Physics, Earth & Space
Study: Planets Capable of Sustaining Photosynthesis Are Extremely Rare
Headlines currently buzzing around the Internet are saying things like “Earth-like worlds capable of sustaining life may be less common than we thought” (CNET) or “There Is Only One Other Planet In Our Galaxy That Could Be Earth-Like, Say Scientists” (Forbes). While some might find it encouraging that there’s one other Earth-like planet in our galaxy, when you consider that there are 100 billion stars in the Milky Way galaxy alone, this certainly makes it sound like habitable planets are pretty special.
The claims are based upon a new study in Monthly Notices of the Royal Astronomical Society, “Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone.” Photosynthesis, of course, is the basis of the biosphere for life on Earth, as the paper explains:
Photosynthesis is the dominant process, as it allows us to produce about 99 per cent of the entire biomass of the Earth biosphere. OP is also essential for providing abundant O2 levels which appear to be necessary for the high-energy demands of multicellular life anywhere in the Universe.” A planet that can sustain photosynthesis thus has the ability to sustain a wide variety of other life forms. The study aims to estimate the ability of a planet to sustain oxygenic photosynthesis (OP) given three parameters: (1) the photon flux (i.e., the amount of light), (2) the “exergy,” which is a measure of the amount of work that can be done given the radiation input, and (3) something that can only be put in the words of the authors: “the exergetic efficiency of the radiation in the wavelength range useful for the oxygenic photosynthesis as a function of the host star effective temperature and planet-star separation.
Earth Has Highest Exergetic Efficiency
Let’s cut to the chase: The paper finds that among a database of planets both inside and outside our solar system, Earth is far and away the planet that is best-suited for life, with only one other planet with a radiation input that could possibly sustain oxygenic photosynthesis:
Earth is … the rocky planet with the largest PAR photon flux and with the highest exergetic efficiency. However, we also find that Kepler-442b receives a PAR photon flux slightly larger than the one necessary to sustain a large biosphere, similar to the Earth biosphere. So, it is likely that a Kepler-442b biosphere would not be light-limited.
Now of course the amount and type of radiation reaching a planet is crucial for its habitability. But there is a whole suite of additional features that are needed for advanced life to exist. This includes the presence of water (as well as the proper distance from the host star for the water to be in a liquid form), availability of necessary elements such as hydrogen, carbon, oxygen, and nitrogen, and the proper balance of other compounds such as carbon dioxide. To give a more complete list, planetary habitability requirements seem to include:
- Rocky planet with active plate tectonics to recycle elements needed for life
- Presence of sufficient water in the crust
- Large moon with right rotation period and distance
- Right planetary mass
- Presence of magnetic field
- Location within circumstellar habitable zone which allows liquid water to exist
- Low-eccentricity orbit to allow for stable climate
- Presence of large Jupiter-mass planetary neighbors in large circular orbits
- Location outside spiral arm of galaxy and far enough from center of galaxy – the Galactic Habitable Zone
- Near co-rotation circle of galaxy, in circular orbit around galactic center
- Stable radiation output of host star
- Atmosphere which can allow visible light to penetrate to surface yet block out harmful radiation
We know that Earth meets all of these requirements, but does Kepler-442b? At this point we simply don’t know.
A Planetary System Fit for Life
In his 2018 book Children of Light, Michael Denton elaborates on the last item in the list above — special properties of Earth’s atmosphere which allow radiation needed for “light-eating” organisms to reach the surface yet block out forms of radiation which are destructive to organic molecules. He explains that the electromagnetic radiation emitted by our sun is especially suited to the needs of life, and the atmosphere of Earth allows the precise wavelengths of radiation that are needed for photosynthesis:
[T]he electromagnetic radiation emitted by the Sun (and that of most other stars) is almost entirely light and heat (or infrared), which have precisely the characteristics needed for life, especially advanced life, to thrive on the Earth’s surface. Light is required for photosynthesis and heat is required to raise the Earth’s temperature to well above freezing and preserve liquid water on Earth.
It is only because of the precise absorption characteristics of Earth’s atmospheric gases that most of the light radiation emitted by the Sun reaches the Earth’s surface where it drives the chemical process of photosynthesis upon which we “light eaters” ultimately depend. And the same atmospheric gases which let the light through for photosynthesis absorb a portion of the infrared (IR) radiation, which warms the Earth and preserves water as a liquid on the Earth’s surface. Adding to the miracle, both the atmospheric gases and liquid water, the matrix of carbon-based life, not only let through the right light but strongly absorb all the dangerous types of radiant energy on either side of the visual and infrared regions of the electromagnetic spectrum, a vital property without which no advanced life forms would grace the surface of the Earth.Children of Light, pp. 15-16
Not only is Earth’s atmosphere precisely suited for the forms of EM radiation needed for life, but Denton explains that there’s a coincidence of chemistry wherein the very wavelengths of light that pass through our atmosphere can activate organic molecules yet not destroy them:
Within this Goldilocks region, the light is not so energetic as to cause chemical disruption of organic matter, but it is energetic enough to gently activate organic molecules for chemical reaction. In other words “just right.” No other EM radiation will do! As Wald points out, it is not that life adapted to the right light but that the right light is the only light that provides the correct energy levels for photochemistry:
“There cannot be a planet on which photosynthesis or vision occurs in the far infrared or far ultraviolet, because these radiations are not appropriate to perform these functions. It is not the range of available radiation that sets the photobiological domain, but rather the availability of the proper range of wavelengths that decides whether living organisms can develop and light can act upon them in useful ways.”Children of Light, pp. 25, emphasis in original
Both Earth and our sun form a finely balanced system that is finely tuned to allow photosynthesis to occur. We don’t know that this overall system exists on Kepler-442b.
Underestimating the Complexity of Photosynthesis
Have you ever heard of those “ghost malls” in China? They were fully built and ready for business — including one that was arguably the largest mall in the world. Everything was just right for business, but they were missing one thing: vendors and customers. In a similar way, having the right kind of sun and planetary atmosphere aren’t the only requirements for photosynthesis. You could have a planet that is perfectly habitable for life yet if life never arises it will be empty. Organisms capable of photosynthesis itself must somehow arise. But how? Reflecting a common form of evolutionary thinking, the new paper discussed above seems to suggest that once the right conditions are present, photosynthesis can evolve quite easily. In one passage it describes the familiar basic chemical equation for photosynthesis and then lauds its “overall simplicity”:
6CO2 + 6H2O + light → C6H12O6 + 6O2 (1)
We conjecture that the chemical reaction (1) should be quite common in the cosmos because of the generally large amounts of radiation received by exoplanets from their host stars, the availability of the input ingredients, and its overall simplicity supported by the fact that OP evolved very early on Earth.
As the paper suggests, photosynthesis is the conversion of light into chemical energy. This is a process that happens in the leaves of plants and requires five highly specialized protein complexes (photosystems I & II, cytochrome bf complex, NADPH reductase, and ATP synthase) for the light reactions and 11 enzymes for the dark reactions. Briefly, in the light reactions, a photon is captured by an antenna pigment, and the energy of excited electrons is then transferred to chlorophyll molecules. A high-energy electron from chlorophyll is passed through an electron transport chain which causes the pumping of protons across a membrane. Those protons are used to power the ATP synthase molecular machine which generates ATP, the energy molecule of the cell. And of course ATP synthase alone is a multicomponent irreducibly complex molecular machine:
The dark reactions of photosynthesis are where you fix carbon by taking in CO2 and make a carbohydrate which the organism can further use. This whole process is called the Calvin cycle and over the series of 11 enzyme-controlled steps you have a tightly controlled process that generates either energy molecules or structural molecules or both depending on what the cell needs.
Michael Denton provides his own nice sketch of the complexity of photosynthesis:
The primary event on which the whole process of photosynthesis depends is the capture or absorption of photons of light by the photosynthetic pigments (chiefly the green pigment chlorophyll) in the thylakoid membranes (which surround the so-called thylakoid discs in the chloroplast). When the chlorophyll molecules situated in these membranes capture photons, the energy imparted activates electrons in the chlorophyll, raising them to higher energy levels. (Each photon absorbed raises one electron to a higher energy level.)
This allows the electrons to escape from the chlorophyll, leaving the chlorophyll molecules positively charged or oxidized. (The loss of electrons is oxidation.) The positively charged chlorophylls draw electrons from water molecules (H2O) in the oxygen-evolving complex (OEC), oxidizing them and releasing at the same time free oxygen (0) molecules, as well as protons (H+) and electrons (e-).
Water [H20] → Oxygen  + protons [H+] + electrons [e-]
The energetic electrons escaping from the chlorophyll find their way to electron transport chains, where they flow “down” in discrete steps, releasing energy at each step, which is used to do work, pumping protons (H+) across a membrane (the thylakoid membrane) into the thylakoid lumen (a membrane-enclosed compartment in the chloroplast). These then flow back through the same membrane, providing energy to drive the synthesis of ATP (the cell’s chemical energy currency) by the enzyme ATP synthase.
Overall, photosynthesis can be seen to occur in two stages. In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage molecule ATP and the reducing agent NADPH. These light-dependent reactions occur in the thylakoid membranes. During the second stage (the Calvin cycle), the light-independent reactions use these products to reduce carbon dioxide.Children of Light, pp. 75-77
Of course each of these steps requires finely tuned enzymes, cofactors, and other biomolecules which facilitate the requisite chemical reactions. It may even represent an irreducibly complex system.
So how did the paper determine that photosynthesis has an “overall simplicity,” despite the complexity just described? Only due, again, to evolutionary thinking: photosynthesis appears early in life’s history, and because they presume that unguided evolution is the only mechanism by which complex biological systems can arise, they therefore conclude that photosynthesis must be simple and easy to evolve. But as we have seen it’s not simple at all. Having the right conditions for photosynthesis to take place doesn’t in any way guarantee that photosynthesis will evolve. Compared to evolving the complexity of photosynthesis, obtaining the rare special conditions where a planet receives the EM radiation needed for photosynthesis seems like a much simpler task — even though it’s apparently very rare in the universe!