Could Uranus and Neptune form by collisions of planetary embryos?

TL;DR

This study uses 3D hydrodynamical simulations to evaluate whether Uranus and Neptune could have formed from collisions of planetary embryos, finding some consistency but also significant discrepancies with observed planetary properties.

Contribution

The paper revisits the planetary embryo collision scenario for Uranus and Neptune formation using detailed simulations, assessing the validity of the perfect-merging assumption and planetary property outcomes.

Findings

Simulations generally support the perfect-merging assumption for mass and obliquity.

Formed planets tend to rotate faster than actual Uranus and Neptune.

Most simulated planets have massive disks and high rotation speeds, challenging the formation scenario.

Abstract

The origin of Uranus and Neptune remains a challenge for planet formation models. A potential explanation is that the planets formed from a population of a few planetary embryos with masses of a few Earth masses which formed beyond Saturn's orbit and migrated inwards. These embryos can collide and merge to form Uranus and Neptune. In this work we revisit this formation scenario and study the outcomes of such collisions using 3D hydrodynamical simulations. We investigate under what conditions the perfect-merging assumption is appropriate, and infer the planets' final masses, obliquities and rotation periods, as well as the presence of proto-satellite disks. We find that the total bound mass and obliquities of the planets formed in our simulations generally agree with N-body simulations therefore validating the perfect-merging assumption. The inferred obliquities, however, are typically different from those of Uranus and Neptune, and can be roughly matched only in a few cases. In addition, we find that in most cases the planets formed in this scenario rotate faster than Uranus and Neptune, close to break-up speed, and have massive disks. We therefore conclude that forming Uranus and Neptune in this scenario is challenging, and further research is required. We suggest that future planet formation models should aim to explain the various physical properties of the planets such as their masses, compositions, obliquities, rotation rates and satellite systems.