Inferring asymmetric limb cloudiness on exoplanets from transit light curves

Abstract

Context. Clouds have been shown to be present in many exoplanetary atmospheres. Cloud formation modeling predicts considerable inhomogeneities of cloud cover, consistent with optical phase curve observations. However, optical phase curves cannot resolve some existing degeneracies between cloud location and cloud optical properties. Aims. We present a conceptually simple technique for detecting inhomogeneous cloud cover on exoplanets. Such an inhomogeneous cloud cover produces an asymmetric primary transit of the planet in front of the host star. Asymmetric transits produce characteristic residuals that are different from standard symmetric models. Furthermore, bisector spans can be used to determine asymmetries in the transit light curve. Methods. We apply a model of asymmetric transits to the light curves of HAT-P-7b, Kepler-7b, and HD 209458b and search for possible cloud signatures. The nearly uninterrupted Kepler photometry is particularly well suited for this method since it allows for a very high time resolution. Results. We do not find any statistically sound cloud signature in the data of the considered planets. For HAT-P-7b, a tentative detection of an asymmetric cloud cover is found, consistent with analysis of the optical phase curve. Based on Bayesian probability arguments, a symmetric model with an offset in the transit ephemeris is still the most viable model. This work demonstrates that for suitable targets, namely low-gravity planets around bright stars, the method can be used to constrain cloud cover characteristics and is thus a helpful additional tool for the study of exoplanetary atmospheres.

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

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Equatorial retrograde flow in WASP-43b elicited by deep wind jets?

“…However, transmission spectra contain not only information about the morning but also from the warmer evening terminator and the polar regions. Disentangling the effect of a cloudy morning terminator from the evening terminator has been difficult so far (Line & Parmentier 2016;von Paris et al 2016). Also, the observations to date do not yield a consistent picture for cloud coverage over the terminator.…”

Aerosols in Exoplanet Atmospheres

“…In addition to composition, JWST's ability to continuously observe targets at high time cadence, as opposed to Hubble's staccato way of observing brought on by its orbit around the Earth, will allow us to probe the east and west limbs separately. This will be critical for deciphering the 3D distribution of aerosols and how asymmetric limbs impact transmission spectra (Fortney et al, 2010;Kempton et al, 2017;Line & Parmentier, 2016;Powell et al, 2018Powell et al, , 2019von Paris et al, 2016).…”