A method for direct testing of hydrostatic equilibrium in exopanet interiors

The characterization of the interior of exoplanets will unveil precious information on their formation, structure and evolution. The Love numbers h2 and k2, which describe the planet's response to external perturbations, contain information on the radial density distribution and other interior parameters (e.g. viscosity). In the case of hydrostatic equilibrium, we simply have h2=1+k2. We summarize how one can measure h2 from transit observations and k2 from periastron precession in eccentric orbits, hence providing a direct test for hydrostatic equilibrium in exoplanet interiors