Do the TRAPPIST-1 Planets Have Hydrogen-rich Atmospheres?

TL;DR

This study investigates the atmospheric compositions of TRAPPIST-1 planets, concluding that they likely lack primordial hydrogen-rich atmospheres due to accretion limits and atmospheric escape, implying secondary atmospheres are more probable.

Contribution

It combines formation modeling and atmospheric evolution simulations to show TRAPPIST-1 planets probably do not have primordial hydrogen atmospheres, contrasting with previous assumptions.

Findings

Accreted hydrogen is minimal, less than 1 wt% for most planets.

Hydrodynamic escape can remove all primordial hydrogen atmospheres.

Planets likely have secondary atmospheres like Venus or water vapor.

Abstract

Recently, transmission spectroscopy in the atmospheres of the TRAPPIST-1 planets revealed flat and featureless absorption spectra, which rule out cloud-free hydrogen-dominated atmospheres. Earth-sized planets orbiting TRAPPIST-1 likely have either a clear or a cloudy/hazy hydrogen-poor atmosphere. In this paper, we investigate whether a proposed formation scenario is consistent with expected atmospheric compositions of the TRAPPIST-1 planets. We examine the amount of a hydrogen-rich gas that TRAPPIST-1-like planets accreted from the ambient disk until disk dispersal. Since TRAPPIST-1 planets are trapped into a resonant chain, we simulate disk gas accretion onto a migrating TRAPPIST-1-like planet. We find that the amount of an accreted hydrogen-rich gas is as small as 10$^{-2}$ wt% and 0.1 wt% for TRAPPIST-1b and 1c, 10$^{-2}$ wt% for 1d, 1 wt% for 1e, a few wt% for 1f and 1g and 1 wt% for 1h, respectively. We also calculate a long-term thermal evolution of TRAPPIST-1-like planets after disk dissipation and estimate the mass loss of their hydrogen-rich atmospheres driven by a stellar X-ray and UV irradiation. We find that all the accreted hydrogen-rich atmospheres can be lost via hydrodynamic escape. Therefore, we conclude that TRAPPIST-1 planets should have no primordial hydrogen-rich gases but secondary atmospheres such as a Venus-like one and water vapor, if they currently retain atmospheres.