A giant solution to the disc mass budget problem of planet formation

The inferred dust masses from Class II protoplanetary disc observations are smaller or equal to the observed exoplanet systems and this has ignited a mass budget problem discussion. Planet formation is directly linked to the birthing environment that protoplanetary discs provide. The disc properties determine if a giant planet forms and how it evolves. We perform numerical simulations of planet formation via pebble and gas accretion, including migration, in a viscously evolving protoplanetary disc, investigating the most favorable conditions for giant planet formation, while tracing the dust mass evolution simultaneously. As expected, the presence of a giant planet in the disc can significantly influence the evolution of the disc itself and prevent rapid dust mass loss by trapping the dust outside its orbit. We find that early planet formation is crucial to forming a giant, along with a high initial disc mass. Larger disc radii ensure a pebble flux for a long time, which is beneficial for growing the cores of giant planets. However, smaller discs with the same mass can allow more efficient gas accretion onto already-formed planetary cores, due to the larger amount of available gas, leading to more massive gas giants. Our findings strengthen the hypothesis that planet formation has already happened or is ongoing in Class II discs. Most importantly, we find that the optically thin dust mass significantly underestimates the total dust mass in the presence of a giant planet and could be the answer to the hypothetical mass budget problem.