JWST Thermal Emission of the Terrestrial Exoplanet GJ 1132b

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 <SUB>eq</SUB> = 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 <SUB> p,dayside</SUB> = 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, <inline-formula> <mml:math overflow="scroll"><mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mi>max</mml:mi></mml:mrow></mml:msub></mml:math> </inline-formula> = <inline-formula> <mml:math overflow="scroll"><mml:msubsup><mml:mrow><mml:mn>746</mml:mn></mml:mrow><mml:mrow><mml:mo>‑</mml:mo><mml:mn>11</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>14</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula> 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<SUB>2</SUB>O, atmospheres of any modeled thickness (10<SUP>‑4</SUP> to 10<SUP>2</SUP> bars) that contain at least 1% CO<SUB>2</SUB>, and thick, Venus-like atmospheres (P ≳ 100 bars) with at least 1 ppm CO<SUB>2</SUB> or H<SUB>2</SUB>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.