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Harvard ADS: The Thermal Emission Spectrum of the Nearby Rocky Exoplanet LTT 1445A b from JWST MIRI/LRS

Paper abstract: The nearby transiting rocky exoplanet LTT 1445A b presents an ideal target for studying atmospheric retention in terrestrial planets orbiting M dwarfs. It is cooler than many rocky exoplanets yet tested for atmospheres, receiving a bolometric instellation similar to Mercury's. Previous transmission spectroscopy ruled out a light H/He-dominated atmosphere but could not distinguish between a bare-rock, a high-MMW, or a cloudy atmosphere. We present new secondary eclipse observations using JWST's MIRI/LRS, covering the 5-12 \mum range. From these observations, we detect a broadband secondary eclipse depth of 41 \pm 9 ppm and measure a mid-eclipse timing consistent with a circular orbit (at 1.7\sigma). From its emission spectrum, the planet's dayside brightness temperature is constrained to 525 \pm 15 K, yielding a temperature ratio relative to the maximum average dayside temperature from instant thermal reradiation by a rocky surface R = T_{\rm day,obs}/T_{\rm max} = 0.952 \pm 0.057, consistent with emission from a dark rocky surface. From an energy balance perspective, such a warm dayside temperature disfavors thick atmospheres, excluding ~100 bar atmospheres with Bond albedo > 0.08 at the 3\sigma level. Furthermore, forward modeling of atmospheric emission spectra disfavor simple 100\% CO_2 atmospheres with surface pressures of 1, 10, and 100 bar at 4.2\sigma, 6.6\sigma, and 6.8\sigma confidence, respectively. These results suggest that LTT 1445A b lacks a very thick CO_2 atmosphere, possibly due to atmospheric erosion driven by stellar activity. However, the presence of a moderately thin atmosphere (similar to those on Mars, Titan, or Earth) remains uncertain.