  In recent years, ALMA has been able to observe large-scale substructures
within protoplanetary disks. Comparison with the predictions from models of
planet-disk interaction has indicated that most of these disk substructures can
be explained by the presence of planets with the mass of Neptune or larger at
orbital radii of $\approx 5 - 100$ au. Better resolution is needed to observe
structures closer to the star, where terrestrial planets are expected to form,
as well as structures opened by planets with masses lower than Neptune. We
investigate the capabilities of a possible extension to ALMA that would double
the longest baseline lengths in the array to detect and resolve disk
substructures opened by Earth-mass and Super Earth planets at orbital radii of
$1-5$ au. By simulating observations of a family of disk models using this
extended configuration in ALMA Bands 6 and 7, we show that an upgraded ALMA
would detect gaps in disks formed by super-Earths as close as 1 au, as well as
Earth-mass planets down to $2-3$ au from the young host stars in nearby star
forming regions.
