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Arxiv: The Thermal Emission Spectrum of the Nearby Rocky Exoplanet LTT 1445A b from JWST MIRI/LRS Published: 10/14/2024 9:15:19 PM Updated: 10/14/2024 9:15:19 PM

Paper abstract: The nearby transiting rocky exoplanet LTT 1445A b presents an ideal targetfor studying atmospheric retention in terrestrial planets orbiting M dwarfs. Itis cooler than many rocky exoplanets yet tested for atmospheres, receiving abolometric instellation similar to Mercury's. Previous transmissionspectroscopy ruled out a light H/He-dominated atmosphere but could notdistinguish between a bare-rock, a high-MMW, or a cloudy atmosphere. We presentnew secondary eclipse observations using JWST's MIRI/LRS, covering the 5-12\mum range. From these observations, we detect a broadband secondary eclipsedepth of 41 \pm 9 ppm and measure a mid-eclipse timing consistent with acircular orbit (at 1.7\sigma). From its emission spectrum, the planet'sdayside brightness temperature is constrained to 525 \pm 15 K, yielding atemperature ratio relative to the maximum average dayside temperature frominstant thermal reradiation by a rocky surface R = T_{\rm day,obs}/T_{\rmmax} = 0.952 \pm 0.057, consistent with emission from a dark rocky surface.From an energy balance perspective, such a warm dayside temperature disfavorsthick atmospheres, excluding ~100 bar atmospheres with Bond albedo >0.08 at the 3\sigma level. Furthermore, forward modeling of atmosphericemission spectra disfavor simple 100\% CO_2 atmospheres with surfacepressures of 1, 10, and 100 bar at 4.2\sigma, 6.6\sigma, and 6.8\sigmaconfidence, respectively. These results suggest that LTT 1445A b lacks a verythick CO_2 atmosphere, possibly due to atmospheric erosion driven by stellaractivity. However, the presence of a moderately thin atmosphere (similar tothose on Mars, Titan, or Earth) remains uncertain.