I do not quite understand the radiation argument against life on red dwarf planets.

    It is possible that such life will never leave the water, and be forever contained in the deep sea, but that doesn’t make it any less interesting. One day the inhabitants may become curious about what lies beyond the deadly barrier keeping them contained, and find ways to venture beyond. Sounds like great material for a story which may not have been done, yet.

    Come to think of it, since such planets are expected to be tide-locked, would not the back side be completely safe from flares? Another reason to expect life not only to form, but also flourish on the surface, flares notwithstanding.

  • Paul Gilster January 17, 2011, 15:16

    ProtoAvatar writes:

    ~75% of the stars in the Milky Way galaxy (in most/all galaxies?) have been and are lethal to life on the planets around them?

    This considerably strengthens the case for the ‘Rare Earth’ hypothesis.

    Yes, not to mention what it would do to our evaluations of the Fermi paradox.

  • andy January 17, 2011, 15:56

    Erik Anderson’s comment about tidal locking freezing out the atmosphere seems to be unduly pessimistic. The minimum requirements for atmospheric stability do not seem to be particularly stringent: the Venusian atmosphere would exceed them by a wide margin.

    I’ve seen studies that suggest that flares may actually be beneficial to life, as M dwarfs typically do not produce much in the way of ultraviolet. Flares could provide the necessary UV for photochemical reactions which may aid in forming biomolecules. And if the atmosphere gets Earthlike levels of oxygen, that photochemistry would drive the formation of ozone, protecting the planet’s surface.

    As for non-oxygenated atmospheres, situation would be worse, but bear in mind the Sun was more active too when it was young: Kappa Ceti which has been noted to put out “superflares” every so often is often cited as a good analogue of the young Sun.

  • Sam M-B January 17, 2011, 16:20

    From the Science Fiction side of things, Larry Niven and Brenda Cooper portrayed this kind of flare/magnetism in their book “Building Harlequin’s Moon”. They presented the idea of “kites” set in space to interfere with the flares, along with hardened underground bunkers and an early warning system so people could take cover during such flares.

  • kurt9 January 17, 2011, 16:29

    If the solar wind and flare effects ablate a planet’s atmosphere over time, the ocean will be lost as well since it will boil off at progressively lower temperatures as the atmospheric pressure declines. Any sea life would die out as well.

    It really does appear that M-stars are not likely candidates for life or habitability. These flares make M-stars unattractive even for the construction of O’neill style habitats as well.

  • Mike January 17, 2011, 17:22

    Maybe Earth-size or larger ocean worlds with a thick ice surface (like Europa but larger) could provide a habitat for the biological kind of life that we are familar with.
    As for more chemically exotic life flourishing on the surface of M-dwarf hosted planets, who knows?
    Europa doesn’t have much of an atmosphere and also is irradiated thanks to Jupiter. But a kilometer or more of ice solves both problems for any life that may exist in Europaen oceans. I think such a world, better if it’s more massive, could provide a good habitat.
    And if not, well, there’s still many billions of nice stable K-dwarfs and G-dwarfs hopefully with planets much better suited for us surface dwelling air breathers.

  • andy January 17, 2011, 18:21

    Regarding atmospheric mass loss on M dwarf planets, this paper analyses the situation for non-magnetic planets and concludes that mass loss should not necessarily be a showstopper, particularly for early-type M dwarfs, which seem to show more of a time-dependence on the flare rate than late-type M dwarfs. In the time-dependent case, even the stellar wind rates of active stars like AD Leonis and YZ Canis Minoris should be survivable.

    Writing off red dwarfs as potential hosts for habitable planets at this stage seems premature to me.

  • ProtoAvatar January 17, 2011, 18:22

    Planets around red dwarfs will be tidally locked;
    Their atmosphere will be steadily ablated away by solar flares/wind – meaning whatever water they have on the night side will be frozen solid and whatever water they have on the sun side will evaporate due to being heated by the sun in a low-pressure atmosphere;
    Ultraviolet, X-rays, charged particles will bombard the surface of the planet steadily – and constant bombardment with these is quite lethal to life as we know it; and to technology also;
    Organic matter and chemistry have a lot of remarkable properties that are not matched by any other class of substances humans know about.

    If life is able to appear on such worlds, then life should be able to appear (not merely survive – APPEAR) 0n any number of frozen deserts in our own solar system – Mars, asteroids, various moons (the conditions there are FAR more favourable than on red dwarf worlds).

    The chances of life appearing near red dwarfs just went from ‘not certain, but not very high (considering life appeared ONLY ONCE on Earth, despite a long period of varied, paradisiac conditions present on our planet)’ to ‘negligible’.

  • NS January 17, 2011, 19:51

    “considering life appeared ONLY ONCE on Earth”

    Quibble: we really don’t know that for sure yet. Hopefully research like that proposed here will give us a clearer picture:

    http://www.ironlisa.com/Davies_etal_Astrobio2009.pdf

  • mike wirth January 17, 2011, 20:02

    Maybe the work has been done on this but:

    I would think that based on a few parameters (ie frequency and intensity of flares and location of release from M dwarf surface combined with a planet’s orbital distance, speed and angle of ecliptic), one could make a useful computer model providing a probability/statistical analysis that would give an idea of the number of flare hits and degree of intensity of a flare hit to a planet.

    This could mitigate or give some perspective on the adversity of such flare effects.

    Does anyone know of articles along these lines?

    BTW, I’m a frequent visitor and first time poster of this site and enjoy the articles and thoughtful comments.

  • Eniac January 17, 2011, 22:01

    I recall similar discussions in which it was said that the notion of tide-locked planets freezing on one side and burning on the other does not hold up in the face of ocean currents and/or strong global winds. Similarly, any “ablation” of the atmosphere (is there really such a thing?) can be negated by a slightly deeper gravitational well.

    I am constantly amazed at how every little difference from our own dear Earth is always interpreted as inimical to life, for the most part without real evidence, as if the ‘Rare Earth’ hypothesis was a postulate rather than a hypothesis. Luckily we are now finding lots of exoplanets which contradict such assertions, and will hopefully weed out all the unfounded ones in time.

    I also disagree that counting out red dwarfs does anything for the Rare Earth hypothesis, since non-red-dwarf stars are hardly rare. Nor does it impact the Fermi paradoxon or the Drake equation, which easily accomodate no life on red dwarfs, and, if I am not mistaken, are often formulated with this assumption already in place.

  • Ron S January 18, 2011, 0:15

    Support for rare Earth? Well, a short while ago we had no idea there were so many M dwarfs hanging about. Now we see that they may be unsuitable for life. Seems that we’re more or less back where we started, with the same (unknown) probability for alien life.

  • James M Essig January 18, 2011, 0:58

    Hi mike wirth;

    Welcome aboard! I started posting at Centauri Dreams a few years ago and fell in love with blogging on interstellar space travel concepts. You will make new friends at TZ CD, and also likely be recorded in the annuls of human history as one of the intellectual pioneers among many who have heeded humanities calling to be a cosmic civilization.

    Regards;

    Jim

  • kzb January 18, 2011, 8:16

    Protoavatar, I think you are being unnecessarily pessimistic at this stage. The article linked above by Andy shows this. Radiation in itself is not a problem, because it can be shielded by a sufficiently thick atmosphere, by water or ice.

    Even in the extreme case you describe, there could be a twilight zone where liquid water occurs (nutation), sufficient for primitive life.

  • spaceman January 18, 2011, 9:31

    Hi Paul,

    So, the flaring of M-dwarfs is even more of a problem than we thought–interesting post. At what point in the main sequence do flares become prohibitive? Is flaring as much of a problem for second most common K-stars as it is for the ubiquitous M-stars? Is there a trend of increasing flaring with decreasing stellar mass? if so, at what point does flaring start to become enough of a problem to put the kibosh on life (or at least life of our biochemical variety)? Thanks.

  • Malcolm Ramsay January 18, 2011, 12:10

    Regarding the magnetic field problem, is there any notion of how fast a planet needs to rotate in order to produce a sufficiently strong field? After all, if the planet orbits close enough to its star to have a short year (a few days?), even tidally-locked rotation may be fast enough to create a planetary dynamo. We have no example in our system of an Earth-mass planet with a rotation longer than ours but shorther than Venus’; could a rotation perion of 20-30 days be sufficient?

  • andy January 18, 2011, 13:03

    Let’s not generalise to all red dwarfs either. For example, the infamous M3 dwarf star Gliese 581 is pretty quiet. Probably it lets off the odd flare now and again, but during the time it has been under radial velocity surveillance it has been boringly stable.

  • Flares Allowed On Condition January 18, 2011, 17:34

    As I read it, the great concern here seems to be all about the rate of flaring.

    I think it is already well understood that the youngest and oldest Red Dwarfs would be the most dangerous to live around. The youngest stars flare too frequently and destroy any ozone layer whereas the older red dwarfs, like Barnard’s star, flare so infrequently that planets around them don’t develop a thick protective ozone layer.

    The few studies that I’ve glanced at, online, all seem to come to the same conclusion that Earth like planets around observed flaring Red Dwarfs would normally develop thick ozone layers, and hence life would be protected.

    See -“The Effect of a Strong Stellar Flare on the Atmospheric Chemistry of an Earth-like Planet Orbiting an M dwarf” http://arxiv.org/abs/1006.0022
    &
    Biosignatures from Earth-Like Planets Around M Dwarfs” http://www.liebertonline.com/doi/abs/10.1089/ast.2005.5.706

    Historical information on UV Ceti, one of the best observed and most violently flaring red dwarf ever recorded, was used as a historical model in the above studies. UV Ceti once flared 7500 percent in its brightness. The studies claimed that the ozone layer on an Earth like planet, in orbit around UV Ceti, survived flaring and was replenished before each violent flaring took place. I think they were saying that Red Dwarfs normally flare in exactly the right radiation spectrum to help promote ozone formation and only during the most violent flares is ozone damaged.

  • Dave Moore January 18, 2011, 19:56

    There is a misapprehension as to the effects of tidally-locking a planets rotation. The effects (at least on the lit side) are quite mild. Here is a site showing the effect on Earths climate of stopping it’s rotation with the subsolar point permanently just off the coast of Ecuador.

    http://www.gps.caltech.edu/~tmerlis/coupled_tidally_locked.html

    You will see that on the sunlit side there are 3 main climatic zones: a cool, dry zone around the terminator, hot, dry deserts in northern Canada and southern Brazil, and a mild zone with heavy rainfall in the southern US and northern parts of South America. (Incidentally, people live in places on Earth with more annual precipitation.) Within these areas the climate is very stable, with minute variation over geologically long periods.. Earth, with its seasonal and daily variations comes off as wildly erratic by comparison. This consistency would be ideal for the evolution and profusion of life, much as it is in the tropical rain-forests on Earth.

  • andy January 19, 2011, 18:08

    @Dave Moore: thanks for that link, the animations seem to reveal some fairly surprising behaviour from the simulation (I wouldn’t have expected the Canadian deserts). Of course, those simulations are of a planet that rotates substantially more slowly than a tidally-locked planet in the HZ of a red dwarf, would be nice to know how much of an effect speeding up the planet would have.

  • Athena Andreadis January 19, 2011, 21:01

    NS: Given Paul Davies’ biological credentials (plus his liking of ID) and the (lack of) rigor that accompanied every step of NASA “arsenic bacterium” I really, really would not quote Lisa Wolfe-Simon or Paul Davies around serious biologists or astrobiologists.

    The Agency That Cried “Awesome!”
    http://www.starshipreckless.com/blog/?p=3668

    As for Rare Earth, the data its authors used to support their hypothesis is riddled with errors (some of which they corrected in the second edition of the book, after people goggled at them) and one of their major consultants was a known evangelical creationist.

    E. T., Call Springer-Verlag!
    http://www.setileague.org/reviews/rarearth.htm

  • Athena Andreadis January 19, 2011, 21:26

    Two mistakes in my previous comment — it should be Felisa Wolfe-Simon, and of course the data is plural so it should be “the data are riddled with errors”.

  • Dave Moore January 20, 2011, 21:16

    Andy,
    If you look at Merlis’s original papers on tidally locked aqua planets
    http://www.gps.caltech.edu/~tmerlis/tidally_locked.pdf
    you can see the effect of rapid rotation in his 1 day orbit simulation.
    However, the rotational effects become very minor over 7 day orbits.

    If you look at the idealized model of the aquaplanet with its bulls-eye like concentric rings of first cool climate with slight positive precipitation over evaporation, then hot dry desert clime, and finally a central convergence region of high rainfall, the results of the stopped-Earth model are in line with what you would expect.

    I tried asking Tim on whether there would be extensive glaciation on the night side, given that unlike with Earth’s polar regions, there is a net evaporation on the night side (Antarctic dry valleys?), but he was not prepared to comment on that.

    This lead me to speculate that the arrangement of the land masses on a tidally locked planet could have an effect on its overall climate. If there was a large land mass at the subsolar point, the high rainfall would cause a lot of erosion, raising oceanic Calcium levels and drawing down CO2 levels by carbonate formation, which could initiate an ice age as in the postulated mechanism of the “Snowball Earth” scenario.

    If the subsolar point and surrounding area of atmospheric convergence occurred exclusively over the ocean, then the opposite would happen. There would be no carbonate formation and a CO2 build up, which would cause global warming.

    Because the climate of a tidally locked planet is much more localized than Earths, the arrangement of the oceans and continents will be a lot more important.

  • Eniac January 21, 2011, 11:42

    This lead me to speculate that the arrangement of the land masses on a tidally locked planet could have an effect on its overall climate. If there was a large land mass at the subsolar point, the high rainfall would cause a lot of erosion, raising oceanic Calcium levels and drawing down CO2 levels by carbonate formation, which could initiate an ice age as in the postulated mechanism of the “Snowball Earth” scenario.

    If the subsolar point and surrounding area of atmospheric convergence occurred exclusively over the ocean, then the opposite would happen. There would be no carbonate formation and a CO2 build up, which would cause global warming.

    Interesting speculation. I would just like to note that here on Earth, most carbonate formation is biological, so this model might only apply if life arises in the first place. Also, I believe that it was precisely the “global warming” due to high atmospheric CO2 that kept Earth warm enough for life to form in the early gigayears, when the sun was 30% cooler and carbon not in such high demand.

  • Eniac January 21, 2011, 12:00

    I think it is already well understood that the youngest and oldest Red Dwarfs would be the most dangerous to live around. The youngest stars flare too frequently and destroy any ozone layer whereas the older red dwarfs, like Barnard’s star, flare so infrequently that planets around them don’t develop a thick protective ozone layer.

    Here, again, we should be cautious about not putting the cart before the horse. Ozone formation, IIRC, relies on the presence of atmospheric oxygen, and atmospheric oxygen, at least here on Earth, is of biological origin. So: no life -> no oxygen -> no ozone -> no flare protection -> no life.

    At least if it weren’t for the other things that have been said, such as sheltered conditions under rock and water, or on the night side of a tidally locked planet.

    An (as yet?) lifeless Earth will likely have no oxygen, but a lot of CO2. Probably a whole lot, like Venus and Mars. I don’t really know, but would not a thick, CO2-rich atmosphere also shield a lot of radiation? Would even the strongest flares be noticeable at all, say, at the surface of Venus?

  • andy January 23, 2011, 16:37

    Also, I believe that it was precisely the “global warming” due to high atmospheric CO2 that kept Earth warm enough for life to form in the early gigayears

    As I understand it, such “super-greenhouse” atmospheres are inconsistent with the mineralogical data. Rosing et al. (2010) argue that the early Earth would had similar carbon dioxide levels to the current atmosphere, but the lower continental area and lack of biological cloud condensation nuclei would have decreased the Earth’s albedo.

  • Eniac January 25, 2011, 0:22

    Interesting paper. It appears, however, that it stands by itself, and that the prevailing view still is that greenhouse gases are responsible (http://en.wikipedia.org/wiki/Faint_young_Sun_paradox). CO2 is a natural for that because of its high prevalence on Mars and Venus.

    I would like to know the reasoning behind this mineralogical evidence, is it really that incontrovertible? Unfortunately the article is not accessible on-line…

  • Ronald January 25, 2011, 11:22

    Interesting post and discussion again.
    The relevant question then becomes: why do some M dwarfs not flare, or at least not too much? Is this indeed age and mass related?

    So it seems, from the paper cited by andy (http://arxiv.org/abs/1006.0021) that excessive flaring is mainly a problem for later M dwarfs (>= M5) and much less so for the earlier types (M1 through M4).
    Also interesting are the papers cited by Flares Allowed On Condition, mentioning that ‘normal’ flaring will not neccessarily strip a ‘mature’ O2 rich atmosphere off an earthlike planet, even adding to its ozone layer.

    However, Eniac is probably right in stating that this is only true once a dense O2 atmosphere is formed by lifeforms, so life has to be able to arise and survive initially high levels of radiation in the first place.
    I read the hypothesis that very high levels of UV (and possibly also X-ray to a lesser extent) of a star in its early youth phase are a major inhibiting factor for life, and these levels have to decline below a threshold level for life to be able to gain a foothold. For our own sun, initial levels of aggressive UV were many tens to even hundreds of times present level in its early youth.

    If this is indeed right, and high levels of aggressive radiation are stellar age and mass related, this would imply that minimum ‘habitability age’ increases with later spectral type/decreasing mass, i.e. earlier spectral types (larger stars) being suitable younger and later spectral types (smaller stars) later.

    Also noting that stars above a certain mass (earlier F, all A, etc.) will probably always remain unsuitable because they will always keep emitting too much aggressive radiation besides having too short a main-sequence lifespan. And at the other end of the spectrum it is possible that the smallest M dwarfs (> M5 orso) will always remain excessively flaring.

  • Astronist January 26, 2011, 11:02

    I should like to disagree with kurt9 above, who is the only correspondent so far to comment on red dwarfs’ potential habitability by a technological civilisation coming in from outside.

    I imagine that the flares would not prove a problem to builders of artificial space habitats (O’Neill style colonies) who use as raw materials the asteroidal matter which we might expect to find in orbit around any main-sequence star. Within our own Solar System, shielding from solar flare particles is very much easier than shielding from GCR, because the latter have much higher energies.

    If this is true, then the Galaxy’s red dwarf population represents a large and ubiquitous ecological niche whose only real chance of hosting intelligent life is through colonisation by a star-faring civilisation. What do other people think about this?

    Stephen
    Oxford, UK

  • Ronald January 27, 2011, 13:01

    Astronist: agree, as a civilization once you find a (potentially) suitable planet in the HZ of a reasonable mature and stable red dwarf, you are set for gigayears.
    On the other hand, later G and early K solartype stars could also provide stable and probably more favorable environments for very long time periods. I cannot imagine that an advanced civilization, which is able to bridge the gap between the stars plus terraform and settle an alien planet, would really bother whether the mother star will remain stable for 0.1 or 1 or 10+ gy. In other words: extremely long-term settlement is only a prerequisite if traveling is prohibitively difficult.

    Therefore, I think that the often mentioned long-term stability of M dwarfs as an asset is not really an issue. The real issue, assuming a planet-dwelling civilization (and avoiding that discussion here for the sake of argumental focus) is the availability of suitable (terrestrial) planets in the HZ of a suitable star.
    And in this respect I still believe that the sunlike stars (latest F, G, early K) win hands-down.

  • Eniac January 28, 2011, 18:07

    Quite a few posters here are still ignoring the simple reality that “agressive radiation” is not much of an impediment to the beginning of life, because there is plenty of shelter underground, underseas, and on the far side.

  • Astronist January 28, 2011, 20:10

    Ronald, thanks, but I am assuming that star travellers will necessarily come from a civilisation based on artificial space colonies, not planetary surfaces (mainly because only a space-based civilisation can grow a large enough economy to be able to afford the energy costs of high-speed (0.05 to 0.1c) interstellar flight).

    Looking for Goldilocks zones and earthlike planets is then irrelevent for colonisation, making M dwarfs prime targets. The value of this class of stars is not so much their long lifetimes but their abundance. When an Earth analogue planet is eventually found, it will then be of more value as an object of non-invasive science rather than resource extraction.

    Stephen

  • Eniac January 31, 2011, 3:35

    Because only a space-based civilisation can grow a large enough economy to be able to afford the energy costs of high-speed (0.05 to 0.1c) interstellar flight

    This energy argument keeps coming up, but it does not make sense. For a starship, you need a fusion drive, and a lot of fuel. Energy plays a minor role.

    A 1 ton starship packing 10,000 tons of lithium deuteride fuel would require about half a year’s worth of the world’s lithium. The deuterium would probably require a production build-up, but nothing that could not be done quite easily here on Earth.