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.

Highlights

  • More than 4,600 exoplanets have been discovered since the discovery of 51 Pegasi b in 1995 (Mayor and Queloz, 1995), with an ever-growing fraction of terrestrial planets that are smaller than Neptune and Uranus

  • We focus on two important aspects that were not seriously considered in previous studies: the asymmetry between the morning and evening terminators and the variability of the transmission spectra

  • We find that there is significant asymmetry in the transmission spectra between the morning and evening terminators, especially for rapidly rotating planets

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Summary

Introduction

More than 4,600 exoplanets have been discovered since the discovery of 51 Pegasi b in 1995 (Mayor and Queloz, 1995), with an ever-growing fraction of terrestrial planets that are smaller than Neptune and Uranus. Astronomers are at the stage of characterizing atmospheric compositions of rocky planets. Transmission spectrum, emission spectrum, and reflection spectrum are three of the main methods used for atmospheric characterizations (Seager and Sasselov, 2000; Hubbard et al, 2001; Charbonneau et al, 2002; Deming and Seager, 2017; Kaltenegger, 2017; Kreidberg, 2017; de Wit et al, 2018; Grimm et al, 2018; Schwieterman et al, 2018). Stellar light is absorbed and scattered by atmospheric species when traveling through the planetary limb; at wavelengths where the molecules in the atmosphere have higher opacity, the planet will block more light from the star, having a higher relative transit depth. The upcoming launch of telescopes such as the JWST may enable us to detect transmission spectra with higher resolution, larger spectral coverage, and longer observing duration, which will be a breakthrough in this area, especially for Earth-sized planets

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