Paper

Possible stratospheric emission in the warm Neptune GJ 436 b from high-resolution spectroscopy

Wang, Ji, Cetre, Sylvain, Schofield, Tobias, Hsu, Chih-Chun, Blake, Geoffrey A, Finnerty, Luke, Xuan, Jerry W, López, Ronald A, Echeverri, Daniel, Yinzi Xin, Inglis, Julie, Benjamin, Calvin, Morris, Evan, Baker, Ashley, Jacques-Robert Delorme, Jovanovic, Nemanja, Wang, Jason J, Fitzgerald, Michael P, Bartos, Randall, Jean-Baptiste Ruffio, Wallace, J Kent, Sappey, Ben, Horstman, Katelyn, Liberman, Joshua, Skemer, Andrew, Wallack, Nicole L, Mawet, Dimitri, Doppmann, Greg, Pezzato-Rovner, Jacklyn

2026

Overview

We present high spectral resolution \\(L\\) band (2.91--3.85 \\(\\)m) observations of the warm Neptune GJ 436 b from Keck II/KPIC. KPIC's single-mode fiber feed reduces the \\(L\\) band sky background by a factor of 100, significantly improving sensitivity compared to a seeing-limited spectrometer and enabling a tentative (\\( SNR = 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 broad-band 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 broad-band 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 Further observations, ideally post-eclipse in order to break the \\(K_p - 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.

• Publisher: Cornell University Library, arXiv.org
• Subject: Correlation detection / Cross correlation / Emission / Extrasolar planets / Gas giant planets / High resolution / Spectral resolution / Spectrum analysis / Upper atmosphere