Exoplanet Exergy: Why Useful Work Matters For Planetary Habitabilty

Total (whole spectrum) exergy (Watts) with arbitrary normalization as a function of stellar blackbody temperature (from M8 dwarfs at approximately 2660K to A5 stars at approximately 8620K) for surface temperatures Ts = 273K and 373K and Teff = 242K and 330K - all curves are effectively indistinguishable on this plot.

The circumstellar habitable zone and its various refinements serves as a useful entry point for discussing the potential for a planet to generate and sustain life.

But little attention is paid to the quality of available energy in the form of stellar photons for phototrophic (e.g. photosynthetic) life. This short paper discusses the application of the concept of exergy to exoplanetary environments and the evaluation of the maximum efficiency of energy use, or maximum work obtainable from electromagnetic radiation. Hotter stars provide temperate planets with higher maximum obtainable work with higher efficiency than cool stars, and cool planets provide higher efficiency of radiation conversion from the same stellar photons than do hot planets.

These statements are independent of the details of any photochemical and biochemical mechanisms and could produce systematic differences in planetary habitability, especially at the extremes of maximal or minimal biospheres, or at critical ecological tipping points. Photoautotrophic biospheres on habitable planets around M-dwarf stars may be doubly disadvantaged by lower fluxes of photosynthetically active photons, and lower exergy with lower energy conversion efficiency.

Caleb Scharf
(Submitted on 13 Mar 2019)

Comments: 8 pages, 2 figures, submitted to ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1903.05624 [astro-ph.EP] (or arXiv:1903.05624v1 [astro-ph.EP] for this version)
Submission history
From: Caleb A. Scharf
[v1] Wed, 13 Mar 2019 17:44:01 UTC (75 KB)
https://arxiv.org/abs/1903.05624
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