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

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.

10 pages, 3 figures, accepted for publication in ApJ

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Cited by in corpus (25)

• A review of possible planetary atmospheres in the TRAPPIST-1 system
• TRAPPIST-1: Global Results of the Spitzer Exploration Science Program {\it Red Worlds}
• The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI). Part III: Simulated Observables -- The return of the spectrum
• Predictions for Observable Atmospheres of Trappist-1 Planets from a Fully Coupled Atmosphere-Interior Evolution Model
• HST/WFC3 transmission spectroscopy of the cold rocky planet TRAPPIST-1h
• Breaking Resonant Chains: Destabilization of Resonant Planets due to Long-term Mass Evolution
• Near-infrared transmission spectrum of TRAPPIST-1 h using Hubble WFC3 G141 observations
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• Formation of aqua planets with water of nebular origin: Effects of water enrichment on the structure and mass of captured atmospheres of terrestrial planets
• Unified simulations of planetary formation and atmospheric evolution: Effects of pebble accretion, giant impacts, and stellar irradiations on super-Earth formation
• JWST-TST DREAMS: NIRSpec/PRISM Transmission Spectroscopy of the Habitable Zone Planet TRAPPIST-1 e
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• Size evolution of close-in super-Earths through giant impacts and photoevaporation
• Impact of M-dwarf Stellar Wind and Photoevaporation on the Atmospheric Evolution of Small Planets
• Rapid-then-slow migration reproduces mass distribution of TRAPPIST-1 system
• Phase Modeling of the TRAPPIST-1 Planetary Atmospheres
• Orbital Evolution of Close-in Super-Earths Driven by Atmospheric Escape
• sunset: A database of synthetic atmospheric-escape transmission spectra for nearly every transiting planet
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• Tidally Heated Sub-Neptunes, Refined Planetary Compositions, and Confirmation of a Third Planet in the TOI-1266 System
• Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions
• Disentangling Atmospheric Compositions of K2-18 b with Next Generation Facilities
• New models of reflection spectra for terrestrial exoplanets: Present and prebiotic Earth orbiting around stars of different spectral types
• Non-detection of Helium in the upper atmospheres of TRAPPIST-1b, e and f