Forward and Inverse Modeling of the Emission and Transmission Spectrum of GJ 436b: Investigating Metal Enrichment, Tidal Heating, and Clouds

TL;DR

This study combines new and existing observations of GJ 436b's spectra with advanced models to explore its atmospheric composition, tidal heating, and cloud effects, revealing high metallicity and long tidal timescales, and emphasizing the planet's potential diversity.

Contribution

It introduces a dual modeling approach analyzing both emission and transmission spectra to constrain GJ 436b's atmospheric metallicity, tidal heating, and cloud effects, highlighting its high metal enrichment and long tidal timescale.

Findings

GJ 436b has a high atmospheric metallicity, possibly over 600 times solar.

Tidal heating suggests an intrinsic temperature of 300-350 K and a long orbital circularization timescale.

High metal-enrichment likely results from rocky material accretion, indicating diverse planetary compositions.

Abstract

The Neptune-mass GJ 436b is one of the most-studied transiting exoplanets with repeated measurements of both its thermal emission and transmission spectra. We build on previous studies to answer outstanding questions about this planet, including its potentially high metallicity and tidal heating of its interior. We present new observations of GJ 436b's thermal emission at 3.6 and 4.5 micron, which reduce uncertainties in estimates of GJ 436b's flux at those wavelengths and demonstrate consistency between Spitzer observations spanning more than 7 years. We analyze the Spitzer thermal emission photometry and Hubble WFC3 transmission spectrum in tandem. We use a powerful dual-pronged modeling approach, comparing these data to both self-consistent and retrieval models. We vary the metallicity, intrinsic luminosity from tidal heating, disequilibrium chemistry, and heat redistribution. We also study the effect of clouds and photochemical hazes on the spectra, but do not find strong evidence for either. The self-consistent and retrieval modeling combine to suggest that GJ 436b has a high atmospheric metallicity, with best fits at or above several hundred times solar metallicity, tidal heating warming its interior with best-fit intrinsic effective effective temperatures around 300--350 K, and disequilibrium chemistry. High metal-enrichments (>600x solar) can only occur from the accretion of rocky, rather than icy, material. Assuming Tint~300--350 K, we find that Q'~2x10^5--10^6, larger than Neptune's Q', and implying a long tidal circularization timescale for the planet's orbit. We suggest that Neptune-mass planets may be a more diverse class than previously imagined, with metal-enhancements potentially spanning several orders of magnitude, to perhaps over 1000x solar metallicity. High fidelity observations with instruments like JWST will be critical for characterizing this diversity.