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M-Dwarfs: Weighing UV Radiation and Habitability

With 250 times more X-ray radiation than Earth receives and high levels of ultraviolet, would Proxima b, that tantalizing, Earth-sized world around the nearest star, have any chance for habitability? The answer, according to Jack O’Malley-James and Lisa Kaltenegger (Cornell University) is yes, and in fact, the duo argue that life under these conditions could deploy a number of possible strategies for dealing with the radiation influx. Their conclusions appear in a new paper in Monthly Notices of the Royal Astronomical Society.

Kaltenegger is director of Cornell’s Carl Sagan Institute, where O’Malley-James serves as a research associate. Modeling surface environments on four exoplanets that are prone to frequent flares — Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b — Kaltenegger and O’Malley-James examined different atmospheric solutions that could suppress UV damage in living cells.

Thin atmospheres and a lack of ozone protection fail to block UV radiation well, no surprise there, and such atmospheres do not measure up favorably when compared to atmospheres like that of the Earth today. But go back four billion years and we find that the modeled planets receive radiation in the UV significantly lower than what the Earth experienced in that era of its development. Earth was at that time uninhabitable by human standards — had any humans been available — but life had indeed emerged and continued to thrive. Thus the authors write that UV radiation “…should not be a limiting factor for the habitability of planets orbiting M stars.”

Image: The intense radiation environments around nearby M stars could favor habitable worlds resembling younger versions of Earth. Credit: Jack O’Malley-James/Cornell University.

The extremophile Deinococcus radiodurans is key to this study, for it is one of the most radiation-resistant organisms known. By varying the UV wavelengths, the scientists assessed the mortality rates of the organism, in which it becomes clear that some wavelengths of UV are more damaging to biological molecules than others. From the paper:

…we use this as a benchmark against which to compare the habitability of the different radiation models. This action spectrum compares the effectiveness of different wavelengths of UV radiation at inducing a 90 per cent mortality rate. It highlights which wavelengths have the most damaging irradiation for biological molecules: for example, the action spectrum in Fig. 4 shows that a dosage of UV radiation at 360 nm would need to be three orders of magnitude higher than a dosage of radiation at 260 nm to produce similar mortality rates in a population of this organism.

Image: This is Figure 4 from the paper. Caption: Relative biological effectiveness of UV surface radiation on Proxima-b. (A) The biological effectiveness of UV on DNA and the radiation-resistant microorganism D. radiodurans (Voet et al. 1963; Diffey 1991) quantifies the relative effectiveness of different wavelengths of UV radiation to cause DNA destruction or, for D. radiodurans, mortality, which increases with decreasing wavelength. Biological effectiveness of UV damage for (B) oxygenic atmospheres and (C) anoxic atmosphere models shown as convolution of the surface UV flux and action spectrum over wavelength (solid line shows flaring, dashed line quiescent star), compared to present-day Earth (red solid) and early Earth (3.9 billion years ago) (red dashed). Credit: Lisa Kaltenegger/Jack O’Malley-James/Cornell University.

We can’t rule out organisms below ground or living in water or rock, not to mention such survival characteristics as biofluorescence or protective pigments. We know of microorganisms that can tolerate full solar UV in space exposure experiments, using protective cells or pigments as effective UV screens. Biofluorescence offers protection against radiation because UV can be upshifted to longer wavelengths that produce less harm. The authors think protective biofluorescence would be at its most useful during the intense UV flux of flares, although a constant level of high UV might produce continuous fluorescence.

Here we have a potential biosignature, cited by the authors in a previous paper:

Because biofluorescence is independent of the visible flux of the host star and only dependent on the UV flux of the star, emitted biofluorescence can increase the visible flux of a planet orbiting an active M-star by several orders of magnitude (O’Malley-James & Kaltenegger 2018) during a flare.

We may get our first look at such atmospheres by observing ozone, which is potentially detectable by the James Webb Space Telescope. On the other hand, a high-enough level of UV could also produce a biosphere below ground that would present, if any, only the weakest of biosignatures. Even so, the authors conclude that nearby planets around M-dwarfs like those studied here are serious candidates for biosignature examination by future observatories.

While a multitude of factors ultimately determine an individual planet’s habitability our results demonstrate that high UV radiation levels may not be a limiting factor. The compositions of the atmospheres of our nearest habitable exoplanets are currently unknown; however, if the atmospheres of these worlds resemble the composition of Earth’s atmosphere through geological time, UV surface radiation would not be a limiting factor to the ability of these planets to host life. Even for planets with eroded or anoxic atmospheres orbiting active, flaring M stars the surface UV radiation in our models remains below that of the early Earth for all cases modelled. Therefore, rather than ruling these worlds out in our search for life, they provide an intriguing environment for the search for life and even for searching for alternative biosignatures that could exist under high-UV surface conditions.

The paper is O’Malley-James & Kaltenegger, “Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems,” Monthly Notices of the Royal Astronomical Society Vol. 485 Issue 4 (June 2019), pp. 5598-5603 (full text).


{ 29 comments… add one }
  • Michael C. Fidler April 10, 2019, 11:51

    There are so many of the M Dwarf stars, some studies give upwards of 80% of all stars, plus brown dwarfs also have UV flares. Maybe we should be looking at it the other way around, that maybe panspermia comes from these types of solar systems. Take a look at these beautiful UV induced Biofluorescence in plants and insects:

    NASA’s OCO-3 Measures How Plants Grow – and Glow.

    Photographer Uses UV Light to Capture Shimmering Shots of Fluorescent Flowers.

    Using Ultraviolet Light to Make Nature Fluoresce in Photos.

    Many if not most insects see in the UV light and many animals also do. What is surprising is that plants receive most of their energy from photosynthesis in the blue/violet and orange/red ends of the spectrum. So what is most of the energy given out in M Dwarfs? The blue to UV in flares and red to infrared in normal light.
    Any comments are welcome!

    • Michael C. Fidler April 11, 2019, 0:27

      Two points I made in the comments about this article under Lisa Kaltenegger (Cornell University) “Shaping the TESS Target List”.

      1. It looks like our nearby Red/M Dwarf exoplanets may not be all that bad after all! The UV levels may be significantly lower then all the doomsayers make it out to be. The possibility that some type of biofluorescence from the UV flaring could also create a photosynthesis process that another recent paper says was to low, because of only low power infrared rays from M Dwarfs. With these planets tidally locked the biofluorescence plant life may be near high noon where the UV radiations is strongest and animal life may be closer to the terminator were the atmosphere filters out the hasher radiations.

      2. Or the other possibility, that a biofluorescence jungle could exist on the red dwarf facing side with plenty of animal life living beneath it in the shade… ;-})

      The problem is, that in most comments we look at the earth as the prime example, but G type dwarfs are more then 20 times less common then M Dwarfs! My intuitive thoughts are seriously looking at our early lifeforms, in animals (Insects) and plants even fish, were somehow brought to earth via panspermia from exoplanets around M Dwarfs
      systems. We will most likely see a wide variety of atmospheric gases because of these life forms ability to adapt to different suns and planets but will be most common and easiest to identify in the Red/M Dwarf systems. So the prediction is that in ten years the results while show an overpowering percentage of life’s biomarkers will be found around these miniscule Red Dwarf Systems. ;-})

  • Bruce D. Mayfield April 10, 2019, 13:46

    There is another way to block or at least significantly decrease harmful radiation on a young planet: Have the remnants of the planetary accretion disk block such irradiation. Much of the harmful radiation from young stars might not even reach the top of a young planet’s atmosphere!

  • H. Floyd April 10, 2019, 16:58

    Tantalizing! The “Purple Earth Hypothesis” speculates that archaic terrestrial life reflected hostile conditions that were the norm on Earth from roughly 3 a.m. to 6 p.m. of our planet’s condensed Cosmic Day:


    If so, optical biosignatures we’d associate with Earth’s contemporary biosphere — shades of green — aren’t evolved for optimal efficiency so much as they were lucky improvisations in marginal niches, on an already marginally survivable world. Our earliest common ancestor would have moved through the Great Filter between simple and complex cellular reproduction clad in secondhand photosynthetic chemistry. Not because green is ideal, but because no one else was using it at the time.

    If so, simple biosignatures may be fairly predictable — maybe fluorescents, purples and pinks — while optical biosignatures of complex life in the universe could be wildly counterintuitive.

    • H. Floyd April 10, 2019, 22:17

      * One of our early common ancestors would

  • Rob Flores April 10, 2019, 17:35

    Yes radiolaris is tough, no one is arguing that red dwarf systems are barren. Because of the metabolic cost of being constantly under Repair, life on a planet in the HZ of a red dwarf on the surface is unlikely to be complex. I grant that burrowing creatures could develop but can they live close enough to surface to use oxygen. Being able to develop the equivalent of ATP is crucial for complexity and oxygen is its most efficient companion.

    • Paul451 April 14, 2019, 4:20

      Complex life didn’t evolve on land, but in water. And water absorbs UV in much the same way as our atmosphere does; the higher the frequency, the quicker the absorption. UV-c is pretty much completely blocked in the first metre, while UV-a would penetrate down to 10m; UV-b in between. Pick your preferred level, strong selection pressure in favour of species able to live higher in the water column.

  • John walker April 10, 2019, 18:03

    Depending on whether the M dwarf is a flare type or quiescent and early, mid or late type the photosynthesis effectiveness ranges from near zero to minimal. However, calculations have shown that at least during flares the flux in the photosynthetically useful range of 400-700 nm increases greatly. These events are brief and episodic. And of course also detrimental to the atmosphere. Whether a balance is possible between atmospheric permanence and sufficient insolation for photosynthesis is unclear.

    In my opinion the primary hazard for the prospect of life on M dwarf planets is atmosphere degradation and subsequent planetary dessication.

  • Alex Tolley April 10, 2019, 21:39

    Are we worrying about UV for terrestrial organisms? Water provides very good UV shielding, so we could expect life to stay principally below the surface of the oceans and lakes. In shallower aquatic zones, burrowing life will evolve. Life around ocean vents will not be affected.

    A key issue is what are the conditions for abiogenesis and are these subject to environments where UV levels are problematic. Once life evolves, then adapting to conditions, like the extreme tolerance of UV by D. radiodurans, is probably a given.

    Rather than assuming UV penetration to the organism, perhaps a better analysis is the shielding necessary in a range of environments to maintain UV at Earth-like levels for various primary producers and assume the consumers will be able to adapt by various means.

    • Ron S. April 11, 2019, 10:16

      Perhaps you can comment on this. If life must evolve in a shielded environment, such as underwater, I would think it would need to progress to multi-cellular organisms before migrating to the harsher environment of the surface. The reason being that a protective layer of dead cells (skin, hide, scales) would be required.

      • Michael C. Fidler April 11, 2019, 10:52

        One of my workers caught a nice rock lobster yesterday, bet it would have no problem on the surface with that nice shell, insects seem to be made for UV.
        New image from Proxima-b:

        Harsh radiation no barrier to life on nearby exoplanets.

      • Alex Tolley April 11, 2019, 15:51

        For metazoa, that is exactly what happened. Life did not emerge onto land until metazoa were capable of making the transition. Our own vertebrate lineage were early fish that initially “crawled” out of the shallows. Vertebrates were preceded by invertebrates and in turn preceded by plants.

        For animals, there are adaptive mechanisms. Molluscs had shells to resist desiccation (e.g. snails), and also could burrow in mud and sand when the tides were out. As I am sure you have observed, animals in rockpools tend to hide under overhanging rocks when they can, although this is not an option for sessile animals like anemones.

        Where the problem with that hypothesis starts however, is that stromatolites (bacterial colonies/films) were exposed to the air during low tides before the great oxygenation event when UV was not yet blocked by O3. Unless these ancient stromatolites were different than modern ones, then the colonies must have been able to tolerate or adapt to that UV in some way. I don’t think they sacrificed the topmost films and I don’t believe they stayed immersed in the shallows either (add to that the early tides were much higher due to the Moon being closer).

        • Ron S. April 12, 2019, 20:29

          Thanks, Alex.

  • Patient Observer April 10, 2019, 22:24

    Slightly off topic. I just don’t understand this article regarding cool subdwarf stars. The article states that, for the first time, the radii of such stars can be accurately measure by taking high speed images (1,000 per second).


    How would high speed imaging help establish a diameter of a point source?

    • Bruce D. Mayfield April 11, 2019, 10:08

      Because a high enough frame rate can help mitigate bad seeing at the earth’s surface. The starlight twinkling effect (same as what astronomers call “bad seeing”) comes from air turbulance, movement of gases in our line of sight to the star. They can then digitally add up all the non blurred images to obtain a very sharp focus.

    • Michael C. Fidler April 11, 2019, 10:09

      Here is the original article in Nature and as usual it not a simple as they make it sound. See the “Methods” section and then the “The light-curve fitting method” under that. The actual subject is a M dwarf/White dwarf eclipsing binary and I belive it may be the ability to get such an accurate light curve (see Figure 3.) with the high speed camera.
      Hope that helps!


      • Michael C. Fidler April 11, 2019, 11:10

        Well I’ll be, they just put a paywall up on it!
        But here is the ArXiv for it: Accurate mass and radius determinations of a cool subdwarf in an eclipsing binary.


        • Patient Observer April 11, 2019, 22:25

          The paper adds the key element missing from the article I referenced – it was an eclipsing binary. High speed imaging, as mentioned above, would allow more precise measurements of eclipses. Thanks for the help.

  • Michael C. Fidler April 12, 2019, 18:53

    A Second Planet May Orbit Earth’s Nearest Neighboring Star!!!

    Possible 2nd Planet Spotted Around Proxima Centauri!!!

    This planet would be perfect for UV flare imaging with the Hubble telescope and even earth based scopes. The distance of 1.5 UA for Proxima Centauri c from Proxima Centauri would give a 12 minute delay in the flash from the flare and should be bright enough for Hubble to image it directly!!! Even smaller ground based telescope then Hubble may succeed in imaging the planet in the UV if a large bright UV flare from Proxima Centauri is put in an alert system for quick action of telescopes. Possibly even scopes down to ONE METER could use LUCKY IMAGING to capture it!!!



    • Rob Flores April 13, 2019, 0:02

      So does this 2nd planet help resolve whether or not Proxima is in
      fact in orbit around the AB centauri twins.

      Highly optimistic calling it a SuperEarth.
      Mini Neptne, more likely with most of it’s H2/HE amosphere still
      Depending on how big the solid core is compared to total size
      I suppose its possible for this, world heated by pressure, to have a liquid ocean above said rocky core.

      • Michael C. Fidler April 13, 2019, 10:03

        Oh, That’s where the swamp-gas aliens come from! :-)

  • Michael C. Fidler April 12, 2019, 20:02

    Was watching Alpha Centauri and Proxima Centauri with my binoculars last night. It is well placed in the evening sky now at our latitude of 9.5 degrees north, here in the island of Bohol, Philippines. What a surprise this morning when Proxima Centauri c was announced!!! (8+ GMT) Life is good… ;-})

    A new super-Earth may orbit the star next door.


    • Michael C. Fidler April 12, 2019, 20:09

      Another Cold Super Earth like Barnard’s star b? Could it have life on it…

      Barnard’s Star Planet May Not Be Too Cold for Life After All.


      • Michael C. Fidler April 12, 2019, 23:13

        Planet Orbiting Barnard’s Star Might Have An Atmosphere.
        APRIL 12, 2019

        The planet around Barnard’s Star, Barnard’s Star-b (“BSb”)
        The astronomers conclude that although today the planet Barnard’s Star b may have a relatively mild space climate (comparable, nevertheless, to bad space weather conditions for Earth), in its early years it probably did undergo significant disruption. Today, however, BSb might retain an atmosphere that could studied.


        So Proxima Centauri c may have a substantial atmosphere also, being 1.5 astronomical units (AU) from Proxima Centauri and it takes 1,936 days to orbit it. This will also make it a good target for when an Ultraviolet (UV) flare occurs to observe the Ultraviolet (UV) spectrum of Proxima Centauri c atmosphere.

        • Harry R Ray April 14, 2019, 14:28

          Proxima C update: In addition to the parameters mentioned above, MsinI is now 6 Earth mass, and the level of confidence of its existence has climbed from 2 sigma to 2.5 sigma. Just one more Red Dots observing campaign could put the confidence level OVER 3 sigma and a possible paper could be submitted.

  • Michael C. Fidler April 12, 2019, 23:57

    Just did some simple calculations on the distance Proxima Centauri c is from Proxima Centauri in arcsecond. It turns out to be 1.147 arcsecond, calculated from Proxima Centauri b distance of 0.0485 AU to Proxima Centauri c distance of 1.5 AU gives 31 times. Proxima b is 37 milliarcsec from Proxima Centauri time 31 equals 1147 (milliarcsec) which is 1.147 arcsecond, this could easily be resolved by a twenty inch (.5 meters) telescope. So any amateur astronomer could image this planet when an UV flare is emitted from Proxima Centauri. All they would have to do is wait 12 minutes after the flare for Proxima c to light up in the UV…

  • ljk April 17, 2019, 11:08

    U(V) Light Up My Life

    By Astrobites on 16 April 2019

    Title: The Surface UV Environment on Planets Orbiting M-Dwarfs: Implications for Prebiotic Chemistry & Need for Experimental Follow-up

    Author: Sukrit Ranjan, Robin Wordsworth, & Dimitar D. Sasselov

    First Author’s Institution: Harvard-Smithsonian Center for Astrophysics

    Status: Published in ApJ

    Note: The reference to “life” in this post refers to “life as we know it here on Earth.” [As opposed to… ?]


    Article is online here:


  • ljk April 18, 2019, 10:53

    Explosion on Jupiter-sized star ten times more powerful than ever seen on our Sun


    Researchers led by University of Warwick could only see the star while it was flaring, when it was 10,000 times brighter than normal

    Superflare is equivalent to 80 billion megatonnes of TNT, ten times as powerful as the Carrington event in 1859

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