In planetesimal formation theory, several barriers have been proposed, which are
bouncing, fragmentation, and radial drift problems. To solve these problems,
understanding the porosity evolution of dust aggregates is a key in planetesimal
formation. 

In this work, we investigate static compression of porous dust aggregates in
protoplanetary disks. We show that dust grains coagulate to form fluffy dust aggregates,
and then they are compressed by static compression due to ram pressure of the disk gas
and self gravity of the aggregates. The mass and porosity of the final products are
consistent with comets, which are believed to be remnants of planetesimals. In this way,
we reveal the overall porosity evolution from dust grains via fluffy aggregates to form
planetesimals. Moreover, we also show that the icy fluffy aggregates circumvent the
three barriers in planetesimal formation. In addition, we also investigate the opacity
of the fluffy dust aggregates. We show that the opacity is characterized by the product
of the grain/aggregate radius and the filling factor. This suggests that sub-mm wave
observations of protoplanetary disks can be interpreted by thermal emission of fluffy
dust aggregates.