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The masses and radii of extrasolar planets are key observables forunderstanding their interior, formation and evolution. While transit photometryand Doppler spectroscopy are used to measure the radii and masses respectivelyof planets relative to those of their host star, estimates for the true valuesof these quantities rely on theoretical models of the host star which are knownto suffer from systematic differences with observations. When a system iscomposed of more than two bodies, extra information is contained in the transitphotometry and radial velocity data. Velocity information (finitespeed-of-light, Doppler) is needed to break the Newtonian $MR^{-3}$ degeneracy.We performed a photodynamical modelling of the two-planet transiting systemKepler-117 using all photometric and spectroscopic data available. Wedemonstrate how absolute masses and radii of single-star planetary systems canbe obtained without resorting to stellar models. Limited by the precision ofavailable radial velocities (38 $ms^{-1}$), we achieve accuracies of 20 percent in the radii and 70 per cent in the masses, while simulated 1 $ms^{-1}$precision radial velocities lower these to 1 per cent for the radii and 2 percent for the masses. Since transiting multi-planet systems are common, thistechnique can be used to measure precisely the mass and radius of a largesample of stars and planets. We anticipate these measurements will becomecommon when the TESS and PLATO mission provide high-precision light curves of alarge sample of bright stars. These determinations will improve our knowledgeabout stars and planets, and provide strong constraints on theoretical models.

Absolute masses and radii determination in multiplanetary systems without stellar models