The role of water in planet formation

Planet formation takes place in gaseous disks around young stars. Exactly how small dust particles in such protoplanetary disks turn into rocky planets and the cores of gas giants is not known. In particular, the formation of planetesimals (kilometer-sized objects akin to asteroids, considered to be an important intermediate step in the planet formation process) is still highly elusive. A promising mechanism for planetesimal formation is the streaming instability. Under certain conditions, dense filaments of pebbles (solid particles that are aerodynamically partly decoupled from the gas disk) can emerge as a result of this instability. Clumps of pebbles may subsequently collapse under their own gravity to form planetesimals.
In this thesis, the growth and dynamics of dust particles and the viability of streaming instability being triggered close to the water snowline — the distance from the star beyond which water freezes out — are explored with numerical models. Additionally, a reported ALMA observation of the water snowline around an outbursting star is discussed in light of these models.
Finally, a model that connects the two main stages of planet formation, from dust to planetesimals and from planetesimals to planets, is developed. This model is applied to the formation of the TRAPPIST-1 system; a compact system with seven Earth-sized planets that are expected to have moderate water fractions of a few mass percent. It is shown that local planetesimal formation at the water snowline followed by inward migration of protoplanets naturally leads to such planets.