Metal loading of giant gas planets

One of many challenges in forming giant gas planets via gravitational disc instability model is an inefficient radiative cooling of the pre-collapse fragments. Since fragment contraction times are as long as 105–107 yr, the fragments may be tidally destroyed sooner than they contract onto gas giant planets. Here, we explore the role of ‘pebble accretion’ the pre-collapse giant planets and find an unexpected result. Despite larger dust opacity at higher metallicities, addition of metals actually accelerates – rather than slows down – collapse of high-opacity, relatively low mass giant gas planets (Mp ≲ a few Jupiter masses). A simple analytical theory that explains this result exactly in idealized simplified cases is presented. The theory shows that planets with the central temperature in the range of 1000 ≲ Tc ≲ 2000 K are especially sensitive to pebble accretion: addition of just ∼5 to 10 per cent of metals by weight is sufficient to cause their collapse. These results show that dust grain physics and dynamics are essential for an accurate modelling of self-gravitating disc fragments and their near environments in the outer massive and cold protoplanetary discs.