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


Transmission spectra and their variability of tidally locked planets, from 0.6 to 5 µm. The left y axis is the transit atmospheric thickness (km) and the right y axis is the relative transit depth (ppm). (a), (c) and (e): results for planets with a rotation period of 6 Earth days (= orbital period); (b), (d) and (f): for planets with a rotation period of 60 Earth days. In (a) and (b), the blue line is for the evening terminator and the red line is for the morning terminator, and the areas shaded with pale blue and pale red show their variability. In (c) and (d), the blue line is for the average value of the asymmetry (evening minus morning), and the areas shaded with pale blue show their variability. In (e) and (f), the contributions of air temperature, water vapor, liquid clouds, and ice clouds to the asymmetry (evening minus morning) are shown in red, blue, grey, and yellow, respectively, and their variability are shaded with the corresponding pale colors. The surface pressure is 1 bar, including only N2 and H2O. Note that the summary of the four lines in the lower panel (such as (e)) is not exactly equal to the difference between the blue line and the red line in the upper panel (such as (a)); this is mainly due to the overlaps among water vapor, liquid clouds, and ice clouds.

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 on 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.

Xinyi Song, Jun Yang

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2107.14603 [astro-ph.EP] (or arXiv:2107.14603v1 [astro-ph.EP] for this version)
Submission history
From: Xinyi Song
[v1] Fri, 30 Jul 2021 12:57:25 UTC (2,032 KB)
https://arxiv.org/abs/2107.14603
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