Possible Stratospheric Emission in the Warm Neptune GJ 436 b from High-resolution Spectroscopy

Abstract We present high-spectral-resolution L -band (2.91–3.85 μ m) observations of the warm Neptune GJ 436 b from Keck II/KPIC. KPIC’s single-mode fiber feed reduces the L- band background by a factor of 30, significantly improving sensitivity compared to a seeing-limited spectrometer and enabling a tentative (signal-to-noise ratio of 3–4) cross-correlation detection of GJ 436 b with a thermally inverted atmospheric model. In contrast with recent results from JWST and high-resolution transmission spectroscopy, our retrieval analysis prefers the presence of H 2 O, and possibly CH 4 , molecular features in emission. The broadband continuum flux associated with the maximum-likelihood model is substantially higher than expected based on both the ∼670 K equilibrium temperature of GJ 436 b and previous results from low-resolution spectroscopy. We demonstrate that the loss of continuum information during the processing of high-resolution spectra makes our analysis effectively insensitive to the absolute continuum level of the planet, and that scaling the maximum-likelihood model to match the broadband flux measured from low-resolution observations of GJ 436 b results in a detection of similar strength in cross correlation. These results could be explained by a thermal inversion arising above a haze layer in the upper atmosphere of GJ 436 b. Further observations, ideally posteclipse in order to break the K p –Δ v sys degeneracy, are needed to clarify this possible detection. This work demonstrates the potential of L- band high-resolution spectroscopy for characterizing significantly smaller and cooler exoplanets compared with hot Jupiters.