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Albedos, Temperatures and Habitability of Rocky Exoplanets

Abstract: The potential habitability of known exoplanets is often categorized by a nominal equilibrium temperature assuming a Bond albedo of either 0.3, similar to Earth, or 0. As an indicator of habitability, this leaves much to be desired, because albedos of other planets can be very different, and because surface temperature exceeds equilibrium temperature due to the atmospheric greenhouse effect. We use an ensemble of 3-D global climate model simulations to show that for a range of habitable planets, much of the variability of Bond albedo, equilibrium temperature, and even surface temperature can be predicted with useful accuracy from incident stellar flux and stellar temperature, two known parameters for every confirmed exoplanet. Earth’s Bond albedo is near the minimum possible for habitable planets orbiting G stars, because of increasing contributions from clouds and sea ice/snow at higher and lower instellations, respectively. For habitable M star planets, Bond albedo is usually lower than Earth’s because of near-IR H2O absorption, except at high instellation where clouds are important. I will apply relationships derived from this behavior to several known exoplanets to derive zeroth-order estimates of their potential habitability. I will also discuss how planet rotation period and incident stellar flux affect the spatial distribution of wet, humid climates that support abundant vegetation. Using this I will consider the conditions that are conducive to “superhabitable” planets and our ability to detect surface features on such planets.

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