Centauri Dreams
Imagining and Planning Interstellar Exploration
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Radiolytic H2: Powering Subsurface Biospheres
Although we’ve been focusing lately on photosynthesis, radiolysis — the dissociation of molecules by ionizing radiation — can produce food and energy for life below the surface and in deep oceans. Our interest in surface conditions thus needs to be complemented by the investigation of what may lie within, as Alex Tolley explains in today’s essay. Indeed, biospheres in a planet’s crust could withstand even the destruction of all surface life. The possible range of microorganisms well beyond the conventional habitable zone defined by liquid water is wide, and while detecting it will be challenging, we may be able to investigate the possibilities in our own system with landers, looking to a day when interstellar probes are possible to explore exoplanet interiors.
by Alex Tolley
“There may be only one garden of Eden here for large life forms such as ourselves. But living beings small enough to populate tiny pore spaces may well exist within several – and perhaps many-other planetary bodies.”
– Thomas Gold, The Deep Hot Biosphere, 1999 [1]
Thomas Gold was probably wrong about subsurface microbes being the source of fossil fuels using fossil methane (CH4), but he was the first to suggest that the newly discovered microbes in the Earth’s crust might be common in other planetary bodies. This essay will explore whether the molecules and energy available from the radiolysis of water (H2O) might support similar biospheres in other worlds in space.
Follow the water, but don’t forget the energy
NASA’s mantra of “follow the water” is important when searching for life, because liquid water is required for carbon-based terrestrial life. Life can still exist if the water freezes, but it will be in a non-metabolizing state and dormant. [Even in frozen water, such as the snow on mountains, a speck of dark material can melt a tiny volume of water around it, allowing microbes to live in these microscopic habitats.]
But while liquid water is necessary, it is insufficient to support life. Inoculate microbes in a dark, sealed flask of distilled water and they will die or go dormant, unable to acquire the energy needed for metabolism. [This is why you can keep containers of distilled water for a long time, even if bacteria contaminate the contents before sealing.]
The rich surface biosphere on Earth is powered by the sun. Photosynthesis fixes the sun’s energy from carbon dioxide (CO2) and water. Before photosynthesis evolved energy was anaerobically harvested from molecules that could liberate energy when respired. Bacteria living in the ocean’s dark, hot smoker vents metabolise the molecules erupting from the mantle and in turn provide the food and energy for the complex life living near these vents.
By the mid-1990s it was accepted that microorganisms discovered kilometers down in the crust were active in the interstices between the mineral grains. Water percolating in these rocks was responsible for keeping these microorganisms actively metabolizing rather than being in a dormant state. But what were they using for food and energy where it was lightless? Carbon was available as CO2 and CH4. The archaea kingdom of anaerobic organisms include all the methanogens that can convert CO2 to CH4 extracting energy and the using the carbon for metabolism. These were the dominant forms of life on and in the early Earth, and possibly the source of the traces of seasonal CH4 detected on Mars.
However, there is another more energetic molecule, molecular hydrogen (H2) that can be used for metabolism. As an electron donor, it can be coupled with an electron acceptor to be part of an energy harvesting metabolism. If H2 is a metabolic energy source, what is its source in the crustal biosphere?
The standard explanation is that some forms of the serpentinization reactions can produce H2 as well as the better known production of CH4.
However there is another source of H2, created by radiolysis of H2O by decaying radioactive elements such as unstable isotopes of potassium (K), thorium (Th), and uranium (U).
Figure 1. Radiolysis of water in the interstices of rock when water is present.
Besides creating H2, radiolysis also produces oxidants which in turn react with the rocks, most notably with sulfides, producing sulfates.
Li et al
“We have demonstrated that the S-MIF-bearing dissolved sulfate in the saline fracture waters at Kidd Creek originates from sulfides in the Archaean host rocks. The most likely mechanism for sulfate production in these anoxic fracture water systems is the indirect oxidation of sulfide minerals by oxidants from radiolytic decomposition of water” [3]
Radiolysis vs Serpentinization
Experiments on the radiolysis of water suggested that radiolysis was not an important source of H2 compared to serpentinization. Serpentinization occurs wherever high iron (Fe) igneous rocks from the mantle, water and heat interact. The ocean ridges between the plates are important zones where this takes place.
However, later experiments with oceanic sediments showed that radiolysis production of H2 was catalyzed by the minerals increasing production of H2 many fold. In the sediments on the ocean floor it was found that radiolysis was the main source of H2 as an energy source for microbes.
Sauvage et al: [9]
“Radiolytic H2 has been identified as the primary electron donor (food) for microorganisms in continental aquifers kilometers below Earth’s surface. […] all common marine sediment types catalyse radiolytic H2 production, amplifying yields by up to 27X relative to pure water. […] Comparison of radiolytic H2 consumption rates to organic oxidation rates suggests that water radiolysis is the principal source of biologically accessible energy for microbial communities in marine sediment older than a few million years.”
Moreover, radiolytic H2 is as dominant a source of food and energy as marine photosynthesis powered by the sun.
Sauvage et al: [10]
“[…] radiolytic H2 production in marine sediment locally produces as much electron donor (food) as photosynthetic carbon fixation in the ocean.”
In summary globally, radiolysis can provide both food and energy comparable to that of the marine photosynthetic organisms.
Methanogens & Sulfur-reducing bacteria
In anoxic environments where both archaeal methanogens and sulfur-reducing bacteria coexist, the biomass and types of the latter are greater than the former. One reason may be that the available energy from the reduction of sulphur from sulfates is greater than the reduction of carbon to CH4 from CO2.
CH4 can be created by the reduction of carbon dioxide.
Serpentinization is a geologic source. However, archaean methanogens are believed the dominant source of methane in the atmosphere on the early Earth reducing carbon dioxide anaerobically to methane [12]. It is the uncertainty of the source of the methane detected on Mars that intrigues astrobiologists.
Sulfate-reducing bacteria such as Desulfovibrio and Desulfobacter use the H2 to reduce sulfates created by the radiolysis oxidants on mineral sulfides to again reduce the sulfur to silfides.
As table 1 indicates, this is a more energetic reaction than methanogenesis and may account for the very many different bacteria utilizing H2 and sulphate as an energy source.The radiolytic oxidants also react with CH4 to form simple organic molecules such as formate and acetate which can be used as food sources by bacteria, further indicating the value of radiolysis in maintaining a subsurface habitat.
Habitability
As I have noted in previous posts, the search for life has typically been focused on surface-living, complex, aerobic life, as the low hanging fruit of detectability. This restricts the search to planets in the habitable zone (HZ). However, as unicellular life dominated Earth’s history and anaerobic respiration was dominant until the evolution of photosynthesis, and the Great Oxidation Event increased the partial pressure of oxygen in the atmosphere, such worlds may give rise to false negatives when analyzing the atmosphere by spectroscopy. Furthermore, as professor Tyrrell has suggested, surface life on Earth-like worlds may have a low probability of being sustained over 3 billion years due to events perturbing the surface temperature into runaway conditions.
Unlike the variable conditions on the surface, subject to wide ranges of conditions and vulnerable to cosmic and geologic disruption that saw 5 major extinctions on Earth, as well as an ongoing 6th extinction in the Anthropocene, conditions in the crust are far more stable, and less vulnerable to the disruptions on the surface. Such crustal biospheres once established may survive even after surface life has been extinguished, especially once the star’s luminosity renders the surface uninhabitable.
Such a biosphere may even allow for an evolutionary reset starting with microorganisms should surface conditions become uninhabitable for a temporary period and subsequently returning to habitability.
So we have evidence that life is in the subsurface crustal rocks and that radiolysis may be an important, if not the most important, source of food and energy for this life. But what about bodies elsewhere?
Other Celestial Bodies
1. Mars
Mars has all the same ingredients as Earth for subsurface microorganisms to live. At some depth below the surface the temperature should be sufficient to create liquid water [13]. While serpenitization can occur, especially to generate CH4, CO2 should be available for methanogens to respire and release CH4. Residual radioactive elements should be able to produce the needed H2 and SO4.for sulfur reducing bacteria. The race is on to determine whether there is an extant microbial biosphere on Mars. Looking for frozen microbes in ejecta from large meteor impacts that have penetrated to the needed depths might be the easiest approach for robotic vehicle discovery. If there are any near surface hot spots from residual volcanism or local concentrations of radioactive elements, these might also be good places to look. Whether earth, Venus, or Mars is the original world where abiogenesis occurred, panspermia between these worlds due to ejecta and microbes propelled by solar radiation, early Mars may have been a home for life.
Tarnas et al: [14]
“We have demonstrated that radiolysis alone produced sufficient quantities of reductants to have sustained a subsurface biosphere during the Noachian for hundreds of millions of years. Given sufficient oxidant availability, this habitat could have fostered chemolithotrophic microbial communities that would have imprinted organic, morphological, and isotopic biosignatures on their habitat’s host rock.”
2. Icy moons
Jupiter’s Europa and Saturn’s Enceladus are 2 icy moons that have subsurface oceans and tidally induced warming. Radiolysis of subsurface water in the core below the ocean sediments should provide the conditions necessary for a microbial biosphere. Analysis of the plumes by a flyby or orbiting probe is one method to search for life, although this is more likely to detect life in the oceans, rather than below the ocean-crust interface. This later will prove a much more difficult target.
3. Titan
Saturn’s moon Titan is believed to have a rocky core, overlaid with a liquid ocean, and topped with hydrocarbons with a dense nitrogen atmosphere. As with the icy moons, below the crust-ocean interface is a possible biosphere.
4. Ceres
Like the icy moons, Ceres has a rocky core overlaid with brines. Evidence of cryovolcanism suggests that these brines must be partially liquid. Unlike the icy moons, there is no tidal heating. This suggests that any biosphere in the core must be powered by radiolysis from any residual radioactive element decay. Catillo-Rogers recently suggested Ceres has the potential to host life as it has the radioactive elements for both heating and radiolysis. [5]
5. Eris
The Trans-Neptunian dwarf planet Eris has a density of 2.52 g/cm^3, indicating that it must be composed of rocky material and ices. If, like Pluto, there is evidence of cryovolcanism then it is possible that a core with radioactive elements and liquid water provides a habitat for a microbial biosphere. If so, then other dwarf planets extending out into the Kuiper belt could also have similar subsurface habitats.
6. Comets and Kuiper belt Objects
Holm focused on serpentinization for the production of CH4 and H2 on celestial bodies [7] He notes that radioactivity could also warm these bodies, making serpentinization possible. However, he did not consider radiolysis that might have been an important contribution to the production of energy rich molecules that could be used for metabolism.
Comets and their parent bodies, such as Transneptunian Objects (Kuiper Belt Objects—KBOs), accreted from a mixture of volatile ices, carbonaceous matter, and rocks in the coldest regions of the protosolar nebula. […] However, the rocky material contained in comets includes radioactive isotopes, whose decay can provide an important source of heat, possibly significantly altering the internal structure of these icy objects after their formation. There is a general agreement that short-lived radioactive isotopes like 26Al and 60Fe could have played a major role during the early evolution of both comets and their parent bodies, possibly leading to the melting of water ice and to the triggering of serpentinization and FTT reactions.
A more recent paper by Bouquet emphasized the importance of radiolysis in icy bodies which not only produced H2, but sulfates to support metabolism [2].
We found that radiolysis can produce H2 quantities equivalent to a few percent of what is estimated from serpentinization. Higher porosity, which is unlikely at the scale of a body’s entire core but possible just under the seafloor, can increase radiolytic production by almost an order of magnitude. The products of water radiolysis also include several oxidants, allowing for production of life-sustaining sulfates. Though previously unrecognized in this capacity, radiolysis in an ocean world’s outer core could be a fundamental agent in generating the chemical energy that could support life.
7. Rogue/Free Floating Planets
Rogue planets ejected from their systems would include bodies similar to those in the solar system. Given the prevalence of conditions needed for a subsurface biosphere, especially in bodies at the edge of our system, there seems every reason to believe that these rogue planets should also host subsurface conditions suitable for a microbial biosphere.
Habitable yes, but inhabited?
The above suggests that if radioactive elements can also heat the surrounding material so that water is kept liquid, then almost any celestial body with a rocky material and water could potentially be a microbial habitat in space, irrespective of whether it is in the HZ or not. As suggested earlier, planets in the HZ that have lost surface habitability could retain refugia for life in the crust.
For other non-Earth-like bodies which may have the conditions for a subsurface biosphere the question becomes whether they are living or sterile. Is abiogenesis possible on these worlds, or must they be inoculated by life from living worlds? We don’t yet know the answers to such questions, but it does suggest that astrobiologists take seriously the possibility that any body with a suitable subsurface environment could be inhabited and therefore instruments to detect such life should be included with exploratory probes. As we increase the exploration of our system, landers and rovers should include technologies to detect life, especially within the habitable zone below the surface.
Could we detect subsurface biospheres on exoplanets?
Detection of subsurface biospheres on exoplanets is going to be very difficult. Seager [6] produced a catalog of possible biosignature molecules, of which hydrogen sulphide (H2S) is primarily of biologic origin and therefore its presence is likely an unambiguous biosignature.
Although H2S is likely to have a very low concentration in the atmosphere it has a distinctive IR signal which could be detectable in principle,
As we can currently only analyze exoplanets spectroscopically, if it becomes possible to detect the very small amounts H2S in an otherwise unpromising atmosphere with possibly unsuitable surface conditions for life, then we should attempt to devise the technology to detect the presence of this gas as an unambiguous biosignature.
Based on the terrestrial history of life, it seems likely that on living exoplanets, extant life will be mostly unicellular, possibly even just prokaryotes. On Earth-like worlds geologic processes will ensure that such life will also inhabit a deep, crustal biosphere. Detecting such life will be very difficult, but perhaps not impossible with suitable technology. In the distant future, interstellar probes with landers should be able to detect such life as we explore our stellar neighborhood and catalog and map the forms of life we find.
References
Gold, T. (2021). The Deep Hot Biosphere: The Myth of Fossil Fuels (November 6, 1998) Hardcover. Springer.
Bouquet, A., Glein, C. R., Wyrick, D., & Waite, J. H. (2017). Alternative Energy: Production of H 2 by Radiolysis of Water in the Rocky Cores of Icy Bodies. The Astrophysical Journal, 840(1), L8. https://doi.org/10.3847/2041-8213/aa6d56
Li, L., Wing, B. A., Bui, T. H., McDermott, J. M., Slater, G. F., Wei, S., Lacrampe-Couloume, G., & Lollar, B. S. (2016). Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks. Nature Communications, 7(1). https://doi.org/10.1038/ncomms13252
Lin, L. H., Wang, P. L., Rumble, D., Lippmann-Pipke, J., Boice, E., Pratt, L. M., Lollar, B. S., Brodie, E. L., Hazen, T. C., Andersen, G. L., DeSantis, T. Z., Moser, D. P., Kershaw, D., & Onstott, T. C. (2006). Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome. Science, 314(5798), 479-482. https://doi.org/10.1126/science.1127376
Castillo-Rogez, J. C., Neveu, M., Scully, J. E., House, C. H., Quick, L. C., Bouquet, A., Miller, K., Bland, M., De Sanctis, M. C., Ermakov, A., Hendrix, A. R., Prettyman, T. H., Raymond, C. A., Russell, C. T., Sherwood, B. E., & Young, E. (2020). Ceres: Astrobiological Target and Possible Ocean World. Astrobiology, 20(2), 269-291. https://doi.org/10.1089/ast.2018.1999
Seager, S., Bains, W., & Petkowski, J. (2016). Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry. Astrobiology, 16(6), 465-485. https://doi.org/10.1089/ast.2015.1404
Holm, N., Oze, C., Mousis, O., Waite, J., & Guilbert-Lepoutre, A. (2015). Serpentinization and the Formation of H2 and CH4 on Celestial Bodies (Planets, Moons, Comets). Astrobiology, 15(7), 587-600. https://doi.org/10.1089/ast.2014.1188
Lollar, B. S., Onstott, T. C., Lacrampe-Couloume, G., & Ballentine, C. J. (2014). The contribution of the Precambrian continental lithosphere to global H2 production. Nature, 516(7531), 379-382. https://doi.org/10.1038/nature14017
Sauvage, J. F., Flinders, A., Spivack, A. J., Pockalny, R., Dunlea, A. G., Anderson, C. H., Smith, D. C., Murray, R. W., & D’Hondt, S. (2021). The contribution of water radiolysis to marine sedimentary life. Nature Communications, 12(1). https://doi.org/10.1038/s41467-021-21218-z
Sauvage, J. F., (2018) Sedimentary Catalysis of Radiolytic Hydrogen Production, Open Access Dissertations. Paper 704. https://digitalcommons.uri.edu/oa_diss/704
Cepelewicz, J, (2021, May 24). Radioactivity May Fuel Life Deep Underground and Inside Other Worlds, Quanta Magazine, Accessed online June 2021 https://www.quantamagazine.org/radioactivity-may-fuel-life-deep-underground-and-inside-other-worlds-20210524/
Tolley, A., Detecting Early Life on Exoplanets. February 23, 2018
https://www.centauri-dreams.org/2018/02/23/detecting-early-life-on-exoplanets/
Tolley, A., Is Most Life in the Universe Lithophilic? – AMT prior article JANUARY 11, 2019 https://www.centauri-dreams.org/2019/01/11/is-most-life-in-the-universe-lithophilic/
Tarnas, J., Mustard, J., Sherwood Lollar, B., Bramble, M., Cannon, K., Palumbo, A., & Plesa, A. C. (2018). Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming. Earth and Planetary Science Letters, 502, 133-145. https://doi.org/10.1016/j.epsl.2018.09.001
E-Mail Subscribers: New Service Now Working
I’ll have something more to say about this later, but I did want to mention that the email subscribers to Centauri Dreams should now begin seeing the regular postings in their inbox via the services of follow.it. I wanted to go ahead and mention this as several people wrote to verify that this was indeed my doing. The answer is yes, the follow.it messages are not a fishing expedition for malefactors, but the new delivery mechanism. Again, this applies to those who subscribe to the site by email. More about all this in about a week once we have things normalized.
TESS: How Many Planets Are We Missing?
Many of the planet-hosting stars being identified by TESS, the Transiting Exoplanet Survey Satellite, may actually be binaries. Unless examined closely, a pair of stars can appear as a single object, requiring high resolution instrumentation to separate into its component parts. As it applies to exoplanet research, this is a problem, for TESS operates by the transit method, tracking the change in a star’s light curve as a planet crosses the face of the star.
Light curves yield precious information, but the presence of a second star unknown to researchers can obscure smaller, rocky worlds, just the kind of object we’d like to eventually identify as an Earth 2.0. The problem seems to be wider than we have realized, given that about half of all stars exist in binary systems. New work has put some numbers on the problem. Conducted with data from the Gemini Observatory and the WIYN 3.5-meter telescope at Kitt Peak by NASA Ames researchers, the study examined TESS host stars using a technique known as speckle imaging.
The method works by combining a series of observations taken at rapid intervals, making it possible to eliminate atmospheric effects like those that cause stars in the night sky to twinkle. Two imaging instruments — named Alopeke and Zorro — came into play in the Gemini work, identical units mounted on Gemini North (Hawai‘i) and Gemini South (Chile) respectively. Both are based on the original Differential Speckle Survey Instrument (DSSI) and are permanently mounted.
At the Kitt Peak site where the WIYN instrument is located, the imaging was done using the NN-EXPLORE Exoplanet and Stellar Speckle Imager. Hundreds of TESS host stars came under scrutiny in this work, all of them nearby (as befits the TESS mission concept), with the result that fully 73 of them observed by Gemini turned out to be binary systems, while an additional 18 binary stars turned up using the WIYN instrument.
The team also noted that around single stars, TESS found exoplanets in a wide range of sizes, whereas around the binary systems, only large planets had been found. Katie Lester of NASA’s Ames Research Center led the work:
“We have shown that it is more difficult to find Earth-sized planets in binary systems because small planets get lost in the glare of their two parent star[s]… Astronomers need to know whether a star is single or binary before they claim that no small planets exist in that system. If it’s single, then you could say that no small planets exist. But if the host is in a binary, you wouldn’t know whether a small planet is hidden by the companion star or does not exist at all. You would need more observations with a different technique to figure that out.”
Image: This illustration depicts a planet partially hidden in the glare of its host star and a nearby companion star. After examining a number of binary stars, astronomers have concluded that Earth-sized planets in many two-star systems might be going unnoticed by transit searches, which look for changes in the light from a star when a planet passes in front of it. The light from the second star makes it more difficult to detect the changes in the host star’s light when the planet passes in front of it. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva.
The planet-hosting binary stars the team uncovered had an average separation of about 100 AU, while binary systems without planets are typically separated by about 40 AU, perhaps pointing to lower rates of planet formation around close binaries. By way of comparison to our closest neighbors, Centauri A and B, which appear as a single bright star with an apparent magnitude of ?0.27 to the naked eye, are separated in their elliptical orbit by 35.6 AU at their furthest point and 11.2 AU (about Saturn’s distance from the Sun) at the closest, orbiting a common barycenter with a period of 79.91 years.
All told, the team observed 517 stars hosting a TESS Object of Interest (TOI) using speckle interferometry methods led by Steve Howell (NASA Ames), who is a co-author of the resulting paper. 29 companions turned up around false positive TOIs. The paper concludes:
Our results are consistent with past studies…in showing that the binary exoplanet host period distribution is narrower and peaks at a larger value than that of field binaries, but our lack of very close companions (< 100 AU) is less dramatic than found by these previous studies due to our improved angular resolution and sensitivity to faint companions. Furthermore, our sample clearly shows the observational bias against detecting small planets in transit surveys like TESS due to the third light from a stellar companion. Both the host star multiplicity and this observational bias must be taken into account when calculating the occurence rates of Earth-sized planets...We did not find any additional correlations in the properties of the planets or companions in our sample.
The authors add that they are currently monitoring exoplanet host binaries with shorter periods to determine their orbital parameters, hoping to test each system for dynamical stability and potential for habitable planets.
The paper is Lester et al, “Speckle Observations of TESS Exoplanet Host Stars. II. Stellar Companions at 1-1000 AU and Implications for Small Planet Detection,” in process at The Astronomical Journal (preprint).
A Long Period Super-Earth around a Nearby Star
Nu2 Lupi is a G-class star not all that far away in astronomical terms (48 light years) in the constellation Lupus, its proximity verified by parallax measurements and firmed up by the Hipparcos satellite. This is one of the closest G-class stars to our own, and it’s a fast mover in other ways, with a high radial velocity. Its age is estimated at roughly 12 billion years, making it one of the oldest stars near our system. HARPS spectrograph data pulled up three planets here in 2019, two of them later found to transit.
And now we have, unexpectedly, a third transit. The surprising nature of the third relates to the distance of the third planet from the star. The two inner worlds, with masses between Earth’s and Neptune’s, take 12 and 28 days to orbit Nu2 Lupi. The third takes 107 days, far enough out that a transit seemed unlikely. The ratio of the diameter of the star to the diameter of the orbit comes into play in determining the probability of a transit.
We have the European Space Agency’s CHEOPS mission (CHaracterising ExOPlanet Satellite) to thank for the surprise discovery, allowing a tight look at the third world. For in combination with radial velocity data, the transit information allows scientists to determine the planet’s density and composition. Nu2 Lupi d turns out to be 2.5 times the radius of Earth and about 9 times its mass. While the inner world at Nu2 Lupi is evidently rocky, the outer two are enveloped by hydrogen and helium atmospheres, with a quarter of the mass of each being composed of water in the form of either high-pressure ice or high-temperature steam.
Image: This infographic reveals the details of the Nu2 Lupi planetary system. This bright, Sun-like star is located just under 50 light-years away from Earth in the constellation of Lupus (the Wolf), as shown to the left of the frame, and is known to host three planets (named ‘b’, ‘c’ and ‘d’, with the star deemed to be object ‘A’). The relative sizes, orbital periods, and possible compositions of these three planets are depicted to the centre and lower right of the frame, while planet d’s comparative position within our Solar System is shown to the upper right (as defined by the amount of incident light it receives from its star, Nu2 Lupi). Credit: ESA.
Nu2 Lupi d turns out to be a useful find, as co-author David Ehrenreich (University of Geneva) explains:
“Due to its relatively long period, the amount of stellar radiation reaching the planet is mild in comparison to many other discovered exoplanets. The less radiation a planet receives, the less it changes over time. Therefore, a planet with a long period could have retained more information about its origin. Since its bright host star is quite close to us, it is easier to study. This makes it a golden target for future study, with no known equivalent.”
I think what Ehrenreich is getting at is the fact that Nu2 Lupi is a G-class star, making its proximity to Earth and its transiting worlds of great interest — the authors point out that this is the first time a planet with an orbital period of over 100 days has been discovered in transit around a star bright enough to be a naked eye object. We can imagine future missions using transmission spectroscopy to analyze its atmosphere.
But let’s look at the paper’s further explanation, for planet formation models are also in play:
With regard to the derived amounts of gas, planetary atmospheric evolution calculations indicate that the innermost planet b was subject to significant atmospheric loss, while planets c and d did not suffer strong evaporation… The two outer planets are indeed sufficiently massive and far away from the host star to be only little affected by mass loss throughout their evolution. Therefore, the current low gas content returned by our internal structure modelling for these two planets is likely of primordial origin.
Core accretion models of planet formation, oft discussed in these pages, see planets accreting large amounts of gas when they reach a critical mass, generally cited as being around 10 times the mass of Earth. As this relates to Nu2 Lupi:
The structure of the two outer planets, as observed today, being likely primordial, these two objects provide a very important anchor for planet formation models, as they indicate that neither of them reached the critical mass during their formation. These two planets, by giving access to both the core mass and gas-to-core ratio for two objects in the same system will provide strong constraints on the understanding of the formation of sub-critical planets. Since the presence of large gas envelopes hinders habitability, the ?2 Lupi system, with its two sub-critical outer planets, also provides an interesting case study for numerical models targeting the emergence of habitable worlds.
The paper is Delrez et al., “Transit detection of the long-period volatile-rich super-Earth ?2 Lupi d with CHEOPS,” Nature Astronomy 28 June 2021 (abstract) / preprint.
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).
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