Theoretical studies, laboratory experiments, and numerical simulations have shown several barriers in planetesimal formation, including radial drift, fragmentation, and bouncing problems. Also, observations of protoplanetary disks have shown the radial drift problems of millimeter-sized bodies at the outer orbital radius. The key to solving these problems is understanding porosity evolution. Dust grains become fluffy by coagulation in protoplanetary disks and this alters both the dynamic and

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... ical properties of dust aggregates. First, we revealed the overall porosity evolution from micron-sized dust grains to kilometer-sized planetesimals; dust grains form extremely porous dust aggregates, where the filling factor is 10 4 , and then they are compressed by their collisions, the disk gas, and their self-gravity. In the coagulation process, they circumvent all the barriers if the monomers are 0.1-m icy bodies. The mass and porosity of the final product are consistent with those of the comets, which are believed to be the remnants of planetesimals. We further performed coagulation simulations including porosity evolution. We found that planetesimals can form inside 10 AU, avoiding the radial drift barrier. Furthermore, we calculated the opacity evolution of porous dust aggregates and found that the observed radio emission could be explained either by compact dust grains or by fluffy dust aggregates. A. Kataoka ( )