Core growth as function of time, for a system with orbital distances taken from the current orbits of the known exoplanets in HR8799. In order to comfortably form the critical core at the widest separation of 68 AU, we increased the disc mass by a factor three compared to the solar system case depicted in Fig. 3. The outer two cores reach the critical core mass before reaching pebble isolation (see also Sect. 3.2).