Standard studies of the internal structure and evolution of stars, even when they include atomic diffusion, forget an important hydrodynamical process induced by local element accumulation or accretion of planetary matter, namely “fingering convection”. This is a double-diffusive instability similar to the “thermohaline convection” which occurs in the ocean. Computing this effect in stars needs precise computations of the radiative accelerations on each element, coupled with the induced hydrodynamical instabilities. Some results will be showed in the case of element accumulation in A type stars and in the case of planetary matter accretion on main sequence stars and white dwarfs. I will also present what could be the implications of these processes for the stellar parameters determinations needed to characterize explonets find by the TESS mission.