Detectability of shape deformation in short-period exoplanets

Abstract

Context. Short-period planets suffer from extreme tidal forces from their parent stars. These forces deform the planets causing them to attain non-spherical shapes. The non-spherical shapes, modeled here as triaxial ellipsoids, can have an impact on the observed transit light-curves and the parameters derived for these planets. Aims. We investigate the detectability of tidal deformation in short-period planets from their transit light curves and the instrumental precision needed. We also aim to show how detecting planet deformation allows us to obtain an observational estimate of the second fluid Love number from the light curve which gives valuable information about the planet's internal structure. Methods. We adopted a model to calculate the shape of a planet due to the external potentials acting on it and used this model to modify the ellc transit tool. We used the modified ellc to generate the transit light curve for a deformed planet. Our model is parameterized by the Love number, hence for a given light curve we can derive the value of the Love number that best matches the observations. Results. We simulated the known cases of WASP-103b and WASP-121b which are expected to be highly deformed. Our analyses showed that instrumental precision ≤ 50 ppm/min is required to reliably estimate the Love number and detect tidal deformation. This precision can be achieved for WASP-103b in ∼40 transits using the Hubble Space Telescope and in ∼300 transits using the forthcoming CHEOPS instrument. However, fewer transits will be required for short-period planets that may be found around bright stars in the TESS and PLATO survey missions. The unprecedented precisions expected from PLATO and JWST can permit the detection of shape deformation with a single transit observation. However, the effects of instrumental and astrophysical noise must be well-considered as they can increase the number of transits required to reach the 50 ppm/min detection limit. We also show that improper modeling of limb darkening can act to bury signals related to shape of the planet thereby leading us to infer sphericity for a deformed planet. Therefore accurate determination of the limb darkening coefficients is required to confirm planet deformation.

Machine-learning Inference of the Interior Structure of Low-mass Exoplanets

“…The deformed shape of a transiting extrasolar planet will modify its transit light-curve, introducing a secondorder effect which can in principle be observed (Akinsanmi et al 2019;Hellard et al 2019). Similarly, for some particular configurations (e.g., a close-in planet with a distant companion, Mardling 2007), absidal precession is mainly driven by the tidal interaction of the planet with its host star (Batygin et al 2009), and a value of the Love number of the planet can be inferred .…”

The CHEOPS mission

“…Planets close to their host star are tidally distorted into an ellipsoidal shape. Potentially, this effect is detectable in the transit light curve which would allow the measurement of the planet's Love number providing further insight into the planet's internal structure [1,18]. The main challenge is to separate ellipsoidal deformation from stellar limb darkening effects.…”

“…The CHEOPS (CHaracterising ExOPlanet Satellite) 1 proposal was submitted in response to the call by a Consortium of research institutes located in ESA member states. It was subsequently selected by ESA's Space Programme Committee in November 2012 out of a total of 26 competing submissions.…”

High-precision photometry with Ariel

“…During the transit, the elongated shape of the planet can also be observed, as deviations from the circular planet silhouette. However, the shape of the actual deviations are different [5,32].…”