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A remarkable discovery of NASA's Kepler mission is the wide diversity in theaverage densities of planets of similar mass. After gas disk dissipation, fullyformed planets could interact with nearby planetesimals from a remnantplanetesimal disk. These interactions would often lead to planetesimalaccretion due to the relatively high ratio between the planet size and the hillradius for typical planets. We present calculations using the open-sourcestellar evolution toolkit MESA (Modules for Experiments in StellarAstrophysics) modified to include the deposition of planetesimals into the H/Heenvelopes of sub-Neptunes (~1-20 MEarth). We show that planetesimal accretioncan alter the mass-radius isochrones for these planets. The same initial planetas a result of the same total accreted planetesimal mass can have up to ~5%difference in mean densities several Gyr after the last accretion due toinherent stochasticity of the accretion process. During the phase of rapidaccretion these differences are more dramatic. The additional energy depositionfrom the accreted planetesimals increase the ratio between the planet's radiusto that of the core during rapid accretion, which in turn leads to enhancedloss of atmospheric mass. As a result, the same initial planet can end up withvery different envelope mass fractions. These differences manifest asdifferences in mean densities long after accretion stops. These effects areparticularly important for planets initially less massive than ~10 MEarth andwith envelope mass fraction less than ~10%, thought to be the most common typeof planets discovered by Kepler.

Effects of Planetesimal Accretion on the Thermal and Structural Evolution of Sub-Neptunes