Atmospheric escape from the TRAPPIST-1 planets and implications for habitability

Abstract

The presence of an atmosphere over sufficiently long timescales is widely perceived as one of the most prominent criteria associated with planetary surface habitability. We address the crucial question of whether the seven Earth-sized planets transiting the recently discovered ultracool dwarf star TRAPPIST-1 are capable of retaining their atmospheres. To this effect, we carry out numerical simulations to characterize the stellar wind of TRAPPIST-1 and the atmospheric ion escape rates for all of the seven planets. We also estimate the escape rates analytically and demonstrate that they are in good agreement with the numerical results. We conclude that the outer planets of the TRAPPIST-1 system are capable of retaining their atmospheres over billion-year timescales. The consequences arising from our results are also explored in the context of abiogenesis, biodiversity, and searches for future exoplanets. In light of the many unknowns and assumptions involved, we recommend that these conclusions must be interpreted with due caution.

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

“…If the initial water content of a TRAPPIST-1 planet is as high as 0.1-1 wt%, they can retain a significant amount of water under a strong XUV radiation field of TRAPPIST-1 (Bolmont et al 2017;Bourrier et al 2017a). In addition, the TRAPPIST-1 planets can survive atmospheric ion escape (O + , O + 2 , and CO + 2 ) driven by a stellar wind over a few 100 Myr to ∼ Gyr (Dong et al 2018). Thus, a Venus-like atmosphere as well water vapor might be a plausible solution to the atmospheric compositions of the TRAPPIST-1 planets, which is favored by their flat and featureless transmission spectra.…”

“…Close-in planets undergo atmospheric loss by stellar X-ray and UV (XUV) radiation and injection of highenergy particles via a stellar wind and coronal mass ejection. Atmospheric loss from the TRAPPIST-1 planets was recently studied: water loss from TRAPPIST-1 planets by XUV irradiation (Bolmont et al 2017;Bourrier et al 2017a) and atmospheric ion escape from the TRAPPIST-1 planets via a stellar wind, assuming Venus-like atmospheres (Dong et al 2018). In this paper, we further estimate mass loss of a hydrogen-rich atmosphere from TRAPPIST-1-like planets (like those in the TRAPPIST-1 system) via the energy-limited hydrodynamic escape (e.g.…”

Global Ten‐Moment Multifluid Simulations of the Solar Wind Interaction with Mercury: From the Planetary Conducting Core to the Dynamic Magnetosphere

“…In recent years, our understanding of terrestrial bodies has been significantly advanced by increasingly sophisticated numerical models. A large number of global models based on either fluid or hybrid (kinetic ion particles and massless electron fluid) approach have been developed for both magnetized planets such as Mercury (e.g., Exner et al, 2018;Jia et al, 2015;Kabin et al, 2008;Kidder et al, 2008;Müller et al, 2012;Richer et al, 2012;Trávníček et al, 2010) and unmagnetized planets such as Mars (Dong et al, 2014(Dong et al, , 2015(Dong et al, , 2018a(Dong et al, , 2018bLedvina et al, 2017;Ma et al, 2014;Modolo et al, 2016) as well as exoplanets (Johansson et al, 2011;Dong et al, 2017aDong et al, , 2017bDong et al, , 2018cDong et al, , 2019. However, none of these global models can accurately treat collisionless magnetic reconnection due to their lack of detailed electron physics.…”

Where can a Trappist-1 planetary system be produced?

“…In particular the habitability of planets in this system is uncertain, even though they reside in the "habitable zone" (e.g. O'Malley-James & Kaltenegger 2017; Alberti et al 2017;Dong et al 2017;Bolmont et al 2017). It has also raised some interesting questions regarding the formation mechanism for such a system.…”