  The first long-baseline ALMA campaign resolved the disk around the young star
HL Tau into a number of axisymmetric bright and dark rings. Despite the very
young age of HL Tau these structures have been interpreted as signatures for
the presence of (proto)planets. The ALMA images triggered numerous theoretical
studies based on disk-planet interactions, magnetically driven disk structures,
and grain evolution. Of special interest are the inner parts of disks, where
terrestrial planets are expected to form. However, the emission from these
regions in HL Tau turned out to be optically thick at all ALMA wavelengths,
preventing the derivation of surface density profiles and grain size
distributions. Here, we present the most sensitive images of HL Tau obtained to
date with the Karl G. Jansky Very Large Array at 7.0 mm wavelength with a
spatial resolution comparable to the ALMA images. At this long wavelength the
dust emission from HL Tau is optically thin, allowing a comprehensive study of
the inner disk. We obtain a total disk dust mass of 0.001 - 0.003 Msun,
depending on the assumed opacity and disk temperature. Our optically thin data
also indicate fast grain growth, fragmentation, and formation of dense clumps
in the inner densest parts of the disk. Our results suggest that the HL Tau
disk may be actually in a very early stage of planetary formation, with planets
not already formed in the gaps but in the process of future formation in the
bright rings.
