Limits on Clouds and Hazes for the TRAPPIST-1 Planets

TL;DR

This study uses updated models and laboratory data to analyze the atmospheres of TRAPPIST-1 planets, suggesting the outer planets likely have secondary, volatile-rich atmospheres with clouds and hazes affecting their transmission spectra.

Contribution

It provides new constraints on cloud, haze, and metallicity properties of TRAPPIST-1 planets using physically-based models and recent laboratory results.

Findings

Haze scattering cross sections of 1e-26 to 1e-19 cm^2 are needed for certain planets.

High-altitude clouds and metallicities of 60xsolar fit the data for some planets.

Outer planets likely have secondary, volatile-rich atmospheres.

Abstract

The TRAPPIST-1 planetary system is an excellent candidate for study of the evolution and habitability of M-dwarf planets. Transmission spectroscopy observations performed with the Hubble Space Telescope (HST) suggest the innermost five planets do not possess clear hydrogen atmospheres. Here we reassess these conclusions with recently updated mass constraints and expand the analysis to include limits on metallicity, cloud top pressure, and the strength of haze scattering. We connect recent laboratory results of particle size and production rate for exoplanet hazes to a one-dimensional atmospheric model for TRAPPIST-1 transmission spectra. Doing so, we obtain a physically-based estimate of haze scattering cross sections. We find haze scattering cross sections on the order of 1e-26 to 1e-19 cm squared are needed in hydrogen-rich atmospheres for TRAPPIST-1 d, e, and f to match the HST data. For TRAPPIST-1 g, we cannot rule out a clear hydrogen-rich atmosphere. We also modeled the effects an opaque cloud deck and substantial heavy element content have on the transmission spectra. We determine that hydrogen-rich atmospheres with high altitude clouds, at pressures of 12mbar and lower, are consistent with the HST observations for TRAPPIST-1 d and e. For TRAPPIST-1 f and g, we cannot rule out clear hydrogen-rich cases to high confidence. We demonstrate that metallicities of at least 60xsolar with tropospheric (0.1 bar) clouds agree with observations. Additionally, we provide estimates of the precision necessary for future observations to disentangle degeneracies in cloud top pressure and metallicity. Our results suggest secondary, volatile-rich atmospheres for the outer TRAPPIST-1 planets d, e, and f.