JWST Thermal Emission of the Terrestrial Exoplanet GJ 1132b

We present thermal emission measurements of GJ 1132b spanning 5–12 μ m obtained with the Mid-Infrared Instrument Low-Resolution Spectrometer on the James Webb Space Telescope. GJ 1132b is an M dwarf rocky planet with T eq = 584 K and an orbital period of 1.6 days. We measure a white-light secondary eclipse depth of 140 ± 17 ppm, which corresponds to a dayside brightness temperature of T p ,dayside = 709 ± 31 K using improved star and planet parameters. This measured temperature is only 1 σ below the maximum possible dayside temperature of a bare rock (i.e., assuming a zero-albedo planet with no heat redistribution, T max = 746 − 11 + 14 K). The emission spectrum is consistent with a featureless blackbody, which agrees with a wide range of possible surface compositions. By comparing forward models to the dayside emission spectrum, we rule out Earth-thickness ( P ∼ 1 bar) atmospheres with at least 1% H 2 O, atmospheres of any modeled thickness (10 −4 to 10 2 bars) that contain at least 1% CO 2 , and thick, Venus-like atmospheres ( P ≳ 100 bars) with at least 1 ppm CO 2 or H 2 O. We therefore conclude that GJ 1132b likely does not have a significant atmosphere. This finding supports the concept of a universal “cosmic shoreline” given the high level of bolometric and extreme ultraviolet (EUV) and X-rays (collectively XUV) irradiation received by the planet.