Abstract We present the first broadband near- to mid-infrared (3–12 μ m) transmission spectrum of the highly irradiated ( T eq = 981 K) M-dwarf rocky planet L 168-9 b (TOI-134 b) observed with the Near-infrared Spectrograph and Mid-infrared Instrument (MIRI) instruments aboard JWST. We measure the near-infrared transit depths to a combined median precision of 20 ppm across the three visits in 54 spectroscopic channels with uniform widths of 60 pixels (∼0.2 μ m wide; R ∼ 100), and the mid-infrared transit depths to 61 ppm median precision in 48 wavelength bins (∼0.15 μ m wide; R ∼ 50). We compare the transmission spectrum of L 168-9 b to a grid of 1D thermochemical equilibrium forward models, and rule out atmospheric metallicities of less than 100× solar (mean molecular weights &lt;4 g mol −1 ) to 3 σ confidence assuming high surface pressure (&gt;1 bar), cloudless atmospheres. Based on photoevaporation models for L 168-9 b with initial atmospheric mass fractions ranging from 2% to 100%, we find that this planet could not have retained a primordial H/He atmosphere beyond the first 200 Myr of its lifetime. Follow-up MIRI eclipse observations at 15 μ m could make it possible to confidently identify a CO 2 -dominated atmosphere on this planet if one exists.

JWST COMPASS: The First Near- to Mid-infrared Transmission Spectrum of the Hot Super-Earth L 168-9 b