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 ( 1-3 ) \times 10 ^ { -3 } M _ { \sun } , 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 .