Vol. 554
In section 10. Planets and planetary systems

Static compression of porous dust aggregates

Understanding the structure and growth of ice particules in circumstellar disks is key in analyzing observations of these disks and inferring the consequences for planet formation. Grains had been considered spherical and compact, with a density equal to that of ice, i.e. about 1 g/cm3, until recent work has pointed out that growth tends to form fluffy, very fluffy aggregates with densities as low as 0.00001 g/cm3 and planetesimal sizes. However, no such objects have been observed today, and it is thus critical to understand how we can transform fluffy aggregates into (relatively) dense planetesimals. The work by Kataoka et al. is a first step in that direction: Using numerical experiments they derive a relation between the pressure that is applied to the aggregates (e.g., due to gas drag in the disk) and their filling factor (i.e., their physical density). They show that the filling factor is equal to the size of the monomers forming the aggregates multiplied by the cube root of the ratio of the pressure that is applied to the roll energy of the monomers. This relation will be crucial for understanding the history of the evolution of grains and planetesimals in disks.