Asymmetry and Variability in the Transmission Spectra of Tidally Locked Habitable Planets

Abstract

Spatial heterogeneity and temporal variability are general features in planetary weather and climate, due to the effects of planetary rotation, uneven stellar flux distribution, fluid motion instability, etc. In this study, we investigate the asymmetry and variability in the transmission spectra of 1:1 spin–orbit tidally locked (or called synchronously rotating) planets around low-mass stars. We find that for rapidly rotating planets, the transit atmospheric thickness of the evening terminator (east of the substellar region) is significantly larger than that of the morning terminator (west of the substellar region). The asymmetry is mainly related to the spatial heterogeneity in ice clouds, as the contributions of liquid clouds and water vapor are smaller. The underlying mechanism is that there are always more ice clouds on the evening terminator, due to the combined effect of coupled Rossby–Kelvin waves and equatorial superrotation that advect vapor and clouds to the east, especially at high levels of the atmosphere. For slowly rotating planets, the asymmetry reverses (the morning terminator has a larger transmission depth than the evening terminator), but the magnitude is small or even negligible. For both rapidly and slowly rotating planets, there is strong variability in the transmission spectra. The asymmetry signal is nearly impossible to be observed by the James Webb Space Telescope (JWST), because the magnitude of the asymmetry (about 10 ppm) is smaller than the instrumental noise and the high variability further increases the challenge.

Bistability of the atmospheric circulation on TRAPPIST-1e

“…In the SJ regime, the terminator differences (eastern minus western) are overall mostly positive, which is similar to the fast-rotating simulation in Song & Yang (2021), but have a much lower magnitude of about 1-2 ppm -closer to the slow-rotating simulation in the same study. The east-west terminator difference has the same sign as that found for cloudy simulations of hot Jupiters, but the magnitude is several orders smaller (e.g.…”

“…For example, a recent study by Rustamkulov et al (2022) reports 14 and 10 ppm noise floors with 3-σ and 1.7-σ confidence levels, respectively, for JWST's near infrared spectrograph (NIRSpec) instrument. The transmission depth differences in our study are even smaller than those simulated for a planet like TRAPPIST-1e by Song & Yang (2021). This is because ExoCAM used in that study tends to have higher cloud decks at the terminators compared to other commonly used GCMs, while the UM tends to have lower clouds (Fauchez et al 2021b).…”

“…The terminator asymmetry in the transmission spectra was found to be non-negligible in previous studies, both for hotter gas giants (e.g. Line & Parmentier 2016;Powell et al 2019) and colder rocky planets (Song & Yang 2021). Here we confirm that the circulation regime differences result in slightly different transmission spectra at the eastern and western terminators.…”

“…We do not extend the model top to lower pressures, as has been tested in e.g. Fauchez et al (2021b), as this is not required for a temperate climate with Earth-like temperatures and a low cloud deck (Suissa et al 2020;Song & Yang 2021).…”

ExoCAM: A 3D Climate Model for Exoplanet Atmospheres

“…ExoCAM has been well-received and well-used since being made public. ExoCAM, and its ancestral versions, have been employed for a wide variety of studies, including the Faint Young Sun Paradox for Earth (Wolf & Toon 2013, 2014a, high-CO 2 atmospheres (Wolf et al 2018;Zhang et al 2021), runaway and moist greenhouse thresholds for Earth (Wolf & Toon 2014b, Earth-like extrasolar planets (Wolf et al 2017;Adams et al 2019;Kang 2019aKang , 2019bKang , 2019c, habitable zone exoplanets around M-dwarf stars (Kopparapu et al 2017;Wolf 2017;Haqq-Misra et al 2018;Komacek et al , 2020aKomacek et al , 2020bWolf et al 2019;Yang et al 2019a;Hu et al 2020;Rushby et al 2020;Suissa et al 2020b;Wei et al 2020;Chen et al 2021;May et al 2021;Song & Yang 2021), planets in circumbinary systems (Wolf et al 2020), and model intercomparison studies (Yang et al 2016(Yang et al , 2019bFauchez et al 2020Turbet et al 2021). ExoCAM results have also been used off-line in projects to drive photochemistry and observability studies (Afrin Badhan et al 2019;Suissa et al 2020a).…”