The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI). III. Simulated Observables—the Return of the Spectrum

Abstract

The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) is a community project that aims to quantify how differences in general circulation models (GCMs) could impact the climate prediction for TRAPPIST-1e and, subsequently, its atmospheric characterization in transit. Four GCMs have participated in THAI: ExoCAM, LMD-Generic, ROCKE-3D, and the UM. This paper, focused on the simulated observations, is the third part of a trilogy, following the analysis of two land planet scenarios (Part I) and two aquaplanet scenarios (Part II). Here we show a robust agreement between the simulated spectra and the number of transits estimated to detect the land planet atmospheres. For the cloudy aquaplanet ones, a 5σ detection of CO 2 could be achieved in about 10 transits if the atmosphere contains at least 1 bar of CO 2 . That number can vary by 41%-56% depending on the GCM used to predict the terminator profiles, principally due to differences in the cloud deck altitude, with ExoCAM and LMD-G producing higher clouds than ROCKE-3D and UM. Therefore, for the first time, this work provides "GCM uncertainty error bars" of ∼50% that need to be considered in future analyses of transmission spectra. We also analyzed the intertransit spectral variability. Its magnitude differs significantly between the GCMs, but its impact on the transmission spectra is within the measurement uncertainties. THAI has demonstrated the importance of model intercomparison for exoplanets and also paved the way for a larger project to develop an intercomparison meta-framework, namely, the Climates Using Interactive Suites of Intercomparisons Nested for Exoplanet Studies.

The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI). II. Moist Cases—The Two Waterworlds

“…While our simulations support May et al's (2021) finding, they provide a multimodel estimate and may prove useful if a similar planet is found closer to Earth or if TRAPPIST-1e is probed by better instruments (Fauchez et al 2022). As discussed in Section 3.1.4 and 3.2.4, ExoCAM has the highest amplitude of cloud variability among the THAI models, and this corresponds to the highest variability in the atmospheric transit depth relative to other GCMs, as shown for different wavelengths of the transmission spectrum in Paper III (Fauchez et al 2022). The periodicity of ExoCAM's cloud variability is approximately 12.5 orbits, which is comparable to the minimum number of transits required to detect CO 2 features and thus may need to be taken into account if TRAPPIST-1e is observed during successive transits.…”

“…This variability will most likely stay below the noise level of JWST's NIRSpec PRISM instrument and not affect retrieved abundances (May et al 2021). While our simulations support May et al's (2021) finding, they provide a multimodel estimate and may prove useful if a similar planet is found closer to Earth or if TRAPPIST-1e is probed by better instruments (Fauchez et al 2022). As discussed in Section 3.1.4 and 3.2.4, ExoCAM has the highest amplitude of cloud variability among the THAI models, and this corresponds to the highest variability in the atmospheric transit depth relative to other GCMs, as shown for different wavelengths of the transmission spectrum in Paper III (Fauchez et al 2022).…”

“…Globally averaged, LMD-G is also consistently less cloudy, both in terms of mean cloud area fraction and column-integrated cloud content. Note that this is not the case at the terminators (for further discussion, see Part III; Fauchez et al 2022). The cloudiest Hab 1 and Hab 2 climates are predicted by ROCKE-3D, which simulates TRAPPIST-1e enveloped by clouds with a high concentration of cloud water at the substellar point.…”

The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI). I. Dry Cases—The Fellowship of the GCMs

“…Transit spectra (computed in THAI part 3; see Fauchez et al 2022) are shown to be sensitive to atmospheric pressures down to 1 and 1 × 10 −2 Pa for the Ben 1 and Ben 2 cases, respectively, in the core of the 4.3 μm CO 2 band (Fauchez et al 2022), and considering the typical spectral resolution of the instruments (R ∼ 10 2 -10 3 ) on board JWST. At the typical spectral resolution of ground-based spectrographs (R ∼ 10 5 ), transit spectra are found to be sensitive to atmospheric pressures down to 1 × 1 −3 Pa and 1 × 10 −7 Pa for the Ben 1 and Ben 2 cases, respectively, in the core of CO 2 lines (of the 4.3 μm CO 2 band).…”

“…We leave the question of why weather systems seem to be more pronounced on ExoCAM than other GCMs for future studies. We also recall that the level of temporal variability of the different models is far below the detectability threshold of JWST and the astronomical observatories of the coming decades (part 3; Fauchez et al 2022).…”