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Constraining the Thickness of the Atmosphere of TRAPPIST-1 b from its JWST Secondary Eclipse Observation Published: 5/17/2023 8:50:04 PM Updated: 6/26/2023 8:39:49 PM

Paper abstract: Recently, the first JWST measurement of thermal emission from a rockyexoplanet was reported. The inferred dayside brightness temperature ofTRAPPIST-1 b at 15 \mum is consistent with the planet having no atmosphereand therefore no mechanism by which to circulate heat to its nightside. In thisLetter, we compare the measured secondary eclipse depth of TRAPPIST-1 b topredictions from a suite of self-consistent radiative-convective equilibriummodels in order to quantify the maximum atmospheric thickness consistent withthe observation. We find that plausible atmospheres (i.e., those that containat least 100 ppm CO_2) with surface pressures greater than 0.01 bar (0.1 bar)are ruled out at 1\sigma (3\sigma), regardless of the choice of backgroundatmosphere. Thicker atmospheres of up to 10 bar (100 bar) at 1\sigma(3\sigma) are only allowed if the atmosphere lacks any strong absorbersacross the mid-IR wavelength range, a scenario that we deem unlikely. Weadditionally model the emission spectra for bare-rock planets of variouscompositions. We find that a variety of silicate surfaces match the measuredeclipse depth to within 1\sigma, and the best-fit grey albedo is 0.02 \pm0.11. We conclude that planned secondary eclipse observations at 12.8 \mumwill serve to validate the high observed brightness temperature of TRAPPIST-1b, but are unlikely to further distinguish among the consistent atmospheric andbare-rock scenarios.