The Surprising JWST Panchromatic Emission Spectrum of GJ 436b

GJ 436b is the archetype warm Neptune exoplanet, with an equilibrium temperature of ~ 700 K. The planet’s thermal emission spectrum has previously been observed via intensive secondary eclipse campaigns with Spitzer. The atmosphere has long been interpreted to be extremely metal-rich, strongly out of chemical equilibrium, and potentially tidally heated. This interpretation was due to a high flux in the Spitzer 3.6 um band and almost no flux in the 4.5 um band, implying a lack of CH4 absorption at 3.3 um and very strong absorption due to CO and CO2. Here we present the first panchromatic emission spectrum of GJ 436b observed with JWST NIRCAM (F332W & F444W grism data) and MIRI (LRS) instruments from 2.4 to 12 um. Our observations include multiple visits of the planet’s eclipse in three bandpasses. The JWST emission spectrum appears significantly different from that implied by short-wavelength Spitzer photometry. We use this panchromatic emission spectrum to put constraints on the temperature-pressure profile, atmospheric metallicity, C/O ratio, and cloud structure using a retrieval framework. We also employ detailed 1D photochemical forward modeling to interpret the chemical composition of the planet’s atmosphere from the emission spectrum and shed light on the interior heat and strength of vertical mixing in its atmosphere. We contextualize the planet’s JWST spectrum in comparison with other Neptune-class exoplanets, at similar T_eq values, including GJ 3470b and WASP-107b.