Formation of terrestrial planet cores inside giant planet embryos

Giant planet embryos are believed to be spawned by gravitational instability in massive extended (R similar to 100 au) protostellar discs. In a recent paper, we have shown that dust can sediment inside the embryos, as argued earlier by Boss in a slightly different model. Here we study numerically the next stage of this process - the formation of a solid core. If conditions are conducive to solid core formation, the centre of the gas cloud goes through the following sequence of phases: (i) becomes grain (and metal) rich; (ii) forms a terrestrial mass solid core via a rapid collapse driven by self-gravity of the grains; (iii) starts to accrete a gaseous atmosphere when the solid core reaches mass of a few to 10 Earth masses. This sequence of events may build either terrestrial planet cores or metal-rich giant planets inside the larger gas reservoir of the giant planet embryo. In a companion letter we argue that tidal and irradiation effects from the parent star should disrupt the outer metal-poor layers of the embryo, releasing nearly 'ready to use' planets. We propose this as an alternative way to build planets.