A JWST Panchromatic Thermal Emission Spectrum of the Warm Neptune Archetype GJ 436b

TL;DR

This study presents the first JWST panchromatic thermal emission spectrum of GJ 436b, revealing weaker molecular features and challenging previous high-metallicity atmosphere models, with implications for atmospheric composition and cloud presence.

Contribution

First JWST spectrum of GJ 436b spanning 2.4-11.9 μm, providing new insights into its atmospheric composition and challenging prior high-metallicity assumptions.

Findings

Spectrum fainter at 3.6 μm than Spitzer data

Weak molecular absorption features, tentative CO₂ detection

Atmospheric models suggest lower metallicity and internal temperature than previous estimates

Abstract

GJ 436b is the archetype warm Neptune exoplanet. The planet's thermal emission spectrum was previously observed via intensive secondary eclipse campaigns with Spitzer. The atmosphere has long been interpreted to be extremely metal-rich, out of chemical equilibrium, and potentially tidally heated. We present the first panchromatic emission spectrum of GJ 436b observed with JWST's NIRCAM (F322W2 and F444W) and MIRI (LRS) instruments between 2.4 and 11.9 $\mu$m. Surprisingly, the JWST spectrum appears significantly fainter around 3.6 $\mu$m than that implied by Spitzer photometry. The molecular absorption features in the spectrum are relatively weak, and we only find tentative evidence of CO$_2$ absorption at 2$\sigma$ significance. Under the assumption of a day-side blackbody, we find $T_{\rm day}$=662.8$\pm$5.0 K, which is similar to the zero Bond albedo equilibrium temperature. We use it to obtain a 3$\sigma$ upper limit on the Bond albedo of $A_B{\le}$0.66. To understand the spectrum we employ 1D radiative-convective models but find that atmospheric constraints depend strongly on model assumptions. If thermochemical equilibrium is assumed, we find a cloudy metal-enriched atmosphere (metallicity $\ge$ 300$\times$solar). We employ 1D photochemical modeling to show that the observed spectrum is also consistent with a cloud-free, relatively lower-metallicity atmosphere (metallicity $\ge$ 80$\times$solar) with a cold internal temperature ($T_{\rm int}$$\sim$60 K). These are much lower metallicities and internal temperatures than inferences from Spitzer photometry. The low $T_{\rm day}$ and non-detection of transmission features at high spectral resolution does suggest a role for cloud opacity, but this is not definitive.