Spectral Signatures of Photosynthesis. II. Coevolution with Other Stars And The Atmosphere on Extrasolar Worlds
Year: 2007
Type: review
Abstract: As photosynthesis on Earth produces the primary signatures of life that can be detected astronomically at the global scale, a strong focus of the search for extrasolar life will be photosynthesis, particularly photosynthesis that has evolved with a different parent star. We take previously simulated planetary atmospheric compositions for Earth-like planets around observed F2V and K2V, modeled M1V and M5V stars, and around the active M4.5V star AD Leo; our scenarios use Earth's atmospheric composition as well as very low O2 content in case anoxygenic photosynthesis dominates. With a line-by-line radiative transfer model, we calculate the incident spectral photon flux densities at the surface of the planet and under water. We identify bands of available photosynthetically relevant radiation and find that photosynthetic pigments on planets around F2V stars may peak in absorbance in the blue, K2V in the red-orange, and M stars in the near-infrared, in bands at 0.93–1.1 μm, 1.1–1.4 μm, 1.5–1.8 μ m, and 1.8–2.5 μm. However, underwater organisms will be restricted to wavelengths shorter than 1.4 μm and more likely below 1.1 μm. M star planets without oxygenic photosynthesis will have photon fluxes above 1.6 μm curtailed by methane. Longer-wavelength, multi-photo-system series would reduce the quantum yield but could allow for oxygenic photosystems at longer wavelengths. A wavelength of 1.1 μm is a possible upper cutoff for electronic transiprotions versus only vibrational energy; however, this cutoff is not strict, since such energetics depend on molecular configuration. M star planets could be a half to a tenth as productive as Earth in the visible, but exceed Earth if useful photons extend to 1.1 μm for anoxygenic photosynthesis. Under water, organisms would still be able to survive ultraviolet flares from young M stars and acquire adequate light for growth. Key Words: Photosynthesis—Astrobiology — Photosynthetic pigments — Oxygenic photosynthesis — Anoxygenic photosynthesis — Atmospheric photochemistry — F stars—G stars — Sun — K stars — M stars — AD Leo — Atmospheric oxygen — Atmospheric radiative transfer — Chlorophyll — Bacteriochlorophyll — Photosystems — Radiation spectrum — Photosynthetically active radiation — Light harvesting — Modeling — Extrasolar planets — Earth like planets — Virtual Planetary Laboratory — Biosignatures. Astrobiology 7(1), 252 – 274.
Source: Astrobiology
Authors: Nancy Y. Kiang, Antígona Segura, Giovanna Tinetti, Govindjee, Robert E. Blankenship, Martin Cohen, Janet Siefert, David Crisp, Victoria S. Meadows
Institutions: Goddard Institute for Space Studies, NASA Astrobiology Institute, Universidad Nacional Autónoma de México, Institut d'Astrophysique de Paris, European Space Agency, University of Illinois Urbana-Champaign, Washington University in St. Louis, University of California, Berkeley, Rice University, Jet Propulsion Laboratory, California Institute of Technology
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W1970799990