Phase and transit curves of non-spherical planets

We are witness to a great and increasing interest in internal structure, composition and evolution of exoplanets (Schneider, 2017). However, direct measurements of exoplanetary mass and radius are insufficient to distinguish between different internal structure and composition models, justifying the need for an additional observable (Zeng et al., 2016; Hatzes & Rauer, 2015). Close-in objects with typical orbital periods less than ten days undergo rotational distortions and strong tidal disruptions coming from their host star, modifying their outer shape from spherical to more complicated ones. We assume both components in hydrostatic equilibrium, hence behaving as a fluid, critical condition to interpret the shape in terms of internal structure. The surficial distortions depend on the body’s internal structure, and may be expressed through the fluid Love number  (Kopal, 1959; Love, 1911), providing additional information on the internal structure (Kellermann et al., 2018). As a result, the stellar eclipsed area or planetary reflecting area will differ, changing the corresponding transit and phase curves. We present a new model that computes transit and phase curves of exoplanets beyond the mass-point approximation (i.e. Roche model (Wilson & Devinney, 1971) or ellipsoidal models (Maxted, 2016)). We discuss detectability of non-sphericity effects in transit curves for hot Jupiters, Neptunes, and super-Earths in the light of dedicated space missions (e.g. Kepler, TESS, JWST, PLATO). Eventually, we show that phase curves are also promising for distinguishing internal structures with PLATO, e.g. between super Earths and mini-Neptunes.

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