Rainbow edges: Light-harvesting by photosynthetic systems over a wide range of stellar ages, masses, rotation periods and metallicities

If, as on Earth, photosynthetic life on exoplanets evolved to optimize its light harvesting quantum efficiency, the colors of reflected light from such a biosphere may be detectable as a surface biosignature (Sagan et al., 1993). Photosynthesizing organisms on Earth have efficient solar power conversion specifically adapted to the Solar spectrum as exemplified by the so-called “Red Edge” (e.g. Seager et al., 2005); a sharp increase in spectral reflectance of wavelengths between the red and very-near infrared (700–750 nm) wavelengths. A noise-cancelling network model derived from optimal absorption qualities for efficient solar power conversion (Arp et al., 2020) successfully predicts the observed wavelength dependent absorption of chlorophyll in green plants on Earth under the Sun, a Main Sequence G2 star of solar metallicity (Z=0.01-0.02). This Noisy Antenna model describes the phenomenon of photosynthesis from a quantum engineering perspective. Photosynthesis is treated in terms of light-harvesting antennae  tuned to  minimize excitation noise in power conversion based solely on the structure of the stellar light spectrum, providing a biology-agnostic prediction about its spectral properties. For planets orbiting stars of different masses and metallicities, different spectral reflectance at different wavelengths from that of Earth’s biosphere should be expected (cf. Lehmer et al., 2021). We may better describe this phenomenon as a Rainbow Edge, where distinctive step function-like reflectance features for different star-planet systems show different order-of-magnitude effects. Here, we investigate what wavelengths of light photosynthetic systems would preferentially reflect and absorb for different Sun-like stars based on bolometric luminosity models from MESA++ (Mojzsis et al. this meeting) and XUV evolution from Scherf et al. (2024). We report the results of our analysis of peak optimum absorbers for photosynthetic biospheres on Earth-like planets around stars ranging in mass and spectral type from M1V (0.5M_SUN) to F3V (1.2M_SUN), and metallicities from Z=(0.006)0.01-0.04.

 

Arp, T.B., Kistner-Morris, J., Aji, V., Cogdell, R.J., van Grondelle, R., and Gabor, N.M. (2020) Quieting a noisy antenna reproduces photosynthetic light-harvesting spectra. Science 368: 1490-1495.

Lehmer, O.R., Catling, D.C., Parenteau, M.N., Kiang, N.Y., and Hoehler, T.M. (2021) The peak absorbance wavelength of photosynthetic pigments around other stars from spectral optimization. Frontiers in Astronomy and Space Sciences 8: 689441. doi: 10.3389/fspas.2021.689441

Sagan, C., Thompson, W. R., Carlson, R., Gurnett, D., and Hord, C. (1993) A search for life on Earth from the Galileo spacecraft. Nature: 365, 715-721.

Scherf, M., Lammer, H., and Spross, L. (2024) Eta-Earth Revisited II: Deriving a Maximum Number of Earth-Like Habitats in the Galactic Disk. Astrobiology: 24, e916

Seager, S., Turner, E.L., Schafer, J., and Ford, E.B. (2005) Vegetation’s Red Edge: A possible spectroscopy biosignature of extraterrestrial plants. Astrobiology 5: 372-390.