Constraints on planetary tidal dissipation from a detailed study of Kepler 91b

Context. With the detection of thousands of exoplanets, characterising their dynamical evolution in detail represents a key step in the understanding of their formation. Studying the dissipation of tides occurring both in the host star and in the planets is of great relevance in order to investigate the distribution of the angular momentum occurring among the objects populating the system and to studying the evolution of the orbital parameters. From a theoretical point of view, the dissipation of tides throughout a body may be studied by relying on the so-called phase or time-lag equilibrium tides model in which the reduced tidal quality factor Q≲p, or equivalently the product between the love number and the time lag (k2,pδtp), describe how efficiently tides are dissipated within the perturbed body. Constraining these factors by looking at the current configuration of the exoplanetary system is extremely challenging, and simulations accounting for the evolution of the system as a whole might help to shed some light on the mechanisms governing this process. Aims. We aim to constrain the tidal dissipation factors of hot-Jupiter-like planets by studying the orbital evolution of Kepler-91b. Methods. We firstly carried out a detailed asteroseismc characterisation of Kepler-91 and computed a dedicated stellar model using both classical and astereoseismic constraints. We then coupled the evolution of the star to the one of the planets by means of our orbital evolution code and studied the evolution of the system by accounting for tides dissipated both in the planet and in the host star. Results. We found that the maximum value for k2,pδtp (or equivalently the minimum value for Q≲p) determining the efficiency of equilibrium tides dissipation occurring within Kepler-91b is 0.4 ± 0.25 s (4.5-1.5+5.8×105). We constrained these factors by computing the evolution of the planetary orbit and by reproducing the current properties of the Kepler-91 system. Conclusions. We developed a new method to constrain the tidal dissipation factors using the observed eccentricity of a given planet. Our new approach showed that Kepler 91b has dissipation coefficients compatible with colder Jupiter-like planets. When applied to other targets, our new method could potentially give more precise boundary values to the tidal dissipation factors, and determine whether planetary tides dominate the dissipation during the stellar main sequence.