Constraining the Thickness of the Atmosphere of TRAPPIST-1 b from its JWST Secondary Eclipse Observation

TL;DR

This study uses JWST secondary eclipse data and radiative models to constrain the maximum possible atmospheric thickness of TRAPPIST-1 b, finding that thick atmospheres are unlikely unless they lack strong mid-IR absorbers.

Contribution

It provides the first quantitative constraints on the atmospheric thickness of TRAPPIST-1 b using JWST data and self-consistent radiative models, ruling out thick atmospheres with common compositions.

Findings

Atmospheres with >0.01 bar surface pressure are ruled out at 1σ.

Thicker atmospheres up to 10-100 bar are only plausible if they lack strong IR absorbers.

Bare-rock surface models can match the observed eclipse depth within 1σ.

Abstract

Recently, the first JWST measurement of thermal emission from a rocky exoplanet was reported. The inferred dayside brightness temperature of TRAPPIST-1 b at 15 $\mu$m is consistent with the planet having no atmosphere and therefore no mechanism by which to circulate heat to its nightside. In this Letter, we compare the measured secondary eclipse depth of TRAPPIST-1 b to predictions from a suite of self-consistent radiative-convective equilibrium models in order to quantify the maximum atmospheric thickness consistent with the observation. We find that plausible atmospheres (i.e., those that contain at least 100 ppm CO$_2$) with surface pressures greater than 0.01 bar (0.1 bar) are ruled out at 1$\sigma$ (3$\sigma$), regardless of the choice of background atmosphere. Thicker atmospheres of up to 10 bar (100 bar) at 1$\sigma$ (3$\sigma$) are only allowed if the atmosphere lacks any strong absorbers across the mid-IR wavelength range, a scenario that we deem unlikely. We additionally model the emission spectra for bare-rock planets of various compositions. We find that a variety of silicate surfaces match the measured eclipse depth to within 1$\sigma$, and the best-fit grey albedo is $0.02 \pm 0.11$. We conclude that planned secondary eclipse observations at 12.8 $\mu$m will serve to validate the high observed brightness temperature of TRAPPIST-1 b, but are unlikely to further distinguish among the consistent atmospheric and bare-rock scenarios.