Can Uranus and Neptune form concurrently via pebble, gas, and planetesimal accretion?

ABSTRACT The origin of Uranus and Neptune has long been challenging to explain, due to the large orbital distances from the Sun. After a planetary embryo has been formed, the main accretion processes are likely pebble, gas, and planetesimal accretion. Previous studies of Uranus and Neptune formation typically do not consider all three processes; and furthermore, do not investigate how the formation of the outer planet impacts the inner planet. In this paper, we study the concurrent formation of Uranus and Neptune via pebble, gas, and planetesimal accretion. We use a dust-evolution model to predict the size and mass flux of pebbles, and derive our own fit for gas accretion. We do not include migration, but consider a wide range of formation locations between 12 and $40\, \textrm {au}$. If the planetary embryos form at the same time and with the same mass, our formation model with an evolving dust population is unable to produce Uranus and Neptune analogues. This is because the mass difference between the planets and the H–He mass fractions become too high. However, if the outer planetary embryo forms earlier and/or more massive than the inner embryo, the two planets do form in a few instances when the disc is metal-rich and dissipates after a few Myr. Furthermore, our study suggests that in situ formation is rather unlikely. Nevertheless, giant impacts and/or migration could potentially aid in the formation, and future studies including these processes could bring us one step closer to understanding how Uranus and Neptune formed.