When a star is born, it is surrounded by a disc composed of gas and dust. It is in this gas and dust that the planets form: it's called a protoplanetary disc. The dust will agglomerate to form larger blocks of solid material, called planetesimals. These planetesimals are a few kilometers in size and will collide and agglomerate to create the cores of planets. If the planet is massive enough, then it will start to attract gas around it. The planet enters then a phase of slow gas accretion. Gas and solids are accreted until the mass of the solid core and the mass of the gaseous atmosphere reach similar masses. At this moment, the atmosphere collapses on itself, allowing more gas to flow in the planet: the planet enters a rapid phase of gas accretion called runaway gas accretion.
If we represent the protoplanetary disc as seen from the edge (focusing on one side of the star), where the gas is represented in blue and the solids in light orange, each step will look like this until the dissipation of the gas:


My work consists in studying each step individually under different circumstances, or how the steps can be linked together. For example, in my first first project, I studied how gas accretion can influence the protoplanetary disc characteristics on a larger scale, assuming that the planetary core is already formed and that the planet just entered the last phase of runaway growth.