2025AJ....170..326M

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 μm, 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 4.8σ 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, $\mathrm{log}g$ = 3.75${}_{-0.37}^{+0.09}$, [M/H] = 0.7${}_{-0.21}^{+0.07}$, C/O = 0.458${}_{-0.09}^{+0.08}$, $\mathrm{log}{K}_{{\rm{zz}}}$ = 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 $\hat{e}$ = 1.74${}_{-0.03}^{+0.02}$. We conclude with a discussion on the lessons learned between the fixed slit and IFU-based high contrast spectroscopic methods from our observing program, including some possibilities to improve the analysis method.