Investigating the future potential of an upgraded ALMA to image planet-forming disks at sub-au scales

The discovery of substructures in proto-planetary disks is arguably one of the most striking achievements of ALMA so far. In particular, high-angular resolution observations of nearby young disks have established that their sub-millimeter continuum emission shows rings, gaps, spirals and other asymmetric structures on spatial scales down to the resolution limit of the observations, i.e. > 5 astronomical units. These recent ALMA observations have spawned a variety of different theoretical investigations in the field of planet formation and interaction with the parental disk that are changing our understanding of how planets come to be. Whereas similar ALMA observations using the current 16 km longest baselines over the next few years will certainly find more of these structures at separations > 2 ? 3 au from the central star, longer baselines by factors of ~ 2 are necessary to resolve the Earth forming zone (~ 1 au) in the dust continuum of nearby disks (~ 150 pc) at wavelengths shorter than 1mm. The main purpose of this Study is to quantify the potential of an upgraded ALMA, with improved angular resolution and continuum sensitivity, to detect the substructures expected in planet-forming disks with sub-au resolution at wavelengths shorter than 1mm, and with ~ 1 au resolution at wavelengths longer than 1mm. Our results can be used as examples in the upcoming ALMA Integrated Science Team report on the ?Extended Baselines for ALMA Science Case?. The proposed study could have high impact also on the scientific community of planet formation and exoplanets, as it would quantify the potential of a future upgraded ALMA to open the door to the observational characterization of Earth-like planets in the act of forming, including their interaction with the natal disk.