TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI): motivations and protocol version 1.0

Abstract

Upcoming telescopes such as the James Webb Space Telescope (JWST), the European Extremely Large Telescope (E-ELT), the Thirty Meter Telescope (TMT) or the Giant Magellan Telescope (GMT) may soon be able to characterize, through transmission, emission or reflection spectroscopy, the atmospheres of rocky exoplanets orbiting nearby M dwarfs. One of the most promising candidates is the late M dwarf system TRAPPIST-1 which has seven known transiting planets for which Transit Timing Variation (TTV) measurements suggest that they are terrestrial in nature, with a possible enrichment in volatiles.Among these seven planets, TRAPPIST-1e seems to be the most promising candidate to have habitable surface conditions, receiving ∼ 66% of the Earth's incident radiation, and thus needing only modest greenhouse gas inventories to raise surface temperatures to allow surface liquid water to exist. TRAPPIST-1e is therefore one of the prime targets for JWST atmospheric characterization. In this context, the modeling of its potential atmosphere is an essential step prior to observation. Global Climate Models (GCMs) offer the most detailed way to simulate planetary atmospheres. However, intrinsic differences exist between GCMs which can lead to different climate prediction and thus observability of gas and/or cloud features in transmission and thermal emission spectra. Such differences should preferably be known prior to observations. In this paper we present a protocol to inter-compare planetary GCMs. Four testing cases are considered for TRAPPIST-1e but the methodology is applicable to other rocky exoplanets in the Habitable Zone. The four test cases included two land planets composed with 1 a modern Earth and pure CO 2 atmospheres, respectively, and two aqua planets with the same atmospheric compositions.Currently, there are four participating models (LMDG, ROCKE-3D, ExoCAM, UM), however this protocol is intended to let other teams participate as well.

TRAPPIST-1: Global results of theSpitzerExploration Science Program Red Worlds

“…In this section, we do not consider stellar contamination and only discuss potential detections of atmospheric features of the TRAPPIST-1 planets. Here, we limit our discussion to include only the cases of TRAPPIST-1b, c, e and g because (a) b and c have the smallest periods -that is to say the most transits and therefore the greatest precision on measurements, (b) planet e is arguably the most promising candidate for habitability, for the reasons given in Wolf (2017Wolf ( , 2018, Turbet et al (2018) and Fauchez et al (2019a), and (c) planet g was the most observed with HST/WFC3 (Wakeford et al 2018).…”

“…(22)) is satisfied. We chose planets b and e because e is the planet that is the most likely to harbor liquid water on its surface (Wolf 2017(Wolf , 2018Turbet et al 2018;Fauchez et al 2019a) and b is the closest to the host star. We note that a planet could lie in the abiogenesis zone while not being in the classical habitable zone (Rimmer et al 2018), yet, by choosing to study planet e we maximize the similarities with Earth.…”

“…In short, if the habitable zone planets originally had primordial H/He envelopes, XUV evaporation may have rendered the planets habitable Owen & Mohanty 2016). Three planets orbit within the habitable zone of the star (Gillon et al 2017a), and planet e is the most likely to harbour liquid water on its surface (Wolf 2017(Wolf , 2018Turbet et al 2018;Fauchez et al 2019a).…”

A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System

“…Note presently underway is the TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) project (Fauchez et al. 2020b ), where multiple 3-D GCMs will be compared for habitable iterations of TRAPPIST-1e. This is illustrated in Fig.…”

“…The figure was taken from Fauchez et al. ( 2020b ) Planet h is further out where the classical CO 2 runaway collapse is supposed to take place. To reach temperate conditions, in addition to a very thick CO 2 atmosphere, the planet would need to retain enough volcanic gases such as CH 4 or H 2 in order to block CO 2 infrared windows and prevent the collapse of the atmosphere (Pierrehumbert and Gaidos 2011 ; Wordsworth et al.…”