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Arxiv: Reliable Detections of Atmospheres on Rocky Exoplanets with Photometric JWST Phase Curves Published: 9/6/2024 7:25:57 PM Updated: 11/19/2024 10:46:16 AM

Paper abstract: The distribution of different types of atmospheres and surfaces on rockyplanets is one of the major questions in exoplanet astronomy, but there arecurrently no published unambiguous detections of atmospheres on any rockyexoplanets. The MIRI instrument on JWST can measure thermal emission fromtidally locked rocky exoplanets orbiting small, cool stars. This emission is afunction of their surface and atmospheric properties, potentially allowing thedetection of atmospheres. One technique is to measure day-side emission tosearch for lower thermal emission than expected for a black-body planet due toatmospheric absorption features. Another technique is to measure phase curvesof thermal emission to search for night-side emission due to atmospheric heatredistribution. In this work we compare strategies for detecting atmospheres onrocky exoplanets using these techniques. We simulate secondary eclipse andphase curve observations in the MIRI F1500W and F1280W filters, for a range ofsurfaces and atmospheres on thirty exoplanets selected for their F1500Wsignal-to-noise ratio. Our results show that secondary eclipse observations arehighly degenerate between surfaces and atmospheres, given the wide range ofpotential surface albedos. We also show that thick atmospheres can supportemission consistent with a black-body planet in these filters. These tworesults make it difficult to unambiguously detect or rule out atmospheres usingtheir photometric day-side emission, except in a subset of CO_{2}-dominatedatmospheres. We suggest that an F1500W phase curve could instead be observedfor a similar sample of planets, allowing the unambiguous detection ofatmospheres by night-side emission.