≡ Menu

New Constraints on Exoplanet Photosynthesis

Most autotrophic organisms on Earth use photosynthesis to work their magic. Indeed, photosynthesis accounts for about 99 percent of Earth’s entire biomass (a figure likely to change as we learn more about what lies beneath the surface). The process allows organic matter to be synthesized from inorganic elements, drawing on solar radiation as the energy source, and providing the oxygen levels needed to drive complex, multicellular life.

Does photosynthesis occur in other star systems? We know that it emerged early on Earth, and can trace its development back to the Great Oxidation Event in the range of 2.4 billion years ago, although its origins are still under scrutiny. In a new paper, lead author Giovanni Covone (University of Naples) and colleagues examine the conditions needed for oxygen-based photosynthesis to develop on an Earth-like planet not just at Earth’s level of stellar flux but throughout the classical habitable zone.

The key to the study is stellar radiation as received by the planet from the host star, with the authors examining the efficiency with which living organisms could produce nutrients and molecular oxygen using oxygenic photosynthesis. Here we are considering what the paper describes as photosynthetically active radiation (PAR). Key to the analysis is the idea of exegy, which the authors explain as follows:

…we estimate the efficiency of the PAR radiation driving OP [oxygenic photosynthesis] as a function of the host-star temperature by means of the notion of exergy. Exergy can be defined as the maximum useful work obtainable from the considered system in given environmental conditions (see e.g. Petela 2008; Ptasinski 2016). In other words, exergy is a measure of the quality of energy (Austbø, Løvseth & Gundersen 2014). Living organisms are dissipative structures away from thermodynamic equilibrium with the environment thanks to the constant input of exergy stellar radiation.

This idea of the quality of energy has been the subject of several exoplanet investigations, most recently that of Caleb Scharf (Columbia University), who studied photosynthetic efficiency as a function of a star’s effective temperature over the entire radiation spectrum. Covone and team keep their focus on photosynthetically active radiation, constructing a table showing the parameters of Earth-analog planets and their host stars, including worlds at Proxima Centauri, Kepler 186 and Trappist-1.

The question is whether living organisms can efficiently produce the nutrients and molecular oxygen they need in these conditions via normal photosynthesis.

Table 1. Parameters of the known Earth analogue planets in the HZ and their host stars. Equilibrium temperature values with * have been derived in this work. For Proxima Centauri b the estimate of the mass is given since the planet is probably not a transiting one. Credit: Covone et al.

Considered in terms of the exergetic efficiency of a star’s radiation within this range, the authors find that only Kepler=442b receives a photon flux sufficient to sustain a biosphere something like the Earth’s. This is an interesting world, a confirmed super-Earth orbiting a K-class star in Lyra about 1200 light years out. It does not appear to be tidally locked and offers what the scientists consider to be a good target for a search for biosignatures. But the other worlds lack the energy in the correct wavelength range to sustain a rich biosphere. The figure below is striking:

Image: This is Figure 1 from the paper. Caption: Photons flux in two differently defined PAR ranges at the surface of planets at the two edges of the HZ (dark blue lines for an upper limit of 800 nm and light blue for an upper limit of 750 nm), as a function of the star effective temperature, in units of 1020 photons s−1 m−2 (HZ inner edge: continuous line; HZ outer edge: dotted line). The green dot and circle show the photon flux in PAR range on the Earth surface, yellow dots and circles the estimated photon flux on the surface of known Earth analogues (see Table 1), respectively, with an upper limit for the PAR range of 800 nm (dots) and 750 nm (circles). The red dotted line shows the average photon flux which is necessary to sustain the Earth biosphere. The green dotted line shows the typical lower threshold for OP on Earth. Credit: Covone et al.

Of the planets cited in Table 1, then, only Kepler-442b comes close to receiving the stellar radiation needed. Indeed, given these findings, many stars in the K-class would be unlikely to supply the radiation needed to support a complex biosphere. Nor would red dwarf stars, which would not deliver enough energy to their planets to activate photosynthesis in the first place. Giovanni Covone comments:

“Since red dwarfs are by far the most common type of star in our galaxy, this result indicates that Earth-like conditions on other planets may be much less common than we might hope.”

And he adds:

“This study puts strong constraints on the parameter space for complex life, so unfortunately it appears that the “sweet spot” for hosting a rich Earth-like biosphere is not so wide.”

A much narrower than expected range for the habitable zone? Perhaps in terms of that exact ‘sweet spot’ that mirrors Earth. But the authors are quick to add that caution is in order in terms of biomass production, which softens the message considerably. This passage receives prominence in the paper’s conclusion (italics mine):

…we should bear in mind that biomass production on Earth is not limited by the quantity neither [sic] the quality of the incoming radiation, but rather by the availability of nutrients. For instance, Lin et al. (2016) found that in ocean phytoplankton populations about 60 per cent of the absorbed PAR solar energy is dissipated as heat. Generally, phytoplankton operate at a much lower photosynthetic efficiency than they are potentially capable of achieving, just because in most situations light is a very abundant resource on Earth. Moreover, OP does not respond linearly to the input photon flux (see Ritchie et al. 2018). For these reasons, it is not immediate to draw consequences on the amount of biomass produced from the estimated PAR photon flux and its exergy content. Exoplanets with lower values of these quantities could host a biosphere comparable with the one on our planet.

We should not, in other words, read this as a definitive statement on habitable zone width but rather a pointer to further work that will need to broaden the investigation. The authors themselves mention “exergy destruction that occurs as consequence of biological conversion taking place after the light harvesting, in the leaf transpiration and metabolism” and also atmospheric absorption that changes the radiation spectrum. But solutions beyond oxygenic photosynthesis are also possible, a direction of study that could point to near-infrared light harvesting on red dwarf planets.

The paper is Covone et al., “Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone,” Monthly Notices of the Royal Astronomical Society, Volume 505, Issue 3 (August 2021), pp. 3329–3335 (full text).


Comments on this entry are closed.

  • Michelangelo Pantaleoni June 29, 2021, 11:09

    Sorry to be so picky, but I think figure 1 in the paper is diferent from what you posted (figure 3). The footnote doesn’t match the image.

    • Paul Gilster June 29, 2021, 16:49

      Not picky at all. I just corrected the mistake — I had linked to the wrong image. Thanks for checking in on this!

  • wdk June 29, 2021, 13:26

    When we consider that our fundamental understanding of exoplanets is based on detection methods, indirect measurements of dimensions or mass providing equilibrium temperatures, this paper is an excellent next logical step of discussion. Whether life as we know it could originate on such planets – or be sustained if we get to visit, oxidation photosynthesis (OP) – that’s a great “What’s next?” for discussion.

    Reading over this thus far, I note that the consequences of OP do not show themselves until about midway through Earth’s lifetime with the
    great oxidation period or event. Also, we have to make some assumptions about what Earth’s atmosphere was like in the past, even surface atmospheric pressure. Because clearly large impact events could
    adjust that value all over the place. After all, volatiles were both accreted over eons and lost. But given all that, we should be able to better understand where carbon based life in O2 environments has its best chances. At this point, we still might even discover there are better places to be than where we are.

  • Abelard Lindsey June 29, 2021, 13:54

    This is believable and would explain why we orbit a G star rather than the more numerous K or M stars.

  • henry cordova June 29, 2021, 20:54

    I’ve always suspected anaerobic microbial life was fairly common in the cosmos (arising <1 Gyr after planetary formation), but the start of photosynthesis was one of the great bottlenecks (2-3 Gyr) life must pass on its way to sentience, along with the rise of multicellular life forms (about 4 Gyr). If these numbers are comparable to other solar systems, then we must expect stars and planets with potential intelligent inhabitants must be at least several billion years old, and that those systems remained relatively stable and hospitable for several billion years.

    Granted, one data point is not totally convincing, but its still better than nothing.

  • Michael J. Sloboda June 29, 2021, 21:04

    Maybe someone could plant rice, lettuce and tomatoes indoors and turn on artificial light. If the light mimics the yellow sun, the plants will grow and yield a normal harvest. Lower the effective temperature of the light to match an orange or a red star. How much does the crop yield fall? At what cooler black body spectrum do the seeds sprout and then wither instead of growing?

    • Alex Tolley June 30, 2021, 9:20

      There have been lots of experiments on overall light illumination, and even using filters to deliver just the light needed for photosynthesis, so that is known well. What has not been done experimentally, AFAIK is to use the light of different spectra to mimic other stars. Having said that, white light LEDs are sold with different peak temperature spectra to mimic different types of incandescent and fluorescent lamps. Those with lower temperatures, e.g. 2700K mimic M_dwarfs, whilst those with 6000K mimic our sun. As most light experiments back in the 1960s using incandescent bulbs which were far yellower than the sun, I imagine that illumination experiments would mimic conditions for M_dwarfs at that spectral range. [I don’t think one can buy LEDs that have a peak spectral temperature of 2000K for the dimmest of M_dwarfs. The depiction of M_dwarfs as having red light is just wrong – most would appear yellow to our eyes, just like fully powered incandescent lamp light ~ 2400-2500K, but dimming them reduces that temperature.]

      • wdk June 30, 2021, 16:52

        Curiously and near forgotten: About 25 years ago was engaged in an effort to simulate performance of spacecraft star trackers, in particular for the Space Shuttle. The avionics lab needed to examine the response to the several dozen brightest stars in its navigational catalog; so the exercise in the lab in those pre-LED days was to mix the light sources enough to get similar blue, green, yellow red light contributions to mimic stars of various temperature and perceived brightnes. It worked rather well. But since we were dealing with very distant starlight, I doubt if we would have been able to grow much wtih G2V stars at our intensities either. But in principle this sort of thing could be done with crops, algae or the most fundamental photosynthesis units. This could be a (sic) growing exercise in exoplanet studies.

    • Ioannis Kokkinidis June 30, 2021, 18:54

      What you are describing is greenhouse agriculture in the Netherlands, and it works great once you know how. On top of photosynthesis there is also the question of photoperiodism, i.e. the day length necessary for a plant to get to the flowering stage. Chrysanthemums in a greenhouse in winter require extra hours of light otherwise they will not flower, not matter what the Photosynthetically Active Radiation they receive

  • Brett June 30, 2021, 0:05

    It looks like you could get the smallest K-stars and very largest M-stars. They have surface temperatures at 3900 K, which (judging by the chart) allows for sufficient photons if the planet is close to the inner edge of the habitable zone. Those stars burn so slowly that any planets there would still likely have tens of billions of years before the HZ moves past them.

    The smaller M-stars are pretty unpromising for earth-like biospheres anyways due to the flaring, the brutal pre-main sequence phase, etc. Just another reason they’re unlikely to have Earth-like planets.

    • John walker June 30, 2021, 15:11

      The limiting habitability factor in systems with M and K class stars is not stellar longevity. The longevity of a planetary magnetic dynamo will be orders of magnitude less. This is especially relevant for biospheres not protected by thick Europa like ice shells. An extant magnetic field to protect surface life and the atmosphere that surrounds the planet will die long before the star leaves the main sequence. Some variation in the speed of core crystalisation exists due to variances in planetary mass and composition, but trillion year stellar lifespans do not predict a planet’s biospheric longevity.

  • Robin Datta June 30, 2021, 0:39

    With just one example, speculation about the adaptability (or otherwise) to ambient environmental conditions is less well guided. Nevertheless the variety seen within just our one example may suggest that other environments may guide the emergence of life along other pathways in The Full Palette of Photosynthesis.

    It would seem that the tetrapyrrole ring is essential to hold biologically active metal atoms: iron in the heme of hemoglobin, magnesium in chlorophyll, nickel in methanogenic bacteria and cobalt in vitamin B₁₂, and might therefore be found in similar roles on exoplanets.

    Perhaps these recurring patterns may have a basis in physics and physical chemistry that may guide the emergence of patterns throughout the universe.

    • Alex Tolley June 30, 2021, 9:30

      As you point out, the tetrapyrrole ring structure is common in biology and the more complex chlorins and chlorophylls based on that structure have been evolved several times. Unless there is something special about terrestrial biology, I would expect that basic structure and its evolved forms to be common on other planets but adapted to the local conditions.

      What would be fascinating is if an other biology found a very different way to trap light and transfer electrons to energy capturing molecules, perhaps mimicking how our solar PV works.

  • Michael June 30, 2021, 2:36

    Does the work take into account our Suns lower luminosity billions of years ago when the average wavelength was more towards the red end of the spectrum.

  • Michael Fidler June 30, 2021, 10:26

    I see the curse of the earth centered universe raises it ugly head again!
    “Life as we know it” is a nice correct way to say prejudice. Lets look at it from the other side. M dwarfs put out radiation that can break water bonds and other oxygen laden compounds. Since they are 20 times more common then G dwarfs they may also populated the universe with panspermia. It took 2 billion years for panspermia to bring something that could produce oxygen on earth because the stars they came from are 20 times less common.
    Red dwarfs planets are likely to be rich in volatile organic compounds and similar to the carbon and water rich asteroids. The energy from flaring alone could start life on these worlds. Evolution would skip a step and start developing intelligent life within a billion years around them.
    Do we find tropical plants in the artic or dinosaurs in are back yard? No and that is the point, we need to uncross are eyes and look at these worlds from their perspective.

    • Alex Tolley June 30, 2021, 14:50

      Do we find (…) dinosaurs in are back yard?

      If you look, you will see those feathered and beaked therapods hopping about everywhere. Larger ones are farmed in factories and are a staple of many meals. A primate even made a fortune covering pieces of them in spicy breadcrumbs and serving them by the bucket. ;)

      • Michael Fidler June 30, 2021, 21:59

        Why, I was telling my wife how unsightly the dead dinosaur was sitting in front of us. What was left of the roasted chicken.

  • Alex Tolley June 30, 2021, 11:54

    I find several problems with the approach of this paper:

    Problem 1.
    Figure 1. Earth PNP (red dashes)
    Irrelevant as we don’t really care what Earth can do, just what is possible in other worlds. Yes, cooler stars are less likely to have as rich biospheres as Earth has as the flux is lower, but this analysis assumes that life on such worlds has not evolved to adapt to this by increasing photosynthetic efficiency for the light flux.
    Figure 1. OP Threshold (green dashes)
    This is more like the relevant value and all planets in the set can achieve this. However, see below.

    Problem 2.
    The photon flux is averaged using 1/4 of the surface area 4*pi.r^2 over the incidence area pi*r^2.
    Plants only need enough light while photosynthesizing during light hours.
    Therefore this averaging makes no sense. Furthermore, if light levels are low at the poles, plants will not exist there, only where light intensity is high, e.g. at the equatorial areas.
    Lastly, for tidally-locked M_dwarfs, the total surface is not relevant, so that the averaging factor should be 1/2, not 1/4.

    Problem 3.
    We know that algae can photosynthesize at depth in the ocean, with the euphotic zone can reach 200m in depth where light intensity is very much lower than at the surface. The authors dismiss this by claiming that net primary production is minimal, although they fudge and do not use consistent units. Their minimum OP threshold looks at about 5% of the Earth-like surface threshold. But, the light level on Earth is far higher than needed for photosynthesis. One can reduce incident light levels significantly. Plants also adapt – those in low light zones increase the density of chloroplasts so that their leaves are darker green than plants living with full sunlight exposure. Other adaptations include larger leaf areas to stems, turning leaves towards the light, etc.

    Problem 4
    I am at a loss to understand the relevance of exergy which seems to be defined as the maximum usable work from the photon energy. Photosynthesis is very inefficient, perhaps capable of using just a few percent of the energy received. [It is also self-poisoned by the oxygen it generates and needs to remove the O2 as fast as possible to ensure CO2 is dominant locally. Increasing CO2 levels in greenhouses do increase productivity.] The references below show marine primary productivity and light (depth) and the light saturation which suggests that the star’s illumination in the HZ and its spectrum is best determined in other ways than exergy.


    This site’s figure 1 shows the light saturation of photosynthesisInfluence of Light on Crop Growth

    • Ioannis Kokkinidis June 30, 2021, 18:56

      Plants also need light for photoperiodism, not just photosynthesis. Many plants time their flowering stage based on day length.

      • Alex Tolley June 30, 2021, 20:50

        Terrestrial organisms evolve to use environments and cues particular to that planet. Day length for planets with seasons. Full moon for grunion mating. Cold snaps, fires, etc, etc.

        Organisms on other worlds will use different cues or no cues like abyssal organisms do without any cues (unless the rain of particles is seasonal).

  • wdk June 30, 2021, 15:27

    Ditto on the issue of latitude dependence in application.
    Ignoring our planet’s moderate obliquity, sun light in polar regions is filtered through a longer transit path. Opacity is wavelength dependent, but the overall effect is that the irradiation at the equator is far different than at high latitudes, enough to compete with stellar spectral features to a degree. Anothe aspect of photosythesis here on Earth is that it still occurs on the floor of twin ( or triple?) canopy rain forests and at significant depths at sea. Likely a fainter or a redder sun would cause retreat in some of those environments, but the lines, I doubt, are sharply drawn as yet.

    Also, the experiment with lighting commonplace plants is intriguing too. It just gives a hands on feeling to astro-biology if nothing else.
    But a priori ( and doing some conventional gardening outside too), the results I would expect would be something akin to what a seed packet label might say about growing seasons, except expanded to an interstellar market. “Shrooms” ought to do well in red dwarf systems and perhaps proliferate. Be careful what you pick though. Cabbage and carrots… Beets….

    Trouble is, one will need some topsoil. Now how do you get started with that if you show up at Barnard’s Star and none has been provided in advance. Send a dirt bag in advance?

    • Alex Tolley June 30, 2021, 20:52

      I believe Mark Watney had a partial solution… ;)

  • Michael Fidler June 30, 2021, 22:13

    Do not forget the prehistoric hard radiation loving giant fungus!

    Mystery prehistoric fossil verified as giant fungus.


  • Ivan Vuletich June 30, 2021, 22:43

    It seems to me that aside from making the assumption of life as we know it. This paper also makes the assumption of photosynthesis as we know it. Even on Earth there is a variety of photosynthetic systems such as red algae and c3 & C4 plants, each of which is adapted for their environments.

  • Michael Fidler July 1, 2021, 7:04

    Some viruses have a totally different genome to the rest of life on Earth. Very interesting material!


    • Alex Tolley July 1, 2021, 11:10

      Not quite “totally”. Just a modified adenine base.

    • Robin Datta July 1, 2021, 11:27

      The genetic code impied will have to conform to the rest of the milieu if it is to produce the structural proteins of new virions, and just as importantly, the enzymatic machinery to produce the variant nucleotide and incorporate it into new copies of its genome.

      A far cry from a differnt set of nucleottides with a different genetic code and a whole different cellular chemical machinery, which would then constitute a true alien. It is perhaps not impossible that we may find one sequestered in remote crevices kilometers underground.