JWST COMPASS: The First Near- to Mid-infrared Transmission Spectrum of the Hot Super-Earth L 168-9 b
Munazza K. Alam, Peter Gao, Jéa Adams Redai, Nicole L. Wallack, Nicholas F. Wogan, Artyom Aguichine, Anne Dattilo, Lili Alderson, Natasha E. Batalha, Natalie M. Batalha, James Kirk, Mercedes López‐Morales, Annabella Meech, Sarah E. Moran, Johanna Teske, Hannah R. Wakeford, Angie Wolfgang
Abstract
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 <4 g mol −1 ) to 3 σ confidence assuming high surface pressure (>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.