Constraining the Thickness of TRAPPIST-1 b’s Atmosphere from Its JWST Secondary Eclipse Observation at 15 μm

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 μm 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 TRAPPIST-1 b's measured secondary eclipse depth 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 CO2) with surface pressures greater than 0.3 bar are ruled out at 3σ, regardless of the choice of background atmosphere, and a Mars-like thin atmosphere with surface pressure 6.5 mbar composed entirely of CO2 is also ruled out at 3σ. Thicker atmospheres of up to 10 bar (100 bar) are consistent with the data at 1σ (3σ) only 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 basaltic, metal-rich, and Fe-oxidized surface best matches the measured eclipse depth to within 1σ, and the best-fit gray albedo is 0.02 ± 0.11. We conclude that planned secondary eclipse observations at 12.8 μm will serve to validate TRAPPIST-1 b's high observed brightness temperature, but are unlikely to further distinguish among the consistent atmospheric and bare-rock scenarios.

Atmospheric Reconnaissance of TRAPPIST-1 b with JWST/NIRISS: Evidence for Strong Stellar Contamination in the Transmission Spectra

“…In addition to being consistent with pre-JWST observations, the conclusions from both the sequential and simultaneous fits of stellar contamination and planetary atmosphere are consistent with the conclusions from the JWST/MIRI secondary eclipse observations (Greene et al 2023;Ih et al 2023) in that the transits reveal no evidence for an atmosphere. Both the transit and secondary eclipse observations agree that TRAP-PIST-1 b does not have a thick atmosphere, at least not one with a low mean molecular mass based on the transit observations.…”

“…Moreover, the nondetection of an atmosphere on TRAPPIST-1 b can provide insight into the evolutionary history of the system. Results from this work and from the eclipse observations (Greene et al 2023;Ih et al 2023) should thus not discourage anyone from monitoring transits of the TRAPPIST-1 planets to search for atmospheric signatures.…”

“…Both the transit and secondary eclipse observations agree that TRAP-PIST-1 b does not have a thick atmosphere, at least not one with a low mean molecular mass based on the transit observations. The MIRI observations provide additional constraints by rejecting certain atmospheres containing CO 2 , as well as atmospheres containing no CO 2 for certain pressures and compositions (right panel of Figure 2 from Ih et al 2023), further ruling out regions of the parameter space allowed by the transmission data. In this sense, the transmission spectra are less constraining in terms of atmospheric characterization for TRAPPIST-1 b, but they partially confirm the emission data and, more importantly, they bring insight into the stellar- (d) Corresponding posterior probability distributions from the joint stellar-contamination and planetary atmosphere retrieval for visits 1 and 2.…”

“…26 K, consistent with little to no heat redistribution to the nightside and a near-zero Bond albedo (Greene et al 2023), ruling out CO 2 -dominated atmospheres at surface pressures of 10 and 92 bar, along with O 2 -dominated atmospheres with 0.5 bar of CO 2 at surface pressures of 10 and 100 bar. A complementary analysis of these emission data (Ih et al 2023) rejected atmospheres with at least 100 ppm CO 2 with surface pressures above 0.3 bar, as well as a Mars-like, pure CO 2 atmosphere with a surface pressure of 6.5 mbar. Secondary eclipse observations of TRAPPIST-1 c with JWST/MIRI also disfavor thick, CO 2 -rich atmospheres (Zieba et al 2023), but certain scenarios involving less CO 2 remain consistent with the data (Lincowski et al 2023).…”

Predictions for Observable Atmospheres of Trappist-1 Planets from a Fully Coupled Atmosphere–Interior Evolution Model

“…This is consistent with reradiation from an airless dayside as substantial atmospheres (>1 bar) would redistribute heat resulting in a lower dayside brightness temperature (Greene et al, 2023). Follow-up work (Ih et al, 2023) modeled the dayside thermal emission of Trappist-1b for a wide range of atmospheric compositions and confirmed that >1 bar atmospheres containing even trace amounts of CO 2 are strongly disfavored. Dense atmospheres (>1 bar) consisting of predominantly non-absorbing or weakly absorbing species (N 2 , O 2 , CO) can potentially be reconciled with the observed brightness temperature, but such atmospheres are geochemically implausible given that complete photochemical conversion of CO 2 to CO is unlikely (Gao et al, 2015) and that even highly reduced silicate melts will degas some carbon as CO 2 (Sossi et al, 2020).…”