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Journal Articles Monthly Notices of the Royal Astronomical Society Year : 2022

Mapping the surface of partially cloudy exoplanets is hard

Nicholas Vieira
  • Function : Author
Elisa Jacquet
  • Function : Author
Juliette Geoffrion
  • Function : Author
Youssef Bestavros
  • Function : Author
Dylan Keating
Nicolas B Cowan


ABSTRACT Reflected light photometry of terrestrial exoplanets could reveal the presence of oceans and continents, hence placing direct constraints on the current and long-term habitability of these worlds. Inferring the albedo map of a planet from its observed light curve is challenging because different maps may yield indistinguishable light curves. This degeneracy is aggravated by changing clouds. It has previously been suggested that disc-integrated photometry spanning multiple days could be combined to obtain a cloud-free surface map of an exoplanet. We demonstrate this technique as part of a Bayesian retrieval by simultaneously fitting for the fixed surface map of a planet and the time-variable overlying clouds. We test this approach on synthetic data and then apply it to real disc-integrated observations of the Earth. We find that 8 d of continuous synthetic observations are sufficient to reconstruct a faithful low-resolution surface albedo map, without needing to make assumptions about cloud physics. For light curves with negligible photometric uncertainties, the minimal top-of-atmosphere albedo at a location is a good estimate of its surface albedo. When applied to observations from the Earth Polychromating Imaging Camera aboard the Deep Space Climate Observatory spacecraft, our approach removes only a small fraction of clouds. We attribute this difficulty to the full-phase geometry of observations combined with the short correlation length for Earth clouds. For exoplanets with Earth-like climatology, it may be hard to do much better than a cloud-averaged map. We surmise that cloud removal will be most successful for exoplanets imaged near quarter phase that harbour large cloud systems.

Dates and versions

obspm-04414290 , version 1 (24-01-2024)



Lucas Teinturier, Nicholas Vieira, Elisa Jacquet, Juliette Geoffrion, Youssef Bestavros, et al.. Mapping the surface of partially cloudy exoplanets is hard. Monthly Notices of the Royal Astronomical Society, In press, 511 (1), pp.440-447. ⟨10.1093/mnras/stac030⟩. ⟨obspm-04414290⟩
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