  A large population of planetary candidates in short-period orbits have been
found through transit searches. Radial velocity surveys have also revealed
several Jupiter-mass planets with highly eccentric orbits. Measurements of the
Rossiter-McLaughlin effect indicate some misaligned planetary systems. This
diversity could be induced by post-formation dynamical processes such as
planet-planet scattering, the Kozai effect, or secular chaos which brings
planets to the vicinity of their host stars. In this work, we propose a novel
mechanism to form close-in super-Earths and Neptune-like planets through the
tidal disruption of giant planets as a consequence of these dynamical
processes. We model the core-envelope structure of giant planets with composite
polytropes. Using three-dimensional hydrodynamical simulations of close
encounters between planets and their host stars, we find that the presence of a
core with a mass more than ten Earth masses can significantly increase the
fraction of envelope which remains bound to it. After the encounter, planets
with cores are more likely to be retained by their host stars in contrast with
previous studies which suggested that coreless planets are often ejected. As a
substantial fraction of their gaseous envelopes is preferentially lost while
the dense incompressible cores retain most of their original mass, the
resulting metallicity of the surviving planets is increased. Our results
suggest that some gas giant planets can be effectively transformed into either
super-Earths or Neptune-like planets after multiple close stellar passages.
Finally, we analyze the orbits and structure of known planets and Kepler
candidates and find that our model is capable producing some of the
shortest-period objects.
