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

Time evolution of the atmospheric mass fraction (top) and semi-major axis (bottom) of a non-migrating (solid curves) and migrating planetary core with mass of 0.3 M⊕ (left: TRAPPIST-1 d [red], 1 h [blue]), 0.7 M⊕ (middle: 1 e), and 1 M⊕ (right: 1 b [red], 1 c [blue], 1 f [green], and 1 g [black]). A planetary core starts to migrate from 0.1 au (dashed ones) or 0.2 au (dash-dotted ones). The locations of a non-migrating planetary core adopt semi-major axes similar to those of the TRAPPIST-1 planets. Mass loss from an accreting planet driven by a stellar XUV irradiation is not included.

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.

Yasunori Hori, Masahiro Ogihara
(Submitted on 12 Dec 2019)

Comments: 10 pages, 3 figures, accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1912.05749 [astro-ph.EP] (or arXiv:1912.05749v1 [astro-ph.EP] for this version)
Submission history
From: Yasunori Hori
[v1] Thu, 12 Dec 2019 03:21:07 UTC (2,070 KB)
Astrobiology, Astrochemsitry

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