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tokens: studied the typical outcomes of close encounters between planets and orbit-crossing planetesimals. Applying their results to the fifth planet shows that it would scatter most of the orbit-crossing material on to bound orbits, thus supporting the fifth planet as the cause of the scattered disc. However, this planet’s mass of is much lower than the mass of the scattered population (). It is impossible that a much less massive planet scattered the much more massive planetesimal population. Read et al. ( 2018 ) furthermore found that planets with a mass in the range from to corresponding to either a semimajor axis from 115 to 130 au or from 140 to 160 au, with the most massive planets being nearest to or farthest away from the star, similarly well explained the observations. Although any planet with a mass in excess of would eject orbit-crossing material, a fifth planet capable of scattering of planetesimals is still possible. As the fifth planet may or may not be real, we shift our focus on to the four confirmed planets. Referring again to the work by Wyatt et al. ( 2017 ), one sees that the gas giants of HR 8799, each for itself, would typically eject their orbit-crossing material in unbound orbits. This material would be on its way out of the system, but, unless this is a continuous process, it should only operate for a relatively short amount of time. For this process to be continuous, a constant flux of material would need to cross the giant planet’s path, which is unlikely. Therefore, we do not consider scattering through the giant planets to be responsible for the scattered disc either. Migration is considered to have occurred in the formation history of HR 8799, as neither core accretion nor gravitational instability could have produced all four planets at their current location (Currie et al. 2011 ). Although we focus in the discussion on outward migration, a similar case can be made for inward migration. Using the formula of Ida et al. ( 2000 ) to calculate the migration rate in our primordial disc, we find a rate of a few 10 s of . Such a fast migration would allow the planets to quickly reach their current location. Taking into account that migration stops when the mass within a few Hill’s radii becomes less than the planet’s mass (Kirsh et al. 2009 ), we find the fully formed planets in HR 8799 to be too massive to have undergone migration within the protoplanetary disc. However, if the planets accreted their gas envelope while migrating or even after the migration ceased, then the core masses rather than planet masses would have to be considered. Migration rate of these, less massive, cores would be lower, and they could easily create a scattered disc of planetesimals instead of ejecting them out of the system. Planet–planet scattering may have also occurred early on in the history of the HR 8799 system. If the planet cores formed closer to the star by core accretion, such a scattering event could have relocated the planets, bringing