On the survival of the long-lived inner disk of PDS 70

PDS 70 remains as the best laboratory to investigate the influence of giant planet formation on the structure of the parental disk. One of the most intriguing discoveries is the detection of a resolved inner disk from ALMA observations, extending up to the orbit of PDS 70b. This inner disk is challenging to explain because most of the dust particles are expected to be trapped at the outer edge of the gap open by PDS 70b and PDS 70c. By performing dust evolution models and radiative transfer simulations that match the gas disk masses obtained from recent thermochemical models of PDS 70, we find that when the minimum grain size in the models is larger than 0. 1m, there is an efficient filtration of dust particles, and the inner disk is depleted during the first million-year of dust evolution. Therefore, to maintain an inner disk, the minimum grain size in the models needs to be smaller than 0. 1m. Only when grains are that small, they are diffused and dragged along with the gas throughout the planets' gap. The small grains transported in the inner disk grow and drift therein, but the constant reservoir of dust particles that are trapped in the outer edge of the gap and that are continuously fragmenting allows refilling the inner disk on million-year timescales. Our flux predictions at millimetre wavelength of these models agree with ALMA observations. These models predict a spectral index of 3. 2 in the outer disk and 3. 6 in the inner disk. Our simple analytical calculations show what the inner disk water emission recently observed with JWST may originate from these ice-coated small grains that flow through the gap, grow and drift towards the innermost disk regions, reaching the water snowline. These models may mirror the history and evolution of our Solar System where Jupiter and Saturn played a crucial role shaping the architecture and properties of planets in our Solar System.