Absence of a Thick Atmosphere on the Terrestrial Exoplanet LHS 3844b

Abstract

The majority of terrestrial planets in the Galaxy orbit small stars with radii less than 60% that of the Sun1,2. Theoretical models predict that these planets are more vulnerable to at- mospheric escape and collapse than their counterparts orbiting Sun-like stars3–5. To deter- mine whether a thick atmosphere has survived, one approach is to search for signatures of atmospheric heat redistribution in a planet’s thermal phase curve6–9. This technique was previously applied to the super-Earth 55 Cancri e, which showed an offset hot spot indicative of atmosphere heat circulation10. Here we report a phase curve measurement for the exo- planet LHS 3844b, a 1.3 R⊕ world in an 11-hour orbit around a small, nearby star. This is the first such measurement for a planet smaller than 1.6 Re, the size marking the transition from rocky to gaseous worlds11. The phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1040±40 K and a nightside temperature con- sistent with zero K (1σ confidence). The data are best fit by a bare rock model with a low Bond albedo (< 0.2 at 2σ confidence), or a tenuous atmosphere with surface pressure below 0.1 bar. These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.