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

TL;DR

This study investigates the simultaneous formation of Uranus and Neptune through pebble, gas, and planetesimal accretion, highlighting the importance of formation timing and initial conditions for their development.

Contribution

It introduces a comprehensive model considering all three accretion processes and explores how formation timing and initial mass affect the planets' development.

Findings

Concurrent formation is challenging with equal initial conditions.

Outer embryo forming earlier or more massive favors successful formation.

In-situ formation is unlikely without additional processes.

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 don't 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 both 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 40au. 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 disk is metal-rich and dissipates after a few Myr. Furthermore, our study suggests that in-situ formation is rather unlikely. Nethertheless, 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.