Excitation Properties of Photopigments and Their Possible Dependence on the Host Star

Manasvi Lingam,Amedeo Balbi,Swadesh M Mahajan

doi:10.3847/2041-8213/ac3478

Abstract

Abstract Photosynthesis is a plausible pathway for the sustenance of a substantial biosphere on an exoplanet. In fact, it is also anticipated to create distinctive biosignatures detectable by next-generation telescopes. In this work, we explore the excitation features of photopigments that harvest electromagnetic radiation by constructing a simple quantum-mechanical model. Our analysis suggests that the primary Earth-based photopigments for photosynthesis may not function efficiently at wavelengths >1.1 μm. In the context of (hypothetical) extrasolar photopigments, we calculate the potential number of conjugated π-electrons (N ⋆) in the relevant molecules, which can participate in the absorption of photons. By hypothesizing that the absorption maxima of photopigments are close to the peak spectral photon flux of the host star, we utilize the model to estimate N ⋆. As per our formalism, N ⋆ is modulated by the stellar temperature, and is conceivably higher (lower) for planets orbiting stars cooler (hotter) than the Sun; exoplanets around late-type M-dwarfs might require an N ⋆ twice that of the Earth. We conclude the analysis with a brief exposition of how our model could be empirically tested by future observations.

Highlights

With the ongoing explosion in the number of exoplanets, there has been a commensurate rise in interest in ascertaining/ establishing (1) the conditions that render a planet habitable, and (2) the traits of the putative biospheres (Lammer et al 2009; Cockell et al 2016; Cockell 2020)
The inner and outer limits of the habitable zone (HZ)—namely, the region surrounding the star where surface temperatures on rocky planets could be conducive for liquid water—is manifestly regulated by the stellar spectral type (Dole 1964; Kasting et al 1993; Kopparapu et al 2013)
By computing the possible number of conjugated electrons that are involved in photosynthesis, we suggest that this quantity might be modulated by the spectral type

Summary

Introduction

With the ongoing explosion in the number of exoplanets, there has been a commensurate rise in interest in ascertaining/ establishing (1) the conditions that render a planet habitable, and (2) the traits of the putative biospheres (Lammer et al 2009; Cockell et al 2016; Cockell 2020). This endeavor, in addition to its theoretical import, will have real consequences in shaping and prioritizing the selection of target planetary systems for in-depth characterization by forthcoming telescopes (Fujii et al 2018; Schwieterman et al 2018).

Model for Electronic Excitation

Peak Absorption Wavelength of Photopigments

Discussion and Conclusions