Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b

Most known terrestrial planets orbit small stars with radii less than 60 per cent of that of the Sun. Theoretical models predict that these planets are more vulnerable to atmospheric loss than their counterparts orbiting Sun-like stars. To determine whether a thick atmosphere has survived on a small planet, one approach is to search for signatures of atmospheric heat redistribution in its thermal phase curve. Previous phase curve observations of the super-Earth 55 Cancri e (1.9 Earth radii) showed that its peak brightness is offset from the substellar point (latitude and longitude of 0 degrees)—possibly indicative of atmospheric circulation. Here we report a phase curve measurement for the smaller, cooler exoplanet LHS 3844b, a 1.3-Earth-radii world in an 11-hour orbit around the small nearby star LHS 3844. The observed phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1,040 ± 40 kelvin and a nightside temperature consistent with zero kelvin (at one standard deviation). Thick atmospheres with surface pressures above 10 bar are ruled out by the data (at three standard deviations), and less-massive atmospheres are susceptible to erosion by stellar wind. The data are well fitted by a bare-rock model with a low Bond albedo (lower than 0.2 at two standard deviations). These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres. ; © 2019 Nature Publishing Group. Received 22 March 2019; Accepted 22 July 2019; Published 19 August 2019. Data availability: The raw data used in this study are available at the Spitzer Heritage Archive, https://sha.ipac.caltech.edu/applications/Spitzer/SHA. Code availability: We processed and fitted the data with the open-source pipeline POET, which is available at https://github.com/kevin218/POET. We used the code version corresponding to commit ID adbe62e7b733df9541231e8d1e5d32b7e2cdad76. L.K. is a Junior Fellow of the Harvard Society of Fellows. J.D. is a 51 Pegasi ...