2024ApJ...973L...8X

We present thermal emission measurements of GJ 1132b spanning 5-12 μm 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 1σ 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% H₂O, atmospheres of any modeled thickness (10^–4 to 10² bars) that contain at least 1% CO₂, and thick, Venus-like atmospheres (P ≳ 100 bars) with at least 1 ppm CO₂ or H₂O. 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.