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‘Red Edge’ Biomarkers on M-dwarf Planets

When we think about the markers of possible life on other worlds, vegetation comes to mind in an interesting way. We’d like to use transit spectroscopy to see biosignatures, gases that have built up in the atmosphere because of ongoing biological activity. But plants using photosynthesis offer us an additional option. They absorb sunlight from the visible part of the spectrum, but not longer-wavelength infrared light. The latter they simply reflect.

What we wind up with is a possible observable for a directly imaged planet, for if you plot the intensity of light against wavelength, you will find a marked drop known as the ‘red edge.’ It shows up when going from longer infrared wavelengths into the visible light region. The red-edge position for Earth’s vegetation is fixed at around 700–760 nm. What we’d like to do is find a way to turn this knowledge into a practical result while looking at exoplanets. Where would we find the red edge on planets circling stars of a different class than our own?

Led by Kenji Takizawa, researchers at the Astrobiology Center (ABC) of National Institutes of Natural Science (NINS) in Japan have taken up the question with regard to M-dwarfs. These stars have lower surface temperatures than the Sun and emit more strongly at near-infrared wavelengths than at visible wavelengths. Assuming vegetation in such an environment evolves to use the most abundant photons for photosynthesis, shouldn’t we expect the red edge to shift accordingly? Perhaps not, argue the authors, as only blue-green light penetrates beyond a few meters of water. Visible light, in other words, may play a larger role than we imagine.

This is a useful study, because we will begin our observations of possible biosignatures on exoplanets around stars like these, using not only upcoming space missions but ground observatories like the European Extremely Large Telescope, the Thirty Meter Telescope, and the Giant Magellan Telescope. The question is, what effect does the radiation of the star itself have on the red edge?

Image: Artist’s impressions of a habitable planet around M-dwarfs (left) and primordial Earth (right). Credit: ABC/NINS.

The authors believe that the first oxygenic phototrophs would have evolved underwater, using light at visible wavelengths. The star AD Leonis (AD Leo), an M-dwarf located 16 light years away, served as their model, with Takizawa and team plugging in a hypothetical planet of Earth’s size and insolation in orbit in the habitable zone there, allowing light conditions on the planet to be estimated and compared with solar irradiation on the Earth. The paper predicts ‘photosynthetic machineries’ that could emerge underwater and on land, and goes on to examine the question of land phototrophs evolving in the direction of infrared use from marine phototrophs that evolved in visible light.

From the paper:

After comparative investigations of light environments on the hypothetical habitable exoplanet around AD Leo and on the Earth, we conclude that two-photon reactions using visible radiation may first evolve in an M-dwarf planet’s ocean as oxygenic phototrophs. Reactions using NIR radiation may then evolve on the land surface. The first oxygenic phototroph is most likely to be established underwater about 10 m or deeper, and may expand its habitat to shallow water after the formation of an ozone layer and/or the cessation of UV emission from the active M-dwarf.

At depths of one meter and less, the authors believe, the abundance of near-infrared radiation may stimulate its evolutionary use, though notice what happened on Earth:

…phototrophs using NIR radiation are exposed to drastically changing light conditions in shallow water, which may be an obstacle to using NIR radiation before land colonization. The formation of the ozone layer on the Earth prior to the emergence of land plants enabled them to quickly colonize the land, safe from the effects of UV. A rapid transition from aquatic algae to land plants was accomplished without a change in the photosynthetic machinery.

Image: Figure 1 from the Takizawa paper. Credit: Takizawa et al.

The authors’ calculations show that green algae can adapt to M-dwarf radiation, though less productively than phototrophs using near-infrared radiation. Indeed:

…oxygenic photosynthesis could be driven by low-energy NIR radiation by improving the redox energy conversion efficiency and/or employing multi-photon reactions.

Thus it is possible that a ‘first wave’ of land plants could exhibit a red-edge position at 700–760 nm, as on the Earth. Using near-infrared radiation underwater would expose phototrophs to strong changes in light intensity and quality when they approach the water’s surface. That makes it more likely that adaptation to near-infrared would occur once on land, where the radiation spectral change is smaller.

To take these possible changes into account, astronomers need to look in a broad range when considering biomarkers on red dwarf planets:

…future missions should prepare to record surface spectra of habitable exoplanets at wavelengths from shorter than 700 nm to longer than 1,100 nm so that they have the capability to address the possibility that the red-edge position may change as the host M-dwarf ages.

The paper is Takizawa et al., “Red-edge position of habitable exoplanets around M-dwarfs,” Scientific Reports 7, No. 7561 (full text).

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{ 18 comments… add one }
  • Scott September 19, 2017, 16:30

    Cyanobacteria/Chloroplasts use the spectrum they do because they had to make do with the (less energetic) remaining spectra than the ones already monopolized by phototrophs that emerged earlier.

    Earth has only looked like “earth” for a very short period of time, and will go back to the way it was in 100 million years when current carbon geochemistry cycles are disrupted. If the planets around M-Class stars are truly as old as suggested in publications, they’re overwhelmingly more likely to resemble earth during one of its slimeball phases.

    As a consequence, you’d do better to try the technique on the absorption spectra of Halorhodopsin or one of the Bacteriochlorophyll variants instead or in addition.

  • Steven Torry Rappolee September 19, 2017, 18:25

    A recent KISScaltech talk on remote sensing of chlorophyll

    https://www.youtube.com/watch?v=gntvibev92A

  • David Hart September 19, 2017, 20:55

    Water is a very good absorber of near infra-red, so much so that monochromatic near infra-red aerial photos show water as black. So unless the plants reach the surface (like kelp) I cannot see any underwater pathway to the use of near infra-red for photosynthesis.

  • Andrew LePage September 19, 2017, 22:54

    We should really beware of claims of using some hypothetical extraterrestrial variation of chlorophyll as any sort of useful biomarker. It was only two-thirds of a century ago that many astronomers were sure that they had detected chlorophyll on Mars along with using a range of other observations to “prove” that Mars possessed primitive plant life.

    http://www.drewexmachina.com/2014/10/05/a-cautionary-tale-of-extraterrestrial-chlorophyll/

  • Ioannis Kokkinidis September 20, 2017, 14:51

    The red edge is a broad region in the red where we have a rapid change in plant reflection response. There are a couple of remote sensing satellites (around earth) with a Red Edge band, but it is not something typically found. In hyperspectral remote sensing very related is the Red Edge Inflection Point (REIP) which is somewhere on the red edge where the reflection curve changes from concave to convex. REIP is dependent on species, hydric condition, nutrient availability, plant health and several other biophysical factors, which is why it is popular. Now is strong reflection on the red, similar to the red edge, a biomarker? While alien plants display the characteristic curve of chlorophyll in the red and a REIP? I dare not guess

  • andy September 20, 2017, 16:21

    I’m always sceptical about this kind of thing. Evolution does not find optimum solutions: everything is contingent on what came before. It is far more likely to end up trapped near some local (rather than global) optimum. So while it may be overall better to follow the trends suggested by the energy balance of the star’s radiation, the pre-existing biochemistry may constrain it to end up following a less optimal path.

  • John walker September 20, 2017, 20:49

    While this paper is interesting, I believe there are major problems which have not been given their due attention when discussing habitability on M dwarf exoplanets. One of which is UV and its relationship to prebiotic chemistry.

    The consequences of early M dwarf high UV luminosity has been often discussed in terms of it’s potentially deleterious effects on extant surface life and it’s massive influence on atmospheric development. However, UV has an important perhaps critical role in prebiotic chemistry as evidenced by recently discovered signs that terrestrial RNA evolved in a high UV environment. And that is the problem. Following the high flux phase of young M dwarfs, there is a paucity of the essential NUV that is necessary for the synthesis of a number of important prebiotic molecules. While acknowledging that we do not presently know with certainty the importance of UV to the emergence of terrestrial life(deep ocean vents) there has been somewhat of a lack of consideration of the surface UV environment of M dwarf exoplanets on prebiotic chemistry. A paper recently submitted to Nature attempts to address this issue. It’s a deserving topic for a future blog entry in my opinion. https://arxiv.org/pdf/1705.02350

  • Alex Tolley September 20, 2017, 22:48

    I also am skeptical of the reasoning. There is little reason to assume that light trapping molecules will work like the chlorophylls on Earth, or if they do, that the absorption peaks will be in the same position. If life remains in the aquatic stage, the technique will be largely useless. By all means look for this signature, but I think we may find a lot of false negatives.

  • Antonio Mario Magalhaes September 20, 2017, 23:29

    Paul,
    Very interesting paper and post.
    One could turn a “possible observable for a directly imaged planet” to hopefully a more concrete observable by using polarimetry. In the case you describe, plants, by being more reflective from ~750 nm down, would be more polarized than at shorter wavelenths.
    One advantage of polarimetry is that one would not need as large an observing time as w/ spectroscopy if one could instead do a broad-band survey.

  • Michael Fidler September 21, 2017, 10:10

    It would be interesting to simulate the actual conditions around a red dwarf in the lab and see how different microbes, fungus and other primitive slimes react to it.

  • ljk September 22, 2017, 10:00

    http://astrobiology.com/2017/09/space-technology-for-directly-imaging-and-characterizing-exo-earths.html

    Space Technology for Directly Imaging and Characterizing Exo-Earths

    Press Release – Source: astro-ph.IM

    Posted September 21, 2017 4:04 PM

    The detection of Earth-like exoplanets in the habitable zone of their stars, and their spectroscopic characterization in a search for biosignatures, requires starlight suppression that exceeds the current best ground-based performance by orders of magnitude.

    The required planet/star brightness ratio of order 1e-10 at visible wavelengths can be obtained by blocking stellar photons with an occulter, either externally (a starshade) or internally (a coronagraph) to the telescope system, and managing diffracted starlight, so as to directly image the exoplanet in reected starlight. Coronagraph instruments require advancement in telescope aperture (either monolithic or segmented), aperture obscurations (obscured by secondary mirror and its support struts), and wavefront error sensitivity (e.g. line-of-sight jitter, telescope vibration, polarization).

    The starshade, which has never been used in a science application, benefits a mission by being decoupled from the telescope, allowing a loosening of telescope stability requirements. In doing so, it transfers the difficult technology from the telescope system to a large deployable structure (tens of meters to greater than 100 m in diameter) that must be positioned precisely at a distance of tens of thousands of kilometers from the telescope.

    We describe in this paper a roadmap to achieving the technological capability to search for biosignatures on an Earth-like exoplanet from a future space telescope. Two of these studies, HabEx and LUVOIR, include the direct imaging of Earth-sized habitable exoplanets as a central science theme.

    Brendan Crill, Nicholas Siegler

    (Submitted on 19 Sep 2017)

    Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

    Cite as: arXiv:1709.06660 [astro-ph.IM] (or arXiv:1709.06660v1 [astro-ph.IM] for this version)

    Submission history

    From: Brendan Crill

    [v1] Tue, 19 Sep 2017 22:04:30 GMT (347kb,D)

    https://arxiv.org/abs/1709.06660

  • Geoffrey Hillend September 22, 2017, 16:28

    There still also transit spectroscopy, but the exoplanet has to move in front of the star and behind it. The problem with that is that if the system is seen from above then it won’t work because one cant subtract the stars plus planet spectral bands from the planet alone revealing the chemical composition of the planet’s atmosphere. There is also the light polarization technique.

  • Geoffrey Hillend September 22, 2017, 17:15

    The idea that we can detect the spectral signature of vegetation in the near infrared will work since it reflects light in the near infra red; vegetation has a unique spectral signature. Some different soils with moisture contents also reflect in the near infrared like very coarse grained sand, but we still might be able to differentiate their infra red spectral bands with an infrared spectrometer, but if there is also an oxygen, water and methane spectral signature, then that would be a good indicator. It also depends on the angle of reflected sunlight. http://www.geol-amu.org/notes/m1r-1-8.htm There would have to a large amount of microbial life in the ocean to make enough oxygen for planets to come onto land so there could be enough oxygen and ozone to protect it. I don’t expect to see the spectroscopic biosigns of life on an exoplanet around red dwarfs stars but I could be wrong.

  • Geoffrey Hillend September 25, 2017, 17:43

    An infrared spectrometer can tell the difference between plants and moist course grained sand since they both emit thermal infrared radiation at a slightly different frequency. Scientists can tell what the different chemical composition is of different surfaces are on a planet since every atom and molecule radiates at a slightly different brightness of infra red. They have a brightness map of different substances, types of rock etc.

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