Direct Spectroscopy of 51 Eridani b with JWST NIRSpec

Abstract We present high-contrast direct spectroscopy of the low-mass, cool exoplanet 51 Eridani b (2–4 M Jup , ∼750 K) using JWST/NIRSpec in a fixed-slit configuration (F290LP/G395H, 3–5 μ m, R ∼ 2700). A cross-correlation analysis between the continuum-subtracted data and atmospheric forward models indicates a detection of molecular signals of planetary origin at 4.8 σ at the expected position and velocity of the planet. The detection of the planetary signal is driven primarily by molecular features from methane and carbon monoxide, providing the first direct confirmation of these two molecules coexisting in chemical disequilibrium in the atmosphere of 51 Eridani b. A new comprehensive atmospheric model analysis shows consistency between the ground-based IFU spectroscopy and the NIRSpec data, with the best-fit model parameters T eff = 800 − 55.5 + 21.5 K, log g = 3.75 − 0.37 + 0.09 , [M/H] = 0.7 − 0.21 + 0.07 , C/O = 0.458 − 0.09 + 0.08 , log K zz = 3 − 0.73 + 0.47 , R P = 1.36 − 0.03 + 0.07 R Jup , and f hole = 0.3 − 0.07 + 0.10 , and the NIRSpec error bar inflation parameter e ˆ = 1.74 − 0.03 + 0.02 . We conclude w