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Dearth of Photosynthetically Active Radiation Suggests No Complex Life on Late M-Star Exoplanets

Joseph J. Soliz, William F. Welsh

TL;DR

This study probes whether a late-M-dwarf planet in the TRAPPIST-1 system could experience a Great Oxidation Event (GOE) akin to Earth's, given TRAPPIST-1e's markedly reduced Photosynthetically Active Radiation (PAR). By first applying a simple linear scaling of oxygen production to PAR photon flux, it predicts an unrealistically long GOE timescale (~63 Gyr). The authors then refine the estimate by incorporating far-red extension of PAR, synchronous rotation effects, and photoinhibition through photosynthesis-irradiance curves, identifying that low-light adapted cyanobacteria could yield GOE timescales as short as ~1.4–3 Gyr, with the best-case ~3 Gyr (Acaryochloris marina). However, considering the dominant advantage of anoxygenic photosynthesis under near-infrared illumination, light harvesting up to 1100 nm could enable anoxygenic phototrophs to dominate, potentially preventing GOE and, consequently, complex multicellular life. Overall, the work suggests that on late-M-star planets, oxygen-rich atmospheres and Cambrian-like biological complexity may be unlikely, with profound implications for biosignature expectations around these stars.

Abstract

The rise of oxygen in the Earth's atmosphere during the Great Oxidation Event (GOE) occurred about 2.3 billion years ago. There is considerably greater uncertainty for the origin of oxygenic photosynthesis, but it likely occurred significantly earlier, perhaps by 700 million years. Assuming this time lag is proportional to the rate of oxygen generation, we can estimate how long it would take for a GOE-like event to occur on a hypothetical Earth-analog planet orbiting the star TRAPPIST-1 (a late M star with Teff 2560 K). Although in the habitable zone, an Earth-analog planet located in TRAPPIST-1e's orbit would receive only 0.9% of the Photosynthetically Active Radiation (PAR) that the Earth gets from the Sun. This is because most of the star's light is emitted at wavelengths longer than the 400-700 nm PAR range. Thus it would take 63 Gyrs for a GOE to occur. But the linear assumption is problematic; as light levels increase, photosynthesis saturates then declines, an effect known as photoinhibition. Photoinhibition varies from species to species and depends on a host of environmental factors. There is also sensitivity to the upper wavelength limit of the PAR: extending just 50 nm increases the number of photons by a factor of 2.5. Including these and other factors greatly reduces the timescale to roughly 1-5 Gyrs for a GOE. However, non-oxygenic photosynthetic bacteria can thrive in low-light environments and can use near-IR light out to 1100 nm, providing 22 times as many photons. With this huge light advantage, and because they evolved earlier, anoxygenic photosynthesizers would likely dominate the ecosystem. On a late M-star Earth-analog planet, oxygen may never reach significant levels in the atmosphere and a GOE may never occur, let alone a Cambrian Explosion. Thus complex animal life is unlikely.

Dearth of Photosynthetically Active Radiation Suggests No Complex Life on Late M-Star Exoplanets

TL;DR

This study probes whether a late-M-dwarf planet in the TRAPPIST-1 system could experience a Great Oxidation Event (GOE) akin to Earth's, given TRAPPIST-1e's markedly reduced Photosynthetically Active Radiation (PAR). By first applying a simple linear scaling of oxygen production to PAR photon flux, it predicts an unrealistically long GOE timescale (~63 Gyr). The authors then refine the estimate by incorporating far-red extension of PAR, synchronous rotation effects, and photoinhibition through photosynthesis-irradiance curves, identifying that low-light adapted cyanobacteria could yield GOE timescales as short as ~1.4–3 Gyr, with the best-case ~3 Gyr (Acaryochloris marina). However, considering the dominant advantage of anoxygenic photosynthesis under near-infrared illumination, light harvesting up to 1100 nm could enable anoxygenic phototrophs to dominate, potentially preventing GOE and, consequently, complex multicellular life. Overall, the work suggests that on late-M-star planets, oxygen-rich atmospheres and Cambrian-like biological complexity may be unlikely, with profound implications for biosignature expectations around these stars.

Abstract

The rise of oxygen in the Earth's atmosphere during the Great Oxidation Event (GOE) occurred about 2.3 billion years ago. There is considerably greater uncertainty for the origin of oxygenic photosynthesis, but it likely occurred significantly earlier, perhaps by 700 million years. Assuming this time lag is proportional to the rate of oxygen generation, we can estimate how long it would take for a GOE-like event to occur on a hypothetical Earth-analog planet orbiting the star TRAPPIST-1 (a late M star with Teff 2560 K). Although in the habitable zone, an Earth-analog planet located in TRAPPIST-1e's orbit would receive only 0.9% of the Photosynthetically Active Radiation (PAR) that the Earth gets from the Sun. This is because most of the star's light is emitted at wavelengths longer than the 400-700 nm PAR range. Thus it would take 63 Gyrs for a GOE to occur. But the linear assumption is problematic; as light levels increase, photosynthesis saturates then declines, an effect known as photoinhibition. Photoinhibition varies from species to species and depends on a host of environmental factors. There is also sensitivity to the upper wavelength limit of the PAR: extending just 50 nm increases the number of photons by a factor of 2.5. Including these and other factors greatly reduces the timescale to roughly 1-5 Gyrs for a GOE. However, non-oxygenic photosynthetic bacteria can thrive in low-light environments and can use near-IR light out to 1100 nm, providing 22 times as many photons. With this huge light advantage, and because they evolved earlier, anoxygenic photosynthesizers would likely dominate the ecosystem. On a late M-star Earth-analog planet, oxygen may never reach significant levels in the atmosphere and a GOE may never occur, let alone a Cambrian Explosion. Thus complex animal life is unlikely.
Paper Structure (16 sections, 2 figures, 2 tables)

This paper contains 16 sections, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. TRAPPIST-1e
  3. Estimating the Timescale for a Great Oxidation Event
  4. The Origin of Oxygenic Photosynthesis and the Great Oxidation Event on Earth
  5. Simple Linear Approximation
  6. Improving the Timescale Estimate
  7. Far-Red Light and the Extended PAR
  8. Synchronous Rotation
  9. Photoinhibition and Photosynthesis-Irradiance (PI) Curves
  10. Revised Timescale Estimates
  11. Discussion
  12. Anoxygenic Photosynthesis
  13. Caveats and Concluding Thoughts
  14. Acknowledgments
  15. Tables and Figures
  16. ...and 1 more sections

Figures (2)

  • Figure 1: The incident photon flux density for the modern-day Earth (black), Archean Earth at 2.65 Ga (blue), and TRAPPIST-1e (red). The spectral resolution has been reduced for clarity. The shaded regions represents three relevant bandpasses for photosynthesis: standard PAR (0.40-0.70 $\mu$m), extended PAR (0.40-0.75 $\mu$m), and anoxic PAR (0.40-1.1 $\mu$m).
  • Figure 2: (a) Normalized P-I curves showing the relative rate of oxygen production as a function of the light level for various cyanobacteria and related photosynthesizing species. Note that the irradiation is shown in logarithmic units. The more important species for this work are shown in thicker curves. The mean surface irradiance that the Archean Earth received from the Sun is shown as the vertical cyan lines, for the PAR (dashed) and the extended PAR (solid). The vertical red lines shows what a hypothetical Earth would receive it if were located in TRAPPIST-1e's orbit. (b) A zoom-in of the normalized P-I curves for low-light irradiance levels.