3.1 Forming giant planets via core accretion
Once the mass of a planetary core reaches a few Earth masses, it starts to build up a gas envelope. This process can lead to a variety of outcomes because, as seen in Activity 5, the speed at which a core grows and its final mass depend on where in the disc it is found.
Figure 6(a) shows that for Jupiter-like gas giants to form, the core needs to reach a critical mass, high enough that the gas envelope cannot maintain hydrostatic equilibrium and start contracting. (Usually, Mcrit is in the approximate range 5 – 20 Earth masses.) Exceeding the critical mass triggers a phase of rapid accretion, which continues until either the gas is dispersed or the planet opens a gap in the disc and the rate of gas accretion slows down. (In the core-accretion scenario, the gas-dispersal timescale is one of the factors that governs the lifespan of the disc and hence the final mass of gas giants.)
Figure 6(b) shows that if the core grows in a region of the disc where accretion is slower than in (a), there will be less gas in the vicinity of the planetary core by the time the critical mass is reached. This will typically occur further out than (a). Therefore, the final mass of the gas envelope will be smaller than it is for gas giants, and the resulting planet will be a core-dominated ice giant, like Uranus and Neptune.
Finally, Figure 6(c) shows that if the timescale for the core growth is much smaller than the gas-dispersal timescale, or if the core grows much closer to the star than gas or ice giants (where very little solid material is available) the planet will only develop a thin hydrogen-dominated atmosphere, like that of the primordial Earth.
Many observed protoplanetary discs show gaps, bright rings, asymmetries, spirals and other structures. Figure 7 provides a stunning example of the variety of configurations that can arise from the interactions of forming planets with the disc.
